The population relaxation rate of the first excited state of the acetylenic C-H stretch is compared for roomtemperature gas-phase and solution-phase samples of 10 terminal acetylenes. The gas-phase sample pressure is less than 1 atm for all measurements, ensuring that the dynamics occur under collision-free conditions. The relaxation rates are measured using two-color transient absorption picosecond spectroscopy. The population of the excited state is monitored directly through the anharmonically shifted V ) 1-V ) 2 excited-state transition. The relaxation rates of isolated and solvated molecules are strongly correlated and follow a relationship expected for a parallel relaxation process in solution: the total rate in solution is the sum of a molecule-dependent rate related to the isolated molecule dynamics and a molecule-independent solvent-induced relaxation rate. For the terminal acetylenes, the vibrational normal-mode frequencies of the acetylene chromophore and their anharmonic interactions are highly conserved for all terminal acetylenes. Therefore, the observation that a single solvent relaxation contribution to the total relaxation rate describes the solution dynamics for all terminal acetylenes is consistent with the idea that solvent-induced energy relaxation pathways are dominated by the vibrational motions that are in close proximity to the excited state.

Ultrafast mid-IR transient absorption spectroscopy has been used to study the vibrational dynamics of hydrogen-bonded cyclic dimers of trifluoroacetic acid and formic acid in both the gas and solution phases (0.05 M in CCl(4)). Ultrafast excitation of the broad O-H cyclic dimer band leads, in the gas phase, to large-scale structural changes of the dimer creating a species with a distinct free O-H stretching band on 20 ps and 200 ps timescales. These timescales are assigned to ring-opening and dissociation of the dimer, respectively. In the solution phase, no such structural rearrangement occurs and our results are consistent with previous studies. The gas phase dynamics are insensitive to both the specific excitation energy (over a span of 550 cm(-1)) and the chemical identity of the dimer.

The pathway for vibrational-energy flow following the excitation of the first excited state of the acetylenic C-H stretch is investigated for a series of 10 terminal acetylenes in room-temperature gases and dilute solutions using transient absorption picosecond infrared spectroscopy. The transient absorption infrared spectra are obtained at three different probe frequencies. These experiments separately detect the population of the excited C-H stretch state, the population of vibrational states with 2 quanta of acetylenic C-H bend excitation, and the population of all other vibrational states with C-H stretch absorption frequencies within the laser bandwidth (25 cm -1 ) of the C-H stretch fundamental frequency. These measurements show that the initial redistribution event for the isolated molecule involves population transfer to vibrational states with bend overtone excitation. The secondary intramolecular vibrational-energy redistribution (IVR) process, which involves population transfer to the remaining near-resonant vibrational states, occurs on a time scale that is about 5 times slower than the initial redistribution event. The same relaxation pathway is observed in dilute solution. The total relaxation rate in solution for the slower process can be quantitatively described using a simple model where IVR and solvent-induced vibrational-energy relaxation (VER) proceed independently. The main effects of the solvent are to increase the extent of population relaxation for the first stage of IVR and to cool vibrational excitation rapidly in the low-frequency acetylene wag normal-mode vibrations produced by the IVR dynamics.