The effect of a laser field on the cross section of photoionization of an atom by probe radiation in the region of closely lying autoionizing resonances is studied theoretically. The cross sections in the region of the overlapping autoionizing Rydberg states 5p 5(2 P 1/2)6d′ J = 1 and 5p 5(2 P 1/2)8s′ J = 1 resonantly coupled with the discrete state 5p 5(2 P 3/2)7p[1/2]1 in the xenon atom are calculated. The behavior of the shape and position of resonances in relation to the frequency and intensity of laser radiation is studied.
The nonresonant, three-photon ionization spectrum of atomic Xe in the region between the
Xe+ 23/2 and
21/2 fine-structure limits is reported. Using linearly polarized light, four autoionizing Rydberg series are observed converging to the
Xe+ 21/2 limit, corresponding to
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)ns'[(1/2)]\ifmmode^\circ\else\textdegree\fi{}
1, (2P\ifmmode^\circ\else\textdegree\fi{}
1/2)nd'[(3/2)]\ifmmode^\circ\else\textdegree\fi{}
1,
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)nd'[(5/2)]\ifmmode^\circ\else\textdegree\fi{}
3, and
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)ng'[(7/2)]\ifmmode^\circ\else\textdegree\fi{}
3; as expected, using circularly polarized light only the J=3 series are observed. These results are in contrast to earlier experimental work in which no resonant structure was observed. Photoelectron angular distributions are reported for the
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)9s'[(1/2)]\ifmmode^\circ\else\textdegree\fi{}
1 and the
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)7d'[(3/2)]\ifmmode^\circ\else\textdegree\fi{}
3 autoionizing resonances. The results are compared with the recent theoretical calculations by P. Gangopadhyay et al. [Phys. Rev. A 34, 2998 (1986)].
Optical activity of atomic medium with interfering laser induced continuum structures is studied theoretically and illustrated by the example of hydrogen. The optically active region in the atomic spectrum is scanned by linearly polarized probe field whose polarization properties are changed by the medium. Analytical equations for acquired ellipticity and polarization plane rotation are obtained. They demonstrate that the optical activity is a non-trivial function of the parameters of the driving laser fields. Numerical results are presented for the VUV probe radiation with two circularly polarized lasers, coupling the hydrogen Ep continuum with the 2s and the 5s, 5d states, respectively.
Nonresonant ac Stark shifts of Rydberg states in Xe have been investigated by probing an atomic transition with a weak UV light in the presence of an intense YAG-laser pulse. Rydberg atoms produced through two-photon excitation, 5p → np (n = 15–48) and 5p → nf (n = 12–49), are subsequently ionized. IR-laser-induced shifts of the resonant ionization profiles are then measured. The measured shift, 3.9 ± 0.3 cm−1 at 1010 W cm−2, is more than a factor of 2 smaller than that expected from the basis of ponderomotive term, 8.5 cm−1 at 1010 W cm−2. In addition, the shift varies linearly as a function of the IR-laser intensity and does not depend on the principal quantum number of the np and nf series investigated here.
Laser-induced autoionization, a form of forced autoionization, is discussed. Highly excited atomic Rydberg states generally have a small cross section for photoionization due to their small overlap with the nucleus. The cross section for photoionization can be strongly enhanced if the Rydberg series is perturbed by an atomic state with a relatively large cross section for photoionization. The photoionization dynamics of such a perturbed Rydberg system shows strong similarities to that of an autoionizing Rydberg series. For intense laser pulses, which strongly couple the perturber to the continuum, the photoionization rate is limited by the coupling between the Rydberg states and the perturber or, in other words, the autoionization rate. The model describing this ionization dynamics is applied to the barium 6snd
(1,3D2) state which is perturbed by the 5d7d
(1D2) state. The calculations show good agreement with experimental results of photoionization of barium Rydberg states with subpicosecond laser pulses of light of 620 nm in wavelength.
The Stark spectra of autoionizing Rydberg states of Xe converging to the second ionization limit 5
p5 2P1/2 have been investigated as a function of applied electric field over the range 26--2362 V/cm. Single-photon excitation from the Xe ground state was achieved using coherent radiation near 92.5 nm, generated by frequency tripling the pulsed output of a frequency-doubled dye laser in a free jet of rare-gas atoms. For the lowest electric field only the ns' and nd' series were observed. As the electric field was increased the np' series appeared because of the mixing of wave functions with different l values. Further increase of the electric field caused the appearance of hydrogenlike Stark manifolds. The Stark spectrum was simulated in a two-step procedure with no adjustable parameters. Using a jl-coupling basis set the Hamiltonian matrix was diagonalized to determine the energy levels and corresponding eigenfunctions for a given electric field strength. The intensities and line shapes were then calculated using the formalism developed by Fano for a many-discrete-level--single-continuum configuration interaction. This treatment was sufficient to reproduce the positions, widths, and shapes of most observed features. At field strengths between 1000 and 2000 V/cm, however, an additional modulation structure with a spacing of about 2.7 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ appeared on top of the \ensuremath{\Vert}M\ensuremath{\Vert}=1 autoionizing resonances, which could not be explained by our simple theoretical model.