Anisotropic dynamics of two-photon ionization: An attosecond movie of photoemission

Abstract

Imaging in real time the complete dynamics of a process as fundamental as photoemission has long been out of reach because of the difficulty of combining attosecond temporal resolution with fine spectral and angular resolutions. Here, we achieve full decoding of the intricate angle-dependent dynamics of a photoemission process in helium, spectrally and anisotropically structured by two-photon transitions through intermediate bound states. Using spectrally and angularly resolved attosecond electron interferometry, we characterize the complex-valued transition probability amplitude toward the photoelectron quantum state. This allows reconstructing in space, time, and energy the complete formation of the photoionized wave packet.

Resonant two-photon ionization of helium atoms studied by attosecond interferometry

“…The first peak corresponding to the 1s3p 1 P 1 resonance exhibits a double structure, that could be due to a continuum structure induced by the interference between direct non-resonant two-photon ionization and ionization from the 1s3p 1 P 1 bound state [40]. Such a feature has been previously observed in photoelectron spectroscopy [18], transient absorption measurements [41], as well in numerical simulations [15]. Figure 6(b) shows the phase variation of the angleintegrated sideband 16 intensity.…”

“…Phase jumps of approximately −π rad can be observed when the 15 th harmonic energy becomes equal to that of the np 1 P 1 states. Likewise, phase jumps of opposite sign and similar magnitude can be seen in between each two successive resonant jumps [17,18]. The splitting of the 1s3p 1 P 1 state is also accompanied by a phase variation.…”

“…Angle detection brings a new dimension, momentum, to this type of measurement, allowing the reconstruction of coherent electron wavepackets in time and space. This has been beautifully shown for two-photon resonant ionization of Ne via the 3p 5 3d 1 P 1 state [16], and recently of He via the 1s3p and 1s4p 1 P 1 states [18]. Photoionization studies with angular resolution [18,[24][25][26] have pointed out that additional phase jumps as a function of emission angle appear due to the interference of angular channels, beside the phase jumps as a function of energy.…”

“…In this implementation of the RABBIT technique, sometimes called "below-threshold RABBIT" [15,17,18,29], the interesting information is the phase change due to resonant two-photon ionization, which is "read" by interferometry with the emission path (absorption of the 17 th harmonic and emission of an IR photon).…”

Probing electronic decoherence with high-resolution attosecond photoelectron interferometry

“…Performing angle-resolved measurements with high spectral resolution for photoelectrons with kinetic energies around 35 eV, as is the case here, is extremely challenging. Nonetheless, such measurements have been demonstrated for slower photoelectrons close to the ionization threshold [52].…”