Gas and solution phase chloroiodomethane short-time photodissociation dynamics in the B-band absorption

30May 1997

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 270 (1997) 506-516

Gas and solution phase chloroiodomethane short-time photodissociation dynamics in the B-band absorption Wai Ming Kwok, David Lee Phillips * Departmentof Chemistry, Universityof Hong Kong, PokfulamRoad, HongKong, HongKong Received 4 November 1996; in final form 26 March 1997

Abstract

Resonance Raman spectra were obtained within and to the red of the B-band absorption spectrum of gas phase chloroiodomethane and chloroiodomethane in cyclohexane solvent. The spectra show the fundamental and overtones of the nominal C-I stretch (nv s) and combination bands of the CH 2 wag (v3), I-C-CI bend (v6), and the CH 2 scissor (v 2) fundamentals with the C-I stretch bands (nvs). The chloroiodomethane B-band short-time photodissociation dynamics have significant substituent effects relative to the B-band of iodomethane due to the presence of the C-CI chromophore n(X)--* g *(C-X) transitions -- 170 nm that are close to the B-band absorption of chloroiodomethane but absent in iodomethane.

1. Introduction

Several of the lower Rydberg states of iodomethane have been extensively studied in the gas phase and in high pressure mixtures using resonance Raman spectroscopy [1-8]. The iodomethane B-state absorption from 191 nm to 201 nm consists of 6s Rydberg vibronic transitions that are rotationally diffuse due to predissociation (however not so fast as to lose vibronic resolution). Thus, the B-state is quasibound instead of unbound like the lower lying A-state of iodomethane. The B-state Rydberg transitions show a strong origin, a moderate progression in the totally symmetric umbrella bending mode (v2), and a few weaker bands corresponding to the C - H stretch (vl), the C - I stretch (v 3) and methyl

* Corresponding author.

rock (v 6) vibrational modes. Careful and extensive Raman polarization and excitation profile measurements [2-7] have found that the B-state origin and the next v6 vibronic band have lifetimes of 0.5 + 0.1 ps (1.2 ps for CD3I). Further studies [6,7] found vibrationally specific subrotational period lifetimes for several other bands: one quantum of the C - I stretch (v 3) has a lifetime of -- 1.5 ps (-- 5.0 ps for CD3I), two quantum of the C - I stretch (2v 3) has a lifetime of --- 0.5 ps, and one quantum of the C - H stretch (v~) has a lifetime of -- 0.06 ps (-~ 0.6 ps for CD3I). These vibrational mode-specific lifetimes (or predissociation rates) were attributed to the multidimensional reaction coordinate in the curve crossing region of B-state and unbound surface [6]. In contrast to iodomethane, there are relatively few vibrational mode specific studies that have examined dihalomethane Rydberg states. Dihalomethanes are interesting molecules to examine

000%2614/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 ( 9 7 ) 0 0 4 0 0 - 4

W.M. Kwok, D.L. Phillips/Chemical Physics Letters 270 (1997) 506-516

how the presence of the second C - X chromophore perturbs the Rydberg states associated with the C - I chromophore. Rydberg orbitals are very sensitive to changes in the surrounding environment due to their extended character, We have obtained preliminary resonance Raman spectra of chloroiodomethane in the gas phase and in cyclohexane solution with excitation in its B-band absorption in order to investigate whether the lower Rydberg states associated with the B-band absorption are noticeably perturbed by the presence of the C-CI chromophore and also whether there are noticeable solvation effects. We have used a simple model and time-dependent wavepacket calculations to simultaneously simulate the resonance Raman intensities and absorption spectra in order to quantitatively investigate the FranckCondon region of the excited states and their associated short-time photodissociation dynamics. We also compare and contrast our results for chloroiodomethane to some previous results for the single chromophore iodomethane.

