Donald S. McClure

Triplet‐Singlet Transitions in Organic Molecules. Lifetime Measurements of the Triplet State

The emission lifetimes of the metastable triplet states (phosphorescent states) of a large variety of organic molecules have been measured. The lifetimes are in the range from 10−4 to about 10 seconds. It is shown that the transition probabilities corresponding to the shorter lifetimes are of the same magnitude as found in the light atoms of which the molecule is composed. The longer lifetimes, on the order of seconds, are found only among the aromatic compounds. A consideration of the perturbing singlet states in aromatic compounds shows that the matrix elements for the intercombination must be very much smaller than those responsible for the intercombination in the free carbon atoms. Direct evidence that the long‐lived states of the aromatic compounds are triplet states is obtained by showing that as the atomic number of chemically similar substituents is increased (e.g. substitute Br for Cl), the transition probability increases approximately in proportion to the increase in the square of the spin‐orbi...

Optical Spectra of Transition‐Metal Ions in Corundum

The polarized optical spectra of the ions Ti3+, V3+, Cr3+, Mn3+, Co3+, and Ni3+ in corundum single crystals have been studied at temperatures from 4.2° to 1200°K. A theory of the band strength based on the point‐charge model and p‐d mixing has been developed and applied to the data with results in fair agreement with experiment. The effects of temperature show that the vibrational‐electronic contribution to band strength is quite small at low temperature but may be appreciable at high temperatures. The crystal‐field parameters have been calculated as convergent lattice sums. The observed trigonal‐field parameter has the opposite sign from that calculated by the point‐charge model if the impurity ion is assumed to occupy an Al3+ ion position in the lattice, but has the same sign as calculated for an ion 0.1 A displaced along the c3 axis toward the empty octahedral site. Details of the spectra have been interpreted as showing that the surroundings of an ion are distorted in some electronic states.

Spin‐Orbit Interaction in Aromatic Molecules

The matrix elements of spin‐orbit interaction are obtained for polyatomic molecules using M.O. wave functions, taking account of configurational interaction. These results are then applied to the calculation of singlet‐triplet transition probabilities in aromatic compounds. It is shown without evaluation of integrals that intercombination transitions in these compounds should be much weaker than in most other classes of organic compounds. This is in agreement with experimental results on the phosphorescence lifetimes of these compounds.

The distribution of transition metal cations in spinels

Crystal field theory in conjunction with spectroscopic data may be applied to give quantitatively the d-shell splitting and the resulting stabilization of a transition metal ion in a crystal site of given symmetry. The thermodynamic stabilization values are found for both octahedral and tetrahedral sites in the spinel lattice. The differences between these values is the site preference energy. The cation distributions predicted from this energy are in good agreement with known experimental data. The occurrence of a large Jahn-Teller effect is correlated with tetragonal phase formation.

Optical Spectra of Hydrated Ions of the Transition Metals

The absorption spectra of crystalline hydrates of the first transition group ions have been measured with polarized light at room temperature, and with unpolarized light at a series of lower temperatures. Some new data on solution spectra have also been obtained. The values of Dq for these compounds are obtained, and are used to explain irregularities in the heats of hydration of the ions. The temperature dependence of the spectra shows that the transitions are vibration‐induced electric‐dipole transitions for the most part. The dichroism has been used to resolve some of the bands and to determine the effect of noncubic components of the crystal field.

Ultraviolet Spectra of Stilbene, p‐Monohalogen Stilbenes, and Azobenzene and the trans to cis Photoisomerization Process

In order to improve our understanding of the electronic states and photochemical reactions of stilbene, we have carried out a spectroscopic and photochemical investigation of stilbene, some substituted stilbenes, and azobenzene. High resolution absorption and fluorescence spectra of the singlet—singlet transition in dilute mixed crystal have been analyzed, and from them it is estimated that the potential barrier to trans‐cis isomerization in the first excited singlet state is about 40 kcal/mole. The absorption spectrum of the first singlet—triplet transition has been observed by enhancement with a heavy atom solvent and is interpreted as showing that the central bond in the lowest triplet state has a very substantial barrier to rotation. An electronic energy level scheme for stilbene has been constructed by treating the molecule as one ethylene molecule interacting with two toluene molecules. This treatment suggests that as many as four triplet states may be of lower energy than the first excited singlet ...

