John C. Keske

Molecular rotation in the presence of intramolecular vibrational energy redistribution

At high energy, the vibrational dynamics of a polyatomic molecule are qualitatively different from the separable normal-mode dynamics that characterize the low energy region of the spectrum. Once the total rovibrational state density exceeds 10-100 states cm-1, the effects of intramolecular vibrational energy redistribution (IVR) are readily observed in the frequency-domain spectrum. In an energy region where IVR occurs, the time scale for the flow of vibrational energy is comparable to the time scale for molecular rotation. The jostling of nuclear positions caused by the IVR dynamics leads to a time-dependent moment of inertia for the molecular rotation. The time-dependent modulation of the moment of inertia, in turn, affects the appearance of the rotational spectrum of the molecule. These effects can be described by the motional narrowing formalism first developed for nuclear magnetic resonance spectroscopy. We present a basic description of the rotational problem for the case where the molecule has a single energetically accessible nuclear geometry and the case where the total energy of the molecule is above the barrier to isomerization. In the latter case, the microcanonical isomerization rate can be obtained from the overall line shape of the rotational spectrum. An example of using rotational spectroscopy to measure the isomerization rate of 4-chlorobut-1-yne at 3330 cm-1 is presented.

Intramolecular vibrational energy redistribution and conformational isomerization in vibrationally excited 2-fluoroethanol: High-resolution, microwave-infrared double-resonance spectroscopy investigation of the asymmetric –CH2(F) stretch near 2980 cm−1

The asymmetric –CH2(F) stretch spectrum of 2-fluoroethanol near 2980 cm−1 has been rotationally assigned using microwave-infrared double-resonance spectroscopy methods in an electric-resonance optothermal molecular-beam spectrometer. The eigenstate-resolved infrared spectrum shows the effects of intramolecular vibrational energy redistribution (IVR) through the fragmentation of each rotational level of the vibrationally excited state into a set of transitions. From the spectrum we determine the IVR lifetime of the asymmetric –CH2(F) stretch to be 275 ps. The measured vibrational state density at 2980 cm−1 is 44 states/cm−1, and matches the value for the total state density obtained from a direct count. This agreement suggests that vibrational states of both the Gg′ and Tt conformers are coupled by the intramolecular dynamics. From measurements of the c-type pure rotational transitions of the Gg′ conformer we determine that the tunneling splitting for the Gg′ ground state is less than 35 kHz. The infrared ...

Rotational spectroscopy of vibrationally excited states by infrared-Fourier transform microwave–microwave triple-resonance spectroscopy

A Fourier transform microwave (FTMW) spectroscopy-based technique for measuring the rotational spectrum of vibrational excited states is demonstrated. A pulsed infrared laser is used to prepare the excited state outside the FTMW cavity. Following laser excitation, the molecules drift into the FTMW cavity region. The FTMW spectrometer is used to monitor a single rotational transition in the excited state. The rotational spectrum of one of the states involved in the transition monitored by the FTMW spectrometer is obtained through the Autler–Townes splitting of the quantum state caused by the application of resonant microwave radiation to the cavity region.

High-sensitivity infrared–microwave–microwave triple-resonance spectroscopy of isomerizing molecules

Abstract We describe a molecular-beam infrared–microwave–microwave triple-resonance spectroscopy technique for measuring the rotational spectrum of single quantum states in energy regions where isomerization occurs. Infrared–microwave (IR–MW) saturation spectroscopy, with a resolution of 300 KHz, has been employed for the excitation of single eigenstates. In a second step, the rotational spectrum of the prepared quantum state is measured using a technique based on the Autler–Townes splitting of states. The technique is illustrated by measurements of the rotational spectrum of single quantum states of 4-chlorobut-1-yne near 3330 cm −1 .

Structure and vibrational dynamics of the halogen-substituted butynes HC=CCH2CH2X (X=F,Cl,Br) from molecular-beam high-resolution microwave and infrared spectroscopy

The pure rotational and high-resolution acetylenic C–H stretch rovibrational spectra of a series of substituted butynes, HCCCH2CH2X (X = F,Cl,Br), are reported. For each of the molecules the pure rotational spectrum of two conformational isomers (trans and gauche) has been assigned. For the Cl and Br substituted compounds the pure rotational spectrum of two isotopic species has been assigned (35Cl, 37Cl and 79Br, 81Br) for each conformer. An analysis of the nuclear quadrupole hyperfine structure in the pure rotational spectrum shows good agreement with structural parameters obtained through electronic structure calculations. The rotational transitions are used to obtain full rotational assignment of the acetylenic C–H stretch vibrational band of the more stable trans conformers through infrared-microwave double-resonance spectroscopy. The rotational band contours are predominantly a-type for all three halobutynes. The rotational structure of the band displays separations characteristic of the trans conformer. This result reflects the fact that the bright state for the vibrational spectrum retains the ground state conformation. From the eigenstate-resolved spectra we determine the timescale for intramolecular vibrational energy redistribution (IVR). The IVR rate obtained from the high-resolution spectrum provides an upper limit to the isomerization rate following coherent vibrational excitation. The IVR lifetimes of the acetylenic C–H stretch have been determined to be 1.5 ns for trans-4-fluorobut-1-yne, 3.5 ns for trans-4-chlorobut-1-yne, and 2.0 ns for trans-4-bromobut-1-yne. In all cases, the upper limit isomerization rate inferred from the vibrational spectrum is three orders-of-magnitude slower than the RRKM rate calculated using the ab initio barrier heights. Although direct dynamical information about the isomerization rate cannot be obtained from the spectrum, evidence is found for conformational isomerization occurring through the J-dependent growth of the measured rovibrational state density.