OPTICAL PROBING OF LASER GENERATED SHOCK LOADED SAMPLES

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OPTICAL PROBING OF LASER GENERATED SHOCK LOADED SAMPLES

A. Campillo, A. Huston, B. Justus, C. Merritt

To cite this version:

A. Campillo, A. Huston, B. Justus, C. Merritt. OPTICAL PROBING OF LASER GENERATED SHOCK LOADED SAMPLES. Journal de Physique Colloques, 1987, 48 (C4), pp.C4-145-C4-162.

�10.1051/jphyscol:1987410�. �jpa-00226642�

JOURNAL DE PHYSIQUE

Colloque C4, supplement au n"9, Tome 48, septembre 1987

OPTICAL PROBING OF LASER GENERATED SHOCK LOADED SAMF'LES

A.J. CAMPILLO, A.L. HUSTON, B.L. JUSTUS and C.D. MERRIlT

Optical Sciences Division, U.S. Naval Research Laboratory, Washington, D.C. 20375, U.S.A.

Rbsum6.- Afin de depasser les limitations du t'ir isole dans les procedes classiques de gen6rations de chocs tels que les fusils a gaz ou les detonations explosives, nous avons etudiit les propri6ti.s des ondes de compression cr66es par laser dans le but d'examiner la possibilite de sonder simultanement 116tat mol6culaire de la matiere en utilisant un second faisceau laser. Deux proc6d6s de chocs par laser sont d6crits ici : (1) creation d'ondes de compression au moyen de plasmas confin6s produits par laser, et (2) creation d'ondes de compression en utilisant des feuilles lamin6es acceler6es par laser. Nos progrbs exp6rimentaux r6alises $i ce jour, nous encouragent A penser qu'une telle approche est grandement prometteuse pour les etudes chimiques et physiques des chocs en phase condensee. Les propriet6s fluorescentes de plusieurs mol6cules telles que longueurs d'onde et temps d'emission, sont affectbes par la pression locale, la temperature, la viscositi., la symktrie cristalline et la nature chimique des mol&cules avoisinantes. Plusieurs experiences ont kt6 effectuees pour 6tudier la fluorescence provoqu6e par l'addition de traces de colorants A des liquides charges par choc. Pour chaque type de colorant ou de solvant choisi pour notre etude, des estimations correlatives de pression de choc, de temperature ou de viscosite pourraient &tre relev6es.

Outre la mesure des parametres dlQtat, il est important d'identifier ce qui se passe au niveau mol6culaire. Dans cet article, on passe rapidement en revue les spectroscopies par laser disponibles adaptees ti

l'identification des esp&ce_s en temps reel (fluorescence, absorption.

processus Raman spontan6s et coh6rents). On les met en comp6tition en fonction de leurs avantages et inconvenients respectifs.

Abstract.- In an effort to bypass the single shot limitations of conventional shock generation schemes such as gas guns or explosive detonation, we have been studying the properties of laser-driven compressional waves to assess the feasibility of simultaneously probing the molecular state of matter using a second laser beam. Two laser-driven shock schenies will be described : (1) generation of compressional waves by confined laser created plasmas and (2) by laser-accelerated flyer plates.

Our experimental progress to date encourages us to believe that this approach has considerable promise in condensed phase shock chemistry and physics studies. The fluorescence properties of many molecules, such as wavelength and emission time are affected by such quantities as local pressure, temperature, viscosity, crystal symmetry, and the chemical nature of nearby molecules. Several experiments have been performed to study the fluorescence from trace quantities of dyes added to shock loaded liquids. Depending on the dye and solvent chosen for study, estimates of the shock pressure, temperature or viscosity could be obtained. Besides measuring state parameters, it is important to identify events occurring at the molecular level. In this paper, a brief survey of available laser spectroscopies, suitable for use in real time species identification, will be presented (fluorescence, absorption, spontaneous and coherent Raman processes). These will be contrasted as to their advantages and disadvantages.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987410

JOURNAL DE PHYSIQUE

I. INTRODUCTION

There are a number of severe experimental difficulties associated with understanding the initial details of pre-detonation chemistry. One limitation has been the very low data rate obtained during shock experiments in the 10 to 300 Kbar pressure range. Often shock experiments are performed a t a rate of one or less per day . Another limitation has been the relative lack of suitable experiments employing molecular level probes, such a s optical spectroscopies. In an effort to bypass the single shot limitations of conventional shock generation such as gas guns or explosive detonation, we have been studying the properties of laser-driven compressional waves in order to assess the feasibility of using such shocks while simultaneously probing the molecular state of matter using a second laser beam.

Driving shocks of this magnitude with a laser is attractive because it requires relatively modest table-top setups and affords higher experimental repetition rates ( 1 to 30 shots/hr.), permitting rapid material surveys. Because optical probing requires little sample area, modest diameter (mm) driver beams can be used. The central region of the compressional wave driven by a Gaussian beam is nearly planar. In the following sections, we describe two laser-driven shock generation schemes and the development of several fluorescent probe techniques that have been successfully employed to determine state parameters such a s pressure, viscosity and temperature. In section IV, we survey available spectroscopies that may be suitable for trace species identification under shock loading and discuss their limitations.

11. LASER-DRNEN COMPRESSIONAL SHOCKS

As a result of recent interest in inertial confinement fusion there has been extensive work reported .in the literature on laser-driven shock generation. In that application, the emphasis has been on generating shocks of immense pressure ( >> 100 Mbar ). In the pressure range of interest here, 10-300 kbar, a scaled down table top version of the experimental apparatus may be used. Most emphasis in previous work has involved studies of laser ablation processes of targets in vacuum. Because the temporal properties of such shocks are unsuitable for use in energetic materials research, we have been exploring two alternative geometries : (A) using confined laser heated plasmas and (B) using laser- accelerated flyer plates.

