Electric-Field-Induced Spectra: Theory and Experimental Study of Formaldehyde

Abstract

An experiment is described which measures the modulation, produced by an applied electric field, of the absorption coefficient of the rovibronic spectrum of a gaseous sample. Phase-sensitive detection provides for measurement of 0.01% modulation at a spectral resolution of 0.1 cm− 1 in the near ultraviolet. A theoretical development of the electric-field-induced spectral line shape and intensity as an expansion in powers of the applied field is presented. The important molecular parameters are the ground-state and excited-state dipole moments and rotational constants; measurement of the excited-state moment is discussed. The dependence of line intensity on quantum numbers is predicted to be such as to emphasize transitions to states of low angular momentum; observation of this emphasis is of importance in assigning spectra. The correspondence between field-induced and absorption spectra in a single experiment lends great confidence to assignments of specific absorption features. Experimental study of formaldehyde confirms many of the predictions of the theory. Assignments are obtained for the low-K subbands in the 3390-, 3430-, and 3520- Å bands, and the asymmetry parameters are presented. Effects of asymmetry on the EFS are shown. A method of determination of the excited-state dipole moment based on measurement of ratios of intensities of EFS in different polarizations is presented. The dipole moment of HCHO in the A 1A″ state is determined to be 1.4 ± 0.1 D.