DYNAMICS OF VIBRATIONAL RELAXATION OF MULTIPHOTON EXCITED SF 6 MOLECULES IN GASEOUS MIXTURES CONTAINING OZONE AND INERT GASES ( He , Ar , Kr )

SF6 at pressure of 0.6 Torr in gas mixtures with ozone (0 + 5 Torr) and inert gases: He, Ar, Kr (0 + 5 Torr) was vibrationally excited with and IR pulsed radiation ofCO2TEA laser at 10P20 laser line (F 0.6 J/cm2). The vibrational energy transfer from multiphoton excited (MPE) molecules SF6 to the components of gaseous mixture was probed by UV absorption of ozone at Z 253.6 nm band. The relaxation process ofMPE molecules SF6 was proposed to occur through parallel V-V’ and V-Tcollisional channels. Based on the kinetic model, collisional rate constants were determined from the time evolution of UV I. SF6-O3 absorption data after laser shot. The obtained values of relaxation rate constants are: ,v-v 7.7 I.SF6-He I.SF6-Ar 10 13 t.SF6-Kr 10-14 I.SF6-SF6 10cmO/S,v_T 3.1 10cmO/S,v_T 2.3 cmO/s, V-T 7.7 cm/s, V-T (4.8 + 6.4) 10-13 cm3/s. The observed decay ofIR fluorescence from MPE SF6 molecules at Z 6.3/zm wave number in SF6-Kr gas mixtures had an exponential time profile with the constant characteristic relaxation time (V-T). The rate of IR fluorescence cluenching allowed us to find an effective V-T relaxation rate constants for 1.SFa--Ar 10--14 1,.SF6--SF6 10--12 collisional partners: ,v_r 7.8 cmS/s. ,,.’r-v 1.0 cma/s.


INTRODUCTION
Description of different physical and chemical processes in gaseous media occurring in the field of intense IR radiation require quantitative data on collision relaxations of multiphoton excited (MPE) molecules.Characteristic relaxation time for the MPE molecules characterizes the energy redistribution in the system and at the same time gives practical information concerning for instance the thermalization rate of the system.The latter is essential for the estimation of the time necessary to achieve the top equilibrium temperature of the gaseous mixture in laser-powered homo- geneous pyrolysis (LPHP), when chemical reaction initiated by pulsed IR sensiti- zation.This method proves quite effective for the study of mechanism and kinetics of high temperature gaseous reactions especially those with participation of thermodynamically unstable compounds such as ozone.
Sulphur hexafluoride (SF6) is known to be one of the most commonly used IR sensitizers.However, the number of studies on collision deactivation of SF6 mole- cules excitated by the CO2-1aser is limited. 2-1For the most part they concern with the investigation V-T relaxation process of SF6 in the absence of chemical reactions.
In the early works the IR-IR resonance and IR fluorescence techniques were used for determination of V-V' and V-T rate constants of the processes in binary mixtures containing SF6 with low level of excitation, 2'4'5 in these experiments vibrationally excited SF6 could be considered only as a small admixture in the "cold" gaseous buffer.
A few works on the relaxation of MPE SF6 molecules on "cold" SF6 molecules are available, 6-1 in which characteristic V-T relaxation times were estimated.It has been proved by interferometry technique that at high energy of IR radiation the characteristic relaxation time for MPE SF6 in pure SF6 and SF6-Ar mixtures is several times lower than for regime of weak excitation. 9There are practically no quantitative data on collision deactivation of MPE SF6 molecules on chemically inert gases as well as on the reactional molecules including the ozone molecules.