2. Experiment Sample solutions of chloroiodomethane in cyclohexane solvent with concentrations ranging from 0.1 to 0.4 M were prepared from CH2IC1 (99%) and spectroscopic grade cyclohexane (99.9 + %) from Aldrich Chemical Company. Excitation wavelengths for the resonance Raman experiments were generated from hydrogen Raman shifted lines of the second, third, and fourth harmonics of a Spectra-Physics GCR-150-10 Nd:YAG laser: 199.8 nm (3rd antiStokes of the fourth harmonic), 204.2 nm (Sth antiStokes of the third harmonic), 208.8 nm (7th antiStokes of the second harmonic), 217.8 nm (2nd anti-Stokes of the fourth harmonic), and 223.1 nm (4th anti-Stokes of the third harmonic). The resonance Raman experimental apparatus has been previously [9-12] described so only a brief description will be given here. A lightly focused laser beam (-- 1 mm diameter and 50-100 I~J) excited a flowing liquid sample and the Raman scattered light was collected with an ellipsoidal mirror ( f / 1 . 4 ) in a backscattering geometry. The collected Raman scattered light was imaged through a polarization scrambler (Oriel) and entrance slits of a 0.5 meter spec-

507

trometer (Acton) onto a liquid nitrogen cooled CCD (Photometrics). The Raman spectrum was obtained by collecting ten to twenty 60 s to 120 s scans from the CCD and adding these scans together. The gas phase resonance Raman experiments used the same source of excitation light, collection optics and detection apparatus as the solution phase experiments. The sample handling equipment for the gas phase experiments consisted of a heated reservoir of liquid and vapor chloroiodomethane was connected to a heated pipette and a stream of dry nitrogen was passed through this sample reservoir to carry some chloroiodomethane vapor out through the pipette nozzle where a lightly focused laser beam excites the gas phase sample. The excitation light was not noticeably attenuated by the vapor phase sample thus the reabsorption of the resonance Raman scattering for the gas phase experiments were minimal. The cyclohexane solvent known vibrational frequencies were used to calibrate the wavenumbers of the solution phase resonance Raman spectra, while mercury emission lines and Raman lines of nitrogen and oxygen were used to calibrate the gas phase resonance Raman spectra. An intensity calibrated deuterium lamp (Optronics) was used to correct the Raman spectra for the wavelength dependence of the detection apparatus. The reabsorption of the resonance Raman scattering was minimized by using a backscattering geometry and the rest of the reabsorption for the solution phase spectra was corrected for using the methods given in Ref. [13]. Sections of the Raman spectra were fit to a sum of Lorentzian functions plus a baseline to find the integrated areas of the Raman peaks. The absolute resonance Raman cross sections for chloroiodomethane in cyclohexane solution were measured relative to previously measured absolute Raman cross sections for cyclohexane [14]. The sum of the C - H stretches near 2900 cm-i or the 802 c m - i peak of cyclohexane were used as internal standards for the absolute Raman cross section measurements. The concentrations of the chloroiodomethane/cyclohexane solutions were measured spectrophotometrically before and after the Raman measurement using a Perkin-Eimer 19 U V / V I S absorption spectrometer. The absorption spectra exhibited changes of less than 5% during the experiment due to either evaporation or photodecomposition. An

508

W.M. Kwok, D.L. Phillips/Chemical Physics Letters 270 (1997) 506-516

average from three separate measurements were used to find the reported absolute Raman cross sections.

3. Calculations We modeled the absolute resonance Raman intensities and absorption spectrum using calculations based on Heller's time-dependent approach to resonance Raman scattering [15-17]. This model is not meant to be a complete description of the short-time photodissociation dynamics and solvent-solute interactions but provides a convenient method to discern the main features of the excited state in the FranckCondon region in a semi-quantitative manner. Our rather simple model may also serve as a reference to which the results of more sophisticated calculations can be compared to assess the importance of different effects such as coordinate dependence of the transition dipole, Duschinsky rotation of the normal coordinates, or the multiple time scales of the solvent-solute interactions relative to the experimental absorption and Raman time scales. The absorption spectrum was calculated using: O'A(EL) = (4"rr eE L M~/3nh2c)

f _ ~ d 6 G( 3 )

oc

× R e f o d/(010(/)} Xexp[i(E L- Eo + eo- 6 )t/h]

×exp[-rt/h],

(1)

and the resonance Raman cross sections were calculated from: O-R.0_f(EL) = (8'rr eaE3 EL M~/9h6 c 4) ×L

daG'(6)

i:0

dt(flO(t))

