Transient electronic absorption of vibrationally excited CH2I2:: Watching energy flow in solution

Transient electronic absorption of methylene iodide (CH2I2) in CCl4, CDCl3, and C6D6 after excitation of two quanta of C-H stretching vibration with a 100 fs laser pulse allows direct observation of the times for intramolecular vibrational relaxation and energy transfer to the solvent. Intramolecular energy redistribution populates vibrational states with larger Franck-Condon factors for the electronic transition, leading to an increased absorption of probe pulses in the wavelength range of 380-440 nm. A model based on the temperature dependence of the electronic absorption coefficient describes the transient absorption well for all wavelengths. In the model, the temperature rises and decays exponentially with time, reflecting the initial redistribution of energy within the excited molecule and the subsequent transfer of energy from the vibrationally excited molecule into the solvent. The intramolecular vibrational relaxation time for CH2I2 is essentially the same in the solvents CCl4 (10.8 +/- 1.5 ps) and CDCl3 (11.2 +/- 2.0 ps) and is only slightly shorter in C6D6 (8.0 +/- 1.5 ps). Energy transfer to the solvent takes longer, occurring with a time constant of 68 +/- 10 ps for CCl4, 51 +/- 10 ps for CDCl3, and 23 +/- 2 ps for C6D6. (C) 2000 American Institute of Physics. [S0021-9606(00)01336-2].