Non-selective photoionization of uranium atoms by the third harmonic of a Nd-YAG laser: experiment and simulation

Time of flight mass spectrometry in combination with laser ablation (LA) ca n be successfully applied to low-isotope-ratio measurements of metallic sam ples. In this connection, the non-selective quasi-resonant photoionization by Nd-YAG pulse laser third-harmonic radiation of uranium atoms created by either LA on a metallic target or by radiative heating in an oven was studi ed in a wide range of laser fluence both experimentally and with computer s imulation. In the latter case both the level structure of the uranium atom and the experimentally obtained profiles of spectral and spatial distributi ons of the third-harmonic radiation of the Nd-YAG laser in the photoionizat ion zone were taken into account. Comparison of experimental and simulated results was made. Experimental and simulated results for the LA case were f ound to be in agreement only with the assumption that the ionization yield from the metastable level of the uranium atom is strongly suppressed. This could be explained by low population of the metastable level in the photoio nization volume due to the expansion and subsequent population cooling of u ranium atom vapour created by LA. The unknown parameters of uranium transit ions (lifetimes and ionization cross sections) involved in the quasiresonan t photoionization process under study are estimated.