Kinetic spectroscopy of the infrared multiplephoton absorption by SF6 and SF6-noble gas mixtures in a CO2-TEA laser pulse

ox‘- 8539/90$3.00+0.00 0 1990Pergmm Pressplc

SpwmchimicaActa,Vol.46A,No. 4,pp.623-626.1990. F'rinted inGreatBritain. SPECTROSCOPY OF THE INFRARED KINETIC NOBLE GAS MIXTURES IN A CO2-TEA LASER

M. Lenai*. x

** ***

E. Molinari'~***.

MULTIPLEPHOTON PULSE

G. Piciacchia**,

V.

ABSORPTION

Sessa***

Avanrate Inorganiche. Metodologie Istituto di 00016 Roma. Italy Scala, Monterotondo Area della Ricerca, C.N.R.. Servizio Laser, P.O.10. Italy Roma, e Tecnologie Chimiche. di Science Dipartimento Italy. 00173 Roma, Via 0. Raimondo,

and

BY

SF6

SFg-

Il. L. Terranova***

C.N.R., Monterotondo II

and

Universita'

P.O.Box

10.

Scala.

00016

di

Roma,

multiple-photon This paper reports the results of a kinetic study of infrared the with Argon and Krypton during in neat SF6 and in SF6 in mixtures absorption set up is 5 ~1s pulse of a TEA-CO2 laser tuned to the P(20) line. The experimental similar to that described in a previous paper (1). The method, which is based on the simultaneous recording by a box-car of the outputs of 3 photon-drag detectors and peak monitor incident energy (JO), residual energy (Ji) continuosly which allows the laser beam. axis of the initially gaussian the (J2) at energy molecule P(20) photons absorbed per SF6 of the average number of determination of time and fluence F (J cm-Z) during the (photons molecule -1) as a function Data reduction yields, as a function of time, the mean values in the laser pulse. the effective radius of the laser beam cell of the following quantities: r(t) = tro 0.025CJl(t)/J2(t)10'510~5, the mean fluence experienced by the SF6 molecules P(t) = (Jo(t) Jl(t))0*5/(nF(t)2), of photons absorbed by a molecule and the gaussian average of the average number = (JO(tl-Jl(t))/(hvlt~(t)LNo) nG(t) d:n > number of SF6. The average number L the cell length and No the density with of photons absorbed by a molecule at the uniform peak fluence F is computed by a proper decomposition procedure which takes into account Lhe functional dependence of on F (2). Differentiation of with respect to F gives the values of the differential cross section (I(t) for the absorption process. laser power Experimental data obtained for SF6 at 1.0 torr and two different we report, as a function of F',
c 25

0.2

0.4 0.6 T (Jcm+)

FIg.l-nm ~~OfPhOtOns~prSFgmlruls~vr.t)*ulflurrr~ lanr 0.75 Y. 10 Hz.Dash-w lfnt:~ 1ur0.28H.10 litautura data: (a)

ids.

(3,4), (b) W.(l).

(c) ref.@).

623

0.8

for1.lNJtm~SFg.~sh&ttsd Hm: Hz. Full Il~~~:~rllwrof

~ulculatd

fracmss

q

xectim

624

M. LENZI er al.

FIG.2 - Ths absarptim dlffarmtial EIWSLsection bvs q~ for 1.00 torr SF& lx&&wad Hm: Cq laser 0.75 w. 10 HZ. Dash-tt~~ d&tad line: + law 0.20 X, 10 Hz. Full lines: literatara data of &sorption csatla~s: (a)refs.(3.4). (b) I+. (0). (c) ref. (0).

refer to: a) systems Literature data (a), (b) and (c) shown in Fig. 1 and 2 absorption (3,4): in Boltzmann equilibrium and essentially unperturbed by photon and b) systems in which vibrational modes are equilibrated but rotation data for pulsed experiments with a translation remain "cold" (5.): c) recommended laser profile derived by interpolation from a selection of literature smoothed collisionless conditions. data (6). which refer to nearly then progressively The experimental curves of Fig. 1 start close to curve c) deviate from it in a more marked way the lower the laser intensity, and after a value of F depending on irradiation conditions, run parallel to curve b). reflected in the trends of the The behaviour illustrated in Fig. 1 is clearly cross sections curves of Fig. 2. where one appreciates the the evolution of values close to those measured in system from curve c) to curve b), i.e from presence of rotational bottleneck to values close to those obtained under conditions The observed departure from vibrational equilibrium. conditions of bottleneck effects and rotational holes determine the effective values of where two different processes eventually leading the cross sections can be ascribed to rotational relaxation and collisional v-v an increase of the cross section, to molecules. As transfer which redistributes vibrational energy among all energy it is to be noted that at 1 torr SF6 process is involved, far as this latter calculated experience about 10 collisions in 1 (4s whereas it has been molecules vibrational (7) that a few collisions are required for a quasi-equilibration of of present experiments both the above mentioned energy. In the conditions equilibrium processes are sufficiently fast to create a condition of vibrational V-R, T transfer is found on the contrary to towards the end of the laser pulse. degrees (a), and equilibration of rotational and translation be less efficient the laser pulse. is not occurring to an appreciable extent during of freedom Conditions corresponding to a system where the vibrational modes are equilibrated should therefore be and translation remain comparatively "cold" rotation but respectively a/ curves of Fig. 1 and 2 with time and / F and approached curves b). in the final stage slopes corresponding to those of assume should from the analysis of the intermediate stage of Unexpected features however emerge which is characterized by the presence of well defined maxima in the the process, those of curve b). U/ curves with experimental values of o largely exceeding consider the order to account for these unexpected results we are led to In responsible for the possible creation of a parallel presence of a third process, the concentration profile of SF6 within the laser region, accompanied by radial consequence increase of the number density of excited SF6 at the beam axis. As a introduced by X.de Hemptinne (9-111 in the description of the of this phenomenon, (LIO) the number of absorption named "laser induced osmosis” mechanism and effective increase by transport of matter and the resonating molecules may therefore decrease. An number of photons per SF6 molecule would absorbed the pressure increase and of the perturbing effect of LIO evaluation of the

process on the kinetics from a model calculation

of

infrared

developed

multiple-photon on

the

basis

of

absorption the

treatment

may of

be ref.(l2).

