Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

Spectroscopic and Theoretical Aspects

BY D. PHILLIPS

1 Introduction

The format for presentation of this section is as in previous years. Attempts have been made this year to reduce the length of the volume, so although coverage of significant new work is believed to be complete, detailed discussion of individual papers is more selective.

2 MO Calculations

All papers discussed under this section are concerned with calculations of energy levels and transition energies. A criterion for Gaussian approximations of Slater-type orbitals, multi-configuration SCF theory for excited states, spin-restricted open-shell SCF theory, and complex MO's in the extended HF scheme have been presented. A one-electron model has been used with some success to predict singlet- and triplet-state energy differences in helium and carbon monoxide, results being within 15% of observed values. The method thus permits reasonable estimation of triplet levels from more easily observed singlet levels. Experimental energy levels of helium have also been compared with values calculated using a method of autocorrelation fields. Theoretical transition probabilities for electronic transitions in beryllium, first-row neutral and singly and doubly ionized atoms, and for the SiI 3s23p23P-3s5p33 D0 isoelectronic sequence in PII, ClIV, CaVII, and FeXIII have been obtained and compared with experimental values.

The use of the CNDO method in spectroscopy has been illustrated with respect to doublet states, and electronic quadrupole moments of excited states have been computed. A simple new function which can be used to represent the intermolecular pair potential energy of the noble gases has been proposed, and this can be compared with the excited state Ar + Xe (3P1) potential obtained from solvent shifts of the vacuum-u.v. spectrum of xenon in gaseous argon. Recent experiments involving microwave optical magnetic resonance induced by electrons (MOMRIE) on the d(3p)3IIu states of ortho- and para-H2 and -D2 have revealed discrepancies in measured fine-structure parameters, which are inexplicable in terms of the Born–Oppenheimer approximation. A general theory which can account for the observations has been presented.

SCF wavefunctions and energies for valence states and excited states of carbon and nitrogen have been discussed, and the momentum distribution in the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [??] Σu+ states have been presented graphically and the results discussed. An exact numerical method for reducing eigenvalues of diatomic molecules to molecular parameters has been described for triplet states, particularly for those with Λ ≥ 2, such as the [??]3Δg and [??]3Φu states of N2 Dipole moments, induced dipole moments, and quadrupole moments for the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [??]2Σ + states of CH have been calculated from accurate ab initio wavefunctions and potential curves. The oscillator strength for the [??]1Σ+ -> [??]2Π system in CH+ has been calculated as 0.001 (excitation energy 2.50 eV) by the equations of motion method. CI calculations on NO [??]2Π and [??]2Σ+ have been carried out but, at least for the [??]2Σ+ state, serious discrepancies were found between the theoretical and experimental values for the dipole moment and quadrupole coupling constant. Spectroscopic constants for the eight lowest electronic states of the NO+ ion have been tabulated. Several calculations on the iodine molecule potential curves, including analysis of the long-range RKR potential of the B3ΠOu+ state have been carried out. Unexpected effects concerning perturbations of the latter state have been discussed.

A number of papers presented during the past year have been concerned with calculations on triatomic species, and several of these have dealt with ozone. Thus ab initio SCF–CI calculations have shown that the ππ* singlet state of ozone has an unsymmetrical equilibrium geometry, with one long and one short O — O bond, and similar behaviour was proposed for the SO2 molecule. Theoretical evidence for strongly bent excited states of ozone has also been given by the same group. A comparison of INDO and ab initio methods for the corrected wavefunctions of ground and excited states of ozone has been carried out, and theoretical evidence for bound excited states of ozone has been presented. This indicates that the 3B2, 3A2, 1A2 and B1 states have binding energies of 0.4, 0.3, 0.1, and 0.0 eV, respectively, with respect to ground-state O2 and O. These results could be of importance if experiment verifies the existence of bound upper states, since ozone is of great...