A single isolated attosecond pulse of 67 as was composed from an extreme UV supercontinuum covering 55-130 eV generated by the double optical gating technique. Phase mismatch was used to exclude the single-atom cutoff of the spectrum that possesses unfavorable attochirp, allowing the positive attochirp of the remaining spectrum to be compensated by the negative dispersion of a zirconium foil. Two algorithms, PROOF and FROG-CRAB, were employed to retrieve the pulse from the experimental spectrogram, yielding nearly identical results.

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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...

The motion of electrons in the microcosm occurs on a time scale set by the atomic unit of time—24 attoseconds. Attosecond pulses at photon energies corresponding to the fundamental absorption edges of matter, which lie in the soft X-ray regime above 200 eV, permit the probing of electronic excitation, chemical state, and atomic structure. Here we demonstrate a soft X-ray pulse duration of 53 as and single pulse streaking reaching the carbon K-absorption edge (284 eV) by utilizing intense two-cycle driving pulses near 1.8-μm center wavelength. Such pulses permit studies of electron dynamics in live biological samples and next-generation electronic materials such as diamond.

Understanding and controlling the dynamic evolution of electrons in matter is among the most fundamental goals of attosecond science. While the most exotic behaviors can be found in complex systems, fast electron dynamics can be studied at the fundamental level in atomic systems, using moderately intense (≲103 W/cm2) lasers to control the electronic structure in proof-of-principle experiments. Here, we probe the transient changes in the absorption of an isolated attosecond extreme ultraviolet (XUV) pulse by helium atoms in the presence of a delayed, few-cycle near infrared (NIR) laser pulse, which uncovers absorption structures corresponding to laser-induced “virtual” intermediate states in the two-color two-photon (XUV+NIR) and three-photon (XUV+NIR+NIR) absorption process. These previously unobserved absorption structures are modulated on half-cycle (~1.3 fs) and quarter-cycle (~0.6 fs) timescales, resulting from quantum optical interference in the laser-driven atom.