(A) CONFJNED LASER HEATED PLASMA GEOMETRY

Since 1970 there have been several important advances in understanding laser shock generation in confined geometries, largely through the efforts of

Anderholm(l1 , Yang (21 and Fairand, Clauer and associates [3,4]. The basic physics of this approach may be understood by viewing Figure 1, which shows a n experimental schematic of laser driver geometry and subsequent particle velocity measurement using interferometric probing 15.61. The shock generator consists simply of an absorbing foil (usually 40 to 250 microns of copper) sandwiched bewteen two windows. The laser driver pulse, shown entering from the left, could be derived from either a low energy (1-50 mJ) picosecond modelocked Nd:Glass laser or a moderate energy (1-5 J), 20 nsec duration Q-switched Nd:Glass laser.

The spatial profile of the driver beam was a smooth Gaussian and would typically

be focused to 0.5 mm diameter in the picosecond case or 3 rnm diameter in the

Q-switched case. Partial absorption of the driver pulse vaporized a portion of the

foil and created a hot plasma. As the trapped plasma attempted to expand, an

intense compressional wave was launched into and through the foil. The

propagating pressure wave set the material behind it into motion and this was

monitored using a Michelson interferometer. The rear window of the sample

CW GREEN Cu

LASER DRIVER

SCOPE

r l

FIGURE 1. Experimental schematic of picosecond laser shock generation and subsequent particle velocity measurement using interferometric probing.

holder had a mirrored surface at the interface with the foil and was an integral part of the interferometer. A single frequency argon ion laser was used to illuminate the interferometer and was focused to 200 microns diameter at the sample. As the interface began to move the output of the interferometer was amplitude modulated a s the reflected probe beam went in and out of phase with the beam from the reference arm. The modulated output was detected with a fast photodiode and the electrical waveform was digitized and stored. The sample assembly was mounted on an x-y stage to enable the foil to be moved transversely after each shot thereby bringing fresh material into focus without the necessity of realignment. Using this arrangement and the picosecond laser driver, over ten shots an hour were obtained. Using the Q-switched laser driver, typically 3 shots an hour were obtained.

A pressure prdile typical of a picosecond laser-driven shock at the rear of a 40 micron copper foil is shown in Figure 2. The 2 ns rlse time results from pressure

"ring-up" in a thin layer of fluid contacting the foil to the witness plate. The profile is also typical of that applied to samples described in Section 111. In those cases, the sample replaces the thin layer sf contact fluid shown here and the witness plate is replaced by a clear window. Peak pressures of 20 kbar were observed in the picosecond case and approximately 60 kbar observed in the Q- switched case.

The obsenrations are resonably consistent with a simple model in which the

laser-generated metal vapor/plasma is treated as an ideal gas expanding against

the confining foil and window [61. In the model, we assume that the plasma

expands a t the velocity, u = P/Z. This ideal gas is heated as a function of time by a

laser pulse of Gaussian temporal form. The subsequent behavior of the system is

determined by the equation of state of an ideal gas and the first law of

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FIGURE 2. Pressure profile of picosecond laser-driven shock a t rear of 40 micron thick copper foil. The 2-ns rise time results from pressure "ring-up" in a thin layer of fluid contacting the foil to the witness plate. TD

represents the delay time of probing pulses used for fluorescence experiments desctibed in Section 111.

thermodynamics. The peak pressure is given by

where 0 ^is the laser fluence in J/cm2, 'T the laser pulse duration, Z the shock impedance, f the fraction of driver laser energy absorbed by the plasma, and Y and

p are constants.

The model predicts that the peak pressure should vary as the 1/2 power with laser fluence, Q , as observed in the Q-awitched case. However, this differs from the linear relationship observed with picosecond excitation [5].

In a related experiment, the structure of the shock front generated by

propagating a multikilobar laser driven compressional ramp wave into water was

probed using laser reflectivity (71. Good agreement between the observed

reflection coefficient of the front and that calculated using w e Fresnel formula

implied that the spatial extent of the leading edge of the shobk was less than 20

PS. The time profile of the pressure wave resulting from the contined laser

geometry presents a problem when simultaneously applying sonie laser

spectroscopies. Many important spectroscopies require fairly long interaction lengths for detectable signals. The nonuniform pressure profile accross thick samples broaden spectra and reduces gain and the ability to see trace species. In order to bypass this difficulty, our group has also been exploring laser accelerated flyer plates as a means of generating shocks.

(B) LASER-ACCELERATED FLYER PLATE GEOMETRY

Flyer plate technology is used extensively in shock studies. When the shock impedance of the flyer is less than that of the target, a compressional pulse having a rectangular time profile is produced. Ripen et a1 [8,91 demonstrated that thin flyer plates could be accelerated using laser pulses to produce high pressure shocks. They have successfully applied a simple 'rocket' model to describe their experimental results. Sheffield et al [10,11] have successfully employed this approach to achieve velocities in excess of 2 km/sec and to initiate HNS and PETN explosives. Figure 3 shows the geometry currently used a t NRL. By

employing a Q-switched laser of 3 J and 20 nsec duration. 125 micron thick flyers of A1 have been accelerated to 1 km/sec. When impacting samples, rectangular pressure pulses of approximately 50 nsec duration are produced.

LASER ACCELERATED FLYER PLATES

OUR GEOMETRY: / ^{/ GASKET} \

I

SIMPLIFIED PICTURE:

F) ki ^PI "

t FLYER

FIGURE 3. Two-foil geometry used a t NRL for generating shocks via laser accelerated flyer plate method.

111. FLUORESCENT PROBES A. INTRODUCTION

Traditionally, shock experiments primarily yield shock and particle velocity

data from which pressure, specific volume and internal energy are inferred using

conservation relations. Direct measurement of the properties of shock loaded

materials has been difficult due to both the extreme conditions and the single shot

nature of shock experiments as well a s the fast response time required (<1

C4-150 JOURNAL DE PHYSIQUE

microsecond). For example, conventional experimental methods for the

measurement of temperature require fabrication of thermocouples or thermistors of less than one micron thickness to insure the requisite thermal response. This scaling down in physical size leads to mechanical and electrical complications that restrict accuracy. In this section, we describe several novel fluorescent probe techniques possessing nanosecond response that we have developed to measure the pressure, temperature and viscosity of shocked materials. We also describe fluorescent studies of shock moderated chemistry.

