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

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 $A^1\mathrm{A″}$ state is determined to be 1.4 ± 0.1 D.