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 first-laser frequency on selected ultraviolet transitions to individual rotational levels in the (000) band of the $3pπ 2Π$ intermediate Rydberg state 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) spectra. Intermediate resonance with this Rydberg state facilitates transitions to the threshold for producing ground-state cations by bridging the Franck–Condon gap between the bent neutral radical and linear cation. By selecting a single rotational state for ionization, double-resonant excitation eliminates thermal congestion. Spectroscopic features for first-photon resonance are identified by reference to a complete assignment of the $3pπ 2Π(000)−X 2A′(000)$ band system of DCO. Calibration with HCO, for which the adiabatic ionization threshold is accurately known, establishes an experimental instrument function that accounts for collisional effects on the shape of the photoionization efficiency spectrum near threshold. Analysis of the DR/PIE threshold for DCO yields an adiabatic ionization threshold of $65 616±3 cm−1.$ By extrapolation of vibrationally autoionizing Rydberg series accessed from the $Σ+$ component of the $3pπ 2Π(010)$ intermediate state, we determine an accurate rotationally state-resolved threshold for producing $DCO+(010).$ This energy, together with the threshold determined 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 (010) autoionization spectrum further yields a measurement of an energy of $4.83±0.01 cm−1$ for the (2-1) rotational transition in the $Σ+1(0110)$ state of $DCO+.$