Transient electronic absorption monitors the flow of vibrational energy in methylene iodide (CH 2 I 2 ) following excitation of five C-H stretch and stretch-bend modes ranging in energy from 3000 to 9000 cm Ϫ1 . Intramolecular vibrational relaxation ͑IVR͒ occurs through a mechanism that is predominantly state-specific at the C-H stretch fundamental but closer to the statistical limit at higher excitation levels. The IVR times change with the excitation energy between the fundamental and first C-H stretch overtone but are constant above the overtone. The intermolecular energy transfer ͑IET͒ times depend only weakly on the initial excitation level. Both the IVR and the IET times depend on the solvent ͓CCl 4 , CDCl 3 , C 6 D 6 , C 6 H 6 , or (CD 3 ) 2 CO] and its interaction strength, yet there is no energy level dependence of the solvent influence.

Wavelength dependent, transient, electronic absorption spectroscopy of methylene iodide (CH2I2) in CCl4, CDCl3, C6D6, and (CD3)2CO following excitation of the fundamental C–H stretching vibration reveals the time scales of intramolecular vibrational energy redistribution and energy transfer to the solvent. In contrast to the case for overtone excitation, state-specific relaxation to one or a few states that are coupled by low order interactions with the initially prepared state dominates the intramolecular vibrational energy redistribution. This mechanism is consistent with previous infrared pump–probe measurements of CH2I2 fundamental relaxation as are the measured relaxation time scales. We also find a previously unobserved relaxation pathway through weakly-coupled states that have several quanta of excitation in the Franck–Condon active modes, primarily C–I stretch and bend. Although this statistical component is a minor channel in the relaxation, it is the only contribution to the signal at the longest probe wavelengths in CCl4 and CDCl3. Time scales for both intramolecular energy redistribution and intermolecular relaxation to the solvent depend strongly on the strength of interaction with the solvent.

Transient electronic absorption spectroscopy monitors the population of intermediate states in the vibrational energy relaxation of the iodo-substituted methanes, CHI3, CH2I2, and CH3I in solution after excitation of the fundamental C–H stretching vibration. These experiments give times for both intra- and intermolecular energy flow for each molecule. The trend in intramolecular vibrational energy redistribution (IVR) times shows that the local vibrational state structure around the initially prepared states in each molecule, not the overall state density, controls IVR. A comparison of experiments in the solvents chloroform and acetone reveals the solvent role in IVR and shows a strong solvent effect on the intermolecular energy transfer.

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