HIGH-RESOLUTION INFRARED LASER SPECTROSCOPY
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Richard H. Schwendeman Professor Emeritus (b. 1929). B.S., 1951, Purdue University; M.S., 1952, Ph.D., 1956, University of Michigan; Postdoctoral Fellow, 1955-57, Harvard University; Postdoctoral Fellow, 1964-65, Uppsala University. High-resolution infrared laser spectroscopy of gases; determination of vibration-rotation parameters and structures; experimental analysis of molecular collisions. 517-355-9715, Ext. 311 |
The goal of the research in our group is the measurement of gas-phase molecular properties that are related to the forces within and between molecules. A wide variety of high-resolution spectroscopic experiments in the infrared, microwave, and radiofrequency regions of the spectrum are performed. Microwave and radiofrequency oscillators and CO2, N2O, and CO lasers are intense, nearly monochromatic radiation sources that allow us to employ powerful spectroscopic methods for analysis of vibration-rotation spectra and for the study of unusual spectroscopic processes such as three-level and four-level double resonance, level crossing, level anticrossing, saturation dips, and the combined effects of collisional narrowing and collisional broadening.
Our current research involves the use of the technique of high-resolution infrared-infrared double resonance to characterize collisionally-induced energy transfer processes. When polar molecules undergo rotational transitions as a result of molecular collision, the transitions occur without substantial change in velocity. By contrast, the only other reasonably probable collisionally-induced transitions - those in which the vibrational energy of one collision partner is transferred to the other (V-V collisions) - occur without retention of velocity. In high-resolution infrared-infrared double resonance, a fixed-frequency infrared laser (the "pump laser") is used to excite molecules that have a narrow range of velocity in the direction of the laser beam from the ground vibrational state to an excited vibrational state. A second tunable infrared laser (the "probe laser") is used to record a vibration-rotation transition that initiates in a rotational state of the excited vibrational state. As a result of the Doppler effect, the lineshapes of some of these transitions consist of a narrow component ("transferred spike") that results from collisionally-induced rotational transitions and a broad Gaussian component that results from V-V energy transfer. Analysis of the lineshape allows estimation of the relative rates of the two processes as well as the rms velocity change upon collision of collisionally-induced rotational transitions in the species under study. In a recent extension of our studies of molecular collisions, we have found that by switching the polarization of the pump laser, while keeping the probe laser polarization constant, it is possible to analyze the lineshapes to determine the extent of reorientation of the molecules during collisionally-induced rotational transitions. We use both plane polarization and circular polarization of the pump and probe to analyze different aspects of the reorientation.
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As samples for our spectroscopic studies, we try to select molecules that are of interest in atmospheric, astrophysical, or far-infrared laser studies.
Representative Publications
Pump laser Offset Frequencies of CH3F, Q. Song and R. H. Schwendeman, J. Mol. Spectrosc., 165, 277 (1994).
The Effect of Initial Velocity on Rotational Energy Transfer in 13CH3F, Q. Song and R. H. Schwendeman, J. Chem. Phys., 98, 9472 (1993).
Polarization Effects in Infrared-Infrared Double Resonance in Methyl Fluoride, U. Shin and R. H. Schwendeman, J. Chem. Phys., 96, 8699 (1992).
Infrared-Infrared Double Resonance of Methyl Fluoride in Foreign Gases, Q. Song and R. H. Schwendeman, J. Mol. Spectrosc., 153, 385 (1992).
Observation of Dynamic Stark Splitting in Infrared-Infrared Double Resonance in Methyl Fluoride, Q. Song and R. H. Schwendeman, J. Mol. Spectrosc., 149, 356 (1991).
