Rotationally resolved photoionization of H₂O

A rotationally resolved one-photon threshold photoionization spectrum of jet-cooled water (H₂O and D₂O) has been obtained by pulsed field ionization of extremely high-n Rydberg states. Observed spectral intensities for both vibrationless (0,0,0) and vibrationally excited (1,0,0) water cation show a strong propensity for ΔN = 0, ± 1 transitions. In contrast to earlier work on O₂ and HCl, the lack of large ΔN transitions suggests that ionization occurs with only small angular momentum transfers between the core and photoelectron. The presence of both type A and type C "symmetric top" transitions varies from the conclusions of a recent MQDT analysis of H₂O photoionization, which predicts only type C transitions. Rotational analysis of the spectra yields improved ionization potentials for both H₂O and D₂O. The ionization potential of the (1,0,0) vibrational level provides a direct measurement of the symmetric stretch fundamental in H₂O⁺ which is in excellent agreement with an earlier indirect determination. Room temperature spectra are presented for ionization into the (0,0,0), (0,1,0), and (1,0,0) vibrational levels of H₂O⁺.