Knowledge Bank

Overtone spectra in solid $H2$ and $D2$ arise from the overlap and quadrupole induced dipole moments and therefore are exceedingly weak. At the same time these spectra are of special interest in interpretation of planetary spectra of outer giant planets, as well as for understanding the dynamics of the simplest of molecular crystals in which quantum effects play a significant role. Pulsed opto-acoustic spectroscopy of condensed phase materials - a technique that we have recently developed for the measurements of optical spectra having absorption coefficients as small as $10-7cm-1$ (from the visible to the infrared) has been successfully applied to yield the first observations of overtone vibrational spectra of solid $H2$ for $\Delta \nu =3$ and 4 near 8200 A and 6100 A respectively. In this review I will give the details of the experimental techniques and describe the new data in solid $H2,D2$ and HD.