Precise Two And Three-Photon Spectroscopy Of H 2

Abstract

Recent progress in the generation of ultraviolet laser radiation has raised several intriguing possibilities for performing very precise spectroscopic measurements in the far ultraviolet, by exciting multiphoton transitions using UV light generated from single-mode lasers. We have recently begun to apply some of these methods to the spectroscopy of the simplest neutral molecule, H 2 . Both two-and three-photon excitation have been used to determine some of the intervals from the ground state to the lowest electronically excited states with considerably higher accuracy than has previously been achieved. The transitions are observed using resonant multiphoton ionization in a collisionless supersonic beam, collimated to reduce the Doppler width. Several vibrational bands of the E,F4-X transition have been measured using two-photon excitation near 220 nm. The observed spectra were free of power broadening, and had linewidths limited only by residual Doppler broadening in the collimated molecular beam. These intervals were measured to 0.01 cm -1 , and much higher accuracy is possible. In the three-photon measurements, we studied transitions to the B(2pσ) and C (2pπ) states. These results were somewhat less precise because shifts arising from the AC Stark effect and from a blue shift that dep-ends strongly on number density, arising from the interference of three-photon excitation and third harmonic generation. A few of the many prospects for future work are discussed briefly.