We present a simple model of time-resolved absorption spectra of molecules that are in the process of dissociating. Within an eeimpact approximation,ee the spectra are shown to be superpositions of Lorentzian and dispersion profiles with relative weights determined by a net phase shift. New methods are proposed for obtaining molecular transition frequencies and relative transition dipole moments as functions of internuclear separation. Experimental data on TlI are compared to theoretical predictions, with good overall agreement. © 1990 The American Physical Society.