The fluorescent properties of many molecules are sensitive to the surroundings in which they are placed. Passage of a compressional shock wave changes the local environment thus causing changes in the fluorescent properties of selected probe molecules. Molecules can be selected whose fluorescent properties respond primarily to a change in viscosity or to temperature or to pressure. Our approach consists of adding probe molecules specific to a given property, in low concentration, to the liquid or solid whose properties are to be measured. In the following paragraphs, shocks of 1 to 20 kbar were generated using the low energy picosecond driven system described in section 11-B. Thin liquid samples (ca.

micron were sandwiched between a copper foil and output window a s shown in figure 4. Due to the thin sample, the compressional wave would ring up in 2 nanoseconds and had a pressure profile a s shown in figure 2. Consequently, the data reported in the following are more appropriately described a s "isentropic" or ring-up shock data. TD in this figure indicates the time the sample is interogated by a probing pulse. The resulting fluorescence could be examined using a

spectrograph/OMA system, a streak camera or a fast photodetector.

GREEN

FIGURE 4. Experimental arrangement used for

picosecond laser shock generation and optical probing. G:

glass window, Cu: copper foil, S: sample, W: sapphire window, P: prism, L: lens, FL: fluorescence, SC: streak camera, V: vidicon, OMA: optical multichannel analyzer.

B. TEMPERATURE

The shock temperature of water was measured [12] in a previously inaccessible pressure range (1-10 kbar) with superior sensitivity and time response by adding the fluorescent dye, fluorescein. Fluorescein shows a red shift in the absorption peak, a s well as band broadening with increasing temperature causing a n increase

in the wing absorption of a green probe pulse. This results in enhanced laser

induced fluorescence from the probe dye following singlet excitiation. The

magnitude of the fluorescence intensity provides a direct measure of the

temperature increase. This can often be quite dramatic. In our work, a 30 degree temperature rise resulted in a fourfold increase in observed fluorescence.

The shock-induced fluorescence enhancement data were corrected for pressure induced shifts in peak absorption wavelength and plotted in figure 5 in the form of temperature change above ambient, AT, versus effective pressure. Also shown for comparison are estimates of the single shock state temperature

increase of shocked water calculated by Rice and Walsh [I31 (dashed line). Their estimates were obtained by comparing the measured P-V Hugoniot and previously measured static pressure isotherms of Bridgman [14] and assuming the shock

SHOCK PRESSURE (kbar)

FIGURE 5, Measured shock-induced temperature rise in water vs shock pressure. Circles are single shot data- The

dashed line follows calculated values from Rice and Walsh 1131.

temperatures to be those of the isotherms at the P.V points of intersection with the Hugoniot. In general, single shock state temperatures are expected to be somewhat higher than ring-up shock temperatures for the same final pressure.

However, comparison of the two in the low pressure regime of these experiments seems justified. Our data agree with the calculated values of Rice and Walsh to within our experimental accuracy (about +4 C at 8 kbar). This preliminary accuracy, although superior to that available using other techniques in this pressure and temperature range, was greatly limited by our method of shock generation and is not indicative of the potentialaf this approach.

Adding dyes also has the advantage of fast response time [ca. nsecs). However, it has the disadvantages of requiring dye solvency in the system of interest and requiring recalibration with each solvent change. It is possible to incorporate a suitable fluorophor in a universal solid which may be added to the sytem of interest in the form of microparticles(for requisite thermal response) or interfaced to optical fibers for use a s a generalized temperature probe.

C. PRESSURE

The absorption and emission spectra of aromatic compounds such as

anthracene often exhibit red shifts of 2000-4000 wavenumbers for applied

C4-152 JOURNAL DE PHYSIQUE

pressures of up to 100 kbar while a t the same time exhibiting little sensitivity to thermal and other induced changes. We studied the fluorescence spectra of anthracene in benzene solution I151 at various shock pressures following excitation with 353 nm laser pulses. The shift in the large 0-1 vibronic peak versus pressure has been found to be nearly linear and equal to 65

wavenumbers/kbar over the range 0-30 kbar. By monitoring this shift, the

pressure can be tracked in real time with a time resolution of about ¹ nanosecond.

Figure 6 shows fluorescence spectra of anthracene in benzene solution a t ambient pressure and at 10 kbar.

WAVELENGTH (nm)

FIGURE 6. Fluorescence spectra of Anthracene in benzene solution a t ambient pressure (solid line) and of shocked solution at 10 kbar (dotted line).

D. VISCOSITY

The dye, crystal violet, was used to directly measure the viscosity of a shock loaded liquid (glycerol). The viscosity of the solution was estimated from streak camera measured fluorescence lifetimes following picosecond green laser excitation. Structurally, the crystal violet molecule has phenyl side groups which are free to rotate in low viscosity solvents. Torsional motions of these side groups are known to greatly affect the fluorescent lifetime of the excited state and are strongly affected by viscous drag which can be directly correlated with the macroscopic viscosity of the solution. Using this approach, it was found that the fluorescence lifetime varied between 100 psec at ambient pressure to 260 psec a t

19 kbar shock loading, corresponding to a viscosity change from 12 to 57 poise [16]. The log of the shock viscosity of glycerol is plotted in figure 7 versus pressure. Also included in the plot for comparison are hydrostatic isotherms obtained by Bridgman[ 171.

E. SHOCK MODERATED CHEMISTRY: DIFFUSION CONTROLLED QUENCHING IN SOLUTIONS AT KBAR PRESSURES

Using laser driven compressional shock loading of up to 20 kbar and

picosecond streak camera fluorometry, we have measured the change in the

bimolecular quenching rate constant for the diffusion controlled electron transfer

6 10 16 PRESSURE Ikbarl

FIGURE 7. The viscosity of glycerol as a function of pressure. Shown are isotherms at 30 and 75 C from Bridgman [17] as well a s the present shock results.

reaction occuring in a solution of photoexcited singlet state Rhodamine 590 and ground state iodide in isopropanol.

