Spectra of transient absorption and stimulated emission are recorded for the styryl dye DASPI, after excitation at 470 nm, with experimental resolution of 100 fs. The evolution of the S1→S0 transition energy distribution is obtained for the solvents methanol and acetonitrile at several temperatures. It is described by the dependence of the mean (first moment), width, and asymmetry (second and third central moments) of the distribution on time. The observed time-dependence of the mean transition energy is simulated by appropriate models for the solvation dynamics. In both methanol and acetonitrile an ultrafast component is observed. Width and asymmetry change most rapidly and characteristically during this initial part of solvation. In the evolution of the higher moments, different relaxation contributions apparently are better distinguished than in the evolution of the first moment. For methanol at 50 °C, an oscillatory evolution is observed mainly in the higher moments which may indicate underdamped coherent solvent motion.

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.

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.

We studied the temperature dependence of the structural relaxation in poly(vinyl acetate) near the glass transition temperature with single molecule spectroscopy from Tg-1 K to Tg+12 K. The temperature dependence of the observed relaxation times matches results from bulk experiments; the observed relaxation times are, however, 80-fold slower than those from bulk experiments at the same temperature. We attribute this factor to the size of the probe molecule. The individual relaxation times of the single molecule environments are distributed normally on a logarithmic time scale, confirming that the dynamics in poly(vinyl acetate) is heterogeneous. The width of the distribution of individual relaxation times is essentially independent of temperature. The observed full width at half maximum (FWHM) on a logarithmic time axis is approximately 0.7, corresponding to a factor of about 5-fold, significantly narrower than the dielectric spectrum of the same material with a FWHM of about 2.0 on a logarithmic time axis, corresponding to a factor of about 100-fold. We explain this narrow width as the effect of temporal averaging of single molecule fluorescence signals over numerous environments due to a limited lifetime of the probed heterogeneities, indicating that heterogeneities are dynamic. We determine a loose upper limit for the ratio of the structural relaxation time to the lifetime of the heterogeneities (the rate memory parameter) of Q<80 for the range of investigated temperatures.