Non-thermal laser induced chemistry at high reactant gas pressures

Volume 60, number 2

NON-THERMAL

CHEMICAL

1 January 1979

PHYSICS LE-JTEXS

LASER INDUCED CHEMISTRY

AT EIIGH REACTANT

R.C. SLATER and J_HH. PARKS Avco Everett Resezrch L&oratory, Inc.. Everett, Mas.wchusetts

02149,

GAS PRESSURES’

USA

Received25 August 1978 The laserinduced decomposition of HCCW2 was studied *undercollisional conditions using time resolved probing of the dominant reaction products. The reaction was shown to be non-thermal, and a lo=er bound of 43% of the absorbed laser energy is charmeled into decomposition_

ly excited HCl was probed via &tie resolved infrared e-mission from the u = 1 state near 3 _6fl.

1. Introduction In this letter we report quantitative measurements of the yield of the high power infrared laser induced

2. Experiment

dissociation reaction of chlorodifluoromethane HCClF, * CF2 + HCl,

E, e53

k&/mole

(1)

measured during and immediately following a 200 ns laser pulse. Extensive research has been reported recently on laser driven dissociation reactions in the collisionless regime [ 1] where multiphoton absorption processes have been shown to be essential to a description of the dissociation mechanism. However, the experiments reported here were performed at HCCIF, pressures up to 60 torr where hundreds of collisions can occur within the laser pulse width. Nevertheless, the data indicate that reaction (1) proceeds by a nonequilibrium mechanism, i.e., the HCClF, vibrational temperature is not equal to the ambient-gas temperature during the reaction. Previous studies [2,3] of CO, laser induced decomposition of HCClF* indicate *Aat reaction (1) as well as the recombination of CF, radicals: CF2 + CF2 --f C2F4

(3

are the dominmt chemical processes_ Ir: the current

experiments the ground vibrational state of the CF2 radical was probed by time resolved W absorption spectroscopy at 249 nm and, in addition, vibrational* Work q-red Inc.

by the Avco EverettResearchLaboratory.

The derails of the experiment will be reported elsewhere [44!_Briefly, a Lumonics CO2 TEA laser was modified to produce single longitudinal mode output [S] (see fig- 1) with a near gaussian spatial profile (l/e full width = 2.5 mm). The laser was tuned to the R34 (9.2 p) transition and irradiated samples of HCCiFz with an intensity varied between 0.3 and 3.0 MW/cm2 (pulse width x200 ns). A uniform interaction volume was obtained by adjusting cell length and reactant pressure to limit the CO, laser energy deposited in the gas to less than 30% of the incident laser energy. Absolute laser energy deposition was obtained from pulse transmission measurements using two Au:Ge detectors (risetime < 20 ns) and a caliirated energy meter. A high pressure cw Hg arc lamp provided a broad band W source which was spectrally resolved with a 114 meter monochromator (th ~5 run) and detected with a lP28 photomultiplier (risetime of UV detection electronics = 10 ns). T%e collinear laser pump and probe beams were directed down the 0.4 cm length of the reaction cell. HCL fluorescence was monitored with an InSb detector (rise time = 140 ns) through an infrared interference filter (&,, = 3 9 CL,AX = 0.35 p) and an HCl cold gas filter cell (path length = 82 cm)_ The interference filter isolates the HCI (uv= I--+ 0) P branch transitions from the C-H stretch and the first over27.5

Volume 6F, number 2

CH EXICAL PHYSICS LJZITERS

tone of the antisymmetricC-F stretch in HCCiF,. The HCI cold gas falterverified that HCl was the emitting product species. TypIcal single shot oscilloscope traces of&e CF2 absorption and HCl emission signalsare displayed in Egs la and lb, respectively_These data resulted from irradiationof 60 torr HCClF* with a laser intensity of = I .5 MW/cm2_The laser pulse whkfr appearson one of the dual beam oscilloscope channeIsservesas a convenient time origin. Note that both products begin to form within the laser pulse wid*& In the case of the IICI emission signal the risetime is convolved with the hSb detector response time. Hence, fig_ 1b represents a iower limit to the HCI production rate_Tbe absorption coeffident (base e) of CF, at the probe waveIength has been reported to be E = 17.5 X 1O3 Q/mole cm at 300 K [6) and 3 X 10: Q/mole cm at 1800 K [7 3_ &kg the 0.4cm path Iength and the absorption cross section 3.05X IO-l7 cm2 (caicufated from ref. [6]), Freer’slaw yields a peak CF2 ground state number density of 8 X 1016 cm-3 generated after 1 jfs at

