Rj. Foltynowicz et al., Double-resonant photoionization efficiency spectroscopy: A precise determination of the adiabatic ionization potential of DCO, J CHEM PHYS, 114(12), 2001, pp. 5224-5232
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.