Vacuum-UV absorption spectrum of a laser-produced chromium plasma: 3p-subshell photoabsorption by Cr²⁺ ions

The dual laser plasma photoabsorption technique has been used to measure the time-resolved vacuum-UV photoabsorption spectrum of a chromium plasma. Resonant photoabsorption cross sections, constructed with the aid of Hartree-Fock calculations, and weighted in accordance with the plasma temperature, have been used to produce the synthetic Cr²⁺ spectra. The relevant plasma temperature and ionization balance are obtained from simple analytical models for various times during the expansion phase of the plasma plume. The experimental spectra taken at delays of 32, 62 and 90 ns compare well with Cr²⁺ spectra computed for corresponding predicted temperatures. It is found that in order to produce synthetic spectra that match experiment well, it is necessary to take into account absorption from many states belonging to the Cr²⁺ ground state configuration 3p⁶3d⁴, while states from the nearest metastable configuration 3p⁶3p³4s make a negligible contribution.