Vibrational energy transfer in CH3I

Chemical Physics 14 (1976) 375-383 © North-Holland Publishing Company

VIBRATIONAL ENERGY TRANSFER IN CH 31 Y . LANGSAM, S .M. LEE and A.M. RONNt The City University of New York, Brooklyn College, Department of Chemistry, Brooklyn, New York 11210, USA

Received 12 January 1976

Previous works have reported vibration-vibration and vibration-translation transfer rates in the methyl halides . Using the technique of laser induced infrared fluorescence we have studied energy transfer in the concluding member of this series, CH 3 I. Following excitation by resonant lines of a Q-switch CO. laser, infrared fluorescence has been observed from the vZ , v s as well as the 2v s , v t , v4 vibrational energy levels of CH 3 I. All the observed states exhibit a single exponential decay rate of 23 t 2 ms7 1 tori t . Measurements have also been made on deactivation of the various modes by rare gases. The risetime of the v; , v5 levels was found to be approximately 101 ± 20 ms-1 for{r, while that of the 2v s , v t , v4 levels was approximately 225 t 45 ms-1 for1 . Fluoresecence was not detected from the v 3 level . These results are discussed in terms of SSH type theoretical calculations, and comparison is made with the results obtained for other members of the methyl halide series, namely CH3F, CH3 C1 and CH 3Br.

1 . Introduction The technique of laser induced infrared fluorescence has been utilized in the study of vibrational energy transfer in a number of polyatomic species [1-61 . Of particular interest is a comparison of a series of molecules which are similar in structure and symmetry, yet exhibit particular differences . One such series which has been studied extensively is the methyl halides [7-9] . In this paper the concluding member of the series is investigated and compared to the other previously studied species. in a polyatomic gas two types of collisional energy transfer processes take place . The first is a transfer of the excess,energy supplied by the laser to vibrational levels other than the excited level (V-V) . Such a V-V transfer is usually more rapid than the second kind of process, i.e., the conversion of vibrational energy into translational and rotational degrees of freedom (V-T/R) . By studying the rates of these processes for each vibrational state as a function of pressure, a detailed kinetic model of the energy transfer mechanisms can often be obtained . Alfred. P. Sloan Fellow.

The rate of equilibration of the various modes suggests, as with the other methyl halides, that energy transfer takes place via a rapid propagation of energy up the vibrational manifold by a series of efficient near resonant collisions . Subsequently, V-V energy transfer distributes this excess energy among all other vibrational levels . Deactivation then proceeds through V-T/R energy transfer of the lowest level . These conclusions can be borne out by examination of the rare-gas dependence of the measured rates . Furthermore, by comparison with theoretical models and the known experimental results available for the other members of the methyl halide series, one is able to discuss the relative importance of vibration to translation (V-T) energy transfer rate as opposed to vibration to rotation (V-R) relaxation in the total deactivation scheme .

2. Experimental A detailed description and block diagram of the laser induced fluorescence apparatus used in these experiments has appeared in earlier works [5,10] . A Q-switch C02 laser, tuned to the P(30) line of the 10.6 u transition of CO 2 was used to pump the CH3 L