The quenching of a n electronically excited molecule M* by a quencher Q in a homogeneous solvent is a well known photochemical problem that has been studied extensively at ambient pressures. The quenching channel competing with the fluorescent decay channel (M* + M + hv) is typically a diffusion controlled reaction

where kr >> and (M----Q)* represents a short lived excited collisional complex. Turley and Offen [18] studied this process a t pressures up to 3 kbar using hydrostatic cells and found that the modified Debye expression

was applicable a t high pressures for several dyes. Equation 3 establishes a

connection between the diffusion rate, the Boltzman constant k, the temperature, T, and the viscosity. 'll . The purpose of this study is to test the applicability of equation 3 under conditions of intense compressional shock loading. Such tests under shock are important to understanding chemistry occuring under detonation where diffusion processes may limit the final reaction rates, as in the 'carbon diffusion problem'. In this experiment. the shock induces both temperature and viscosity changes which subsequently modify the kinetics of the photochemical reaction.

We observe that the fluorescence lifetime varies nearly linearly from a value of

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SHOCK PRESSURE (kbar)

FIGURE 8. Rate constant versus shock Dressure for the

- - . -

diffusionicontrolled electron transfer relaction between rhodamine 590 and iodide in isopropanol.

1.16 nsec a t ambient pressure to 1.53 nsec at 20 kbar [19]. From the pressure dependence of the measured fluorescence lifetimes we can determine kdiff in equatidn 3 (see Figure 8 ). This has enabled u s to extract values for T/ ll for shock loaded isopropanol and using previously published data we have been able to estimate both shock temperatures and viscosities as a function of pressure. The good agreement of temperature with independent estimates and the overall consistency of the results is a strong argument for the validity of equation 3 and for a n Einstein-Smoluchowski model of diffusion under shock loading.

IV _{SURVEY OF} OPTICAL SPECIXOSCOFY USAGE IN SHOCK CHEMISTRY

An important experimental problem in energetic materials science involves

determining the initial chemical reaction pathways and rates occuring under

condensed phase shock loading. The tremendous successes resulting from optical

spectroscopy usage in gas phase combustion processes has naturally led to the

hope that such molecular level probes could be employed in condensed phase

shock chemistry studies as well. Unfortunately, the difficulties in studying

condensed phase systems coupled to problems unique to shock generation have

resulted in rather slow progress to date. In this section, we survey available

optical and laser spectroscopies and contrast these as to their relative advantages and disadvantages. An earlier survey discussing Raman spectroscopies was published by Schmidt et al 1201.

To gain an appreciation of the difficulties, lets first consider how a typical shock experiment differs from optical studies in a gas flame. First, the shock is essentially a single shot event with temporal features occuring on a

submicrosecond time scale. As a result, many attractive cw techniques cannot be used and it is necessary to employ nanosecond or faster pulsed sources. Insuring that the resulting signal contains a detectable number of photons during the pulse interval usually requires that intense probe lasers be used. This in turn leads to difficulties from competing nonlinear optical processes such as stimulated Brilloiun, backward wave stimulated Raman, multiphoton absorption etc. which are dependent on high electric fields. Many spectroscopies developed for flame studies rely on the luxury of slowly tuning the probing laser wavelength during data collection. This isn't possible in shock studies and broad spectral probes must be used. Generating the broad intense probing pulse is often not

straightforward and usually results in limited bandwidth. In gases, the spectral signatures of various species are often narrow and easily identified. Consequently, when species A and B react to form C . it is often possible to follow two of these by virtue of their unique spectra. However, the high pressures and temperatures occuring in condensed phase shocks broaden and complicate the spectral features and often make them unidentifiable. Condensed phase studies also necessitate noncollinear geometries when using coherent schemes requiring phase matching.

Shocks often introduce index of refraction gradients as well as birefringence to further complicate optical probing. Because the shocks travel a t velocities of only a few microns per nanosecond, adequate temporal resolution requires high spatial resolution. For example, when performing coherent anti-Stokes Raman

spectroscopy, if the noncollinear geometry results in a beam overlap region lrnm thick in the direction of the propagating shock, then the rear portion of the probed region will have seen the shock arrive approximately 200 to 500 nsec before the front of the probed region. Note, that this sets the effective time resolution of the geometry even if picosecond duration pulses are used. This coupling of spatial and temporal resolutions further complicates the tradeoff between signal strength, bandwidth of observation and spatial and temporal resolution. Another serious problem concerns gases trapped in bubbles or voids.

When compressed, these yield background emissions that mask the desired signal.

Before discussing individual spectrocopies, we first summarize desirable features which must be present in suitable candidates: (a) the material must be transparent a t wavelengths of interest, (b) the specroscopy must be specific; i.e.

able to differentiate species ^A from B even when [BI >> [A1 (this usually implies narrow spectral features for species of interest), (c) the spectroscopy must be sensitive (able to detect minority species concentrations of a t about 1%). (d) the spectroscopy must be broadband to enable as much information to be obtained on one shot as is possible, and (e) the spectroscopy must be pulsed.

Table 1 compares a number of schemes. By far, the most sensitive scheme in practice is laser induced fluorescence. This was amply illustrated in several of the studies described in section 111. A principle drawback, however, is that many important energetic intermediate species are, unfortunately, poor fluorescers.

Therefore , use of this spectroscopy is rather limited. The broad spectral features of fluorescence are another drawback that works against discriminating one species from another. Because of the large signals associated with fluorescing species, rather thin samples may be used and high temporal resolutions achieved.

In table 1, the column labled spatial/temporal resolution lists typical spatial

resolutions and the corresponding highest temporal resolution that may be

achieved consistent with a n 'end-on' probe geometry. In the case of fluorescence

spectroscopy, it is possible, in principle, to achieve true picosecond resolution

when using a monolayer sample.

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TABLE 1 : OPTICAL SPECTROSCOPIES

...

