Vibration–rotation spectroscopy by stimulated emission pumping in a supersonic beam: a-Axis Coriolis coupled v4=1 and v8=1 levels of X 1A g glyoxal

In combination with supersonic cooling, stimulated emission pumping (SEP) can be used to obtain vibrational spectra of medium-size molecules in their electronic ground state with single rotational level selectivity. The greatly reduced rotational congestion in the spectra of 0.04 cm−1 resolution makes the vibration–rotation assignment straightforward. Two vibrational levels, C–C stretching v4=1 with vibrational term values determined at 1065.80(1) cm−1 and CH wagging v8=1 at 1048.07(1) cm−1, of glyoxal were studied. These vibrational levels are symmetry forbidden in the IR spectroscopy, but can be detected by SEP. Their rotational constants are determined from primarily the low J, K levels, as A=1.9394(25), B=0.1594(25), and C=0.1426(25) cm−1 for 41 and A=1.7335(14), B=0.1594(10), and C=0.1464(8) cm−1 for 81. The A constants and the symmetries indicate that the two vibrational levels are strongly coupled to each other through the a-axis rotation. A deperturbation analysis yielded the A constants as 1.8414(126) for 41 and 1.8317(127) for 81, as well as the Coriolis coupling constant ζa48 =0.3642(226). These, and other previous observations, indicate that Coriolis interactions are important in promoting intramolecular vibrational relaxation in the glyoxal molecule.