Two-photon excitation spectrometer for dilute gases

Abstract

A two-photon excitation spectrometer which can be used with dilute molecular gases is described. The spectrometer is based on a standing wave tunable dye laser synchronously pumped by a mode-locked Ar+ laser. Two-photon excitation is enhanced in two ways. First, excitation is carried out intracavity at a focus produced by a pair of spherical mirrors. Second, nonlinear absorption is further enhanced by temporal compression of the laser power through mode locking the Ar+ pump laser and synchronously pumping the dye laser. The performance of the synchronously pumped intracavity focused (SPIF) dye laser spectrometer is analyzed by measuring the two-photon excitation spectrum of naphthalene vapor and comparing it with previously measured spectra. Two-photon excitation spectra of naphthalene measured with SPIF have signal-to-noise ratios that significantly surpass those of spectra found in the literature. It is estimated that for 1 Torr of gas whose fluorescence yield is unity, a two-photon absortivity of order 10−56 cm4 s photon−1 molecule−1 would produce a photon count rate of 1 Hz. This high sensitivity is partly due to the stability of the cw design of the dye laser and the high intracavity powers. This stability and the ease of its operation makes SPIF an especially useful spectroscopic tool.