SPATIAL/TEMPORAL

SCHEME RESOLUTION ADAVANTAGES DISADVANTAGES ---

LASER-INDUCED

FLUORESCENCE Extremely sensitive Many species are weak fluorescers.

ABSORPTION

< micron Simple.

10 psec Competition from thermal

Background.

Broad spectral features (Often lacks specificity).

Universal. Visible absorption usually requires thick sample.

microns Simple.

nsec Suitable pulsed broadband

IR source not readily available.

Majority species wing absorption usually greater than trace signal.

Heating and pressurizing sample complicates interpretation.

RADIOMETRY microns Yields temperature Limited information.

microsecs a t high pressures.

SPONTANEOUS Excellent specificity Weak effect.

RAMAN (Narrow spectral

200 microns features available Competition from fluore- 100 nsec over wide range). scence. blackbody emission

and stimulated processes.

Nearly universal for

molecular species. Requires thick samples.

RESONANCE Enhanced Raman Competition from self-

RAMAN 200 microns signal. fluorescence.

100 nsec Nearly universal.

RAMAN GAIN Good specificity Two light sources reqd.

100 microns Large signals Nonlinear signal.

50 nsec Nearly universal. Competition from majority species.

No phase matching.

Competing nonlinear Directed signal effects.

beam.

TABLE 1 : OPTICAL SPECTROSCOPIES (Cont .)

...

SPATIAL/TEMPORAL

SCHEME RESOLUTION ADVANTAGES DISADVANTAGES

INVERSE RAMAN Good specificity. Appears as small signal on noisy broadband probe.

200 microns Nearly universal.

Thick samples required.

100 nsec No phase matching.

Two light sources reqd.

Directed signal beam.

Competing nonlinear effects.

COHERENT Nearly universal. Two light sources reqd.

ANTI-STOKES

1 mm Good sensitivity. Phase matching reqd.

(noncollinear geometry) 500 nsec Good specificity.

Limited bandwidth.

Directed signal beam.

Nonresonant background.

Interpretation problem.

PHASE CONJUGATE Nearly universal. Phase matching reqd.

RANIAN (noncolinear geometry)

1 rnm Good sensitivity

Two light sources reqd.

500 nsec Good specificity.

Directed signal.

Broader bandwidth than CARS

RAMAN INDUCED (Similar advantages Sensitive to shock induced KERR EFFECT to above two). birefringence (appears

1 mm impractical).

500 nsec Competition from optical

Kerr effect.

OTHER NONLINEAR --- Complicated.

Competition from other nonlinear processes.

Irnpracff cal.

C4-158 JQURNAL DE PHYSIQUE

Absorption is a simple universal spectroscopy. It is universal because all species absorb light. The effect is strong and linear. Unfortunately, in the uv/vis ible region, absorption bands are rather broad, leading to specificity problems. TO date, Duvall's group a t Washington State University have, nevertheless, obtained promising results on carbon disulfide and on nitromethane using this approach

[21]. Ideally, the IR ^is the best molecular fingerprint region. Unfortunately, there currently does not exist a convenient pulsed broadband IR source for such studies.

A nonlinear optical IR generation scheme called TRISP [22] has been applied to shock absorption studies with rather limited results[23l. This scheme, although promising, is technologically sophisticated and laser equipment intensive. In addition, further development work is required to access additional bands in the mid-IR.

Radiometry, of course, has been used extensively in the past. The primary utilization has been in temperature estimates. When coupled with infrared detectors, it appears that temperatures as low a s 300 C may be measured. The primary drawback of this scheme is that the information obtained using it is rather limited.

Raman schemes 1201 constitute a class of promising spectroscopies for use in shock studies. Vibrational modes in molecules may be infrared active. Raman active or both. Consequently, Raman spectroscopies provide information complimenting infrared spectroscopy. In the case of spontaneous Raman spectroscopy, a fvred frequency visible laser may be used to inelastically scatter light off these modes into new frequencies which contain information a s to the identity and quantity of molecular species present. In this process, a pump photon at frequency VL is inelastically scattered off a virtual state to generate a lower frequency v s Stokes wave and excite a n available Raman active mode at the difference frequency, v, = VL - v s . Using a simple visible spectrograph/OMA combination we have been able to view the region between 700 and 3500 wavenumbers with a s i g ~ a l to noise of 30:l in benzene under light shock conditions. In principle, it should be possible to obtain higher signal to noise ratios. To obtain complimentary information in the IR, broad band light between 3 and 14 microns would have been needed. As a result, Raman spectroscopy is highly attractive. Unfortunately, spontaneous Rarnan is also a very weak effect .

High peak power pump beams are required. The danger exists of competition from other nonlinear processes; in particular, backward wave stimulated Raman and stimulated Brflloiun. In addition, a t higher shock intensities there should be sigfiificant competition from black body radiation. Interesting work on measuring both the temperature and chemistry using this scheme is ongoing[24-261.

Raman signals may be enhanced significantly by positioning the virtual state near a n existing electronic transition. This is called Resonance Raman. A

disadvantage is that in order to utilize this approach short wavelength excitation is used and this often results in a competing fluorescence background.

Furthermore, use of uv light may photolyze the sample and introduce spurious chemistry.

In Raman gain, a pump beam sufficiently intense to generate significant gain a t

the Stokes wavelengths is used. If a broadband probe having wavelengths in this

range is mixed with the pump, then those frequencies corresponding to Stokes

wavelengths will be amplified. The spectrum of the broadband probe will have

superimposed several sharp Rarnan features. If the gain is too high, one of the

frequencies will grow to strengths comparable to the pump. This case and those

involving growth of the Stokes from noise, are called stimulated Raman scattering

(SRS). The disadvantages of forward and backward SRS [27] is that the frequency

of the intense Stokes wave corresponds to the strongest ground state majority

species and very little new information is conveyed. Raman gain, on the other

hand has the potential to interogate other modes. Unfortunately, the signal

intensity is nonlinear and one is normally forced to operate dangerously close to

the SRS threshold.

