High-resolution photoionization spectrum of water molecules in a supersonic beam

We have obtained high-resolution (∼1.5 cm-1) photoionization spectra of supersonically cooled (Trot∼50 K) H2O and D2O in the 1000-900 Å range. The light source, which used the technique of frequency tripling in a pulsed free jet of gas, is described briefly. Spectra are rotationally resolved. Vibrationally excited autoionizing Rydberg series converging to the ground electronic [X̃; (1b 1)-1] state of the molecular ion are detected. This may well be the first example of a highly resolved Rydberg spectrum of a stable polyatomic molecule. From the convergence limit, the ionization potential H 2O is determined to be 101 777 ± 7 cm-1. Intensities of the Rydberg state autoionization signals are smaller than predicted with known Franck-Condon factors, indicating that predissociation is a competitive decay channel. Rydberg state lifetimes are ∼1 ps, deduced from homogeneous linewidths. Autoionizing features from Rydberg states associated with the ion's quasilinear à (3a1)-1 state are observed with linewidths above 10 cm-1, indicating that their lifetimes are less than ∼0.5 ps. Rotational assignments of some of the bands in this linear ← bent transition show that the Rydberg and ionic state geometries are nearly identical. A consistent assignment of the controversial bending (υ2) quantum number and Rydberg series quantum defect δ = -0.037 have been provided. © 1988 American Institute of Physics.