O-H TORSIONAL VIBRATIONS IN THE S-0 AND S-1 STATES OF CATECHOL

A spectroscopic study of supersonic jet-cooled catechol (1,2-dihydroxy benzene) and its d(1)- and d(2)-isotopomers, deuterated at the hydroxy groups, was performed by resonant two-photon ionization (R2PI) and fl uorescence emission techniques, and supplemented by molecular-beam hol e-burning experiments. The latter prove that one single rotamer of cat echol is dominant under molecular beam conditions. The complicated vib rational structure in the S-0-->S-1 spectrum from the O-0(0) band to 4 00 cm(-1) above is not due to three different rotamers, as previously thought, but is due to the excitation of a vibrational progression ass ociated mainly with the torsion of the hydroxy groups. The torsional b ands are very prominent in the R2PI spectra, but are weak in the emiss ion spectra. Detailed analysis of the torsional bands was based on a f it to the S-1 and S-0 state frequencies and the Franck-Condon factors in absorption and emission, using a double-minimum potential for the S -1 state and a harmonic potential for the S-0 state. In the S-1 state one of the two -O-H torsional mode frequencies is lowered from tau(2) approximate to 250 to approximate to 50 cm(-1), and the molecule is on ly quasiplanar with respect to the -O-H torsional coordinates.