X exp[i( E L -- E o + e o - 6) t/h] × exp[ - / ' t / h

]12 ,

(2)

where E L is the incident photon energy, f is the final state for the Raman scattering, E s is the scattered photon energy, E 0 is the central zero-zero energy, n is the solvent index of refraction, M 0 is the transition length evaluated at the equilibrium geometry, the term e x p [ - F t / h ] is a homogeneous

damping function which has contributions from excited state population decay and pure dephasing, 10(t)) = e - i m / ~ D ) which is 10) propagated on the excited state surface for a time t, H is the excited state vibrational Hamiltonian and G ( 6 ) and G'(6) are the inhomogeneous distribution of zero-zero energies for the absorption and Resonance Raman processes which were modeled as Gaussian functions. The ground and excited states associated with the B-band transition were modeled as harmonic oscillators with their minima set apart by an amount A with units of dimensionless ground-state normal coordinates. The model we used had no Duschinsky rotation of the normal modes, the ground and excited state harmonic oscillators had the same vibrational frequency, and only the thermal population in the v = 0 states of the molecule were used. The structureless solution phase absorption spectrum of B-band chloroiodomethane suggests that the total electronic dephasing a n d / o r population decay occurs fairly fast and may even occur prior to the first vibrational recurrence of any Franck-Condon active modes. The homogeneous damping term, e x p [ - F t / h ] , is a function which has contributions from excited state population decay and pure dephasing which can cause the Raman overlap function to approach zero fairly rapidly. The calculations truncate the time integral in Eqs. 1 and 2 at a time after the wavepacket has moved far enough from time zero so that the wavepacket no longer has appreciable overlap with any of the final states that are observed in the resonance Raman spectrum. The bound harmonic oscillator model for the excited-state potential energy surface only provides a convenient way to mimic the part of the excited state surface in the Franck-Condon region that the resonance Raman and absorption spectra depend on and does not mean that the excited state is necessarily bound. We note that our separation of the solvent induced absorption spectral broadening into 'homogeneous' and 'inhomogeneous' components is somewhat arbitrary and a fairly gross approximation since there is probably not any truly static contributions to the spectral broadening in liquids. However, it appears that most of the breadth of the B-band absorption band is due mostly to a very large geometry change along mainly the C - I stretch and I - C - C I bend coordinates (see 'Results and discussion' section) and thus the solvent-solute

W.M. Kwok, D.L. Phillips/Chemical Physics Letters 270 (1997) 506-516

interactions most likely make only a small contribution to the spectral width. Thus, our crude treatment of the solvent-solute interactions will likely have little effect on the main features of the excited state potential surface in the Franck-Condon region which we obtain from simulating our experimental absorption and resonance Raman spectra.

4. Results and discussion

Fig. 1 shows the absorption spectra of chloroiodomethane and iodomethane in the gas phase and in cyclohexane solution. Our gas phase absorption spectra (which have instrument limited resolution) were obtained with very low resolution and as vapor in 1 atm of air. Upon solvation the B-band of chloroiodomethane appears to red-shift substantially while the B-band absorption of iodomethane appears to blue-shift. This observation suggests that the B-

,

,~

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.

.

.

.

i

,

- Gas Phase ........ in Cyclohexane

coo

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2001

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03 600

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,q,,,

~ ",,~

200

=

CH,,IC1

,,~

0

. 200

210

220

Wavelength (rim) Fig. 1. Absorption s p e d r a o f the B-band o f c h l o r o i o d o m e t h a n e

and iodomethane in the gas phase and in cyclohexane solution taken with low resolution. The excitation wavelengths for the resonance Raman experimentsare given as numbers in nm above the chloroiodomethanespectra.