Inverse Raman spectroscopy is energetically quite similar to Raman Gain, except that the intense laser pulse is applied instead at the Stokes frequency and the broadband probe is applied at shorter wavelengths. Like Raman gain, this coherent process is self phase matching and does not require a particular geometry. In inverse Raman, the broad band probe is observed to have dark frequency regions corresponding to a Raman induced absorption process. The signal to noise ratio of this process is usually limited by variations in the spectral content of the broad band probe. Most broad band generation schemes yield such inherent noise and in this process the signal from a minority species would therefore appear a s a small signal on a noisy background. Interesting feasibility studies using this scheme have been reported by Von Holle [28].

Coherent Anti-Stokes Raman spectroscopy [29] is a four wave process in which an intense laser pulse and Stokes pulse from a tunable dye laser are mixed in a sample to generate a coherent vibrational wave. A third wave, usually from the original laser is scattered off the coherent vibration to generate a new wave a t frequency, 2vL - v s = vm . This scheme has many attractive features. The anti- Stokes beam is usually a strong directed beam at a frequency higher than those used to generate it. Hence, it is usually very easy to detect and free of background fluorescence. In shock studies, a broad band version is used in which the stokes wave is replaced with a braodband probe. Phase matching necessitates

noncollinear geometry in condensed phase systems. This results in fairly large interaction regions which limits the achievable temporal and spatial resolutions.

Phase matching also limits the usable bandwidth of the probe. There is also a nonreasonant background signal due to the majority species that often limits sensitivity to minority species to about 1 %. A final disadvantage is that the resulting spectrum is often not a s easily interpreted a s . for example, spontaneous Rarnan. The particular choice of any of the frequencies may lead to fortuitous and unexpected resonances. The group of Schmidt and Moore and coworkers a t Los Alamos [27,30,31] have explored this scheme and report promising results.

Phase Conjugate Raman spectroscopy is a four wave process that was originally suggested by Saha and Hellwarth 1321. This scheme has not been applied to shocked systems as of this date but L. Goldberg of NRL has conducted feasibility studies that show that it is a n attractive candidate. In his studies, Goldberg introduced a n intense green beam at 3 degrees to a broad band probe beam in liquid Benzene. Because Benzene is Raman active at the difference frequency, several moving coherent vibrational phase gratings were generated . ^{A second}

green beam was brought into the sample from a direction opposite to the broadband beam. This beam is coherently scattered into several red frequencies and directed 180 degrees to the first green beam. Although this is also

noncollinear process, it has greatly relaxed frequency and angular requirements.

In particular, Goldberg observed a bandwidth of 600 cm-1 in benzene under the same conditions that using CARS, he previously observed a 200 cm-1 bandwidth.

In addition, the phase conjugate scheme is free from a nonresonant background signal under certain polarizations. In addition, the phase conjugate nature of the process should compensate somewhat for refractively distorted shocked media.

Other nonlinear schemes do not at this time appear to be attractive for use in shock studies. Schmidt et al 1301 explored the feasibility of using Raman induced Kerr effect spectroscopy but this scheme appeared to be especially sensitive to shock induced birefringence. Chemiluminescence or the light emitted a s a result of a chemical reaction may yield useful data [33] on systems of interest but suffers from background blackbody emissions.

In conclusion, it is safe to state that no single optical spectroscopy appears entirely suitable as a universal probe of shocked energetic materials chemistry.

On the other hand, depending on the material to be studied and the precise

conditions encountered, several spectroscopies appear to offer considerable

promise. These include spontaneous Raman, absorption and fluorescence

spectroscopy. Over all, coherent anti-stokes Raman spectroscopy and phase

conjugate Raman show the greatest promise for species identification.

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REFERENCES 1. N. C. Anderholm, Appl. Phys. Lett., 16, 113 (1970) 2. L. C. Yang, J. Appl. Phys., 45, 2601 (1974)

3. B.P. Fairand, B.A. Wilcox, W.J. Gallagher a n d D.N. Williams, J. Appl. Phys., 43,

3 8 9 3 (1972)

4. B.P. Fairand a n d A.H. Clauer, J. Appl. Phys., 50, 1497 (1979) 5. P.E. Schoen a n d A.J. Campillo, Appl. Phys. Lett., 45, 1051 (1984)

6. R.D. Griffen, B.L. J u s t u s , A.J. Campillo a n d L.S. Goldberg, J. Appl. Phys., 59,

1 9 6 8 (1986)

7. A.J. Campillo, R.D. Griffin a n d P.E. Schoen, Optics Comm., z, 301 (1986) 8. B.H. Ripin, R. Decoste, S.P. Obenschain, S.E. Bodner, E.A. McLean, F.C. Young,

R.R. Whitlock, C.M. Armstrong, J. Grun, J.A. Stamper, S.H. Gold, D.J. Nagel, R.H. Lemberg a n d J.M. McMahon, Phys. Fluids, 23. 1012 (1980)

9. M.D. Rosen, D.W. Phillion, R.H. Price, E.M. Campbell, S.P. Obenschain, R.R.

Whitlock, E.A. McLean a n d B.H. Ripen, i n Shock Waves in Condensed Matter- 1983, J.R. Asay, R.A. Graham a n d G.K. Straub, eds. (North-Holland,

Amsterdam 1984). p 3 2 3

10. S.A. Sheffield a n d G.A. Fisk, in Shock Waves in Condensed Matter-1983, J.R.

Asay, R.A. Graham a n d G.K. Staub, eds (North-Holland, Amsterdam 1984) p 2 4 3

11. S.A. Sheffield, J.W. Rogers, Jr. and J.N. Castaneda, i n Shock Waues in

Condensed Matter -1985, Y.M. Gupta, ed. (Plenum, New York, 1986) p. 541 12. B.L. J u s t u s , A.L. Huston a n d A.J. Campillo, Appl. Phys. Lett., 47, 1 1 5 9 (1985) 13. M.H. Rice a n d J.M. Walsh, J. Chem. Phys.. 26, 8 2 4 (1957)