The dynamics of vibrational energy transfer from the MPE sensitizer (SF6) to the other molecules of gaseous mixture may be observed not only by registration of the energy transformation of the sensitizer itself (e.g. by means of IR-IR, IR-UV resonance and IR luminescence) but by kinetics of excitation of the collision partners as well.In the present work an UV probing technique has been suggested for the kinetic study of excitation of the partner molecules (03) on collisions with SF6.The technique is based on the registration of the change in the UV spectrum of the ozone molecule in the region of Hartly band (200-300 nm) which is caused by the alteration of the extinction coefficient in the course of the transfer of vibrational energy into vibrational modes of ozone molecules Vl arid v3 which manifest themselves in the UV absorption spectra.It has been shown that the extinction coefficient of ozone (el) can be represented as follows: 12 ex (Pooo + Polo) ex + (Ploo + P ool) e (1)   where e e 10 is the extinction coefficient for the 0 3 (000) and 03 (010) states; e; is the mean weighted (according to population) extinction coefficient of the O3 (001) and O3 (100) states; Pooo, PlOO, Polo, Pool are the relative populations of the corresponding vibrational states of the ground electronic term of ozone (1A1).It is well known, that if < 271 nm, the inequality el* < e is true.For instance e/e 0.17 for .254 nm. 12 The alteration of the optical density (AA) due to vibrationally excited ozone molecule (O3') is related to the concentration of the latter by equation: AA where [O3*] [O3(100)] q-[O3(001)], is the optical thickness of the absorbing gas 000 layer (in cm), Aex el* el.It is evident from Eq. (2) that the time dependence of dynamics of the vibrational relaxation of MPE molecules SF6 after the pulse of the IR radiation (F 0.6 J/cm2, :1/2 70 ns, 3. 10.6/zm) was obtained from the depend- ence of the intensity of the continuous UV radiation (k 253.6 nm)-I(t), crossed the gas mixture.I(t) I + AI(t), where I is the constant component which was measured by a digital voltmeter and AI(t) is the variable component which was registered by oscilloscope.The experiment was carried out in a cylindrical quartz cell 18 x 60 mm equipped with BaF2 windows.The gaseous mixture contained SF6 (0.6 Torr) 03 (1-5 Torr), buffer gases He, Ar, Kr (0-5 Torr).
It is necessary to note, that in our experiments was completely excluded the direct vibrational excitation of ozone molecules by IR laser pulse in gas mixture.It was provided with considerable distance in IR absorption spectrum (---100 cm-1) between vibrational mode v3 of ozone, centered at 1042.1 cmand vibrational mode '3 of SF6, which was excited at to 944.19 cm -1 laser line.The linewidths of these vibrational modes does not cover each other in linear IR spectrum.The absence of direct excitation of ozone molecules was confirmed in series of experiments, when irradiated gas mixture was not contained SF6 molecules.In that case there were not any obvious temporal changes in UV absorption of ozone after the laser shot, 13rn4 Moreover, we could not define in such conditions any absorption of IR laser beam in the cell.

RESULTS AND DISCUSSION
Figure 2 represents the initial part of the oscillogram with the AI(t) value changing from 0 to A/max for 20-30/zs after the completion of the IR pulse.Filtre 2 Picture of initial portion of detected UV transmission (/% 253.6 nm) through the gas cell, containing SF6-O3-M mixture, after laser shot, SG6 (0.6 Torr), O3 (1 Torr), M (0 Torr).
According to Eq. ( 2) and Lambert-Beer law: AA A(t) A lg {Io/[I + AI(/)]} lg (Io/I) (3) where from taking into account that for all the experiments Al(t) ,/o, one can get: The [O*3] and AI(t) are apparent to be given by one and the same time dependence.
We connect the dynamics of the [0*3] increase in the system with a simple collisional model of the relaxation.The excess of vibrational energy of the system after the IR laser exposure will be characterized by (n)--the mean number of quants (h(o) absorbed by one SF6 molecule from the IR field.The determinated value of parameter (n) 8 was still constant for all gas mixtures under investigation.It was controlled by accurate measurements of IR energy absorbed in the cell from laser beam by calorimetric detector with 5 % error of absorbed energy.Thus we supposed, that the initial average energy of SF6 molecules after the laser shoot does not change with the addition of small amounts of ozone and buffer gases in whole series of our experiments.
