LASER OPTOGALVANIC EFFECTS CAUSED BY FORMATION OF NEGATIVE IONS

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LASER OPTOGALVANIC EFFECTS CAUSED BY FORMATION OF NEGATIVE IONS

I. Beterov, N. Fateyev

To cite this version:

I. Beterov, N. Fateyev. LASER OPTOGALVANIC EFFECTS CAUSED BY FORMATION OF NEGATIVE IONS. Journal de Physique Colloques, 1983, 44 (C7), pp.C7-447-C7-454.

�10.1051/jphyscol:1983743�. �jpa-00223300�

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L A S E R O P T O G A L V A N I C E F F E C T S CAUSED BY F O R M A T I O N O F N E G A T I V E I O N S

I.M. Beterov and N.V. Fateyev

I n s t i t u t e o f Thermophysics, Academy of Sciences o f t h e U.S.S.R., Siberian Branch, 630090 Nouosibirsk, U . S . S. R.

Resum6 - Dans ce rapport deux types d'effets optogalvaniques sont pr6sentGs et Btudies. En premier lieu, il a et6 d6montr6 la possibilit6 d'obtenir un effet optogalvanique dans les gaz mol6culaires 6lectron6gatifs (SF6, CCL2F2, 12). L'effet du au pi6geage volumique d161ectrons est observe 2 bas voltage dans une diode fonctionnant en r6gime de charge d'espace et dans un magn6tron 2 triode. Les mol6cules SF , CCR F sont ex- 2 2 . cit6es vibrationnellement par absorption d'u8e transltlon d'un laser 2 CO ou par un processus Raman r6sonnant ( I 2 ) sous l'action c o m b i h e des rayonnements du deuxisme harmonique du laser 2 YAG et du laser 2 colorants organiques. La possibilit6 d'uti- liser ces effets li6s au pi6geage volumique d161ectrons et leurs propri6t6s sont 6tudi6es en spectroscopie mol6culaire optogalvanique.

En second lieu, la physique de la chimionisation laser-induite par collisions thermiques entre des atomes de m6taux alcalins et quelques mol6cules 6lectron6gatives, est pr6sent6e. Les ex- p6riences ont 6t6 faites sur le sodium dont les atomes sont excit6s par un laser 2 colorants organiques. Nous avons mesur6 la vitesse absolue de l'ionisation collisionnelle pour 116tat 4d de Na avec des gaz-cibles tels que 02, SF CH Br, CH I.

Les donn6es exp6rimentales sont compar6es

a !?A

thJorie paur des transitions nonadiabatiques dans la th6orie de Landau-Zener mo- difi6e par Zembekov. I1 est montr6 que le modSle des croisements multiples est valable et que le maximum Landau-Zener de la section efficace est atteint.

Abstract

-

^Inthis paper two kinds of optogalvanic effects are biscussed and investigated. First ,it was demonstrated the possibility of obtaining optogalvanic effects in electronegative molecular gases (SF CC1 P2,J2) due to volume electron capture.

Effects have been o6;ervgd at low voltage in space charge limi- ted diode and triode magnetron. The molecules were vibrational- ly excited at absorption of CO -laser radiation (SF ,GC1 P ) o r at the resonance Raman-like prgcess (J2) under secoed hahngnic Y A G l a s e r and dye laser radiation. The properties and the probability using of these effects for molecular optogalvanic spectroscopy are discussed.

At second, the physics o f laser-assisted chemo-ionization at thermal collisions between alkali metal atoms and some electronegative molecules are considered.Experiments were made with sodium atoms whose were excited by dye laser radiation.We measured the absolute rate of the collisional ionization for 4d Na state with gas-target as 0 ,SF ,CH Br,CH J. The experimental data are compared with ~ a n d g u - ~ 6 n e r ~ t h e o r g o f nonadiabatic transitions modified by Zembekov. It is discovered that the model of multiple-crossing are valid and the Landau-Zener maximum of the cross-section i s realized.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983743

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JOURNAL DE PHYSIQUE

As is known, negative ions are important in a low-ionized gas. The transition of a negative charge from a free electron to a heavy negative ion gives a sharp decrease of the plasna conductivity, changes a recombination rate of positive and negative charge.Under certain conditions negative ions almost completely determine an impe- dance of the low-temperature plasma. Naturally, if laser excitation influences the process of the negative ion formation we can expect the appearence of a new type of the optogalvanic effects and use it for laser optogalvanic spectroscopy.