509

band of chloroiodomethane is noticeably perturbed by the presence of the C-C1 chromophore relative to the single chromophore iodomethane. Chloroiodomethane's A-band absorption band (peak ~ 270 nm and an extinction coefficient of about 540 mol -~ l c m - 1 in cyclohexane solution) [18] is red shifted and more intense than the corresponding A-band absorption of iodomethane which is ~ 258 nm and has an extinction coefficient of about 430 mol -I l cm -~ in cyclohexane solution. The interaction between the C-C1 and C - I chromophores may be responsible for the red shifting and enhancement of the A-band absorption in chloroiodomethane. A similar interaction has been observed for the bromoiodomethane A-band absorption (maximum ~ 268 nm which has been assigned to a n ( I ) ~ cr*(C-I) transition) and the B-band absorption (maximum ---213 nm which has been assigned to a n(Br) ~ cr * (C-Br) transition) [10,19,20]. The A-band absorptions of bromomethane (maximum ~ 202 nm) and iodomethane (maximum = 258 nm) also arise from n ( X ) ~ cr * ( C - X ) transitions. The A- and B-bands of bromoiodomethane are redshifted and substantially more intense than the corresponding n(X)--* cr*(C-X) transitions in bromomethane and iodomethane which is likely due to a degree of coupling of the C - I and C - B r chromophores in bromoiodomethane. The A-band absorption of chloromethane (n(X) --* or* ( C - X ) transitions) is ,-~ 170 nm and is closer to the B-band absorption of chloroiodomethane than the A-band absorption. Thus the C - C I transitions would be expected to perturb the excited states of the B-band absorption of chloroiodomethane more than the Aband absorption. Fig. 2 gives an overview of the gas and solution phase B-band resonance Raman spectra of chloroiodomethane. Fig. 3 shows the 199.8 nm resonance Raman spectra of gas and solution phase chloroiodomethane with the tentative assignments shown above the larger peaks. The resonance Raman spectra of Figs. 2 and 3 have been intensity corrected and the solution phase spectrum has been solvent subtracted. The fundamentals, overtones, and combination bands of four Franck-Condon active vibrational modes account for most of the spectral intensity of the resonance Raman spectra displayed in Figs. 2 and 3. Table 1 shows the Raman peak

510

W.M. Kwok, D.L. Phillips/Chemical Physics Letters 270 (1997) 506-516

positions and intensities of the resonance Raman spectra depicted in Figs. 2 and 3. The biggest series of Raman peaks are the fundamental and overtones of the nominal C - I stretch ( n v 5) which we observe up to 7v 5. These C - I stretch bands ( n v 5) also form significant combination bands with the CH 2 wag (v 3) fundamental, the I - C - C 1 bend ( v 6) fundamental and overtone and the CH 2 scissor ( v 2) fundamental. Our B-band resonance Raman spectra have similar intensity patterns compared to the A-band resonance Raman spectra of chloroiodomethane [12]. However, the B-band resonance Raman spectra have more intensity in the v 2 + n v 5 combination bands relative to the A-band resonance Raman spectra. The similarity of the B-band and A-band resonance Raman spectra is somewhat surprising since the A-band and B-band resonance Raman spectra of iodomethane are very different from one another with the A-band

CH2IC1 Gas Phase :::::::::::::::::::::::::::::::::::::::

CH21CI in Cyclohexane 199.8 nm

~)

204.2 nm

1

0

, .~ /

208.8 nm

I

217.8 nm

500 1000 1500 2000 2500 3000 3500

Raman Shift (cm -1) Fig. 2. Overview of the B-band resonance Raman spectra of chloroiodomethane in the gas phase and in cyclohexane solution obtained with the excitation wavelengths given in Fig. 1. The resonance Raman spectra are intensity corrected and the solution spectra are solvent subtracted. The asterisks mark regions where solvent subtraction artifacts are present and # labels features due to stray or ambient light artifacts.

nV 5 v~v 5

2v~v,

vc.v, vc.v,

1

i o

i i i

2 :: 2

4

7

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. ::

li :: ~

:i ~ !: :

21 ii 2

i !

3

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~ ! !G:

:

~ :

[

CH2IC1 (199.8 nm)

II

in Cyclohexane

i ....

500

, ....

i ....

i ....

i

. . . .

i

. . . .

i

. . . .

