Early-time photodissociation dynamics of chloroiodomethane in the A-band absorption from resonance Raman intensity analysis

We have obtained resonance Raman spectra and absolute Raman cross sections for h2-chloroiodomethane (fourteen excitation wavelengths between 200 nm and 355 nm) and d2-chloroiodomethane (for 282.4 nm excitation) in cyclohexane solution. Most of the intensity in the A-band resonance Raman spectra appears in the nominal C-I stretch overtones progression and combination bands of the nominal C-I stretch overtones with the fundamentals of the CH2wag, CH2scissor, and the Cl-C-I bend or C-Cl stretch fundamentals. The A-band absorption and absolute resonance Raman intensities were simulated using a simple model which included preresonant contributions to the fundamental Raman peaks and time-dependent wave packet calculations. The motion of the wave packet on the excited state surface was converted from dimensionless normal coordinates into internal coordinates using the results of normal coordinate calculations. The A-band short-time photodissociation dynamics of chloroiodomethane shows that the C-I bond lengthens, the I-C-Cl and H-C-I angles become smaller, and the H-C-Cl angles become larger. These internal coordinate motions which are associated with relatively low frequency modes are consistent with a simple impulsive "soft" radical model of the photodissociation and the CH2Cl group changing to a more planar structure. However, the C-H bond length does not change much and the H-C-H angle (associated with higher frequency modes) becomes slightly smaller which is inconsistent with the "soft" radical model and the CH2Cl group changing to a more planar structure. This suggests that an impulsive "semirigid" radical model may be more appropriate than the "soft" radical model to qualitatively describe the chloroiodomethane photodissociation. An ambiguity in the assignment of the 724 cm-1Raman peak and its associated combination bands to combination bands of the nominal C-I stretch overtones with the fundamentals of the Cl-C-I bend or C-Cl stretch fundamentals limits what we are able to determine about the C-Cl bond length changes during the initial stages of the photodissociation.