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; (1b1)−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 H2O 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 (v2) quantum number and Rydberg series quantum defect δ=−0.037 have been provided.