Absorption and photoionization cross sections for H2O and D2O in the vacuum ultraviolet

The total absorption, photoionization and neutral product cross sections for H2O and D2O have been determined over the wavelength range 580 to 1050 Å. The helium continuum was used as a background with a bandwidth of approximately 0.5 Å. Previously observed Rydberg series converging to the (000) level of the ionic ground state have been extended and utilized to obtain ionization thresholds of H2O (982.47 ± 0.5 Å, 12.619 ± 0.006 eV) and D2O (981.16 ± 0.5 Å, 12.636 ± 0.006 eV). At wavelengths shorter than 1000 Å, the total cross section curves for H2O and D2O exhibit a number of previously unreported bands superimposed on broad continua which have peaks at approximately 720 and 925 Å. These bands are most likely due to progressions belonging to Rydberg states converging to the first and second excited states of the molecular ion. There is evidence from photographic spectra of H2O16 and H2O18 that the Rydberg states converging to the first excited state of the ion (Ã2A1) have an effectively linear conformation.