Determination of optical properties by third-harmonic generation in noble gases in unfocused laser beams

A new laser technique involving phase-matched third-harmonic generation in unfocused laser beams has been used to determine vacuum ultraviolet (VUV) oscillator strengths in xenon, VUV indices of refraction for the buffer gas (krypton or argon), and the VUV absorption coefficients were determined through total ionization measurements in the frequency regions of phase matching resulting from the interaction of unfocused linearly polarized light with mixtures of xenon and positively dispersive buffer gases. Since the multiphoton ionization mechanism is sharply dominated by the dimer absorption of third-harmonic photons, the ionization signal provides an excellent probe for determination of these optical parameters. Absolute ionization measurements were made through use of a proportional counter which was calibrated by the ionization resulting from an internal ⁵⁵Fe source.

The determination of these parameters involves the accurate measurements of the partial pressures of the two gases, the widths of the phase matching ionization curve, and the detuning of the phase matching curve from the xenon resonance. In the intial studies the accuracy of these measurements was such that the parameters could be determined to within ± 10 percent. An improved experimental arrangement provided determinations to within ± 3 percent.

This technique has the capability of measuring these optical parameters for wavelengths below 100 nm. This region is not accessible by the spectrometer methods, since there are no transparent materials available for the windows. In this phase matching technique the third-harmonic does not need to leave the generation chamber.