1 January 1979

an incident Iaser intensity of 1S MW/crC2. CF2 ground state yields as a function of incident Iaserflux at 60 torr HCCIF2 are displayed in fig 2 under a variety experlme~tal conditions. Expeiiments

of

were performed using either collinear pump/probe

geometry or else a SO” intersection of the two beams,

and l ,he monochro.mator b~d~d~

was reduced to I -2 nm in some experiments. As indicated in fig. 2, the experimental data were independent of these changes. The estimated total error in each data point is about f 15%_ Also displayed in fig. 2 is the upper l&nit to the possible CF2 ground state yield. This limit is based upon the measured absorbed laser energy and the heat of reaction (I) (=50 k&mole). The contributicn to the reaction yield from vibrationallyexcited CF2 was estimated by assuminga CF2 vibrationaltemperature of = 1000 K. The InitialCFz vibrationaldistribution (2-v =S1160 K) was measured under collisionlessconditions [8] _We approximate the distribution at I p assuming vibrationalrelaxation is not significant. Using the measuredlaser energy absorbed under conditiors similarto fig. 1, IOV2 J, and the total peak CF2 density, I .8 X lOI7 crne3 an averageof 240 CO2 quanta were required for ea& CF2 formed as compared to

PEAK

lV

‘3

CF2

YIELDS

ov f

EXPERIMENTAL CONDITIONS

--THERMAL HEATING

MODEL

Fig. f . (a) Top Qace: zero Ievefof the UV absorption signal. Middle trace: time resoWedW absorption ofCF2, Bottom trace: CO, Iaserpuke. (b) Top trace: time resolvedWCS IR emissionsign& Noise fromtheCOa Iaserdisctirgeappears prior to the CO2 laser pulse. Bottom trace: CO2 laser pulse. AU tracesswept at 500 ns[division_Experimentalconditions: = 15 ~/cm* and +&-cUr, = 60 tax rCe2 276

._

0

2

I

I

5 _ LHWlcm*:

Fig. 2. A comparisonof the measuredCF+ )iefds and the predictionsof a thermalheatingcahxdation(see text). The dotted fine is tha upper &nit to the metsus&CF2 production.

Volume 60, number 2

CHEMICAL PHYSICS LE-I-IERS

the thermal activation energy equivalent of 17 quanta. A dissociation yield of this order of magnitude creates a substantial local pressure rise in the interaction volume_ These time resolved measurements reveal that the CF, density inside the interaction volume decreaSes at 2 rate consistent with g2s-dynamic expansion following dissociation.

3. Results and discussion It is observed in fig. 2 that lowering the incident laser fhrx, at constant HCClF, pressure, from 1 S MSV/ cm* to 0.38 MW/cm* (energy deposited drops by a factor of 3) reduced the peak CF2 number density to 3 X lOI cmd3. To emphasize the discrepancy between this result 2nd a thermal mechanism, 2 rate eqluation analysis was performed to estimate the thermal reaction yield assuming all the laser energy was converted into heat_ Rate equ2tions for the CF2 and C2F4 number densities:

as well as an energy balance relation derived from the heat equation:

AE(r’ - cx *V

--

N

&f(T)dT

x3t,,,

(5) were solved using the measured thermal rate constants for KL [9], K_, [9] and K2 [6] _ Instantaneous V-T rates were assumed; thermal diffusion was neglected on this timescaie; and the driving term, *E(t), absorbed laser energy, was assumed to be 2 linear ramp in time. As 2 consequence of these assumptions, the model predicted upper bounds to the thermal reaction yield. In these equations C$(T) is the temperature dependent heat capacity [lO] of each reactant 2nd product species x; AHi is the standard heat of reaction for eqs. (1) 2nd (2), 50 kcaljmole 2nd -70 kcal,!mole, respectively; and AV is the inte,-action volume. Eqs_ (3), (4) and (5) were solved simultaneously to obtain NCFz and NCZE4 at I p. Reaction (2) contributes a negligible amount of energy to the overall decomposition of HCCIF, since its rate is at least an order of

