Attosecond Electron Spectroscopy Using a Novel Interferometric Pump-Probe Technique

Abstract

We present an interferometric pump-probe technique for the characterization of attosecond electron wave packets (WPs) that uses a free WP as a reference to measure a bound WP. We demonstrate our method by exciting helium atoms using an attosecond pulse (AP) with a bandwidth centered near the ionization threshold, thus creating both a bound and a free WP simultaneously. After a variable delay, the bound WP is ionized by a few-cycle infrared laser precisely synchronized to the original AP. By measuring the delay-dependent photoelectron spectrum we obtain an interferogram that contains both quantum beats as well as multipath interference. Analysis of the interferogram allows us to determine the bound WP components with a spectral resolution much better than the inverse of the AP duration.

Reconstruction of an excited-state molecular wave packet with attosecond transient absorption spectroscopy

“…The mechanism of the laser perturbation can be further classified by scrutinizing the absorption dynamics within a particular excited-state absorption line: resonant processes involving coupling of neighboring states through the absorption or emission of one NIR photon typically result in relatively slowly varying spectral features such as absorption line splitting (analogous to Autler-Townes splitting [39]) and perturbed free induction decay [40], whereas nonresonant couplings involving two or more NIR photons result in fast oscillations with periodicity shorter than the dressing laser optical cycle [4]. These features reveal information about the field-free evolution of the electronic wave packet in the interval between the two pulses [2,41,42]. In the case of molecules, dipole absorption reflects nuclear dynamics as well [18].…”

“…The analytical expression for A (2) v B ←v B is immaterial to the present discussion. Here, we simply note that the short duration of the NIR pulse allows us to factorize this amplitude into an electronic component A (2) B←B and a Franck-Condon overlap,…”

“…In the absence of external degrees of freedom, electronic wave packets excited in atoms by an isolated attosecond pulse [2][3][4] or by strong-field ionization [5,6] exhibit a high degree of coherence [7] and can be probed via the photoelectron or transient absorption spectrum. In molecules [8][9][10], electronic excitation is accompanied by redistribution of charge and the subsequent rearrangement-rotation, torsion, bond elongation-of the nuclei, on a time scale of femtoseconds to picoseconds, which can ultimately lead to the breaking of chemical bonds.…”

Reconstruction and control of a time-dependent two-electron wave packet

“…phase locking 23 ) present in a highharmonic spectrum, the observation of a well-defined phase evolution υ(t) is possible 24 even in the absence of carrier-envelope-phase stabilization and without knowing the number of attosecond pulses in our few-cycle generated attosecond-pulse train (Methods -Effects of the Attosecond Pules Configuration and the Carrier Envelope Phase‖; Extended Data Figs. [4][5][6]. The images (Fig.…”

Double ionization probed on the attosecond timescale

“…After a decade where attosecond light sources 4,5 were characterized and their potential demonstrated, the next phase will include the exploration of correlated electron dynamics in complex systems. A series of ground-breaking studies on single ionization (SI) in atoms using attosecond light pulses sheds light on the escaping electron and its interaction with the residual ion 6,8 , and the resulting coherent superposition of neutral bound states 9,10 . Double ionization (DI) by absorption of a single photon is an inherently more challenging phenomenon, both experimentally and theoretically 1-3 .…”