Autoionization of very-high- n strontium Rydberg states

Abstract

We study, using a combination of experiment and theory, the excitation and decay of very high-n (n ∼ 280-430) strontium autoionizing Rydberg states formed by near-resonant driving of the 5s 2 S 1/2 → 5p 2 P 1/2 core-ion transition. The branching ratio between decay through radiative transitions and through autoionization is explored. Autoionization rates are measured as a function of both the n and ℓ quantum numbers of the Rydberg electron. The non-stationary decay dynamics is studied by creating and manipulating Rydberg wave packets and by varying the laser pulse that drives the core excitation.

Autoionization rates of core-excited magnesium Rydberg atoms in electric fields using the core fluorescence as a reference

“…Measurements of core-excited Rydberg states in other alkaline-earth-metal atoms have also revealed very fast autoionization (see, e.g., Refs. [8,46,47]). From separate experimental and theoretical studies of the Rydberg series converging to the upper spinorbit component (3p 3/2 ) of the first excited state of Mg + , we conclude that the fastest autoionization process is the spin-orbit autoionization of the 3p 3/2 nd 3/2 (J = 3) Rydberg series [48], which leads to autoionization rates given by (1.9 × 10 16 /n * 3 ) s −1 .…”

“…Briefly, the calculation is based on a twoactive-electron, configuration-interaction description of the Mg atom, similar to that used for high-lying, doubly excited states of the helium atom [55], combined with the exterior complex-scaling method [56]. A similar approach was used by Fields et al to calculate the energies and autoionization rates of the core-excited 5p 1/2 np and 5p 1/2 n f Rydberg states of Sr [8]. The two valence electrons of the Mg atom are assumed to evolve in an effective, l-dependent potential V l (r) that describes the effect of the Mg 2+ (1s 2 2s 2 2p 6 ) closed-shell core [57], and their mutual Coulomb interaction is treated exactly.…”

“…Such states are reminiscent of two-planet solar systems, in which one planet would be Venus-like, the other Pluto-like, although often the term planetary atom is used for atoms with excited electrons in nonpenetrating high-l orbits, see, e.g., Refs. [2][3][4][5][6][7][8]. Coreexcited Rydberg states, denoted [M + * ]nl below, play an important role in plasmas by resonantly enhancing electron-ion radiative-association reactions through the process of dielectronic recombination [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24],…”

“…They can also be accessed by exciting the ion core of a Rydberg atom and be observed as autoionizing resonances in photoionization spectra (see Ref. [8] for a recent example)…”

“…These applications rely on the decoupling of the Rydberg electron from the ion core, which forms the basis of the isolated-core excitation (ICE) method [34][35][36]. The decoupling can be achieved in three ways: (i) by increasing the principal quantum number n, as illustrated in the recent experiments of Fields et al [8], (ii) by increasing the orbital angular momentum of the Rydberg electron, as demonstrated, e.g., by Pruvost et al [37] and more recently by Lehec [38], and (iii) by using the Stark effect to enhance the nonpenetrating character of the Rydberg electron through l mixing [8]. In terms of the competition between core fluorescence and autoionization discussed above, perfect ICE conditions imply a suppression of autoionization.…”