Absolute photoabsorption cross sections for H2O and D2O have been measured photoelectrically from λλ 180 to 790 Å using synchrotron radiation. The cross sections increase smoothly with wavelength to ∼λ610 Å, with both H2O and D2O displaying a broad absorption band extending above a nearly linear background from λλ 400 to 490 Å. The continuum has a maximum of ∼ 22.5 Mb at λ 640 Å. Above λ 615 Å, superimposed on the continuum, a diffuse structure appears which is similar to the vibrational structure of the 2B2 states of H2O+ and D2O+ as observed in photoelectron spectra. The structure is believed to arise from excitation of a 1b2 electron to the vibrational levels of a Rydberg orbital with n∗ ≈ 2.64.
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The photoionization cross sections of several gases were obtained by making simultaneous measurements of the total absorption cross sections and ionization efficiencies in the wavelength region from 473 A to 1100 A. In each case the total absorption cross sections were in agreement with those reported by others. For O2, the ionization cross sections were found to be between 5 and 30×10-18 cm2, while those for N2 were close to 23×10-18 cm2 over almost the entire range studied. The ionization cross section section of argon at the onset of ionization was compared with the theoretical estimate made by Dalgarno. The long-wavelength limits for photoionization were determined and were found to yield ionization potentials in good agreement with those obtained by spectroscopic and electron impact methods.
