Electric‐Field‐Induced Spectra: Excited‐State Dipole Moment from Line‐Shape Analysis

A theoretical development, including instrumental effects, is presented for the shape of a single experimental line in the electric‐field‐induced spectrum (EFS). The absorption coefficient of a line is defined by incorporating the Stark shifts and transition probabilities (to second order in the electric field) and a shape function for the degenerate components of the line. This treatment results in a model that explicitly contains the molecular parameters: ground‐ and excited‐state rotational constants and dipole moments and the half‐width at half‐height of the line. This explicit dependence on the excited‐state dipole moment leads naturally to employing a search procedure which compares the theoretical EFS line to the experimentally determined line shape. The rotational lines ${\mathrm{pQ}}_3\text{(4)}$ and ${\mathrm{pP}}_5\text{(5)}$ of the 3390‐Å band of formaldehyde were measured under various experimental conditions and fit using this technique, yielding a value of 1.53 ± 0.11 D for the excited‐state dipole moment. The accuracy of this result was found to be limited by the experimental accuracy of measuring the field strength, which is amenable to significant improvement. The accuracy of a method, previously used, which determines excited‐state dipole moments from ratios of line intensities, is discussed in the light of the present results.