Showing papers by "David L. Andrews published in 1987"

Resonant excitation transfer: A quantum electrodynamical study

Abstract: A new quantum electrodynamical (QED) method is presented, based in the Schrodinger representation, for the calculation of the rate of energy transfer between identical molecules. In contrast to existing methods in this representation, the new treatment gives explicitly causal and energy‐conserving results. By returning to perturbation theory the correct, complex form for the electric dipole–electric dipole interaction tensor is obtained, without recourse to the physical, ‘‘outgoing wave’’ arguments of quantum scattering theory necessary if the Fermi rule is used. This method also allows a new interpretation for the role of the time‐ordered diagrams involved, which may be useful in the rigorous treatment of higher order cooperative processes. The QED treatment uses virtual photon coupling, and incorporates both the Coulombic, R−6 dependence, and the R−2 dependence characteristic of two‐step radiative transfer.

Cooperative mean‐frequency absorption: A two‐beam two‐photon process

Abstract: It is shown that by a cooperative process involving virtual photon coupling between molecules, pairwise two‐photon absorption can lead to molecular transitions associated with the mean of the laser frequencies in a two‐beam experiment. The process is represented by A+A+ℏω1+ℏω2→A*+A*, and the selection rules are those normally applicable to two‐photon absorption. The interaction may involve the excitation of chromophore pairs in polyatomic molecules, van der Waals molecules, or more usually discrete molecules in a gas or liquid. In the former case, however, dissymmetric juxtaposition of the chromophores produces a chirality which is manifest in a circular dichroism associated with the absorption process. The appropriate absorption rates are calculated using the methods of quantum electrodynamics, and the dependence on the separation of the interacting pair is examined in detail. Methods for observing the mean‐frequency interaction are also outlined, and attention is drawn to a resonance condition which should appreciably enhance the absorption rate.

A new magneto-optical effect in Raman spectroscopy

Abstract: It is well known that the application of a static magnetic field can result in a change in both Raman spectral intensities and line positions. this effect is generally associated with a removal of spin degeneracy, and leads to a manifestation of chiral discrimination in both optically active and inactive samples. However, other magnetic field effects can be conferred upon the Raman process by virtue of direct magneto-optical interactions. It is here shown that for molecules belonging to one of the cubic or icosahedral point groups, any polarised Raman transition associated with a totally symmetric vibration becomes depolarised on application of a magnetic field directed along the usual right-angled direction of observation. Hence with a suitable polarisation filter, the corresponding Raman line can be “switched” into the spectrum by application of the field.