1000 1500 2000 2500 3000 3500

Raman Shift (cm "1) Fig. 3. Expanded view of the 199.8 nm gas and solution phase resonance Raman spectra of chloroiodomethane. The spectra are intensity corrected and the solution phase spectrum has been solvent subtracted. The tentative assignments of some of the larger Raman peaks are given. The asterisks label regions where solvent subtraction artifacts are present.

dominated by the nominal C - I stretch overtone progression and the B-band dominated by the nominal CH 3 umbrella mode overtone progression (in the = 200 nm region) [2-8,21-24]. We have simulated the B-band resonance Raman intensities and the absorption spectra of gas and solution phase chloroiodomethane using the parameters of Table 2. Fig. 4 compares the calculated and experimental B-band absorption spectra. Fig. 5 and Table 3 shows a comparison of the calculated and experimental resonance Raman intensities for the B-band of chloroiodomethane. There is reasonable agreement between the calculated and experimental absorption spectra and resonance Raman intensities shown in Figs. 4 and 5. We should note some caveats about our simulations of the resonance Raman intensities and absorption spectra. Our simulations were fairly insensitive to changes in the inhomogeneous broadening parameter (probably because inhomogeneous broadening contributes only a small portion to the absorption band width) and we are not able to obtain a good estimate of the 'inhomogeneous' contributions to the spectral width. The homogeneous broadening parameter was sensitive to fitting the absolute cross sections of the solution phase B-band spectra. The 100 c m - J H W H M for F in the solution phase combined with the multidimensional excited state suggests that vibrational recurrences of the Franck-Condon active modes will not

W.M. Kwok, D.L. Phillips/Chemical Physics Letters 270 (1997) 506-516 b e v e r y i m p o r t a n t to d e t e r m i n i n g man

the resonance Ra-

intensities (at least for the solution phase

band). The E 0 parameters

absorption

B-

spectrum.

511

Inspection

of Table

that the gas and solution phase B-band

2 shows

parameters

in Table 2 should not be

are somewhat

different with the gas phase parame-

t a k e n l i t e r a l l y s i n c e t h e y c a n n o t b e e x t r a p o l a t e d to a n

ters generally

larger. However,

excited-state potential minimum

very

r e l i a b l y (i.e. t h e p a -

rameters are mainly fitting the multidimensional in the Franck-Condon

region

of the excited

slope

significant

difference

this may

in view

of

not be a the

fewer

constraints on the gas phase B-band parameters (only

state

a portion of the experimental

absorption

spectrum

when vibrational recurrences are relatively unimpor-

and no absolute cross section measurements).

tant). Our parameters

pears that the gas and solution B-band excited states

for the B-band

cited state are not as well determined ters for the solution

phase

since we

gas phase exas the parameonly have

of chloroiodomethane

a

a large

general

It a p -

a r e s i m i l a r to o n e a n o t h e r w i t h

solvation

effect

that

substantially

portion of the B-band absorption spectrum along the

r e d - s h i f t s t h e s o l u t i o n p h a s e e x c i t e d s t a t e r e l a t i v e to

red-edge

the gas phase but only moderate solvation effects on

to c o m p a r e

with our calculated gas phase

Table 1 Resonance raman intensities of chloroiodomethane in the gas phase and in cyclohexane solution Raman peak a

Gas

Solution

Raman shift a

199.8 nm int. b

Raman shift a

199.8 nm int. b

204.2 nm int. b

208.8 nm int. b

217.8 nm int. b

223.1 int. b

535 1060

202 100

532 1061

231 100

215 100

251 100

364 100

668 100

absolute Raman cross section for 2v 5 (A2/molecule): 3 vs 1583 74 1583 4v s 2107 59 2101 5 v5 2625 43 2614 6v s 3137 22 3116 7v 5 3653 20 3630

6.45 X 10-8 68 50 40 12 8

4.3 × 10-8 67 49 42

2.0 × 10- s 65 45 27

3.0 X 10- 9 50 27 15

8.6 × 10- 1o 40 17 -

/-'6 + v6 + v6 + v6 + v6 + v6 +

v5 2v 5

v5 2v 5 3v 5 4v 5 5v 5 6v 5

724 1247 1769 2287 2799 3315

76 56 42 41 28 29

725 1255 1777 2293 c 3303

87 55 38 34 c 16

73 47 30 29 c

69 42 26 18 c

72 33 14 11 c

86 28 11 c

v 5 + 2v 6 2v 6 + 2v 5 2v 6 + 3v 5

908 1441 -

14 6 -

921 1435 1962

15 14 3

14 10 2

14 10 3

9 10 2

8 14 3

v3 v 3 + v5 v 3 + 2 v5 v 3 + v6 + and (v 2 + v 3 + v6 + and ( v2 + v2 /:6 2v 6

1183 1715 2240 1928

21 19 5 (56)

