Predissociation of H₃ Rydberg states

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 H₃ 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 nsA₁^′ and npE^′. For the 3sA₁^′, 4sA₁^′, and 5sA₁^′ 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 H₃ are selectively subject to strong predissociation. This process appreciably shortens their natural lifetimes. In the high principal quantum number ns- and nd Rydberg series converging to the lowest ionization threshold of H₃ 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^′ and 5pE^′ states. The relevance of the H₃ predissociation mechanisms for the dissociative recombination of H₃⁺ is discussed.