KINETICS OF ENERGY RICH SYSTEMS

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KINETICS OF ENERGY RICH SYSTEMS

J. Brooke Koffend

To cite this version:

J. Brooke Koffend. KINETICS OF ENERGY RICH SYSTEMS. Journal de Physique Colloques, 1987,

48 (C7), pp.C7-325-C7-330. �10.1051/jphyscol:1987779�. �jpa-00227083�

JOURNAL DE PHYSIQUE

Colloque C7, suppl6ment au n012, Tome.48, dgcembre 1987

KINETICS OF ENERGY RICH SYSTEMS

J. BROOKE KOFFEND

Aerophysics Laboratory, The Aerospace Corporation, El Segundo, CA 90245, U.S.A.

I. INTRODUCTION:

The chemistry and kinetics of NF, and N F radicals with various atoms and molecules have been the subject of a number of studies1-". Systems that involve NF2 or NF are of interest since they are among the few that produce electronically excited products in high yield. These systems are attractive because of their potential use in chemically pumped lasers. In this paper we will present results of several studies performed in our laboratory. These include the kinetics of the H2/NF2 system, an in- vestigation on the BiF AO' state, and the tunable U V laser photolysis of NF,. The H2/NF2 reaction system is important as an efficient chemical source of the metastable NF(~'A) radicals. The kinetics and radiative properties of the BiF AO' state were studied since it has been shown to be produced via reactions with NF(~'A)." The results of Ref. 12 have spurred interest in the development of a chemi- cally pumped BiF A

-

^X laser with emission at a number of lines near 457 nm. Following the UV photolysis of NF,, the N F ( ~ ' A ) fragment is observed to have a short but finite rise time, contrary to what one would expect from continuum photodissociation. The N F ( ~ ' A ) yield from the tunable UV photolysis of NF, was examined to more fully characterize the electronic structure of NF, and explain this anomaly.

11. KINETICS O F T H E H2/NF2SYSTEM:

The reaction of hydrogen atoms with NF2 was studied in a flow tube using a pulsed K r F laser to initiate H2/NF,/Ar mixtures. The photolysis quantum yield f o r N F ( ~ ' A ) at 249 nm was measured as well as the rate coefficients for several important reactions in this system."

The critical reactions under our experimental conditions are:

NF2

+

hv (A = 249 nm) -+ N F ( ~ ' A , X ~ C )

+

F, F

+

Hz -+ HF(v)

+

H,

F + NF,

+

Ar -+ NF3

+

Ar,

H + NF, -+ NF(~'c, alA, x 3 c )

+

HF,

~ N F ( X ~ C ) -+ N,

+

2F, N F ( ~ ' A )

+

M -+ N F ( X ~ C )

+

M, NF(~'A)

+

HF(v)

-

^NF(~'c)

⁺

^HF(v-2).

Reaction (4) is known to produce the N F ( ~ ' A ) state with a branching ratio greater than 0.90.' The rapid disproportionation of ground state N F ' ~ in reaction (5) regenerates fluorine atoms and a chain reaction occurs under certain experimental conditions. Hence, this system has been of interest as a chemical source of metastable N F radicals which could be used as an energy donor to a suitable recep- tor species. We note that the three body recombination of fluorine atoms with NF2 is crucial since it serves as the chain termination step in the reaction sequence.

An analytic model describing the N F ( ~ ' A ) time behavior was developed" which allowed us to determine k3, the three body recombination rate coefficient. Essentially, this model accounts for the fluorine atom source from reaction (5) as well as the competition for them by reactions (2) and (3). A

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987779

C7-326 JOURNAL DE PHYSIQUE

much more complete computer model was created for this system using the

NEST'^

code. Both models exhibit good agreement with experimental data as shown in Fig. 1. The squares, circles, and dots represent the analytic model, the NEST model, and experiment, respectively. The non-zero intercept in Fig. 1 is due to the 10% N F ( ~ ' A ) photolysis quantum yield. The results of this study are collected in Table 1.

