GLYOXAL TRIPLET'S SPECTROSCOPY BY ANTICROSSINGS IN STRONG MAGNETIC FIELD

$^{1}$ T.A. MILLER and R.S. FREUND, Magnetic Induced by Electrons in Advances in Magnetic Resonance. Editor J. WILEY and SONS. $^{2}$ P. DUPRE, R. JOST, M. LOMBARDI, to be submitted to Chem. Phys.Author Institution: Laboratoire de Spectrom\'{e}trie Physique, Service National des Champs Intenses - CNRSWe excite single rovibronic $S_{1}$ levels of the Glyoxal molecule (CHO CHO) with an $Ar^{+}$ pumped CW ring dye laser. The Glyoxal is cooled in a supersonic jet. We monitor unresolved fluorescence as a function of magnetic field up to 8 T (homogeneity $\delta B/B \sim 10^{-5}$ in $1 cm^{3}$). Anticrossing accurs when rotational levels of high vibrational states of $T_{1}$ are tuned by the field (Zeeman effect) across the singlet level excited by the laser. Selection rules for anticrossings are nearly identical to those for optical electric dipole transition ($\triangle N = 0, \pm 1,\ldots)^{1}$. The positions in magnetic field of the anticrossings enable us to deduce the triplet rotational structure and then to determine rotational $constants^{2}$. The resolution Doppler free spectroscopy is limited in principle only by the widths of the anticrossings. These are related to the singlet-triplet coupling $V_{st}$ which varies between the natural linewidth and $\sim 300$ MHz. In fact these anticrossings have fine and hyperfine structure. When the various peaks due to these structures overlap $(V_{st}>25 MHz)$ we use a microwave-optical double resonance method to resolve them. We are thus able in all cases to measure the fine and hyperfine structures of the triplet levels. We have noticed that the hyperfine structure is nearly constant for several hundreds of observed levels $(83 \pm 3 MHz)$. This contrasts with the fine structure which varies in an erratic manner. Experimental uncertainties are due entirely to the inhomogeneity of the field which we measure with an NMR probe. We have thus determined the rotational constants of several highly excited triplet vibrational levels located around $2 800 cm^{-1}$ and $3 500 cm^{-1}$ above the fundamental vibrational level of the electronic state $T_{1}$