Photoabsorption to Even-Parity Autoionizing Levels of Na I from the Laser-Populated 3P^2p States

Extract

A column of sodium vapor with a significant fraction of the atoms in the 3p^2p state (excited state density of the order of 10¹⁵ atoms/cm³ ) was prepared by axially irradiating a sodium heat-pipe oven with a pulsed dye laser tuned to the 3s-3p resonant frequency. This technique has recently been shown to produce nearly complete ionization of the illuminated column in ∼ 600 nsec. By passing continuum VUV light down the pipe 150 nsec after initiating the laser pulse, an absorption spectrum of neutral excited state sodium is obtained in the region of autoionizing levels containing the 2p⁵3s3p and 2p⁵3d3p configurations. The continuum is produced by a BRV-type triggered spark that gives a light pulse of ∼ 100 nsec duration in the wavelength range of observation of ∼ 200-400 A. About 25 absorption lines are attributed to these configurations based on predicted wavelengths obtained from an intermediate coupling calculation of the autoionizing levels, including configuration interaction. This appears to be the first observation of these even parity levels, which are inaccessible to photabsorption from the ground state by electric dipole transitions.