Quantum State-Resolved Probing of Strong-Field-Ionized Xenon Atoms Using Femtosecond High-Order Harmonic Transient Absorption Spectroscopy

Abstract

Femtosecond high-order harmonic transient absorption spectroscopy is used to resolve the complete |j, m quantum state distribution of Xe + produced by optical strong-field ionization of Xe atoms at 800 nm. Probing at the Xe N 4/5 edge yields a population distribution ρ j,|m| of ρ 3/2,1/2 : ρ 1/2,1/2 : ρ 3/2,3/2 = 75 ± 6 : 12 ± 3 : 13 ± 6%. The result is compared to a tunnel ionization calculation with the inclusion of spin-orbit coupling, revealing nonadiabatic ionization behavior. The sub-50-fs time resolution paves the way for table-top extreme ultraviolet absorption probing of ultrafast dynamics.PACS numbers: 42.50. Hz, 32.80.Rm, 42.65.Ky, Studies of laser-atom interactions in the nonperturbative, strong-field regime elucidate novel phenomena such as above-threshold ionization [1,2], nonsequential double ionization [3], and high-order harmonic generation [4,5,6]. While these processes are extensively studied both experimentally and theoretically, details remain unknown about the |j, m state distribution of the photoion produced by the initial photoionization step (m is the projection quantum number associated with the total angular momentum j of the hole). Moreover, experimental tests of theoretical models for strong-field ionization mostly rely on measuring the ion yield as a function of laser peak intensity [7]. The strong dependence of ionization yields on the orbital angular momentum and its direction relative to the laser polarization axis, as predicted by most theoretical models (e.g., the Ammosov-Delone Krainov (ADK) rates [8]), suggests that knowledge of the complete |j, m state distribution can be used as an additional benchmark for theory. Young et al. recently reported the use of synchrotron x-ray pulses to probe the hole-orbital alignment of Kr + photoions generated in the strong-field ionization of Kr [9]. The unresolved finestructure transitions prevented retrieval of the complete |j, m state distribution. However, the observed degree of alignment is reproduced by the |j, m state distribution obtained by tunnel ionization calculations with the inclusion of spin-orbit coupling [10].Here we investigate the experimental and theoretical strong-field ionization of xenon to extract the complete |j, m quantum state distribution. Femtosecond extreme ultraviolet (EUV) transient absorption spectroscopy is demonstrated with a laser-based, high-order harmonic probe source for the experiments; results are compared to tunnel ionization calculations that incorporate spinorbit coupling. The resultant angular momentum distribution and hole-orbital alignment of the Xe + photoions are measured by probing the transition from the 4d core level to the 5p valence shell. These measurements allow the determination of the complete |j, m quantum state distribution, which is compared to theory.The schematic of the experimental setup is illustrated in Fig. 1. Briefly, the amplified output from a commercial Ti:sapphire laser system (2.4 W, 800 nm, 45 fs, 1 kHz) is sent to a 20 : 80 beamsplitter to produce the optical pump an...

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