The photolysis of NF,/Ar mixtures without H, present results in NF(~'A) time profiles depicted in Fig. 2. The short but finite rise time does not follow the K r F photolysis pulse (15 nsec duration, < I nsec risetime). We performed several experiments that provided evidence that this effect is due neither

Typical NF(a) Time Profile (248 nm Photolysis)

24000r

gsec sec

N F ( a ) FORMATION BEllAVlOR FROM KrF PIIO'IOLYSIS O F NF2/1I2

0

I

I I I I I I I

I

0 125 250 375 500 625 750 875 I000 TIME Ipsecl

to an impurity nor from secondary reactions of the photofragments. This time behavior is not expected considering t h e diffuse nature of the NF, absorption band near 260 nm.15316 This leaves NF,*

dynamics or NF2 electronic structure as the probable cause for the NF(~'A) temporal behavior. Further discussion is given in section 111.

11. BiF A 0' KINETICS STUDY:

The development of a BiF A - X laser requires an understanding of the kinetics of the BiF AO+

and X1O+ states. Although these states have been spectroscopically characterized,17 there have been few kinetic studies OIL the BiF molecule. Only a brief description has been presented1' of the reactions of NF(~'A) with BiF. Prior to this study, essentially nothing was known about the kinetics and rada- tive lifetime of the BiF A state. A Broida-type 0 v e n ~ ~ 7 ' ~ produced BiF from the reaction of Bi with fluorine and a pulsed dye laser was used to populate selected BiF A state vibronic levels. The radiative lifetimes of several BiF A state vibrational levels were measwed from time resolved BiF A

-

^{X laser}

induced fluorescence under collisionless conditions. Rate coefficients f o r electronic quenching and vibration-to-translation relaxation were determined by adding He or Ar buffer gas and monitoring time resolved fluorescence from the initially pumped A state level as well as adjacent levels.

Time resolved LIF curves from BiF levels under collisionless conditions were fit to a single ex- ponential decay to obtain the radiative lifetimes. The data could be f i t for times greater that three radiative lifetimes in all cases. Total pressures ranged from 20 - 50 mtorr for these experiments. For the A state levels studied (v'=O-3), rrad was constant (1.4

+

0.1 ysec).

With the addition of buffer gas, time resolved A - X emission traces were recorded from nearby levels as well as the laser pumped level. The rate coefficients for quenching and VT transfer were ex- tracted by fitting the data with a four level computer model, described i n Ref. 20. Table 2 contains the radiative lifetimes and rate constants determined in this study.

An examination of Table 2 reveals that Ar is a slightly more efficient quencher than helium.

The greater polarizability and size of argon may account for this difference.,' The increase in the

quenching rate constant with increasing v' has also been observed in other species, such as and IF". Classical scattering arguments predict that the electronic quenching depends upon the hard sphere collision diameter which can be related to the critical impact parameter, b,.24 This parameter grows with vibrational quantum number in a harmonic oscillator and this increase is expected to be greater for the anharmonic BiF AO' state.

One would expect that the V-T rate constants, kV,v-l, would scale with v for low vibrational levels from a classical standpoint. However, that data in Table 2 indicate that the V-T rate constants for both He and Ar scale as vl.' rather than as v.'.' To date, there does not seem to be a satisfactory model that explains the strong v dependence observed in fast V - R,T processes. A recent study2' on the vibrational relaxation of HF/DF support our results.

The V-T rate constants for all levels studied are at least an order of magnitude greater that those for electronic quenching. The V-T rates effectively compete with spontaneous emission for pressures greater that 3 torr. At moderate pressures, rapid equilibrium within the AO' manifold will be achieved by a slower decay of the total A state population, governed by quenching and radiation. Hence, the BiF A state is a good candidate for an upper laser level because population will be concentrated within the lower vibronic levels leading to higher gain on transitions out of these levels.

