TWO PHOTON (VUV AND VISIBLE) RESONANT IONIZATION SPECTROSCOPY OF ATOMS AND MOLECULES.

Two-photon absorption allows the examination of atomic and molecular states that lie in a higher-energy region than is normally accessible by a one-photon process and yields more easily reproduced data than nonresonant multiphoton spectroscopy. In the experiment reported the third harmonic (355 nm) from a Nd:YAG laser is used to simultaneously pump a dye laser and produce coherent vacuum ultraviolet (VUV) light (118 nm) in the generation cell. The dye laser is tuned so that visible and VUV photons excite the species of interest in the detection cell. The excited species is subsequently ionized by an additional photon, and the ejected electron is detected by parallel plates. The laser schemes for VUV generation in xenon and the 4p double prime left bracket 1 1/2 right bracket J equals 2 two-photon transition in argon are shown. With commercially available narrow-bandwidth pulsed lasers, the energy levels of many gaseous atoms and molecules can be measured as accurately as with the longest VUV monochromators. The results obtained with broad-bandwidth lasers agree well with measurements by other researchers. Two-photon rates were also obtained. These results agree well with theoretical estimates. Two-photon rates are, therefore, useful parameters for predicting the signal size in future experiments.