Knowledge Bank

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 $D2$, HT, DT, and $T2$ have been determined. The Raman shifts agree with theoretical energy levels for $D2$ (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 $H2,D2,T2,$ HD, HT, and DT with an order of magnitude increase in precision and accuracy over earlier measurement on $H2$ and $D2$. 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 $D2$ and the Q(11) transition in $T2$ were seen and the predicted positions using the most recent molecular constants were in error by $1.5cm-1$. 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 $H2)$ used in their determination.