First experiments were made with SF molecules and CO -laser for vibrational excitation of 3%. Laser opt6galvanic effects Bere observed by Avriller and Scheman using electron modulation ; l / , by us in a triode magnetron /2/ and by Chen and Chantry / 3 / , who used very slow electrons. These experiments have demonstrated a possibility of obtaining optogalvanic effects due to negative ionization and the phy- sical mechanism of their appearence.Firstly, these effects are important for infrared molecular LOG spectroscopy. Secondly, effects arise only with electronegative molecules and radicals. Thirdly, LOG signal can be obtained at very low electron energy when fragmentation in pla m a is rather low.

The fig.1 shows our experimental scheme for observing the laser optogalvanic effects in a triode magnetron.

31 4

I

Fig. 1

4

-

CO -laser radiation 1 was passed through a specially designed cell

-

tr3ode 2, placed into an axial magnetic field

4.

The mechanical chop- per provided an amplitude modulation of the laser intensity at 130 Hz.

The 130 Hz optogalvanic signal in negative ion current from the collector 6 that is under the positive voltage,is recorded by lock-in- amplifier. Electrons were produced by thermoelectron emission from a heated tungstex cathode 5,then accelerated and arrived to collector 6 or net 7. The 10 to 100 Oersted magnetic field was used to separate an ion current component from an electron one.The experimental results are given at fig.2 and fig.3

.

F i g . 2 F i g . 3

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galvanic effect at a level of 10 per sent of the total current is observed at extremely low accelerating voltages of about 2

-

³^V,when

an electron energy is insufficient for the positive ionization. The dependence on a wavelength is approximately similar to the behaviour of intercombination absorption bands.

At collisions of slow electrons with a molecule the primary act is the formation of a long-lived autoionization state of a negative molecular ion,( SF ) * for example. Along with decay of the autoioni-

zation state into in initial electron and a molecule, several channels of formation of stable negative ions are possible, These are the formation of a stable "parentv negative ion SF at collisions and the process of dissociative attachment of an el6ctron.bs is known,for SF the relation between the channels strongly depends on the temperatur6 of gas. An activation energy for process of dissociative capture in SF is about 0.2-0.4 eV. As an electron transition is vertica1,this inkicates the influence of vibrational excitation on the process of dissociative attnchment.

The most probable explanation of the optogalvanic effect is connected with an increase of the dissociative electron capture rate due to laser vibrational excitation of molecules.This process is essenti- al. Table 1 shows the mass-spectrum of negative ions in the cell,

- that has been obtained by

using the quadrupolar mass- spectrometer.Ions were pull- Table 1. ed through a small hole.

INTENSITY

19.5 11

The change of gas conductivity is connected with the difference of ion mobilities for SF and SF ,as we measure the total ion current.

We operate in the re&on of dhfusion limitations and this confirms the dependence of a pulse shape of LOG signal and delay time on gas pressure.

Experiments with SP and other molecules show the influence of vibrational excitation

06

the process of negative ion fcmation and a possibility of using such optogalvanic effect in molecular spectroscopy. Cross-sections, resonance energy of electrons,channels of process will change depending on the character of potential curves for basic molecules and negative molecular ions. The optogalvanic signal in plasma appears to be due to the change of mobility of the negative charge and the rate of recombination.An introduction of negative ions into the mass-spectrometre with subsequent amplification by using a channeltron can provide a high sensitivity of spectroscopic experiments.The development of the technique is undoubtedly connected with the use of tunable infrared laser.

FOP nonpolar two-atom molecules there are no active infrared vibrational bands in absorption spectra, though we can hope to observe an optogalvanic effect due to vibrational excitation too.(For instance, 0

,

where the strong temperature dependence of cross-section for digsociative electron attachment was observed / 4 / ) . These molecules can be therefore excited to vibrational states only in the Raman-like processes,including quazi-resonant ones. In this case such type of the optogalvanic effects may be observed by using lasers tun- ed in visible and ultraviolet spectral region and used in spectroscopy of electron transitions or optogalvanic Raman spectroscopy of molecules when laser-induced processes take place.

This kind of experiment have been made by us in J .The scheme of the experiment is shown at fig.4. We used a Q-switcged YAG-laser

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JOURNAL DE PHYSIQUE

N I : VAG LASER

,-- - - - - - - - - - - ₁_DOUsLER L - - - ; U - J

0SCILOSM)PE U

F i g . 4

F i g . 5

T , , . , . I . I .

