THE HCO (B)TILDE(2)A'[--](X)TILDE(2)A' SYSTEM - FLUORESCENCE EXCITATION AND STIMULATED-EMISSION PUMPING SPECTRA

The 000 band of~ the HCO B̃2A′ ↔ X̃2A′ system, observed at 0.06 cm-1 resolution in a ∼300 K fluorescence excitation spectrum, is rotationally analyzed. The principal molecular constants (in cm-1, 1σ uncertainties in parentheses), derived from a fit to K′a = 0 and 1 levels only, are in cm-1: [formula] Rotational congestion combined with irregularity of the K′a, K′a + 1 intervals, presented definitive assignments for K′a > 2 levels. A vibronic level at T0 = 41 284 cm-1, previously assigned as the zero-point level of a "C̃2A″" electronic state, is conclusively shown by its hybrid a, b-type rotational contour to be the B̃2A′ (1, 0, 0) level. From the relative intensities of a-type and b-type rotational transitions, the B̃2A′ - X̃2A′ 000 band electric dipole transition moment is found to lie (in the plane of the HCO molecule) at an angle of 31 (4)° relative to the a-inertial axis. Stimulated emission pumping (SEP) spectra (0.06 cm-1 resolution) of two X̃2A′-state vibrational "resonances," (vCH, vCO, vBEND) = (0, 5, 0) and (0, 4, 1) located at Tv = 9091.97 (3)and 8478 (1) cm-1, respectively, both lying well above thc H-CO dissociation limit, exhibited typical asymmetric top rotational structure and highly vibrational mode specific linewidths: Γ(0,5,0) = 0.7 cm-1 (broadened by unresolved spin-rotation splittings) and Γ(0,4,1) = 2.1 cm-1. Despite the lower energy of the (0, 4, 1) level, its much larger linewidth than that of (0, 5, 0) implies that the bending vibration (ν″3) is much more effective than the CO stretch (ν″2) in promoting dissociation. © 1993 Academic Press, Inc.