The Absorption Spectrum of Shock-excited Barium

By the introduction of BaCl 2 into a pressure-driven shock in argon, and the use of a flash-tube as a source of background continuum, a well-developed spectrum of Ba I and the stronger lines of Ba II have been obtained in absorption. The spectrum contains all but the weakest lines of Ba I in the spectral range studied. The majority of the lines start on the 5d6s 1,3 D metastable or on the 6s6p 1,3 P 0 levels, which lie between 1.1 and 1.7 eV above the ground state. Contrary to expectation, the auto-ionizing 5d8p 1 P 1 0 and higher 5d n p levels, which converge on Ba II 5 2 D and are known to produce strong absorption lines by double-electron transitions from the ground state, make no detectable combinations with the metastable levels. The spectrum contains the first recorded example of `forced auto-ionization'. A depression of the normal ionization potential by ionic microfields causes the level 5d8p 3 P 1 0 to auto-ionize into the 6sE(p) continua. Correspondingly, the series 6 1 S 0 -6s n p 1 P 1 0 breaks off at n = 12. A number of hitherto unclassified lines of Ba I and several new lines ascribed to forced dipole transitions are present in the spectrum.