Y. Langsam et ail: f Vibrational energy transfer in CH3I

376

This laser line is most likely absorbed by the RQ 6 line of the v 6 (methyl deformation) band of methyl iodide 1111 . The laser beam traverses through a cell contain-_ ing a specific pressure of methyl iodide, and infrared fluorescence is detected at right angles to the beam axis. Intensity of fluorescence is monitored as a function of time after excitation . By the use of suitable combinations of interference filters and infrared detectors, different spectral components of the fluorescence signal can be isolated . The detector output is them amplified, averaged on a waveform eductor, and displayed on a X-Y recorder for analysis . Methods of analysis have been discussed elsewhere . Fluorescence was detected from CH-, in three distinct regions . An InSb photovoltaic detector, cooled to 77 K, was used to observe the -3 p fluoresecnce, while a photoconductive Au :Ge, cooled to 77 K, was used to study the -7 s emissions . For both regions, a MgF 2 fluorescence window was utilized . An attempt was made to study the ? 9 u fluorescence using a Ge :Cu detector cooled to 4 K . At this wavelength, a KBr window, a combination of two filters, a 13 u longpass and a 14-22 u bandpass, to suppress scattered laser radiation, were used . Unfortunately, for reasons to be discussed later, the signal to noise ratio was too low and therefore pressure dependence measurements of 191! emission were not feasible- The close proximity of the v2 to v5 and 2v5 to v l and v4 levels did not allow complete isolation of individual vibrational states, even with the use of appropriate filters, and thus the 3 p and 7 p signals referred to are due to all states at the respective region . The electronic response time for the entire system was <1 .0 µs. All measurements were made at room temperature (295 ± 3 K) . Gas pressures were measured with a capacitance manometer attached directly to the fluorescence cell . Typically, the leak rate of the entire gas handling vacuum system was less than 10 mtorr/h . Reagent grade methyl iodide was obtained from the Allied Chemical Company with a stated purity of 99.99%. Further purity was achieved by vacuum distillation of the sample prior to each measurement . With the exception of 314e, rare gases were Matheson research grade with impurities of greater than 1 ppm listed as He(Ne < 5) ; Ne(He < 10, H2 < 4, Ar < 5); Ar(N2 < 5); Kr(Xe < 25, N 2 < 25, 02 < 4, Ar < 4, H2 < 5, hydrocarbons < 10) ; Xe(Kr < 50, N 2 < 10, 02 < 5, Ar < 5, H 2 < 10, hydrocarbons <10) . The

1.

4

3 He

was obtained from Mound Laboratories and was specified as having 99 .5% isotopic and 99.9% molecular purities . Activation and deactivation measurements on the pure sample were made by adding incremental amounts of CH3 I to the cell . The CH 3I-X (X= He, He, Ne, Ar, Kr, Xe) deactivation rates were obtained by adding small amounts of these gases to a set partial pressure of CH3 I already in the cell . Sufficient time was allowed for complete mixing of all gases before measurements were begun . Repeated runs on freshly distilled samples yielded consistent results over a long period of time .

34

3. Results A partial energy level diagram of CH 3 1 [11,121 is shown in fig . 1, wherein we have indicated the relative position of the pumped level, v6, (heavy arrow) and the levels from which fluorescence was observed (curly arrows) . Subsequent to irradiation of CH31 with the P(30) line of the 10 .6 p CO2 laser band fluorescence is detected at three distinct wavelengths, 4000

v,+vy

v,~

3000

2v=

E

tt t

vjivevs

v5+vs

3±5 W 2000

z

Vs

TT I'll

1000

2v,

vs T 3 COt LASER

0 CH 1 I ENERGY LEVELS Fig. 1 . Partial vibrational energy level diagram for CH 3 I'



V rnnarnm at al IVihmtinnal enernr tramfer in

CHI

377

Fig. 2. Oscilloscope trace of 3 u fluorescence after signal averaging .

19 g, -7 g, and ^3 p which corresponds to emission from the v3 ; v2 , v5 ; and 2v5 , v r , v4 levels respectively . A typical oscilloscope trace of fluorescence intensity (3 u) versus time for pure CH 3 I is shown in fig. 2 . The

Fig. 3 . Plot of 7 u fluorescence activation rate as a function of CH 3 I pressure .

Fig. 4 . Plot of 3 p fluorescence activation rate as a function of CH 3I pressure.



-

Y. Langsam et at /Vibrational

3 78

energy transfer in CH31

160

140

120

S 0100 q E Z 0

a

so

fv a w UO 60 0 W F Q z 40

Scope = 23 .8 t 24 msec-'torr - '

O O 0 0

20

00

0

6 PPHs Fig . 6. Plot of 3 p of CH3 I pressure .

Slope =22.1 t 2 .2 mseetorr'

1 2

I 4

1 6

4 8

PCH,I (Torr)