1

January 1979

magnitude slower than the CF, thermal production rate. The results of the thermal heating model are displayed in fig. 2. These equations indicate that 2 factor of three decrease in the absorbed laser energy would result ln a decrease in the absolute temperature from 1500 K to 700 K. As 2 consequence, the corresponding CF, yield would decrease by five orders of magnitude s&e the thermal activation energy for process (1) is about 53 kcaljmole. The dependence of the measured yields upon incident laser intensity is not described by a high temperature equilibrium model. In 2 related series of experiments the overall heat capacity per unit volume of the sample was increased 2 factor of two by the addition of 460 torr of argon to 60 torr HCClF, _ The absorbed laser ener,T was the same in both samples yet the peak CF, number density in the mixture was within 10% of the peak number density obtained in the pure samples_ A purely thermal reaction would have produced 2 decrease of three orders of magnitude in the yield due to the argon addition. In addition, the risetimes in both samples were identical_ Compa_mble results were obtained at lower 2nd higher Ar/HCClF, mole fractions_ Finally, an estimate of the HCI (u = 1) number density of = 10ls cmm3 based on the magnitude of the peak IR fluorescence signal, the collection efficiency of the infrared optics 2nd the responsivity of the InSb detector is also inconsistent with the equilibrium temperature predicted by the rate analysis for a thermal reaction mechanism. The translational/rotational temperature duriig the first few microseconds of reaction was obtained from a quantitative analysis of the HCi infrared fluorescence transmitted through an HCl cold gas filter cell. A theoretical fit of the measured transmission was obtained by including fluorescence contributions from each of the HCl P branch transitions, P(4) to P(17), and accounted for pressure broadening of these transitions in the emitter as well as the cold gas absorber. Measured linewidths [ 111 were used for the absorber, 2nd the emitter linewldths were scaled from theory [12] to include foreign gas broadening by HCClF,_ The only temperature dependent term i-r the transmission expression was the fraction of molecules in each emitting HCl(u = 1, J) state. The distribution was assumed to be Boltzmann at the rotational temperature which was taken to be equal to the translational temperature at these pressures. Our best esti277

V&me

60, number 2

CHEMICAL

PHYSICS

LETTERS

1 January 1979

mate for the translational/rotationaltemperaturemeas-

Acknowledgement

ured under the condition described in fig_1 is 300509 K. The quantum GeId result descriied above indicated

The authors would Iike to thank Dr. Chas. von Rosenberg, Dr_ Mark Kovacs, and Dr_ Alexander

that an averageof 17j40 {43%) of the iaser energy was accounted for directly by CF2 product. The partitioning of the remainderof the absorbed laser energy, ==57%, has not been established. One possibIe expianation for the remainderof the energy is that the gasdynamic expansion introduces an uncertainty in the

BaIIantynefor many stimulating conversationsduring the course of this work and Mr. Paul FaIkos for performing the experiments.

measurement of the peak CF2 density_ Initial experi-

References

ments designed to confine the interaction volume with rigidwalls indicated that the correction to ‘ihe CF, density could be as great as a factor of two. A second possi%iIityis that at the upper &-nit of the temperature rangeindicated by the cold gas filter result, as much as 20% of the laser energy could have been converted into thermaIenergy. Finally, it should be pointed out that a fraction of the remainingenergy is converted into directed motion of the gas inside the interaction volume as it expamis againstthe surroundings. Additional experimentsare underwayto providea more defiitive understandingof the energy partitioning. In conclusion, the Iaserinduced decomposition of

HCClF2 has been studied under collisional conditions via time resolved probing techniques. These experiindicate that the amount of product formed during reaction is not describedby typical t.hermaI Arrheniusbehavior [9] assumingthe Iaserto be nothing more than a nonseiectiveheat source. The coIIisionaI and multiphoton contriiutions to the reacrion mechanism are currently under investigationments

[ 11 M-3. CoggioIa, P_A_ Schultz, Y-T. Lee andY.R Shen, Phys- Rev. Letters 38 <1977> 17;

J-G. Black,E. Yablonovitch, N. Bloembezgen andS. hfukaxmel,Phys. Rev. Letters38 (1977) 1131. [Z] AaS_ Sudbo.P.A_SchuIz.E-R. Grant.Y-R. Shenand Y-T. Lee,J.Chem.Phys_68(1978) 1306. [3 ] E. GrunwaId, K J. Olszyna, D.F_ Dever and B. Knkhkowy, J. Am. Chem- Sot_ 99 (1977) 6515. [4] R-C_ SIater and J.IL Parks, unpublished. [S] Bf.Xf_T_ Loy and P-A. Roland, Rev_ Sci. Instr. 48 (1977) 554_ i6] W-J-R. Tyerman, Trans. Faraday Sot. 65 (1969) 1188. 171 AS. hfodica, J. Phys. Chem. 72 (1968) 4594. [8] D-S_ King and J-C- Stephenson, J. Chem. Phys. (1978), to be published_ [9] G-R. Barnes. R.A_ Cox and R.F. Simmons,J. Chem. Sot. B (1971) 1176. JANAF Thermochemical Tables, 2nd Ed., NSRDS-NBS 37 (National Bureau of Standards, Washington, 1971). I-L Babrov, G. Ameer and W_ Benesch, J. %foL Spectrf. 3 (1959) 18.5; R-A. Toth, R-H. Hunt andE-K. Plyler,J. hfoL Spectry. 35 (1970) 110. P-W. Anderson, Phys_ Rev_ 76 (1949) 647.