Double-resonance spectroscopy of the high Rydberg states of HCO. IV. Vibrational autoionization dynamics as a function of bending amplitude

Double-resonant ionization-detected absorption experiments extend an investigation of the autoionization dynamics of HCO to include the second overtone of the bend. In these experiments, first-photon transitions to the 3p pi (2)Pi Rydberg state select single rovibrational levels for second-photon scans of vibrationally autoionizing high Rydberg series that converge to specific rovibrational limits of HCO+. Line shapes reflect coupling widths that join discrete states built on vibrationally excited cores with underlying, vibrationally relaxed continua. Scans of series converging to HCO+ with one and two quanta of bend show narrow linewidths, reflecting relatively long autoionization lifetimes. However, for cores excited to the second harmonic of the bend, certain series abruptly broaden, indicating ultrafast decay. The implications of these results are discussed in terms of a qualitative extension of multichannel quantum defect theory to triatomic molecules. (C) 2000 American Institute of Physics. [S0021-9606(00)01004-7].