V. Yakhot

The absorption and fluorescence properties of pyrene crystal: a theoretical approach. I. harmonic approximation

Abstract On the basis of the structure of pyrene crystal we argue that excitation in the crystals localized at sites consisting of pairs of close-spaced molecules, i.e., that we are dealing with a Frenkel exciton. We postulate potentials for ground- and excited-state pairs in which both in-plane and out-of-plane vibrations play a role. Using these potentials and selected experimental data we are able to estimate the values of several features of the absorption and fluorescene spectra of, and the energy migration in, cyrstalline pyrene. Agreement with experiment is satisfactory.

Theory of excimer vibrational relaxation and U.V. emission in solid neon

Abstract A quantitative explanation is provided for the observed UV emission from Ne 2 * molecules in solid neon. An expression involving only known molecular quantities and lattice parameters is given for the rate of vibrational relaxation of an excimer, initially formed in a highly excited level, by interaction with the phonons. This expression is used to show that relaxation cannot proceed to the ground vibrational level of Ne 2 * during the radiative lifetime of the excimer (at the low temperatures at which the experiments are done). The calculation brings out that the vibrational population is concentrated at the n = 3 level when radiation takes place. The lineshape for dissociative UV emission from the n = 3 state of the excimer is computed, and found in very good agreement with experiment.

Pair-states in α-perylene crystal. A theoretical study

Abstract The low- and high-temperature emissions from α-perylene are interpreted as originating in, respectively, loosely- and tightly-bound pair complexes. The change-over from one to the other results from thermal expansion and deformation of the lattice and the resulting change in contact distances between the complexing pair and the surrounding molecules. With this model the binding energies, Stokes shifts, lifetimes, and emissive bandwidths associated with the complexes are estimated, as is the activation energy for excimer-exciton migration; the values agree well with the available experimental data.

Librational relaxation of molecular impurities in host crystals

A study is made of phonon-assisted transitions between closely spaced librational levels of an impurity molecule in a host crystal. The contributions to librational relaxation of direct, of Orbach-type and of Raman processes are evaluated as a function of temperature. It is found for a model case of NH(3 π) in solid Ar, that below T = 16 K, the direct process dominates, while for T > 20 K relaxation takes place mainly by the Raman mechanisms. The calculation predicts a nanosecond time scale for the relaxation at 4 K, and a picosecond time-scale for the relaxation at T = 24 K.

Radiationless transitions. Quantitative interpretation of some experiments.

Abstract The Green function method is used to calculate the rate of radiationless transitions. The obtained formula is applied for quantitative interpretation of the fluorescence quenching of anthracene, pyrene, methylanthracene in various solvents. The vibrational relaxation rates of small molecules substituted to the noble gas crystalline hosts are evaluated. The agreement with experiment is satisfactory.

The absorption and fluorescence properties of pyrene crystal: a theoretical approach. III. Relaxation from highly excited vibrational levels

Abstract It is shown that the model based on vibrational relaxation to the bath, while invalid for highly excited vibrational levels at low temperatures, is valid at high temperature. Thus relaxation time of ∼10 −13 s previously estimated is acceptable. Using this time, known radiative lifetimes, and a reasonable value for relaxation times of the vibrations of the isolated molecules, a modified model for excimer absorption and emission is proposed.

Optical properties of a crystal with nonequilibrium phonon distribution

Abstract The optical properties of the nonequilibrium solid can differ drastically from that in the equilibrium crystal even if, as a whole, the system is very close to equilibrium. The qualitatively new features appear in absorption—emission spectrum which allow to obtain narrow lines with intensity increased by few orders of magnitudes. The processes described in this work are relevant for Mossbauer spectroscopy, X-ray scattering, raditionless transitions.

Effect of Temperature and Crystal Packing on Some Excimer Emitting Crystals

Abstract The compounds 9-chloro-anthracene and 2,4-dichloro-3′-methyl-trans-stilbene are dimorphic, the two crystal forms having “pair” and “stack” structures. Both forms of the above compounds show excimer emission; this emission is of higher energy and with greater temperature sensitivity in stack than in pair structures.