111. TUNABLE UV PHOTOLYSIS OF NF,

Collins and ~ u s a i n ~ explored the vacuum UV (126 - 140 nm) absorption of NF2 and photolysis products were monitored using transient absorption spectroscopy. They observed a series of diffuse ab- sorption bands which they attributed to NF2 Rydberg transitions. ElectronicaIly excited N F was also detected via time resolved absorption. Studies on the first UV absorption band of NF2 at 260 nm have been limited to measurments of the spectrum. The work of ~ o o d f r i e n d " and Woods as well as that of Kuznetsova et all6 show that the "260 nm" band exhibits diffuse structure under moderate resolution of 3 cm-'. Under our higher resolution of 0.25 cm-', no additional structure is perceived. Considering the nature of the absorption spectrum, arguments could be made for a short lived excited state or direct dissociation. The appearance time of the N F ( ~ ' A ) f;agment indicates that dissociation of N F ~ * to NF(~'A)

+

F is

not

direct.

Figure 3 pres6nts the NF2 absorption spectrum and N F ( ~ ' A ) quantum yield. The NF(~'A) quantum yield is a decreasing function of wavelength. At the peak of the NF, absorption near 260 nm the yield is only I%, suggesting that the upper state in this transition leads primarily to ground state NF (dissociation to N F b ' ~ is endoergic for the wavelengths studied).

NF2 Absorption Spectrum

Absolute ~ ~ ( a ' n ) Ouantum Yield from UV Photolysis of NF2

0 250 WAVELENGTH 255 (nm, vacuum) 0

245

PHOTOLYSIS WAVELENGTH lnrn. VaCuulll)

JOURNAL DE PHYSIQUE

mooo

,

^{I F S F} R S ~ L T S : B A N , A T A T

,

F-N-F ANGLE (degrees)

NF, RESULTS: DISSOCIATION COORDINATE

70000

0

R,,-, (Angstroms)

Analysis of the NF(~'A) appearance times and structure of the NF, has been aided by ab-initio calculations on this system. Our preliminary results reveal that the initial excitation of the NF, 'B,

-

,A, transition (the NF, "260 nm" band) produces NF,* with a large amount of energy in the 2 ~bend- 1 ing mode. Such vibrational excitation may cause the diffuse structure observed in the absorption spec- trum through interference effectsz6. Our initial SCF results for the NF, electronic structure are depicted in Fig. 4. A curve crossing between two states of A' symmetry along the dissociation coor- dinate can be seen in Fig. 4b. We are continuing these calculations, increasing their accuracy and transition moments and the non-adiabatic coupling between these states will be determined. Such in- formation will be used to model the dissociation process and may lead to a more complete understand- ing of the NF(~'A) quantum yield as well as its formation kinetics.

TABLE 1. Experimental Results from H,/NF, Study

Reaction Rate Constanta

NF,

+

hv (A = 249 nm) -+ N F ( ~ ' A )

+

F F

+

NF,

+

Ar + NF3

+

Ar

NF(~'A)

+

NF, -+ NF(X'C)

+

NF, HF(v=2)

+

NF, + HF(v<2)

+

NF, HF(v=3)

+

NF, + HF(v<3)

+

NF,

aEstimated errors given in parentheses (20) b~hotolysis quantum yield

TABLE 2. Radiative lifetimes and rate constants f o r BiF (AO')

Level ',ad

(psec)

BiF (AO+,v

-

AO+, v-l)b BiF AO' ~ u e n c h i n g ~

(cm3/molecule-sec) (cm3/molecule-sec)

Ar He Ar He

aEstimated error 0.10 psec

b ~ r r o r in all rate constants is estimated to be ?; 50%

ACKNOWLEDGEMENTS:

This work was conducted under the U.S. Air Force Space Division (AFSD) Contracts F04701-84-C- 0085 and F004701-85-C-0086. The author wishes to acknowledge his co-workers, whose efforts were integral in these experiments. They include Dr. R.F. Heidner 111, Dr. H. Helvajian, Dr. A. Woodin, and J.S. Holloway.

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