Fig. S. Plot of 7 p fluorescence deactivation rate as function of CH3 I pressure. signal consists of a fast risetime followed by a slower exponential decay . Rates of activation and deactivation are obtained by calculating the exponential constant, 1/r, of the corresponding portion of the signal . The failure of the falltime portion of the signal to return to the initial baseline is attributed to translational heating of the gas . Upon the addition of a heavy rare gas this heating is greatly reduced . The rare gas, by increasing the heat capacity of the mixture, allows the hot CH3 I molecules to return to ambient temperature through heat and mass transport to the cell walls . Plots of rates of rise and fall times versus CI1 3I pressure, of the various signals are shown in figs . 3-6. The slopes of these lines, calculated by the least squares method, are the corresponding V-V and V-T/R rate constants. Limits of error are assigned at

I

10

(Tory)

fluorescence deactivation rate as a function

20% for risetimes and 10% for fall-times . All of the plots, within experimental error, intercept through zero at zero pressure consistent with a collisional mechanism . Very poor S/N for the 19 g emission due in part to the low dipole moment matrix element and in part to the very long wavelength of the fluorescence precluded a precise measurement of the rate constant for the v 3 decay . Fluorescence from the v2 , v5 levels (7 p) indicate an activation rate constant of 101 ± 20 ms -1 torr - 1, while the 2v 5, v1 , v4 levels show an activation rate constant of 225 ± 45 ms -1 torr -1 . Table I Vibrational relaxation rates of

CH3 I (ms'r

torr" r )

7u-v2 ,v5

3µ-2v5 ,v 1 ,v4

22.1 ± 2.2 7.4 ± 0.7

23.8 ± 2.4

4 He

5 .6±0 .6

4 .8 ± 0 .5

Ne Ar Kr Xe

1 .9±0.3 2.2±0 .2 1 .6±0.2 1 .3±0.1

1 .5±0.2 1.6±0 .2 2.1±0.2 3.2±0.3 .

CH 31 3 He



Y. Langsam et al.I Vibrational energy transfer in CH3I

379

Table 2 Probability of deactivation Collision

partner 3 He 4 He

Ne Ar Kr Xe CH3 I

p(CH 3 1-X) (amu) 2.94 3.89 17 .67 31 .17 52 .69 68_21 70.97

A112 1.71 1 .97 4 .20 5 .58 7.26 8.26 8.42

aa) (A)

Rateb) (ms't torr't )

Z * x 103 c)

Pexp X 104 d)

Ptheor X 104 e)

2 .60

7 .4 5.6 1 .9 2.2 1.6 1.3 22.1

3.57 4 .00 4.56 4 .35 4 .76 5.56 0.452

2 .8 2.5 1 .8 2.3 2.1 1 .8 22.1

297 216 11 .6 2 .19 0.224 0 .0574 0.0701

2.60 2.80 3.42 3.60 4.05 5.73 0

a) Ref. [151 . b) Experimental relaxation rate of 7 p fluorescence . c) Z * = 1/Pexp . (1) See eq. (2) in text. e) Relative theoretical probability/collision, calculated using SSH theory . f) Ref. [161 .

The V-T/R relaxation rates were measured for pure methyl iodide as well as with 3 He, 4 He, Ne, Ar, Kr, and Xe as collision partners . These V-T/R relaxation rates are the same (for the same collision partners), within the assigned 10% limits of error, for all fluorescence wavelengths. For CH3 I-CH3 I deactivation the rate constant is 23 ± 2 ms - I torr-1 . Relaxation rates of all the collision partners are presented in table 1 . Plots of deactivation rate versus rare gas pressure at constant CH3 I pressure intercept at the rate which would be obtained for a given pressure of pure methyl iodide . In almost all cases the rates obtained from the intercept data are in good agreement with the data obtained for the pure gas . Table 2 summarizes all other relevant parameters which will be useful in comparing different members of the methyl halide series from both long and short range force theoretical points of view . The gas kinetic collision rate, Z, is defined as : 6 Z = 2 .56 X 10

o2µ i/2 s 1 tort 1

,

(1)

where a is the mean of the radii of the colliding pair, and p is their reduced mass . The probability that energy will be transferred per collision is therefore given by : Pexp = Rate/(Z X 10'3) ,

(2)

in which the constant accounts for the change in units . Finally the theoretical probabilities have been calculated using SSH theory [13,141 . These will be discussed more fully below.

