We report a preliminary picosecond Stokes and anti-Stokes time-resolved resonance Raman ～267nm pump and 400nm 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 ;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 4-50ps 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 300-500nm region over the 0.3-3ps time scale.

We have obtained A-band resonance Raman spectra of dibromomethane in the gas and solution phase and nanosecond time-resolved resonance Raman spectra of dibromomethane photoproducts. The A-band resonance Raman spectra suggest the short-time dynamics are similar to other dihalomethanes that are known to have direct photodissociation reactions in the gas phase. Several power dependent A-band resonance Raman bands were tentatively assigned to the C-Br stretch overtone progression of the CH2Br radical which has a strong absorption band that is coincident with the A-band resonance Raman excitation wavelength. Two-color nanosecond time-resolved resonance Raman spectra (266.0nm pump/341.5nm probe) were obtained and comparison of the vibrational frequencies to the results for density functional theory calculations indicate that the iso-CH2Br-Br species is mainly responsible for the transient photoproduct absorption band 360nm. Our preliminary results for A-band photoexcitation of dibromomethane in conjunction with previously reported diiodomethane results suggest that solvation effects (probably via recombination of the CH2Br and Br fragments within a solvent cage) lead to noticeable production of the isodibromomethane photoisomerization photoproduct observed in the nanosecond time-resolved resonance Raman spectra.

Resonance Raman spectra were obtained for nitrobenzene in cyclohexane solution with excitation wavelengths in resonance with the charge-transfer (CT) band absorption spectrum. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion mainly along the nominal NO2 symmetric stretch mode (v11), the nominal benzene ring stretch mode (v7), accompanied by a moderate degree of motion along the nominal ONO symmetry bend/benzene ring stretch mode (v23), the nominal C-N stretch/benzene ring breathing mode (

We report transient resonance Raman spectra and density functional theory calculations for the photoproducts produced after ultraviolet excitation of CHBr2Cl and CCl3Br in cyclohexane solution. Comparison of the computed vibrational frequencies to the experimental Raman frequencies revealed that the iso-CHBrCl-Br and iso-CHClBr-Br species are mainly responsible for the transient resonance Raman spectrum observed following ultraviolet excitation of CHBr2Cl. Similar comparisons for CCl3Br showed the iso-CCl2Cl-Br species is mainly responsible for the transient resonance Raman spectrum observed following ultraviolet excitation of CCl3Br. Additional density functional theory computations were done to examine the chemical reactions of iso-CH2Br-Cl and iso-CH2Cl-Br with ethylene to give cyclopropane and Br-Cl product. We briefly discuss the possibility for release of reactive halogens into the atmosphere via the photochemical and chemical reactions of iso-polyhalomethane molecules formed after ultraviolet excitation of polyhalomethanes in condensed phase environments.

Conformation selective resonance emission spectra are reported for gas-phase iodoeyclohexane with excitation wavelengths of 199.8 and 204.2 nm. These spectra reveal seven Franck-Condon active vibrational modes for the axial conformation and eleven Franck-Condon vibrations for the equatorial conformation, and indicate that the initial dynamics on the B- and C-states have a complex multidimensional character with contributions from the CCC, CCH and HCI bending motions, torsional motions, and C-I, C-C, and C-H stretching motions for both conformations. The resonance emission spectra are generally consistent with the previously reported assignments of the far ultraviolet absorption spectra.