Using fast-neutral-beam photofragment spectroscopy we have made a detailed analysis of the spectroscopy and dissociation dynamics of the triplet gerade n=3 Rydberg states in high (v>3) vibrational levels. Several hitherto unobserved levels have been found near the n=2 dissociation limit. The experimental energy levels are compared with an L-uncoupling model which is extended to include both nonadiabatic effects associated with the breakdown of the pure precession model and off-diagonal vibrational interactions. The kinetic energy and angular distribution of the fragments have been studied with translational spectroscopy using a time- and position-sensitive detector. The kinetic energy distribution not only confirms spectroscopic assignments but is also used to investigate structure in the excitation of the vibrational i ${}_{}{}^3{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$ and $g^3$${\Sigma }_g^{+}$ continuum. Measurement of the angular distribution allowed us to determine the ratio for tunneling through the $i^3$${\Pi }_g$ and g ${}_{}{}^3{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ barriers for some v=4 levels. This showed that tunneling through the $i^3$${\Pi }_g$ barrier is the most efficient dissociation mechanism.

• Received 20 April 1988

DOI:https://doi.org/10.1103/PhysRevA.39.590