4 . Discussion 4.1 . V--V energy transfer

The ultimate goal of energy transfer experiments is the understanding of the mechanisms by which internal energy is distributed among the various levels of the moelcule. Unlike CH3 Br, in which only one risetime was measured for all levels above the pumped state [7-91 , CH 3 I exhibits two distinct risetimes . Examination of table 3 suggests a possible explanation for these observations . A probable mechanism must be reasonably direct, i.e., number of vibrational quanta exchanges must be small, and the energy deficit, €E, must be low. With the exception of resonant collisions, we may reasonably state that all collisions occur with a ground state molecule as a collision partner due to the relative population of the various levels . In general, the collisional population of an overtone by its fundamental, such as process (a), (b), and (d), is expected to be very rapid . Such an "up the ladder" energy transfer accounts for the almost instantaneous activation of the entire v6 vibrational manifold . A likely crossover from the vibrationally hot v 6 manifold into the v4 level, process (c), is then a candidate for the observed rate of rise of the v 1 and v4 levels near 3000 cm -1 at a rate of 225 ± 45 ms -1 -tort-1 . Energy may also be transferred directly from the pumped state v 6 to levels v 2 , v5 , which are presumed to be coupled by Coriolis resonance [201 via processes such as (d) and (e). Again the energy is



Y. Langsam et at f Vibrationatenergy transfer in CH3I-

380 Table 3

Collisional pumping process - Process

AE(cm`' ) a)

CH 3 1(v6 ) +CH3I(v6) : CH3 1(2v6 )+CH3I(0) CH 3I(2v 6) + CH3 10,6 ) 7_ CH 3 I(3u6 ) + CH3 I(0) CH3 I(3v6 )+CH 3I(0) : CH3l(v4) +CH 3 I(0)

"-10

a b

440

c

371 189

d e

-19

f

109

g

-10 '--10 241 109 90 19 -189

h i j k 1 m n

CH3 I(v 2) +Ch31(0) .--CH3I(v5) +CH3 1(0)

371 189

0 p

CH3 I(v3 ) + CH 3 I(v5 ) t CH3I(2v5 ) + CH 3 I(0) CH 3 I(2v 5 )+CH 3 [(0) t CH 3 1(v t ) +CH 3 1(0) CH 3I(v i) +CH3I(0) 2CH31(v2 ) +CH3 I(0)

-19 109 90

q r s

CH3I(v6) +CH 31(0) 2CH 3 1(v 2) +CH3I(0) CH3 I(v2 ) +CH 3 I(0) tCH 3 I(vs ) +CH3 i(0) CH3 I(v5 ) + CH3 I(v 5 ) t CH 3 I(2v5 ) + CH 3 I(0) CH3 I(2v5 ) + CH3 I(0) t CH 3 I(v t ) + CH3 1(0)

CH31(2v6) + CH 3 1(v6) : CH 3 I(2v6 ) + CH 3 I(0) CH 3 I(2v6 ) + CH3 I(v (,) t CH 3 I(3v6 ) + CH 3 I(0) CH 3 I(3v6)+CH3 I(0) t CH3I(2v5)+CH31(0) CH 3 I(2v5 )+CH 3 I(0) tCH 3 I(u t ) +CH 3 1(0) CH 3 I(v t } +CH 3 I(0) tCH 3 I(v4) +CH3 I(0) . CH 3 I(2v5 )+CH 3 I(0) FCH 3 I(v5 ) +CH 3 '(v s ) CH 3 1(vs ) +CH 31(0) CH3 I(v 2) +CH3 I(0) CH 3 I(v 6 ) + CH 3 1(0) t CH 3 I(v2 ) + CH 3 I(0)

-10

a) Amount of energy given up to translation/rotation.

