Spectroscopy and Lifetime Measurements of Some N=3 and N=4 Levels of Hydrogen Molecule.

A versatile laser-molecular beam method has been applied to the study of highly excited states of molecular hydrogen. A thermal beam of H(,2) was excited by electron bombardment to the lowest triplet excited state, which is metastable. Selective, Doppler-free excitation of higher states was accomplished by crossing the molecular beam transversely with the beam from a cw dye laser. The decay of the excited states can be detected by various means, depending upon the predominant decay channel. A photomultiplier was used to observe the decay of the 3s, 3d states in the vacuum ultraviolet. By externally chopping the laser into short pulses and using time-resolved detection, the excited state lifetimes can be measured directly. Measurements of 14 lifetimes in the 3s, 3d complex are reported, with typical accuracies of a few percent. The observed lifetimes range from 11.1 to 48.8 nsec. The apparatus was also used to make spectroscopic measurements of the triplet 4s and 4d states. Preliminary assignments are given for levels of the 4s state, which has not previously been observed. It was found that the v = 1 and v = 2 vibrational levels of 4s state or predissociate in less than 1 nsec. A Channeltron electron multiplier was used to detect the products of this predissociation. A simple theoretical model for Rydberg state structure was developed, and has been applied with considerable success to the optical structure of the 4d states. Future applications of the experimental method to the high Rydberg states of H(,2) and the excited states of N(,2) are discussed.