18 9 2 (44)

15 18 8 (37)

15 12 9 (31)

28 14 5 19

55 9 13

2432 2467 1398 196 385

18 19 52 60 I1

1183 1713 2237 (1905) 1921 (2424) 2450 1396 190 389

(31)

(29)

(21)

6

-

36 67 13

38 65 12

38 83 13

35 108 11

v5 v 5) 2v 5 2 v 5)

42 210 17

a Estimated uncertainties are about + 4 c m - ~ for the Raman shifts. b Relative intensities are based on integrated areas of peaks. Estimated uncertainties are about 10% for intensities 50 and higher, 20% for intensities 10 to 50, and 30% for intensities lower than 10. c This peak is obscured by C - H stretch solvent subtraction artifacts.

512

W.M. Kwok, D.L. Phillips/Chemical Physics Letters 270 (1997) 506-516

the m u l t i d i m e n s i o n a l slopes in the F r a n c k - C o n d o n region. The parameters that we h a v e used p r e v i o u s l y to fit the A - b a n d absorption and resonance R a m a n intensities o f c h l o r o i o d o m e t h a n e in c y c l o h e x a n e are also s h o w n in Table 2. A c o m p a r i s o n o f the solution phase A - b a n d and B-band excited state parameters g i v e n in Table 2 shows that the B - b a n d has normal m o d e - d i s p l a c e m e n t s that are larger along the nominal C H 2 scissor m o d e , smaller along the n o m i n a l C - I stretch m o d e , and about the same for the n o m i nal C 1 - C - I bend and nominal C H 2 w a g modes. This suggests that the B-band excited state has m o r e m o t i o n along the nominal C H 2 scissor vibration and less m o t i o n along the nominal C - I stretch vibration than the A - b a n d excited state in the F r a n c k - C o n d o n region. This is s o m e w h a t similar to the A- and B-bands o f iodomethane in that the A - b a n d has m u c h m o r e C - I stretch motion than the B-band and the B - b a n d has much more C H 3 u m b r e l l a m o t i o n (similar to the C H 2 scissor in c h l o r o i o d o m e t h a n e )

than in the A-band. W e have noted p r e v i o u s l y that c h l o r o i o d o m e t h a n e A - b a n d short-time photodissociation d y n a m i c s are significantly m o d i f i e d by substituent effects relative to the A - b a n d o f iodomethane. Similarly, the B - b a n d of c h l o r o i o d o m e t h a n e B-band short-time photodissociation d y n a m i c s are strongly perturbed by substituent effects relative to the B-band o f iodomethane. The substituent effect appears to be greater for the B-band o f c h l o r o i o d o m e t h a n e and this is likely due to the presence of the C - C I chrom o p h o r e n(X) ~ (r * ( C - X ) transitions --~ 170 n m which is closer to the B - b a n d absorption o f c h l o r o i o d o m e t h a n e than the A - b a n d absorption. A recent v e r y detailed study o f the C H 3 I B-band absorption and resonance e m i s s i o n spectra in high pressure C H 4 and Ar by Z e i g l e r and c o - w o r k e r s [25] s h o w e d that a dipole correlation function ( D C F ) c o u l d describe an i n h o m o g e n e o u s b r o a d e n e d absorption lineshape and simultaneously an ultrafast h o m o geneous electronic pure dephasing rate due to the relevant experimental time scales of the absorption

