| Intermolecular Potentials in Crystals |
Professor Dows' research interests include molecular crystal spectroscopy, phase transitions, anharmonic processes in lattice dynamics, spectroscopy of orientationally disordered crystals, and molecular dynamics as applied to crystal systems, all with concern for intermolecular potentials in crystals.
Other fields of interest are (1) the spectroscopy of excited electronic states using absorption, fluorescence, multiphoton ionization and magnetic field effects to investigate molecular structures, lifetimes and vibronic interactions, (2) molecular fragmentation photochemistry, and (3) the theory of scattering of molecules from a crystal surface using quasiclassical trajectory methods.
The properties of solid state materials are influenced by their structures and the vibrational motions of their atoms, affecting heat capacities, electrical conductivity, acoustic properties, brittleness, phase transitions, etc.
Energy transfer among the vibrational modes and relaxation and vibrational dephasing are of fundamental importance. This energy transfer comes about because of the intermolecular potential energy or intermolecular forces which occur when molecules are in close proximity to one another, and which are also responsible for gas non-ideality, for example.
The class of molecular crystals includes organic and organic polymeric materials as well as inorganic crystals which contain molecular ions. Our studies include theoretical work on lattice vibrations and relaxation processes in these crystals. We use the methods of lattice dynamics and molecular dynamics (computer simulation). Effects of temperature, pressure and lattice disorder on the theoretical lattice frequencies and spectroscopic lineshapes are investigated.
The intermolecular potential energy function is fundamental to such calculations; at the present time there is considerable uncertainty about the potential, particularly concerning its more anharmonic aspects. Thus, one objective of our work is a more complete description of this important property. Another is to develop methods of handling the many-body perturbation theory for anharmonic lattice effects; because of the very large density of lattice states, approximate methods are required to reduce calculation time to a manageable level.
We have an active collaboration with experimentalists in other laboratories where we are involved in high resolution Raman and CARS studies (including picosecond CARS) of molecular crystals.
Currently, we are actively interested in crystals such as carbon dioxide, sulfur, imidazole, and alkali chlorates, all of which are currently under both experimental and theoretical examination. Carbon dioxide is a test case involving a small molecule, very well characterized, where extensive experimental data are available. Sulfur offers an interesting opportunity to probe the effect of impurity levels (the availability of isotopically pure sulfur and the changes in vibrational motion due to isotopic substitution). Imidazole is of course a relatively simple hydrogen bonded crystal, offering entry into relaxation processes in biological systems. The alkali chlorates (and related solids) are cases chosen for their quite different intermolecular potential properties, typical of a large class of inorganic materials.
Selected publications Phonon dynamics and
relaxation processes in isotopically pure 35Cl2
and natural crytalline chlorine, S.
Bussotti, M. Becucci, S. Califano, E. Castellucci, D. Dows,
J. Chem. Phys. 102 9191 (1995).
Phonon dynamics and relaxation processes in isotopically pure 35Cl2 and natural crytalline chlorine, S. Bussotti, M. Becucci, S. Califano, E. Castellucci, D. Dows, J. Chem. Phys. 102 9191 (1995).