Knowledge Bank

When run on a single longitudinal mode, the 454.5 nm line of an $Ar+$ laser is resonant with only a few Doppler broadened rovibronic lines in the 0-0 band of the absorption of glyoxal vapor (CHOCHO). With this excitation, the 5$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow><mrow\; data-mjx-texclass="ORD">0</mrow>}$ fluorescence band at low pressures contains only about 30 lines, there being six lines from each $J\prime K\prime$ state pumped in excitation. These lines are easily resolved (B.F. Rordorf, J. Mol. Spectry. 69 365 (1978)) so that quantitative intensity measurements are possible. The intensities of a set of lines from a single $J\prime K\prime$ state do not fit the standard H""{o}nl-London patterns expected for a near symmetric top. They can be explained quantitatively, however, by inclusion of the Hanle effect, which is a quantum mechanical interference between the degenerate Zeeman levels of the excited rovibronic state. The perturbations on H""{o}Unl-London intensities are large so that the relative line strengths in absorption are appreciably different from those in resonance fluorescence.