0 L B iz is h

ELECTRON CURRENT,W

at 1 .Q6 p or its second harmonic at 532 nm and a dye 1aser.Measur ing current-voltage characteristics of a gas-filled diode we fixed the region of ionization amplification (V

>

¹⁵^V)as well as the phenomena caused by electronegativity of fir8$ecules J .At 0.1 torr the main process resulting in the formation of negatsve ion in J is a dissociative electron capture.Pig.5 also presents the results gf measurments of the negative ion current made by the method of magnetron. Estimates of the effective cross-section for electron capture in J are in a good agreement with Truby data / 6 / .

~fg.6 shows the LOG signal arising under the action of 1.06 pm radiatior. The pulse has a simple shape. It vanishes at small voltages(1ess than 20 V) and is connected with the positive ionization of The appearence of the optogalvanic effect at 1.06 p is surpris- if one taces into account that a spectral boundary of absorption for strong B-X band from lower vibrational states ranges in visible.

An analysis shown that the most probable process is an absorption in a weak diffusion A-X band from the levels with v=6,7,that are popu- lated by themal or electron excitation. The absorption is a one-photon process. The d2pendence on intensity shows the saturation.Howev- er,abouve 20 kLV/cm one can observe a deviation of experimental data from the theoretical curve for one-photon pr0cess.R is likely that the optogalvanic effect at 1 . 0 6 p is comlicated. A contribution to it is provided also by multiphoton processes and transitions between highly excited states.

t

^)r.lOBlr f

6 #2 /8 24

LASER INTENSITY. k ~ l c m ~ F i g . 6

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o~88rved with 1.06 p and the second pulse with large time delay, The first pulse is connected with an electron current resulting from the

Az532nrn ( X - 8 band J2) VC =40v

- 1.Dissociative attachment J2+ e-J+J-

2.IONIZATION ~ ~ + e - ~ ; + 2 e

Z

a . 35-psec

i vc=20v 3.Formation of ion pair J ~ + ~ v - J + + J - Ac2CK)nrn

> I !

1.Photodetachment J-+ hv-J t e A<L05nm

C.R.Webster et .al.

0 5.EIectron attachment to atom J +e

M

J-

LASER PULSE ~h~topredissociation J2+ hv-J + J Fig. 7

positive ionization of J molecules excited to B ~ K a (v= 33,34) by 532 nm radiation.The secgnd delayed pulse of negat!?v@ly charged parti- cles is connected with nggative ions.The delay time agrees with typi- cal values of mobility J

.

With a decrease of the voltage V the ion signal increases,while the electron signal falls and ev&8 changes sign.Thus the laser radiation resultes in decreasing the electron current.

:Jive basic mechanisms may be considered for explaining optogalva- nic effects in J plasma. These are dissociative attachment 1,positi- ve ionization 2,2fomation of ion pairs 3,photodetachment 4, electron attachment to atom J 5.The processes 3 and 4 are not probable at 532 nm /7/. The process 5 needs a more high pressure as it is a triple collision.

The observed pffects can be interpreted by using the potential curves for J and ^J /5/, After excitation to B (v= 33,341 state due to resonance ?luoresgence or induced, Raman process the high vibrational levels of ground electron state X G a r e excited.If the cross-section of dissociative capture process grows with increase of vibrational quantum number, the LOG effect is quite likely to be connected with this phenomenon, A simultaneous decrease of the electron current also confirms this mechanism,as the decrease may be caused only by an increase of the probability of electron capture.

The same dynamical picture was observed when the tunable dye la- ser was used.We have obtained the optogalvanic spectrum of J in the diode.This spectrum is similar to that recorded by using a c$ dye laser and a discharge /8-lq/,but processes in discharge are more compli- cated. The observed optogalvanic effects in J are interested not only for molecular spectroscopy. Firstly ,thay c& be a research method for the negative ionization of excited molecules.The use of pulsed laser radiation enables one to study the dynamics of ionization process, to examine the channels and intermediate long-lived states,

We should like also to discuss the optogalvanic effects associat- ed with formation of negative ions but more closely related to atomic optogalvanic spectroscopy and analytical applications.This is the

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cess of laser-assisted cherno-ionisation under thermal collisions oerO

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C7-452 JOURNAL DE PHYSIQUE

alkali atoms with electronegative molecules /12/. We observed this process experimentally and measured the absolute rates of chemo-ionization at collisions of excited sodium atoms with some electronegative molecules,for instance 02,SF ,CH Br,CH J and other?.