Excited States of the Naphthalene Molecule. I. Symmetry Properties of the First Two Excited Singlet States

The detailed polarization properties of the bands in absorption and fluorescence spectra of the naphthalene molecule have been determined by a method which makes use of a substitutional solid solution of naphthalene in durene. A single crystal of about 0.1 percent naphthalene in durene was cut so that the short axes of the naphthalenes lie strictly parallel to the surface and in one direction. The spectra were then determined in polarized light at 20°K under moderately high dispersion. The results clearly indicate that the first state is 1B3u (the transition from the ground state is long‐axis polarized) and the second 1B2u. Vibrational‐electronic interaction involving b1g vibrations apparently couples the first state with the second, and this results in a large short‐axis polarized transition moment. This fact has been at the root of most of the previous difficulties with analysis of the spectra.

Optical Spectra of Exchange Coupled Mn++ Ion Pairs in ZnS:MnS

The optical spectra of several ZnS:MnS (1 to 10 mole%) mixed crystals were obtained at temperatures from 4.2° to 300°K. A concentration dependence was found which proved that certain bands were due to pairs of Mn++ ions in nearest‐neighbor cation sites. Temperature—dependent absorption bands were found which were interpreted to give an interionic exchange integral in the ground state of —9 cm—1 in the spin—spin Hamiltonian H = —2JS1·S2. This agrees with the value obtained from the analysis of the susceptibility of pure MnS. The narrow bands due to transitions to the 4A1, 4E(G) states were shown to fit an isotropic spin—spin coupling law with J = +6 cm—1 for each state. The inversion of sign from the ground state is not fully understood. The selection rules for the transitions do not follow those for the total spin (ΔS = 0).

Comparison of the Crystal Fields and Optical Spectra of Cr2O3 and Ruby

The polarized optical absorption spectrum of Cr2O3 in thin single‐crystal plates has been obtained at temperatures from 4.2°K upward. The crystal field at the Cr ions in Cr2O3 has been calculated in the same way as for Cr:Al2O3 since their crystal structures are very similar. The experimental results for Cr2O3 were used to derive empirical values for the quantities 〈r2〉 and 〈r4〉 which appear in the expressions for the cubic and trigonal fields of Cr2O3, and these were substituted into the expression for the trigonal field of Cr:Al2O3. The result is a calculated trigonal field much smaller than is observed. The Cr ion in Al2O3 may therefore be displaced from the Al position as was suggested in a previous study. The details of the Cr2O3 spectrum, particularly the lines, are illustrated and commented upon, but no detailed analysis is presented. The spectrum does show many evidences of exchange coupling, however.

Electronic Spectra of Molecules and Ions in Crystals Part I. Molecular Crystals

Publisher Summary This chapter discusses electronic spectra of molecules and ions in crystals. Radiative transitions involving quanta having energies of the order of volts are referred to as optical transitions to distinguish them from other electronic transition processes such as those which occur in the microwave or x-ray region. The optical spectra of molecules and ions in solids normally are observed in the range from a few thousand to about a hundred thousand wave numbers. The purpose of this chapter is to show the extent to which optical spectra have been, and can be, interpreted and the useful information they contain. The parts of this chapter labeled theoretical are limited to the theory needed for the interpretation of spectra and for the derivation of useful empirical constants from the spectra. For example, crystal field theory is developed in sufficient detail to show the methods of finding crystal field constants from spectra; however, the interpretation of these constants is considered to be another subject and is not discussed in any detail. The main emphasis is, therefore, on the results of group theoretical considerations and on the approximate magnitudes of the numbers to be expected. The units of a molecular crystal are held together by weak van der Waals forces, which affect the electronic states of each molecule only slightly. Typical molecular crystals include organic compounds such as the aromatic hydrocarbons or inorganic compounds, of which VCl4 is an example.