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 CCl 4 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 molecularbeam 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.

The rotational spectroscopy of single molecular eigenstates has been used to measure the conformational isomerization rate in 2-fluoroethanol. Eigenstates in the asymmetric –CH2(F) stretch spectrum of the Gg′ conformer near 2980 cm−1 are prepared with an infrared laser. These eigenstates are approximately 2000 cm−1 above the barrier to Gg′−Tt conformational isomerization. The rotational spectrum is measured using an infrared-microwave double-resonance technique based on the Autler–Townes splitting of states in a strong microwave field. This technique does not require saturation of the infrared preparation step. Two types of rotational transitions are observed. These are assigned to rotational transitions from vibrational states with Tt conformation (near 15.8 GHz) and to “isomerization states” (near 17.1 GHz) where the torsional wave functions are above the isomerization barrier. The isomerization kinetics are obtained from the linewidth of the ensemble eigenstate rotational spectrum. The lifetime for the Tt conformer is 2.7 ns. The isomerization states relax at approximately twice the rate of the Tt states (1.5 ns lifetime). This result is consistent with a kinetics model where the isomerization proceeds by “over-the-barrier” pathways. Both lifetimes are longer than the bright-state IVR lifetime (275 ps) indicating that the intramolecular dynamics occur on two distinct time scales. The isomerization rate for the Tt states is three orders-of-magnitude slower than predicted by a simple RRKM rate expression.

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The high-resolution (6 MHz) infrared spectra of the asymmetric ethylenic hydride stretches of both cis and gauche allyl fluoride have been measured. Rotational assignments of the eigenstates are made using the groundstate microwave-infrared double-resonance capabilities of an electric resonance optothermal spectrometer (EROS). The cis vibrational band near 3114 cm -1 is characterized by sparse, narrow IVR multiplets resulting from weak Coriolis or cross conformer interactions between the vibrational bath states. The IVR lifetime of the cis vibrational band is approximately 2 ns (2000 ps). The gauche vibrational band near 3100 cm -1 is qualitatively and quantitatively quite different from the cis band. The gauche band is characterized by significantly fragmented IVR multiplets with an average IVR lifetime of about 90 ps. The measured anharmonic state density of the gauche band is about 30 states/cm -1 . The disagreement between the measured state densities of the gauche vibrational band and the calculated values suggest that the gauche vibrational states do not interact with cis vibrational states. For two IVR multiplets of the gauche band, the transitions were assigned according to the parity of the rovibrational bright state. The two parity states show roughly the same dynamical behavior. Also, RRKM calculations of the unimolecular isomerization rate are performed and compared to experimental results. The RRKM calculations overestimate both the cis and gauche isomerization rates by orders of magnitude.