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 resonance Raman (267 nm pump and 400 nm probe excitation wavelengths) investigation of the initial formation and vibrational cooling of the iso-CH2I-I photoproduct species produced after ultraviolet excitation of diiodomethane in room temperature solutions. A comparison of the picosecond resonance Raman spectra with previously reported nanosecond transient resonance Raman spectra and density functional theory computations shows that the iso-CH2I-I photoproduct 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. Similar results were obtained in both nonpolar solution (cyclohexane solvent) and polar solution (acetonitrile) solvent. The picosecond resonance Raman spectra confirm that the iso-CH2I-I photoproduct species is formed vibrationally hot within several picoseconds and then subsequently undergoes vibrational cooling on the 4-50 ps time scale. This is consistent with the absorption bands changes occurring over similar times in a recent femtosecond transient absorption study. We discuss a possible qualitative scenario for the formation of the iso-CH2I-I species that is in agreement with the available gas phase experimental results for the ultraviolet photodissociation reaction of diiodomethane and gas phase collisional deactivation studies of the CH2I radical. The proposed hypothesis is consistent with the lack of distinct resonance Raman bands in the first few picoseconds of our solution phase spectra of the iso-CH2I-I photoproduct as well as previously reported femtosecond transient absorption bands that are broad and weak in the 300-500 nm region over the 0.3-3 ps time scale. (C) 2000 American Institute of Physics. [S0021-9606(00)02241-8].