RAMAN SPECTROSCOPY OF HYDROGEN ISOTOPES: DETERMINATION OF EFFECTS OF CHANGES IN POLARIZABILITY ANISOTROPY ON INTENSITIES
Date
1983
Authors
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Publisher
Ohio State University
Abstract
Accurate rotational-vibrational relative intensities of each of the isotopic species of hydrogen are determined from gas phase Raman spectroscopy. Previously unreported line positions for $D_{2}$, HT, DT, and $T_{2}$ have been determined. The Raman shifts agree with theoretical energy levels for $D_{2}$ (theoretical values for Tritium containing species are not available), but differ from published spectroscopic constants at high J. The results have implications for the application of Raman-based spectroscopies of hydrogen as a temperature or state-population probe in high temperature systems. An external cavity configuration for an $Ar^{+}$ laser was constructed which delivers 160 Watts at 488 nm for gas phase Raman spectroscopy. A Spex 1403 double monochromator with photon counting and digital data collection allots accurate intensity measurements to be made. O- and S-branch rotational-vibrational line intensities are measured for $H_{2}, D_{2}, T_{2},$ HD, HT, and DT with an order of magnitude increase in precision and accuracy over earlier measurement on $H_{2}$ and $D_{2}$. The first derivative of the polarizability anisotropy with respect to internuclear distance may be extracted using first-order perturbation theory (it is expected, however, that first-order perturbation theory is inadequate to describe the rotational-vibrational Raman intensities of hydrogen isotopes). Neglecting the variation in polarizability anisotropy can lead to 10\% errors in the determined temperatures or state-populations when using Raman spectroscopy of hydrogen as a probe in high temperature systems (above 1000K). Line positions up to the pure rotational S(8) transition in $D_{2}$ and the Q(11) transition in $T_{2}$ were seen and the predicted positions using the most recent molecular constants were in error by $1.5 cm^{-1}$. For high J values the line positions are best determined from theoretical calculations as the molecular constants cannot be accurately extrapolated beyond the highest J values (above J-6 in $H_{2})$ used in their determination.
Description
Author Institution: Chemistry Division Los Alamos National Laboratory, Mail Stop C348. Los Alamos; Chemistry Division Los Alamos, National Laboratory