transferred almost instantaneously to 2v 5 , see process (t). Thus the observed rate of 101 ± 10 ms-1 torr-1 measured for the risetime of both v 2 and VS is most likely to be the rate of the kinetic limiting step, i.e., the transfer of energy from v 6 to v2 , v5 . Processes (a)-(g) can therefore account for our observation of two distinct rate constants . On the other hand, were energy transfer to take place by an exchange of energy between v4 and 2v5 , v 1 either in a mechanism such as process (h)-(n), i.e ., up the v6 manifold and then down to all other levels, or by a mechanism such as process (o)-(s), i.e ., up to v 4 by way of v 2 , v5 , the same risetime rate should have been measured for the v 2 , VS and the v1 , v4 levels . Since no direct measurement of the rise time of 2v 5 is experimentally feasible, we distinctly favor mechanism I, namely the rapid "up the ladder" population of the v 6 manifold to a direct crossover into the v4 state, yet cannot completely rule out the competitive step described by equations (h)-(s) . It is in fact quite reasonable to expect both processes -to occur simultaneously, as the rate limiting step, the :risetime of v4 and v l will ultimately be controlled by the population of both the v 2 , v5 and v6 manifolds .

Yet, the dominant mechanism must be process (a)-(g) in order for our having observed a rate of 225 ± 45 ms -1 torr 1 rather then 101 ± 20 ms -1 torr-1 for the filling of the v 1 , v4 levels. 4.2. V-T/R energy transfer An examination of table 1 reveals that the rate of deactivation of the pure species as well as deactivation in the presence of rare gas is equal within experimental error for all vibrational modes . As with the other methyl halides [7-9] this indicates a fast V-V equilibration of all the excited vibrational levels followed by a slower V-T/R decay through the lowest fundamental, v3, to the ground state . Vibrational equilibration may be considered to be a Boltzmann equilibration at an elevated vibrational temperature . This vibrational temperature relaxes back to equality with the translational temperature by means of processes such as : CH 3 I(vi) + M t CH3 I(0) + M + AE= Ei ,

(3)

in which the deactivating molecule may be in any state or species . The probability'of a vibrational relaxation process - is related to the amount of energy which must



Y. Langsam et al, /Vibrational energy transfer in CH3!

be given up to or taken out of the translational and rotational degrees of freedom . We would therefore expect the deactivation to proceed by way of the lowest level . In an earlier work [9,19] the importance of the socalled "breathing-sphere parameters" in determining the probability of a vibrational relaxation process is discussed . The magnitude of this parameter is related to the average Cartesian displacement of the surface atoms of the molecule for a unit change in a given normal coordinate . Using the normal coordinate analysis for the methyl halides of ref. [18] and the method of ref. [191, the breathing-sphere parameters for CH 3 I were calculated to be (A 2)=0 .24, (A2) _ (A4) _ (A $) = 0 .22, (A3) = 0 .04, and M6) = 0.34, all in amu -1 . According to Stretton's modified SSH [19] theory of

100

av

c

°o

381

collisional energy transfer the probability of deactivation is directly proportional to the magnitude of the breathing sphere parameters . As with the other methyl halides these parameters are approximately equal for all modes except v 3 , while (A 3) is several times smaller . It might therefore be expected that v6 makes a significant contribution to the V-T/R rate . An examination of table 2 reveals that the CH 3 I rare gas rates decrease gradually from 3 14e to Ne and level off in the Ne to Xe range . Schwarz, Slawsky, and Herzfeld (SSH) [13] and later Tanczos [14] predicted that in a vibration-translation (V-T) deactivation the relative probability decreases with increasing mass of the rare gas. According to SSH theory, the excess vibrational energy is transferred into the translational degrees of freedom and therefore it is only the relative translational velocity of the colliding molecules that determines the probability of deactivation, since it is dependent on the reduced mass of the collision pair for AE > kT. In an alternate theory [17] excess vibrational energy is taken up by the rotational degrees of freedom . Moore proposes that V-R energy transfer becomes important, as opposed to V-T, when the rotational velocity (VR) of the system is greater than the translational velocity . (VT) . One would therefore expect, on the basis of V-R theory, the probability of deactivation to be insensitive to the reduced mass of the system. Thus V-R theory predicts that the rate of deactivation does not depend on the nature of the rare gas atom, and is essentially the same for all such collision partners . Fig. 7 shows a plot of the experimental probability of deactivation versus the square root of the reduced mass of the collision partners . The theoretical curve, calculated using SSH type V-T theory weighted for the Boltzmann population of the lower two deactivating levels, v3 and v6 , is also shown . 4.3. Comparison with CH3F, CH3CI and CH3Br