For a two component mixture O3-SF6 the relaxation of MPE SF6 in the scope of such model may be considered as a uniquantum V-V' energy transfer process from MPE SF6 to 03 (000) with the formation of the 03 (100) and 03 (001) states and also a parallel V-T relaxation channel.A schematic representation is as follows: To a first approximation this process may be described by a kinetic scheme of two parallel processes with the first order rate constants k -v, and k V-T.
It is obvious, that the experimental data of (n) produce no information about the initial shape of vibrational energy distribution of SF6 molecules after the excitation by high-power IR laser field.The detailed investigation of multiphoton excitation demonstrates the existence of two groups of SF6 molecules" the "hot" ensemble disposed in quasi-continuum, and the "cold" ensemble, which includes some mole- cules located on the low vibrational levels and in the ground state.The first step in the temporal process of vibrational energy transfer in MPE SF6 is the very first fast V-V and V-V' energy exchange among these two groups, which lead to produce single Boltzman distribution between vibrational degrees of freedom in SF6 gas with very high initial vibrational temperature (Ti 1500 + 1600 K in our case).The characteristic time of such fast process for SF6 molecules was estimated 6 (p: 1/z s Torr).The technique of UV probing allowed us to detect only the further process of vibrational energy transfer from SF6 molecules to O, lowering the vibrational temperature of SF6 and exciting ozone.We consider, that this process is more complicated and occurs in collisions according to kinetic scheme (*).
Finally, such relaxation kinetics results in equal value of vibrational temperature of all components and translational temperature of the gas, so, at last the mix- ture in the cell becomes completely thermolizated at equilibrium value of tem- perature.
This scheme is supported by a series of different considerations.First, of all the relaxation of MPE SF6 occurs in the medium of excess of "cold" ozone 03 (000).Alongside with uniquantum excited states 03 (001) and 03 (100) the multiquantum energy tranfer is possible to produce the O3 (00K) and O3 (N00) states.However, on collisions with 03 (000) molecules the "hot" ozone molecules will quickly lose their energy in quasi-resonance uniquantum V-V' processes, thus transferring to 03 (100) and O3 (001) states manifesting themselves in the UV absorption spectra of ozone.
The energy transfer from MPE SF6 into the v2 mode of ozone molecule gives no alteration in the ozone spectrum, however the transfer from the v2 mode of ozone molecule to the 03 (100) and O3 (001) levels will contribute to the change of optical transmission in the UV range of the absorption spectrum of 03.Thus the k -v' is the bottom limit of the specific rate of the vibrational energy transfer from SF6 molecules to 03.The V-V' intermolecular exchange between SF6 molecules does not change substantially vibrational energy of SF6 molecules as a whole ensemble and conse- quently cannot be considered as an effective decay of vibrational energy of MPE SF6.
Only a small part of vibrational energy can be transferred into the translation as a result of the energy defects in the course of intermode transformations in SF6.That is why the V-T relaxation process from the lower vibrational levels SF6 with the rate constant k _: is the lowest limit of the rate transformation of vibrational energy from MPE SF6 into translation.On the other hand, the V-T relaxation process of MPE SF6 should not effect greatly the 03* concentration, because in conformity with the principle of detailed equilibrium, the 03 excitation as a result of T-V process occurs considerably slower than the V-T process for O3*, occurs according to work 7 from the v2 mode of molecule 03 (010) with the rate constant kv-: 1.01 10 -3 cm3/s.Thus in the time scale under investigation (20-30/xs) the T-V excitation process of 03 can be neglected.
