Predissociation ofH3Rydberg states

Abstract

A fast-beam spectrometer is used to investigate the photofragmentation and autoionization dynamics of laser-excited triatomic hydrogen molecules. The yield of neutral products from rapidly predissociated states of ${\mathrm{H}}_{3}$ is monitored in parallel with the field ionization signal. Using this spectrometer, we have measured the natural lifetime of low principal quantum number Rydberg states ${\mathrm{nsA}}_{1}^{\ensuremath{'}}$ and ${\mathrm{npE}}^{\ensuremath{'}}.$ For the ${3sA}_{1}^{\ensuremath{'}},$ ${4sA}_{1}^{\ensuremath{'}},$ and ${5sA}_{1}^{\ensuremath{'}}$ electronic states, we find lifetimes decreasing with increasing principal quantum number. This is unexpected for a Rydberg series and confirms the observations of a previous investigation. In addition, our experiments give direct evidence that even very high principal quantum number Rydberg states of ${\mathrm{H}}_{3}$ are selectively subject to strong predissociation. This process appreciably shortens their natural lifetimes. In the high principal quantum number $\mathrm{ns}$- and $\mathrm{nd}$ Rydberg series converging to the lowest ionization threshold of ${\mathrm{H}}_{3}$ we find a significant variation of the predissociation yield with principal quantum number and electronic angular momentum. Windows with enhanced predissociation in the Rydberg series are explained by nonadiabatic couplings of the laser-prepared initial states with the repulsive ground states surface mediated by vibrationally degenerate mode excited ${4pE}^{\ensuremath{'}}$ and ${5pE}^{\ensuremath{'}}$ states. The relevance of the ${\mathrm{H}}_{3}$ predissociation mechanisms for the dissociative recombination of ${\mathrm{H}}_{3}^{+}$ is discussed.