Table 2 Parameters for time-dependent wavepacket calculations used to simulate resonance Raman intensities and absorption spectra of chloroiodomethane B-band parameters (this work) vibrational mode

ground and excited state frequency/cm- i gas phase IAI

solution phase a [AI

v5 (C-I stretch) v6 (CI-C-I bend) v3 (CH 2 wag) v 2 (CH 2 scissor)

gas 535, soln. 532 gas 196, soln. 190 gas 1183, soln. 1183 gas 1398, soln. 1396

4.05 6.00 0.45 0.75

transition length E0 homogeneous broadening F (HWHM)

gas M = 1.0/~ gas phase = 42550 cm- i gas = 10 cm- J (0 to 30 cm- i range) solution = 100 cm- ~ _+ 10 cminhomogeneous broadening G (HWHM) gas = 10 cm- i (0 to 30 cm- l range) solution = 60 cm- ~ + 20 cm-

5.00 6.66 0.713 1.032 solution phase M = 0.717 ,~ solution phase = 43270 cm-

A-band parameters (from Ref. [12]) vibrational mode

ground and excited state frequency/cm- ~ solution phase a t A]

v5 (C-i stretch) v6 (C1-C-I bend) *'3 (CH2 wag) v2 (CH z scissor) v9 (CH 2 twist)

532 190 1183 1396 800

transition length E0

solution phase M = 0.186 ,~ solution phase = 27030 cm-

4.90 6.00 0.48 0.22 0.47

W.M. Kwok, D.L. Phillips / Chemical Physics Letters 270 (1997) 506-516

513

oo

× =_

.=,

ar~ e~

× .=,

0 ~d

e-

.=_

co o

x

i,i e~

x I .r-

0 t~

~

^~ [..,

~

~

~

~

~

~ t " , l ¢ ~ ' ~ ' ~ ' ~

++++++

~ eqq__

~+ l_

+

+

+

+

~L .m

514

W.M. Kwok, D.L. Phillips/Chemical Physics Letters 270 (1997) 506-516

•

,

.

.

.

,

.

.

.

,

.

.

.

CH2IC1 3

b2 v

......... Experimental

e 0 ¢o •~ 2

lcu|~/',//

o( / ) ..Q

<

1

Gas Ph~e ,

i

,

46000

~ ,

i

,

,

i

mental observables of B-band CH2CII with the Bband of CH 3I. We do not observe appreciable fluorescence (or fluorescence-like emission) in our Bband resonance Raman spectra of CHzCII (at least up to 4000 cm-~ Raman shift) while the B-band emission spectra of CH3I in high pressure CH 4 has a large amount of intensity in fluorescence like emission (up to --- 95% of the emission on the blue edge of the absorption) relative to Raman-like emission. This suggests that predissociation or motion of the initially excited wavepacket out of the Franck-Condon region for the B-band of chloroiodomethane is substantially faster than solvent dephasing collision times. The B-band resonance Raman intensity pattern of chloroiodomethane is very similar to that found for the directly dissociative A-band resonance Raman intensity pattern. We also note that the gas and solution phase B-band chloroiodomethane absorption spectra appear to be broad and featureless

,

4aooo 50000 Energy (cm -1)

Fig. 4. B-band experimental (solid line) and calculated (dashed line) absorption spectra of gas and solution phase chloroiodomethane. The calculated absorption spectra made use of the parameters of Table 2 in Eq. 1 and the model described in the calculation sections.

240 I 220, it 200~ 100~

nV5 V6+nV 5

in Cyclohexane

V3+nV~i 50

and resonance emission processes. The transition energy correlation function found from MD simulations for CH3I in 1200 psi CH 4 decayed on the --- 120 fs time scale which is slow compared to the DCF decay ( = 30 fs) suggesting the absorption spectrum is inhomogeneously broadened [25]. On the other hand, the transition correlation function decay (---120 fs) is much faster than the vibrational linewidth decay (1-2 ps) which implies that the same DCF decay is to be used as a homogeneous decay for the spontaneous resonance emission for the CH3I in high pressure CH 4 systems. For the B-band CH31 molecular system, the bath-solvent fluctuations and their appropriate time scales relative to the experimental time scales appears to determine much of the broadening of the experimental absorption band and the homogeneous partitioning of the Raman like and fluorescence like resonance emission

[25].

It is useful to compare and contrast the experi-

II

2v6+"v5

I..

