Stark Effect on the Electronic Bands of a Symmetric-top Molecule

Abstract

Calculations have been performed on the energy matrix of a rigid symmetric-top molecule, HCHO, in order to determine the transition energies and intensities of the rotational sub-bands of the electronic spectrum under various magnitudes of external electrostatic fields. Though HCHO is an asymmetric-top molecule, the rotational structure of the 3390 Å band of this molecule is close to that of a perpendicular band of a symmetric-top molecule, at least, for not too small values of K. The variation in the band structures with the applied fields can be illustrated by the behavior of several selected bands, pP, pQ, rR, and rQ. Below 100 kV/cm, the overall Stark splittings have been linearly dependent on the field strength, regardless of the branch types. Over 100 kV/cm, however, a deviation from the linear dependence has been found for a certain branch. Changes in the band shape have appeared at such small values of external fields that no deviation from the linear field dependence of the overall Stark splitting has been observed. Such changes are dependent on the branch types, especially in the cases of branches with small J values and with K<J. The intensity enhancement of some forbidden bands has also been observed, the magnitude of which is proportional to the square of the field strength below 100 kV/cm. However, the relation between the intensity enhancement and the electric fields has been found to be complicated at fields over 100 kV/cm, the enhanced intensity being proportional to a coupled sum of E2 and higher terms. At a fixed electric field, a linear relation has been found between the overall Stark splitting and the various μ values assumed for the excited state. The analysis of the data has enabled us to determine the electric dipole moment of the lowest n→π* excited state of HCHO as 1.65 D.

A symmetry analysis of electric-field-induced spectra

“…For two-photon absorption, we have which is typical for use in two-photon studies,-and an experimentally feasible [8,9] electric-field strength of 5 × 10 7 V m-t, the ratio is approximately unity. The viability of EFIS is further increased, if one considers its quadratic dependence on the applied field.…”