Picosecond time-resolved resonance Raman observation of the iso-CH2I-I photoproduct from the "photoisomerization" reaction of diiodomethane in the solution phase

We report a preliminary picosecond Stokes and anti-Stokes time-resolved res onance Raman (267 nm pump and 400 nm probe excitation wavelengths) investig ation of the initial formation and vibrational cooling of the iso-CH2I-I ph otoproduct species produced after ultraviolet excitation of diiodomethane i n room temperature solutions. A comparison of the picosecond resonance Rama n spectra with previously reported nanosecond transient resonance Raman spe ctra and density functional theory computations shows that the iso-CH2I-I p hotoproduct species is predominantly responsible for the similar to 385 nm transient absorption band observed from several picoseconds to nanoseconds after ultraviolet excitation of diiodomethane in the solution phase. Simila r results were obtained in both nonpolar solution (cyclohexane solvent) and polar solution (acetonitrile) solvent. The picosecond resonance Raman spec tra confirm that the iso-CH2I-I photoproduct species is formed vibrationall y hot within several picoseconds and then subsequently undergoes vibrationa l cooling on the 4-50 ps time scale. This is consistent with the absorption bands changes occurring over similar times in a recent femtosecond transie nt absorption study. We discuss a possible qualitative scenario for the for mation of the iso-CH2I-I species that is in agreement with the available ga s phase experimental results for the ultraviolet photodissociation reaction of diiodomethane and gas phase collisional deactivation studies of the CH2 I radical. The proposed hypothesis is consistent with the lack of distinct resonance Raman bands in the first few picoseconds of our solution phase sp ectra of the iso-CH2I-I photoproduct as well as previously reported femtose cond transient absorption bands that are broad and weak in the 300-500 nm r egion over the 0.3-3 ps time scale. (C) 2000 American Institute of Physics. [S0021-9606(00)02241-8].