I _2_ " ~ 220 200. 100'

345670

0

2346 60123012012

CH2IC1 (199.8 rim) nm

1 [

Experimental ] Calculated

Gas Phase 50

0

2345670123460123012012

Vibrational Mode Fig. 5. Comparison of 199.8 nm experimental (solid bars) and calculated (open bars) resonance Raman intensities for gas and solution phase chloroiodomethane. The calculated resonance Raman intensities made use of the parameters in Table 2 in Eq. 2 of the 'Calculations' section.

W.M. Kwok, D.L. Phillips / Chemical Physics Letters 270 (1997) 506-516

(at least to -~ 192 nm) similar to the gas and solution phase A-band chloroiodomethane absorption spectra. Our previous gas and solution phase A-band chloroiodomethane resonance Raman studies found that the A-band absorption bandwidth is almost completely determined by the shape of the excited state potential relative to the ground state potential and that there were very small contributions to the absorption bandwidth from static and/or dynamic environmental effects for either gas or solution phase in the Franck-Condon region. The general similarity between the B-band chloroiodomethane absorption and resonance Raman spectra with that of A-band chloroiodomethane absorption and resonance Raman spectra suggests that the observed B-band absorption bandwidth could be similarly dominated by FranckCondon factors of the potential energy surfaces. Thus the static a n d / o r dynamic solvent bath fluctuations ('inhomogeneous' and 'homogeneous' broadening contributions) most likely have a minor role in the short-time (i.e. Franck-Condon region) photodissociation dynamics of B-band chloroiodomethane. It would be very interesting to do a more in depth study of the environmental effects on the absorption and emission spectra of the B-band of chloroiodomethane (similar to work reported for the B-band of iodomethane in high pressure gases) in order to pin down the moderate contributions of the solvent-solute bath fluctuations to the absorption and emission spectra. Unfortunately, our preliminary results presented here for the B-band of chloroiodomethane in the gas and phase and in cyclohexane solution do not allow us to determine this very well. The fairly large red shift of the B-band absorption of chloroiodomethane (with possibly some additional spectral broadening) upon solvation does suggest that environmental factors do make a moderate contribution to the absorption and emission spectra. Further high pressure gas a n d / o r solution studies investigating the B-band absorption and emission spectra of chloroiodomethane could also address how the environment perturbs the interaction of the states associated with the C-C1 and C - I chromophores. We have reported the first B-band resonance Raman spectra of chloroiodomethane (to our knowledge) in both the gas and solution phases. The resonance Raman spectra display most of their intensity in the fundamental and overtones of the nominal

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C - I stretch (nv 5) and a smaller amount of intensity in the combination bands of the CH 2 wag (v 3) fundamental, the I-C-C1 bend (v 6) fundamental and the CH 2 scissor (v 2) fundamental with the C - I stretch bands (nvs). We observed no appreciable fluorescence like features in our resonance Raman spectra (up to 4000 cm -I Raman shift) and the B-band absorption spectra were structureless down to -- 192 nm (gas and solution). This suggests that motion of the wavepacket out of the Franck-Condon region the excited state is faster than the time needed for a significant amount of solvent dephasing collisions to occur and possibly faster than recurrence times of any Franck-Condon active vibrational mode. The B-band resonance Raman spectra have an intensity pattern very similar to that found for the directly dissociative A-band resonance Raman spectra of chloroiodomethane. This implies that there is a very large geometry change along the C - I stretch and I - C - C I bend coordinates in the Franck-Condon region of the B-band of chloroiodomethane. We have used a simple model and time-dependent wavepacket calculations to simulate the absorption spectra and resonance Raman intensities o f B - b a n d chloroiodomethane and characterize the multidimensional slope of the excited state in the Franck-Condon region. We find that the presence of the C-CI chromophore significantly perturbs the B-band state of chloroiodomethane relative to the B-band of iodomethane and gives rise to much larger geometry changes along the C - I stretch coordinate.

Acknowledgements This work was supported by grants from the Committee on Research and Conference Grants (CRCG), the Research Grants Council (RGC) of Hong Kong, the Hung Hing Ying Physical Sciences Research Fund, and the Large Items of Equipment Allocation 1993-94 from the University of Hong Kong.

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