Autoionization of argon atoms was studied experimentally by transient absorption spectroscopy with isolated attosecond pulses. The peak position, intensity, linewidth, and shape of the 3s3p 6 np 1 P Fano resonance series (26.6-29.2 eV) were modified by intense few-cycle near infrared laser pulses, while the delay between the attosecond pulse and the laser pulse was changed by a few femtoseconds. Numerical simulations revealed that the experimentally observed splitting of the 3s3p 6 4p 1 P line is caused by the coupling between two short-lived highly excited states in the strong laser field. DOI: 10.1103/PhysRevLett.105.143002 PACS numbers: 32.70.Jz, 32.80.Zb, 78.47.JÀ Bridging the gap between atomic physics and the complex systems that make up the world around us requires indepth study of electron correlation. While rotation and vibration of molecules can be studied by femtosecond lasers [1], observation of the electron-electron interaction requires attosecond time resolution [2]. One of the most interesting processes governed by electron-electron correlation is autoionization [3]. The Fano profile, which is the signature of the autoionization process, has widespread significance in many scientific disciplines [4][5][6][7]. For decades, spectral-domain measurements with synchrotron radiation have served as a window into the rich dynamics of autoionization [4]. However, the synchrotron pulse duration is too long (100 fs to 100 ps) to time-resolve the Fano resonances since the autoionization lifetimes can be as short as a few femtoseconds.Since the generation of the first isolated attosecond pulses in 2001 [8], it was theoretically proposed [9][10][11][12][13] and experimentally demonstrated [14] that time-resolved Fano profiles can be studied using the attosecond streaking technique. To date, most theoretical and experimental investigations of autoionization processes have scrutinized Fano profiles as a function of the photoelectron energy. However, made possible by significant recent progress in short-pulse laser technology [15], timeresolved transient XUV photoabsorption measurements have become feasible, which gives access to complimentary studies of atomic autoionization in the time regime [16,17]. Photoabsorption measurements typically have higher data collection efficiency and better energy resolution than what can be obtained by detecting photoelectrons. The setup is all-optical, much simpler than the attosecond streak camera. Here we demonstrate the first transient absorption experiment using isolated attosecond pulses to probe the autoionization of atoms and show that the autoionization process is strongly modified by an intense laser field.Fano resonance profiles in the absorption spectrum are the result of interference between the direct ionization and the decay from an autoionizing state due to configuration interaction [3]. It is characterized by the resonance energy E r , its width that is related to the lifetime of the autoionizing state by ¼ @=À, and the q parameter, which represents the ratio...

, "Quasi-monoenergetic and tunable X-rays from a laser-driven Compton light source" (2013). Donald Umstadter Publications. 92.

When a terawatt-peak-power laser beam is focused into a gas jet, an electron plasma wave, driven by forward Raman scattering, is observed to accelerate a naturally collimated beam of electrons to relativistic energies (up to 10(9) total electrons, with an energy distribution maximizing at 2 megaelectron volts, a transverse emittance as low as 1 millimeter-milliradian, and a field gradient of up to 2 gigaelectron volts per centimeter). Electron acceleration and the appearance of high-frequency modulations in the transmitted light spectrum were both found to have sharp thresholds in laser power and plasma density. A hole in the center of the electron beam may indicate that plasma electrons were expelled radially.

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