obtained

Kinetic The

relevant

output

of

this

kinetic

spectroscopy

scheme

is the

625

ratio

number :*= Nq the / No number density of vibrationally excited molecules Nq to the gas density NO. Values of q obtained for SF5 under present conditions are plotted vs in the earlier stage time in Fig. 3. and found to range from values of 0.3-0.5 the of the process to values significantly larger than unity as 1.2 and 1.5 in intermediate stage while towards the end of the pulse the expected value q = 1 is attained. Values of q > 1 may be thought as due to a LlO process leading to a net inflow of SF5 molecules into the irradiated volume and thus to an increase of value NO. Within the actual number density with respect to the initial the when. at laser framework of a LIO process the final value of q = 1 is reached molecules are released and NqNO, intensity sufficiently low, the inflown

F1E.S- Ihsq wlua VI tim fa 1.00 tag SF~ mbdomd Hs.

llm: I+ her

0.75W. 10 HZ.0ash-e

Mm: e0, 148~ 0.24 W. 10

means of

the same experimental technique we are carrying on a of kinetic IRMPA by SF5 in the presence of a large excess of noble gas. We here the results obtained for Ar and Kr at pressures ranging between 250 and 750 torr. Fig. 4 reports the time dependent trends of beam the effective radius after the cell r(t) for Kr at different pressures. A roughly constant value depending on the pressure is attained after an apparent focusing effect occurring during the early stage of the runs, a behaviour very similar to that reported for Ar (1). BY

analysis report

FIG.4 - TI* wmtin

rdi~

or th I~U bw * thr 4b80m0n 031148l furtim d pm4sum:duMlim24Otorr:dnMamd llm376tu-r: a it 0.15 m.

PCIF0)- 0.2taT. m InitCl nlu

Experimental plotted in Fig. fluwncw.

ttr. cq IU~ 1.4 Y, 10 dmh-bc4atm lhw670 tax

O values for 0.2 torr of SF5 in mixture with Ar and 5a as a function of time and in Fig. 5b as a function of

oh. llm

Kr are the mean

626

M.LazteetuI.’

I

0

1.0

2.0

3.0

4.0

0

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I

0.2

0.4

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In the presence of a noble gas the total absorbed energy per molecule of SF6 vibrational is made up of two terms: the energy stored Per molecule as the absorber to transferred molecule of the energy and that Per translational modes of the buffer bath. In this experiment a quantity relevant to energy which deposition process is << AE>>, the average amount of the energy noble vibrationally excited SF6 molecules transfer, Per collision, to the gas atoms. A linear dependence of << dE>> on the vibrational energy content has been observed for the system SFg-Ar (13) with << do>> = 4.10~~ (<< AE>>: photons molecule-l). Based on this linear dependence a kinetic model calculation of the type , reported in ref(l) has been performed which enables one to evaluate , provided that the and tnT> under various experimental conditions and << AE>> functional dependence of (r on is known. reference A proper selection of the Cross section leads to more curve b) of Fig. 2 as the appropriate one in describing a like that under investigation characterized from the sys tern beginning by the absence of 1 imiting bottleneck effects, and thus in a and rotational vibrationally equilibrated situation, whereas the translational modes remain “cold”‘. A comparison between calculated and experimental data, made with the appropriate strongly points decomposition (2) shows however some anomalies and procedure again to the presence of radial gaussian concentration profiles along the beam A value of << dE>>/ = i.5*10-4 has been obtained for Kr by taking into axis. this parallel and perturbing effect. for account Preliminary results obtained effjciency of V-T and Xe seem to indicate a well defined trend of the He, Ne Infact << dE>> atomic weight of the noble gas. relaxation rates of SF8 with the decreases regularly from He to Xe.

REFERENCES 1) 2) 31 4) 5) 6)

71 8) 9) 10) 11) 12) 13)

M. L. Terranova, Spectrochim. E.Molinari, G.Piciacohia, V. Sessa and M.Lenzi. 137 (1987). Acta && V.Sessa and M.L.Terranova. Opt.Comm. (in press). M. Lenri. E.Molinari, A.V.Novak and J.L.Lymann, J.Quant. Spectroscopy Rad. Trans. l5, 94.5 (1975). E.Yablonovitch and N.Bloemberger, Phys.Rev. a. 3094 (1981. 1 H.S.Kwok, Chem.Phys.Lett. 62. 443 (1979). U.Schmailzl. Multiple-photon excitation and J.L.Lyman. G.P.Guigley 0. P. Judd in: and C.D.Cantrel 1 (Springer Verlag, dissociation of polyatomio molecules, ed. 1988). E.Tzidoni and I. Oref, Chem.Phys. &4. 403 (1984). W. Braun. M.D.Sheer and B.J.Cveranovic, J.Chem.Phys. @., 3715 (1988). X. de Hemptinne. IEEE Journal of quantum Electronics, OE-21. 755 (1985). X.de Hemptinne, J.Chem.Phys. m, 1824 (1987). X.de Hemptinne, Spectrochim. Acta 43A, 155 (1987). Chem.Phys. G.Piciacchia, V. Sessa and. M. L.Terranova, M.Lenri, E.MoJinari, (submitted). K.M.Beck and R.J.Gordon, J.Chem.Phys. z, 5881 (1987).