14. P.W. Bridgman, J. Chem. Phys., 3, 5 9 7 (1935)

15. A.L. Huston, B.L. J u s t u s a n d A.J. Campillo, Chem. Phys. Lett., 118, 2 6 7 (1985) 16. A.L. Huston, B.L. J u s t u s a n d A.J. Campillo, Chem. Phys. Lett., 122, 6 1 7 (1985) 17. P.W. Bridgman, Proc. Am. Acad. Arts Sci., 61. 57 (1926)

18. W.D. Turley a n d H.W. Offen, J. Phys. Chem., 88, 3605 (1984)

19. B.L. Justus, A.L. Huston a n d A.J. Campillo, Chem. Phys. Lett., 128, 2 7 4 (1986) 20. S.C. Schmidt, D.S. Moore a n d J.W. Shaner, i n Shock Waves in Condensed

Matter- 1983, J.R. Asay, R.A. Graham a n d G.K. Straub,eds. (North-Holland, Amsterdam, 1984) p 2 9 3

21. G.E. Duvall. R.H. Granholm, P.M. Bellamy a n d J.E. Hegland, in Shock waves in Condensed Matter-1985, Y.M. Gupta, ed. (Plenum, New York, 1986) p201 22. Ph. Avouris, D.S. Bethune, J.R Lankard, J.A. Ors a n d P.P. Sorokin, J. Chem.

Phys., 2, 2 3 0 4 (1981)

23. A.M. Renlund, S.A. Sheffield a n d W.M. Trott, in Shock Waves in Condensed Matter- 1985, Y.M. Gupta, ed. (Plenum, New York, 1986) p 2 3 7

24. A. Delpuech a n d A. Menil, i n Shock Waves in Condensed Matter-1983, J.R.

Asay, R.A. G r a h a m a n d G.K. Straub, eds. (North-Holland, Amsterdam, 1984) p 3 0 9

25. A. Delpuech, A. Menil a n d B. Pouligny, in Shock Waves in Condensed Matter- 1985, Y.M. Gupta, ed. (Plenum, New York, 1986) p 8 7 7

26. N.C. Holmes, A.C. Mitchell. W.J. Nellis, W.B. Graham a n d G.E. Walrafen, in Shock Waues in Condensed Matter-1983, J . R Asay, R.A. Graham a n d G.K.

S t r a u b , eds. (North-Holland. Amsterdam, 1984) p 3 0 7

27. S.C. Schmidt, D.S. Moore. D. Schiferl a n d J.W. Shaner, Phys. Rev. Lett., 50,

661 (1983)

28. W.G. Von Holle, i n Shock Waves in Condensed Matter-1983, J.R. Asay, R.A.

G r a h a m a n d G.K. Straub, eds. (North-Holland, Amsterdam, 1984) p 2 8 3 29. W.B. Rah, P.W. Schreiber a n d J.-P.E. Taran, Appl. Phys. Lett., 29. 1 7 4 (1976) 30. D.S. Moore, S.C. Schmidt, J.W. Shaner, D.L. Shampine a n d W.T. Holt, in Shock

Waves in Condensed Matter-1 985, Y.M. Gupta, ed. (Plenum, New York, 1986)

p 2 0 7

31. D.S. Moore, S.C. Schmidt and J.W. Shaner, Phys. Rev. Lett., 5Q, 1 8 1 9 (1983) 32. S.K. Saha and R.W. Hellwarth, Phys. Rev. A, 27. 9 1 9 (1983)

33. M.L. Johnson, M. Nicol and N.C. Holmes, in Shock Waves in Condensed Matter-1 985 , Y.M. Gupta, ed., (Plenum, New York, 1986) p 2 0 1

C o m m e n t - ^J.C. M I A L O C Q

I t appears t o me t h a t your fluorescence spectroscopy method should be a p p l i e d t o the probing o f shock induced p o l a r i t y changes i n nitromethane.

Some dyes as merocyanine dyes (DCM l a s e r dye) show very l a r g e s o l v e n t induced s p e c t r a l s h i f t s due t o e l e c t r o n i c p o l a r i z a t i o n and nuclear p o l a r - i z a t i o n e f f e c t s , i n going from non p o l a r t o p o l a r solvents. Therefore u s i n g l a s e r induced fluorescence probing and a b s o r p t i o n spectroscopy o f such dyes, you should o b t a i n pol'arity changes i n nitromethane submitted t o a l a s e r d r i v e n shock.

Your suggestion i s q u i t e good and should a l l o w important new i n f o r m a t i o n t o be obtained on nitromethane. The p a r t i c u l a r dyes t h a t I discussed were chosen because each o f these showed fluorescence behavior t h a t depended on o n l y one parameter; e.g. fluorescence, v i s c o s i t y o r pressure e t c ...,

w h i l e a t the same time was r e l a t i v e l y immune t o o t h e r changes. The n e x t s t e p w i t h regard t o your suggestion would be t o examine various merocyanine dyes t o i n s u r e they do n o t a l s o show l a r g e changes w i t h temperature and pressure, f o r example. I f t h e p o l a r i z a t i o n c o n t r i b u t i o n dominates, and most l i k e l y i t w i l l f o r one o r more dyes, then an experiment i n nitromethane should be attempted a t moderate pressures.

Q u e s t i o n - E. L E E

Have you some p a r t i c u l a r suggestion f o r t h e q u a n t i t a t i v e measurement o f minor chemical species i n a shock wave environment ?

O f s p e c i a l i n t e r e s t would be NH3, CH4, NO, HCOOH.

Rdponse -

This i s a question t h a t I have asked m y s e l f many times before. U n f o r t u n a t e l y I s t i l l do n o t have a s a t i s f a c t o r y answer and I h e s i t a t e t o make a recom- mendation. The a c t u a l d e c i s i o n as t o what spectroscopic probe should be used denendsstrongly on f a c t o r s such as the c h a r a c t e r o f t h e parent molecule and

o t h e r product species, t h e temperature and pressure range and t h e d e s i r e d temporal and s p a t i a l r e s o l u t i o n . There are regimes where i t may n o t be p r a c t i c a l t o employ any form o f o p t i c a l spectroscopy. The species you mentioned, as I r e c a l l , are n o t f l u o r e s c i n g . However, o t h e r products might be s t r o n g l y f l u o r e s c e n t and i n t e r f e r e i n subsequent measurements.