10 -T

1 I I I I 1cr* 0

2

4

6

8

10

IL lit Fig. 7. Plot of theoretical SSH(e), experimental 3,u(&) and 7 p(o) deactivation probabilities versus the square root of the

reduced mass of collision partners .

Table 4 summarizes the methyl halide deactivation rate constants as well as the energy of the lowest level . As has been mentioned previously the primary factor in accounting for the increase in rate of deactivation is the decreasing energy of the lowest level . The rare gas deactivation rates for each member of the methyl halides given in table 4 are plotted versus the reduced mass of the collision partners_ in fig . 8. Pure V-T theory predicts that the data of fig . 8 should for each methyl

382

Y. Langsam

et al. /Vibrational energy transfer in C!!37

Table 4 Methyl halide deactivation rate constants (ms-I tort1 ) Collision partner

Ee) lowest level

Self

He

Ne

Ar

Kr

Xe

0.78 3 .1

0.065 0 .51

0.039

0.022

0.019

1.24

0.54 1 .01

0.53

4.15 5.6

0.54 1 .29

1.9

2.2

1 .6

1 .3

Molecule CH 3F a) CH3C1 b) CH3 Brc)

1043.2

0.59

732.1

7.5

611 532.8

CH31d)

20

22

1.04

a) Ref. (21 . b) Ref. [8] . C) Ref. [91 . d) Experimental relaxation rate of 7 p fluorescence . e) v 3, all values in cm -1 . l0-s

Table 5 Values of R a)

0

z 0 (n

P

o~

CH3 F CH 3C1 CH 3 Br CH 3 1

o % O

O°

10 -4 0 w cc

A

a

He

Ne

Ar

Kr

Xe

0.99 1 .1 1 .1

1 .9 2.2 2.3

2.2 2 .7 3.0

2.6 3 .2 3 .8

2.7 3.5 4.3

1 .1

2 .4

3.2

4.2

4.7

a) R = VR/ VT = (pd 2/I) 1"2 , calculated from data of ref . [171 .

0

J

with the translational degrees of freedom almost exclusively by a V-R energy transfer . The relative importance of V-R to V-T energy transfer is clearly increasing as we go down the methyl halide series . The ratio [171,

m

a m Er 10-8 Q.

a

a

R = VR/VT = (Ecd2/I) 1 J 2

a

(4)

0

10-8 0

2

4

6 h u

9

10

:

Fig. 8 Results on deactivation of methyl halides by are gases given as probability of deactivation vs the square ,oot of the reduced mass of the collision partners . Points a*e experimental results. CH 3 F-X (o) ; CH3CI-X (o) ; CH3Br-X (0); CH3 I-X (0). halide, lie on a straight line of negative slope . Pure V-R theory, on the other hand predicts an essentially horizontal line for each methyl halide . The deactivation of methyl fluoride by rare gas can be explained almost entirely by a V-T energy transfer . Methyliodide, on the other hand, decays back to equilibrium

where 11 is the reduced mass of the collision pair, d is the radius of the rotor and I the molecular moment of inertia, becomes significantly greater than unity as we progress from CH 3 F to CH3 I as is evidenced by referring to fig . 8 and table 5 . The former, in fact, reveals that the probabilities of deactivation of all halides by helium shows less dependence on the reduced mass of the collision pair than do the heavier rare gases . This may be explained by the facts that the very rapidly moving helium atom itself essentially determines the collision speed . Examination of table 5 shows, in support of such assumption, that R for CH 3 X-He is essentially constant for X=F, Cl, Br, I . Moore's theory postulates that when the parameter R > 1, vibration-rotation relaxation predominates ;



