Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

Spectroscopic and Theoretical Aspects

The work reported in this section is categorized as in Volume 2 together with an additional section on Chemically Induced Dynamic Nuclear Polarization, a subject which is of increasing interest to photochemists. The restriction on the number of papers considered in Section 1 has been even more severe this year, and those selected for inclusion must inevitably represent a subjective selection by the authors from the very large number available. Nevertheless, it is hoped that those included are of interest to photochemists, and that those excluded on the grounds of space-saving are of less current general interest, although inevitably some of those excluded will have been of interest to particular groups and individuals working in the field.

1 Absorption Spectra and MO Calculations

The absorption spectrum of diatomic argon has been investigated in the 780–1080 Å region with a 6.65-m normal incidence vacuum spectrograph using the helium and argon continua as background sources. Nine discrete band systems were identified. The analysis shows that the ground state is stable, has a dissociation energy D00 = 76.9 cm-1, and six vibration levels, v= 0 — 5. A number of previous calculations of intramolecular potentials agree with these experimental results. Definite dissociation products are assigned for some of the upper states of these systems. A triplet potential surface for argon has been calculated, and Wanner-type impurity excited states in liquid rare gases reported.

It has been shown that the gross shifts in the spectrum of the Hg 3P1<- 1S0 transition of Hg in solid rare gases can be correlated by considering the difference in the interaction potential between the lattice and the Hg atom in the 3P1 and 1S0 states, providing the distortion of the lattice caused by the Hg atom is taken into account. In Ar, Kr, and Xe the spectrum can consist of three separate components. It was quantitatively shown that one component arises from relatively isolated Hg atoms, the other two from Hg atoms having nearest neighbour Hg atoms. This interpretation has been criticized, and an alternative suggested in which the two additional absorption components arise from splitting of the 3 P1 state of Hg by the asymmetry of the crystal field caused by a vacancy adjacent to the Hg atom. The latter interpretation is itself subject to several criticisms. The intensities of the two additional components relative to the main component depend on the Hg doping concentrations as predicted by the Hg–adjacent-Hg hypothesis but not by the Hg–vacancy hypothesis. In addition, there is no obvious reason why the equilibrium concentration of a Hg–vacancy complex should greatly exceed the fractional concentration of vacancies in pure rare gases at equilibrium. On energetic grounds the reverse would be more likely. Since the equilibrium mole fraction of vacancies is about 0.015 at the triple point of Ar and Kr, and since for Ar the equilibrium value of about 10-4 can be attained rapidly at 50 K, it would be surprising if the ratio of substitutional Hg to Hg -adjacent vacancy exceeded 0.1, whereas at least this fraction is necessary to explain the observed relative intensities.

A semi-empirical treatment has generated potential energy curves of the hydrogen molecule in the ground 1Σg and excited 3Σu states. A spectroscopic study of deuterium photolysis has been reported.

Hartree–Fock (HF) electronic transition moments have been calculated in both the position and momentum representations, and were presented as a function of the internuclear separation R for the BeH, MgH, OH, and SH (A–X) systems. The vibrational averages of these quantities were obtained and the results used to calculate some absorption band oscillator strengths. For the OH (A–X) system several independent experimental determinations of the 0–0 band oscillator strength have been reported in the literature, and the theoretical value of 20.6 x 10-4 differs from experiment by a factor of 2.5. Consideration of the united and separated atom limits and the region of the equilibrium internuclear separation for the states involved leads to an abbreviated discussion of the effect of correlation on the HF transition moments. HF transition moment calculations at a single value of R were also reported for the BH+, AlH+, HF+, and HCl+(A–X) systems.

By means of a photoelectrical technique, the absorption cross-section of the O2 continuum in the region 2350–1814 Å and the absorption cross-section of CO2 in the region 2160–1718 Å have been measured. The cross-section of the O2 continuum is 3.8 x 10-24 cm2 at 2350 Å; it slowly increases towards shorter wavelengths and reaches 10.7 x 10-24 cm2 at about 1980 Å, then increases very rapidly and reaches 7.1 x 10-22 cm2 at about 1814 Å. In the case of CO2, numerous discrete bands were found overlapping a weak continuum in the wavelength region below 1980 Å. The absorption cross-section of the CO2 continuum is about 2 x 10-24 cm2 at 2100 Å; it gradually increases toward the shorter wavelength side, and reaches about 4 x 10-24 cm2 at 2000 Å. The continuum rises rapidly at 2000 A and its value is 1.19 x 10-20 cm2 at 1718 Å.

Absorption coefficients of 1Δ gO2 have also been measured in the far u.v. Measured lifetimes of rotational and vibrational levels of electronic states of N2 have been reported. Vacuum-u.v. spectroscopy and photo -electron spectroscopy have been used to determine the electronic...