Spontaneous Raman Spectroscopy appears a t t r a c t i v e because i t i s r e l a t i v e l y simple technique and r e c e n t r e s u l t s o f Renlund and T r o t t a t Sandia Lqbs.

have shown t h a t species having C-N, C-C, C-H bonds a r e e a s i l y interngated a t e a r l y times a f t e r the a r r i v a l o f the d e t o n a t i o n f r o n t . I b e l i e v e i t would a l s o be s t r a i g h t f o r w a r d t o see species having N-0, N-H and C-0, which would i n c l u d e a l l t h e molecules you mentioned.

However, depending on the explosive, t h e chemiluminescence and blackbody

r a d i a t i o n r e s u l t i n g from t h e d e t o n a t i o n may obscure t h e Raman spectrum.

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Q u e s t i o n - S . O D I O T

Chimie temperee par chocs.

Can you discuss more t h i s i n t e r e s t i n g ~ o i n t ?

I s i t c o r r e l a t e d w i t h a p o s s i b i l i t y t o c o n t r o l t h e energy r e l e a s e ?

Rdponse -

The purpose o f t h i s study was t o t e s t the a p ~ l i c a b i l i t y o f t h e usual Einstein-Smoluchowski formalism o f d i f f u s i o n c o n t r o l l e d r e a c t i o n s under c o n d i t i o n s o f i n t e n s e shock. Such t e s t s are important i n understanding t h e p o s s i b l e chemical pathways occuring under detonation where d i f f u s i o n processes may l i m i t t h e f i n d l r e a c t i o n r a t e s . One example o f t h i s i s t h e w e l l known ' c a r b o n - d i f f u s i o n ' ~ r o b l e m . I n our study, t h e shock induces both v i s c o s i t y and temperature changes and these subsequently modify t h e k i n e t i c s o f a photochemical process. Ye choose t o study t h e d i f f u s i o n c o n t r o l l e d e l e c t r o n t r a n s f e r r e a c t i o n occuring i n a s o l u t i o n o f ~ h o t o e x c i t e d s i n g l e t s t a t e Rhodamine 590 and ground s t a t e i o d i d e i n isopropanol because t h i s i s a system t h a t photochemists have e x h a u s t i v e l y s t u d i e d a t ambient oressure and i s w e l l understood. Our r e s u l t s under shock l o a d i n a were c o n s i s t e n t w i t h the usual models and assumptions t h a t t h e o r e t i c i a n s make t o e x p l a i n detonation, so t h i s was reassuring.

This work was n o t r e a l l y aimed a t c o n t r o l l i n g energy release. A more a p p r o p r i a t e experimental t o o l f o r t h a t a p p l i c a t i o n i s our temperature fluorescence probe. You can, o f course, estimate t h e energy released a t each i n s t a n t o f t i m e d u r i n g a shock-induced chemical r e a c t i o n using our molecular thermometer technique. Such i n f o r m a t i o n would be necessary t o determine t h e proper course o f a c t i o n i n any attempt t o modify o r c o n t r o l t h e energy released from an explosive.

As an example, consider a Gedanken experiment i n v o l v i n g nitromethane.

A f l u o r e s c i n g dye having a temperature dependent quantum e f f i c i e n c y would be added t o s o l u t i o n . One n e x t conducts a s e r i e s o f shock experiments of i n c r e a s i n g pressure and from t h i s t h e temperature vs. P c h a r a c t e r i s t i c s may be p l o t t e d . Deviations from temperatures expected from t h e equation of s t a t e estimates may be assumed t o be due t o any chemistry occuring. A t each pressure, t h e temperature would a l s o be determined as a f u n c t i o n o f time and energy r e l e a s e r a t e s estimated i n t h e pre-detonation regime, From t h i s i n f o r m a t i o n , one may a l s o deduce t h e s p e c i f i c chemical r e a c t i o n occuring from i t s c h a r a c t e r i s t i c kcal/mole y i e l d . I n p r i n c i p l e , i t should be p o s s i b l e t o chemically a l t e r t h e molecular s t r u c t u r e o r add another soecies t o e x p e r i m e n t a l l y determine t h e e f f e c t on energy release.

F i n d i n g a s u i t a b l e dye thermometer f o r t h e l i q u i d e x p l o s i v e o f i n t e r e s t presents some d i f f i c u l t i e s . The dye must go i n t o s o l u t i o n , must be r e c a l i - b r a t e d f o r each system o f i n t e r e s t , must n o t degrade w i t h temperature and must n o t r e a c t w i t h t h e explosive. For t h i s reason, we are c u r r e n t l y l o o k i n g a t f i t i n g t h e dye i n a p l a s t i c o r glass. Fluorescein, f o r example, goes i n t o PMMA very e a s i l y . A f t e r c a l i b r a t i o n , t h e d o ~ e d f l u o r e s c e i n may be considered t o be a u n i v e r s a l thermometer. A submicron l a y e r o f t h i s m a t e r i a l c o u l d be added t o a window i n c o n t a c t w i t h t h e nitromethane. I t i s necessary t o use a t h i n , submicron l a y e r i n order t o i n s u r e t h a t t h e thermal conduction time i s on t h e o r d e r o f a few nanoseconds. Another p r a c t i c a l geometry would apply t h i n f l u o r e s c i n g l a y e r s t o t h e end o f a f i b e r o p t i c probe. I n t h i s instance, the sample need n o t be transparent.

Most o r g a n i c dyes decompose a t temperatures above 300°C. However t h e therno-

meter concept I ' v e o u t l i n e d i s v a l i d f o r many i n o r g a n i c f l u o r e s c i n g ions,

as w e l l , and these may be added t o glass probes.