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 ${\mathrm{nsA}}_1^{\prime }$ and ${\mathrm{npE}}^{\prime }.$ For the ${3sA}_1^{\prime },$ ${4sA}_1^{\prime },$ and ${5sA}_1^{\prime }$ 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 ${4pE}^{\prime }$ and ${5pE}^{\prime }$ states. The relevance of the ${\mathrm{H}}_3$ predissociation mechanisms for the dissociative recombination of ${\mathrm{H}}_3^{+}$ is discussed.

• Received 13 September 2000

DOI:https://doi.org/10.1103/PhysRevA.63.042711