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 intramolecul ar vibrational relaxation and energy transfer to the solvent. Intramolecula r energy redistribution populates vibrational states with larger Franck-Con don factors for the electronic transition, leading to an increased absorpti on of probe pulses in the wavelength range of 380-440 nm. A model based on the temperature dependence of the electronic absorption coefficient describ es the transient absorption well for all wavelengths. In the model, the tem perature rises and decays exponentially with time, reflecting the initial r edistribution of energy within the excited molecule and the subsequent tran sfer of energy from the vibrationally excited molecule into the solvent. Th e intramolecular vibrational relaxation time for CH2I2 is essentially the s ame 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 s olvent takes longer, occurring with a time constant of 68 +/- 10 ps for CCl 4, 51 +/- 10 ps for CDCl3, and 23 +/- 2 ps for C6D6. (C) 2000 American Inst itute of Physics. [S0021-9606(00)01336-2].