The following processes we& staded: 3~

+

XY -r Ill

+

XY- , As is known, in the reaction an electron is transfered at the point of intersection of ionic and covalent curves of potential energy. A cri- tical radius R is connected with the endothermici&y of the reaction

AE, = I(M) -'E~(xY) by the following rela8ion e /R WAE,

.

^The

cross-section is proportional to the X R

.

By exc?ting atoms one can considerably decrease the effective pot'&tial of ionization and hence observe the collisional ionization with formation of ionic pe- irs with a large effective cross-section. Besides, a threshold of the reaction due to the endothemicity A Eoo may be made very low.

Thus the atomic optogalvanic signal may be extended by adding electronegative molecules in flame,discharge,plasma. Ionization amplification may be real if the process is followed by the decay of negative ion due to electrical field or collisions.

Pig.8 shows the scheme of sodium atom levels and the potential curves for SF and SF;

.

The two-step excitation of sodium atoms to the 4d state Gas made by using two wavelength. The ionization potential from the 4d state is very close to an electron affinity for 0

,

SF6 and other molecules.We used the sodium atomic beam which colli8- ed with a molecular gas-target in the chamber. The sodium atoms were excited by pulse dye laser radiation (Pig.9). The formed negative ions and electrons were recorded by using the channeltron and the electronic system.

Pig.10 shows the signal on the oscilloscope screen, vrhich appears under laser excitation. The first pulse is the electron current due to the process of resonant three-photon ionization of sodium atoms. It is also obtained with no gas target. By adding 8rl electronegative gas we can observe a new pulse. It is connected with the formation of negative ions.We have studied the mass-spectra of the formed negative ions and the dependence of the ionization rate on the atomic guide temperature (Fig.11). The simultaneous observation of tvio processes,namely: photoionization and collisional ionization of excited 4d sodium atoms, enable$ us to measure the absolute values of the chemo-ionization rate without the determination of absolute population of the excited 4d state.

A detailed analysis of the experimental data / l 3/ has shown that they are explained within the frames of Landau-Zener's theory

Jet stream dye laser

- -

A,:S@&,., f.)m~~.Fb-d~'*~f~

F i g . 9

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F i g . 10 F i g . 11

for nonadiabatic transition in the region of quazi-intersection of ionic and covalent potential curves modified for molecules by Zem- becov /14/. This theory predicts that the threshold of the chemo-ionization is determined by the adiabatic electron affinity of molecule. At the same time the probability of nonadiabatic transition in the point of quasi-intersection is proportional to the vertical Frank-Condon transition between the terms of the molecule and the negative ion but only energy splitting. An analysis of the temperature dependences has shown that the threshold of laser-induced chemo -ionization are really determined by the adiabatic affinity but only for the process without dissociation of negative ion.This fact confirms the model of multy---,rossing. From experimental data we have obtained the cross-section of the process near the threshold. These values are very close to Landau-Zener's maximum of the cross-section.

The table 2 shows oqJr experimental results for various molecules. If we know the electron affinity and the effective ionization potential of an excited atom, we can estimate now the chemo-ionization rate under various conditions. It is important to estimate espe- cially in flames,where molecular oxygen and the products of decompo- sition of the compounds of the analysed substances are present.

TABLE 2

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C7-454 JOURNAL DE PHYSIQUE

REPERrnCES

1.AVRILLER S.,SCHERMAN J.P.,Optics comm.,19,(1976)87.

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3.CHEN C.L.,C@NTRY P.J.,J.~hem.Plz~s.~(1979)3897.

4.0t&:2ALLEY T.F. ,~~s.~ev.~(1966)14.

5.BETEROV I,Pb. ,FATEYW N.V. ,Optics comm.g(1982)425.

6.TRUBY P.X.,Phys.Rev.ll_2(1968)24.

7,WEBSTER C.R. ,NCDERblID I.S.

,

~.~hem.~hys.~(1983)646, 8.RETTNER C.T.,WEBSTER C.R.

9,DWNCK C. ,DESTOMBES J.L. ,IEEE 10.BETEROV 1,Ifi. ,FATEYEV N.V.

I1 ,\EBSTER C.R. ,FLAAlAl!?T P.H. ,MCDEFiliIID I.S. ,chem.Phys.~ett .96(1983)302 12,BETEROV 1.N. ,KUROCHKIN V.L. ,FATEYEV 1J.V. ~ov.~hern.~h~s.~1982)957. - 13,BETEROV I.M. ,FATEYEV N.V., JETP (in pint)

14.ZENLBEKOV A.A. ,Chem.Phys.Lett.~(1971)415.