Vibrational Dynamics of Terminal Acetylenes:  III. Comparison of the Acetylenic C−H Stretch Intramolecular Vibrational-Energy Redistribution Rates in Ultracold Molecular Beams, Room-Temperature Gases, and Room-Temperature Dilute Solutions

The population relaxation rate of the first excited state of the acetylenic C−H stretch is compared for a series of isolated and solvated terminal acetylenes. The isolated molecule relaxation rate for ultracold molecules is measured using high-resolution infrared spectroscopy in a molecular beam. These measurements use a microwave−infrared double-resonance technique to obtain rotationally resolved spectra that originate in the vibrational ground state. The relaxation rates in room-temperature gas and dilute CCl4 solution (0.05 M) are measured using two-color transient absorption picosecond spectroscopy. Although the molecule-dependent contribution to the total relaxation rate in solution is proportional to the population relaxation rate measured for the isolated molecule under molecular-beam conditions, a large scale factor (27) is required to reach quantitative agreement. Part of the reason a large IVR scaling rate is observed can be attributed to the fact that the intramolecular vibrational-energy redistribution (IVR) dynamics of the terminal acetylenes occur on two distinct time scales. The faster time scale produces only partial redistribution of the excited-state population. The experimental limitations of high-resolution infrared spectroscopy make it likely that this time scale is undetected in the molecular-beam measurements. Instead, the slower time scale, which is about 5 times slower than the initial IVR rate, is more closely related to the IVR time scale measured using high-resolution molecular-beam infrared spectroscopy. In addition, a thermal factor is expected when comparisons are made between ultracold molecular-beam and room-temperature sample conditions. A comparison of the measured IVR rates under these two conditions suggests that the rate enhancement at room temperature is related to the average thermal energy of the molecule. Most of the molecules in this study have about the same thermal energy, and this energy provides a factor of 5 increase in the IVR rate over the value obtained under ultracold conditions. These two factors together explain the large increase in the isolated molecule rate when the molecular-beam IVR rate is compared to the solution-phase relaxation rate of the room-temperature sample.