Double-resonant photoionization efficiency spectroscopy: A precise determination of the adiabatic ionization potential of DCO

We report the first high-resolution measurement of the adiabatic ionization potential of DCO and the fundamental bending frequency of DCO+. Fixing a f irst-laser frequency on selected ultraviolet transitions to individual rota tional levels in the (000) band of the 3p pi (2)Pi intermediate Rydberg sta te of DCO, we scan a second visible laser over the range from 20 000 to 20 300 cm(-1) to record double resonance photoionization efficiency (DR/PIE) s pectra. Intermediate resonance with this Rydberg state facilitates transiti ons to the threshold for producing ground-state cations by bridging the Fra nck-Condon gap between the bent neutral radical and linear cation. By selec ting a single rotational state for ionization, double-resonant excitation e liminates thermal congestion. Spectroscopic features for first-photon reson ance are identified by reference to a complete assignment of the 3p pi (2)P i (000)-X (2)A'(000) band system of DCO. Calibration with HCO, for which th e adiabatic ionization threshold is accurately known, establishes an experi mental instrument function that accounts for collisional effects on the sha pe of the photoionization efficiency spectrum near threshold. Analysis of t he DR/PIE threshold for DCO yields an adiabatic ionization threshold of 65 616 +/-3 cm(-1). By extrapolation of vibrationally autoionizing Rydberg ser ies accessed from the Sigma (+) component of the 3p pi (2)Pi (010) intermed iate state, we determine an accurate rotationally state-resolved threshold for producing DCO+(010). This energy, together with the threshold determine d for the vibrational ground state of the cation provides a first estimate of the bending frequency for DCO+ as 666 +/-3 cm(-1). Assignment of the (01 0) autoionization spectrum further yields a measurement of an energy of 4.8 3 +/-0.01 cm(-1) for the (2-1) rotational transition in the (1)Sigma (+)(01 (1)0) state of DCO+. (C) 2001 American Institute of Physics.