ABSOLUTE DENSITY-MEASUREMENT OF A PULSED H-2 GAS-JET USED FOR EXTREME-ULTRAVIOLET GENERATION BY NONLINEAR FREQUENCY-CONVERSION

A pulsed molecular hydrogen gas jet is an efficient nonlinear medium f or generation of extreme ultraviolet radiation via two-photon resonant third harmonic generation or four-wave mixing. The absolute density o f hydrogen molecules in the gas jet was measured by comparing the lase r-induced fluorescence signal from the jet with that from a gas cell f illed with a known density of H-2 gas. This information can be used to improve the efficiency of frequency tripling of an ArF laser.