Kinetics of O* 3 formation according to the scheme (*) can be expressed as follows: l.. 5F6-O3 is where klv_T rV-TI" SF6-SF6 [SF6], k v_v ,x =/Cv-v'' SF6-O3 [03 and ,v-v' .s6-sFtheeffectiVeis rate constant of the vibrational relaxation SF6 on ozone molecules, ,v-a- an effective rate constant of SF6 vibrational self-relaxation.The value k_v, (n) is the top concentration of the vibrationally excited ozone ([O3*]max) which can be achieved by the completion of relaxation of MPE SF6.In accordance with Eq. (4) [O3*]max corresponds to the A/max value on the oscilloscope record.Combining the Eq. ( 4) and ( 5) one can get: where k kv-v, [O3] + ,v-r It is obvious from (7), that at [SF6] const, the k values calculated from the experimental data according to (6)change linearly with [03] changes.demonstrates that the k values vary in direct proportion to the ozone partial pressure.The slope of the corresponding experimental points gives for the constant k2rv, the value of 7.7 10 -13 cm3/s (pr 40/zs Torr).Extrapolation of the slope to zero value of ozone oressure gives the value of (4.8 + 6.4) 10 -13 cma/s (px 49 + 65 1. SF-SF /zs.Torr) for ,V-T The same procedure was applied for the calculations of relaxation rate constants for the O-SF systems with added buffer gases.One rnorc V-T relaxation channel of MPE SF on the buffer molecules appears.It means that one more component k__'r [M] enters into the equation for k.Varying the content of the buffer gas in the system with constant [SF] and [O] one can obtain a linear correlation k with which is represented in Figure 4.The relaxation rate constants of MPE SF on He, .SF-Hc Ar, Kr molecules obtained from these data cornc to ,W-T 3.1 10cm/s, 2.3 10 -13 cm3/s, kSvF_67r Kr 7.7 10 -14 cm3/s respectively.P(1), Torr Figure 4 Rise of effective rate constant (k) of vibrational relaxation of MPE SF6 molecules in SFo-O3 M mixtures (M He, At, Kr) versus inert gas pressure, SF6 (0.6 Torr), Oa (2 Tort), O -He, A Ar, [] Kr.
An investigation of V-T relaxation of MPE SF6 in mixtures with Kr by direct method of IR luminescence quenching 4 in the region of 6.3/m was carried out in order to estimate the reliability of the data obtained in the present study by UV probing.IR emission was registered by a photodetector Ge-Au (77 K) and attri- buted to the vibrational transfer in MPE SF6 from the combination v2 / v3 levels to the ground state and v2 / v3 + v6 v6.TM The temporal decay of the IR lumine- scence intensity represented in Figure 5 can be adequately approximated by the exponential function such as" Ie I exp (-t/) (8) where If is the IR luminescence signal in maximum, r is the characteristic time of I, ARBIT.UNITS V-T relaxation in the gaseous mixture SF6-Kr under investigation.It has been shown that in SF6 mixtures with inert gases the parameter r is given by the equation" I,SF6-Kr where ,V-T is the rate constant of V-T relaxation of MPE SF6 on Kr.
As may be inferred from (9) at [SF6] const, the ()-1 values should depend linearly from the [Kr,].The experimental data are given in the Figure 6 in (x) -- [Kr] (%)-1 103, s-1 ISF6-Kr coordinates.The value ,w-v 7.8 10 -14 cm3/s was obtained from the slope of the experimental data.Extrapolation of the slope to the zero pressure of Kr gives the 1.. SF6-SF6 12 value of ,v_T, 1.0 10-cm3/s.It is worthwhile to note good agreement between the values of the effective rate constants of V-T relaxation of MPE SF6 molecules on the Kr molecules obtained by different methods.The corresponding values for the effective rate constants of vibrational self-relaxation of MPE SF6 are in good agreement as well.
The rate constants of V-V' and V-T relaxational processes obtained in our work exceed in several times similar rate constants determined for one-photon excitation of SF6 molecules. 2This fact is in good agreement with the conclusion of considerable acceleration of relaxational kinetics for MPE SF6 molecules. 9'1 But, in both regimes of high and weak excitation of SF6 molecules V-T relaxational rate constants exhibit the same dependence on the mass of collisional partner in the file of He, Ar, Kr.This dependence is well predicted in SSH theory 19 by breathing-sphere approximation 2 '21 shows, that the rate constant of V-T process becomes lower with increasing the mass of collisional partner.
Figure I Block-scheme of experimental setup for investigation IR laser induced vibrational relaxation of MPE SF6 molecules, PRmphotoresistor Ge-Au (77 K),PM--photomultiplier.