Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

Spectroscopic and Theoretical Aspects

BY D. PHILLIPS

Introduction

The format here is the same as that used in Volume 6 of the series. Some special emphasis has been given to interesting developments in simultaneous two-photon absorption and excitation spectroscopy and photofragment spectroscopy, but otherwise reportage in this section has been kept to a minimum.

2 Calculations

As before, this section is confined to estimates of energy levels, transition energies, and oscillator strengths obtained through calculation. Papers dealing with potential-energy surfaces in relation to photochemical reaction and non-radiative decay are found in later sections.

A brief account has been given of the applications of qualitative MO theory, and the use of a least-squares method in CI calculations has been discussed. A new vector method has been proposed to describe the electronic states of atoms and molecules, and the use of one-electron MO theory for energy-level determination discussed. Ab initio computations of spin-orbit interactions in polyatomic molecules using gaussian orbitals and optimal orbitals for SCF CI calculations on excited states have been considered. Polynomial expansion methods for energies and excitation strengths have been outlined. Two new methods, the multiconfiguration electron-hole potential method and a time-dependent variation perturbation approach, have been used to calculate electronic transition parameters in a variety of molecules.

The generally accepted basis for the widely used Hund's rules for predicting the ordering of electronic states has been challenged in recent years, yet the rules appear to be valid. A reformulation of the rules in a strict SCF approximation in which many of the elements of the traditional theory are retained has been proposed, to surmount this difficulty. Electron repulsion in the singlet and triplet states of the helium atom, natural orbitals of several excited states of this atom, and the application of the SCF method to 1P and 3P excited states of two-electron atomic systems have been discussed recently. Among papers appearing concerned with the spectroscopy and energy levels of atomic species are included those on the subjects of Li (II), K+ (I), the Be (1s2s2 -> 1s22s2p) and B (1s22s22p -> 1s22s2p2) transitions, the C (2s2p3) 3S0 ->1D0 isoelectronic sequence, the vacuum-u.v. lines in N (I) to N (IV), the Na S states, oscillator strengths in elements of the iron group, isotope shifts in the arc spectrum of xenon, excitation energies in other atomic systems, and van der Waals interactions for atoms in excited states.

There have been several studies on diatomic species, ions in particular, which indicate the growing current interest in the experimental study of such molecular entities. Theoretical studies have been carried out on H2+, HD+, HeH+, Ne2+, N2+, Li2+, and other ions.

Neutral species investigated include H2, He2, Ne2, Ar2, LiH,> LiF, N2, NO, OH, BH, MgH2, Na2, C2, and CH4.

Triatomic species investigated are relatively few. Several papers have investigated the excited states of the water molecule using ab initio methods, including the improved virtual orbital and CI treatments. Good agreement with experiment was obtained in these studies. In one case it was shown that the 9.81 eV triplet state observed in electron-impact studies corresponds to the 10.17eV singlet (1A1[b1 -> 3px]) rather than the expected 10.00 eV singlet (1B1[b1 -> 3pz]) owing to the larger splitting between the and states of 0.46 eV compared with that between the 4B1 and 3B1 states of only 0.08 eV which rises from the magnitudes of the corresponding exchange integrals. It is of interest to compare these results with those obtained in a photo-fluorescence excitation spectroscopic study of H2O described in a later section. Rydberg states in superexcited states of H2O and NH3 and HF have been discussed.

The existence of a 3A" state of the HNO radical at 5485 cm-1 lying between the 1A" excited and 1A" ground states has been predicted from an equation-of-motion-method study. The usual assumption that electrons are further apart in triplet states than in singlet states has been challenged on the basis of calculations performed on the NH and CH2 species, in which it is shown that electron repulsion can be greater in the state of lowest energy for a number of systems, in accord with statements made earlier concerning Hund's rules.

Methylene (carbene) is a species of some photochemical interest, and has also been much studied from a theoretical standpoint. It possesses a triplet ground state, whereas diflu or omethylene is reported to have a singlet ground state, with CHF having a singlet ground state if electron correlation is included in computations, but a triplet ground state within the SCF approximation. Computed triplet-singlet energy separations including electron correlation were 10, 6, – 11, and – 47 kcal mol-1 for CH2, CHCH3, CHF, and CF2, respectively. The spectroscopy of CH2 requires that the first excited triplet state of CH2 be highly...