The 3s3p ^P^0 level of Mg I has been selectively excited by the second harmonic of the output of a high-power, narrow-band, frequency tunable dye laser. The excited-state absorption series SsSp X P f-Ssnd 1 D 2 nas been recorded to n = 24. A strong autoionization resonance has been obtained at A = 300.9 nm, with a photoionization cross section of ~5 x 10** 16 cm 2 , in very good agreement with the values predicted by calculation.While Bacher showed in 1933 1 that the interaction of 3p 21 D 2 with the 3snd 1 D 2 series of Mg I could account for the location of the 3s3d x D term below the corresponding triplet level 3s3d 3 D, the X D 2 and 1 S 0 displaced terms of the 3p 2 configuration have not been previously observed. The 3 P 0 j 2 levels of this configuration lie just below the normal ionization limit and give rise to a strong multiplet at X = 278 nm 2 in the Mg I spectrum. The presence of 3p 2l D 2 , l S 0 in the continuum will affect the dielectronic recombination rate and the ultraviolet radiation balance in, for example, astrophysical sources. Calculations of the photoionization cross section from the lowest excited singlet level 3sZp 1 P 1° have been carried out, 3 using the approach of Henry and Lipsky. 4 The continuum functions required for the crosssection matrix elements were obtained from a program of Burke and Moores 5 and the 3s, 3/>, and 3d states of Mg II were included in the representation of the 3p 21 S 0 state. The bound 1 P 1° function was calculated using a configuration interaction program of Hibbert. 6 Each configuration had the same (Is 2 , 2s 2 , 2p 6 ) core, and the valenceelectron functions for the ten configurations were the Slater orbitals (3s, 3/>), (3 P ,3d), (4s, 3p), (4p,3d), (4/, 3d), (3s, 4£), (3/>,4d), (4s, 4/>), (4p,4d), (4/, Ad), optimized with respect to the energy. For these bound and continuum functions the photoionization cross section for the 3p 2l S 0 resonance was found to have maximum values, at X = 297.7 nm, of 12.9xl0" 16 cm 2 in the dipole-length approximation and 12.5 xlO" 16 cm 2 in the dipole-velocity approximation.Higher series members and autoionization levels are normally studied in absorption, but transitions from the Mgl 3s 21 S 0 ground state to X S 0 , 1 D 2 states are forbidden. Fortunately, the new possibilities for atomic 7 " 10 and molecular 11 spectroscopy opened up by the development of highpower, frequency-tubable dye lasers include the selective excitation of atomic singlet states to population densities sufficient to obtain, in absorption, transitions to levels not reached from the ground state. Thus, stimulated electronic Raman scattering from excited alkali vapors 12 has been obtained. Extended excited-state absorp-
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