Y. Langsam etaL/Vibrational energy transfer in CH3I while when R < 1 vibrational relaxation is totally translationally dependent . Fig . 8 exemplifies the trend of V--R becoming more and more important as the halide becomes heavier. The series undergoes a dramatic change from the V-T dependence of CH 3 F to the V-R

383

Leonard Gamss for many valuable discussions . This work was supported by the National Science Foundation under grant number GP-43982X and by the City University of New York Faculty Research Award Program .

dependence of CH 3 I. References 5. Conclusion CH 3 I has been vibrationally excited by Q-switch C0 2 -laser pumping of the v 6 mode . Fluorescence is observed from the v 1 , v4 level having a rate constant of 225 ± 45 ms- I torr` 1 , and from the v2 , v5 levels with a rate constant of 101 ± 20 ms -1 tore1 . Energy transfer in the molecule proceeds via at least two independent pathways ; v4 being activated from upper overtones of the v6 manifold while v2 , v5 are excited directly from the pumped state . The general scheme of V-T/R deactivation found in the other methyl halides operates in this case also namely rapid V-V equilibration, followed by a slower deactivation by way of the lower levels . Methyl halides, without exception show one rate of deactivation for the pure gas as well as for the rare-gas deactivators, for each level measured . A unified V-T/R theory is successful in qualitatively predicting the relative efficiencies of deactivation by rare-gases for each of the methyl halides. Considerations of parameters such as energy deficits and breathing sphere parameter are used in understanding the nature of deactivation. Work in progress on all four tri-deuterated methyl halides will add much needed data for support of such general behaviour . I Acknowledgement The authors are grateful to Drs . Boyd Earl and

[11 E. Weitz and G .W. Flynn, Ann . Rev. Phys. Chem. 24 (1974) 275 . [2) E. Weitz and G .W. Flynn, J . Chem . Phys. 58 (1973) 2679 . [31 F.R . Grabiner and G.W. Flynn, J. Chem. Phys. 60 (1974) 398 . [4] S .M. Lee and A .M . Ronn, Chem. Phys . Letters 22 (1973) 279. [5) R.D. Bates, Jr., G.W. Flynn, J .T. Knudtson and A.M. Ronn, 1. Chem. Phys. 53 (1970) 3621 . [61 E.W. Jones, R.J.L. Popplewell and H.W. Thompson, Spechochimica Acta, 22 (1966) 647 . [7) E . Weitz and G .W . Flynn, 1 . Chem. Phys. 58 (1973) 2781 . 181 J.T. Knudtson and G.W. Flynn, J . Chem. Phys. 58 (1973) 2684 . 191 B.L. Earl and A.M. Ronn, Chem . Phys. 12 (1976) 113 . (101 G.W. Flynn, Chemical and Biochemical Applications of Lasers; ed. C.B. Moore (Academic Press, New York, 1974) . j 111 W.T. King, J.M . Mills and B . Crawford, Jr., J. Chem. Phys. 27 (1974) 455 . [12] E.W. Jones and H .W. Thompson, Proc. Roy. Soc . A288 (1965) 50 . [131 R.N. Schwartz, Z .I . Slawsky and K .T. Herzfeld, J . Chem. Phys . 20 (1952) 1591 . [14] F.R . Tanczos,1 . Chem_ Phys . 25 (1956) 439 . [151 J.0. Hirschfelder, C .F . Curtis and R .B . Bird, Molecular Theory of Gases and Liquids (Wiley, New York, 1954) . [16] N. Dyson and A .B. Littlewood, Trans. Faraday Soc . 63 (8) (1967) 1895 . [171 C.B. Moore, J . Chem. Phys.43 (1965) 2979 . [18] J . Aldous and J .M . Mills, Spectrochimica Acta 18 (1962) 1073. [19] J .L. Stretton, Trans. Faraday Soc . 61 (1965) 1053 . [20] A.G. Maki and R .M. Hexter, J . Chem. Phys. 53 (1970) 453.