Knowledge Bank

The zero-point level, the fundamental of the torsional vibration $v1$ and the overtone of $v7$ have each been selectively excited in the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Au(S1)$ state of glyoxal with various lines from an $Ar+$ laser. The - $0.5cm-1$ linewidth of the laser pumps only 10-15 rotational levels in each state. The very initial stages of rotational relaxation in the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Au$ state can be observed in the fluorescence spectra at low (ca. 0.001 Torr) pressure.It is observed in pure glyoxal that rotational and vibrational relaxation have competitive cross sections, and propensity rules are seen for each. The curious vibrational propensity $\Delta \nu 7=\pm 2$ after excitation of the overtone is readily apparent. The restriction $\Delta k=2$ accompanies rotational relaxation. Extensions to other collision partners will be discussed.