Photoionization and Absorption Cross Sections of H_2 and D_2 in the Vacuum Ultraviolet Region

Abstract

The photoionization and absorption cross sections of H2 and D2 have been measured in the 1000 to 580 Å region with a resolution of 0.5 Å. Simultaneous measurements of ion current and absolute photon flux were made in a chamber 40.6 cm long with a parallel-plate ion collector and a platinum photocathode or a sodium-salicylate-coated photomultiplier tube. The absolute yield of the photocathode was measured by calibrating the chamber with the rare-gas atoms A, Kr, and Xe.Weak band absorption with k ≤ 20 cm−1 was observed in the 1000 to 860 Å region, followed by an increase of k to 320 cm−1 at 844.8 Å. Between 844.8 and 803.7 Å, an underlying continuum k about 150 cm−1, and broad predissociated D–X bands were evident. A maximum k value of 680 cm−1 was observed in the (3–0) band of D–X. The underlying ionization continuum reached a broad maximum at about 730 Å with k = 300 cm−1. Band structure was superimposed on the continuum down to about 710 Å. Coefficients for photoionization were less than those for absorption down to about 700 Å, but, from 700 Å to shorter wavelengths, the ionization efficiency approached 100%. Strong absorption in the D–X bands was observed to be less efficient in producing ionization than neighboring bands of weaker intensity. Ion current was detected at wavelengths longer than the ionization potential reported by Beutler and Jünger at 803.7 Å. The current is attributed to an appreciable population at room temperature of the lower rotational levels of the ground state. The absorption and photoionization spectra of D2 were similar to those of H2, except for the expected isotope shift of the bands. The experimental chamber was arranged for the observation of gas fluorescence. Although the fluorescent intensity was very weak, edges were observed at 850, 755, and 720 Å.