Phase-resolved attosecond near-threshold photoionization of molecular nitrogen

Abstract

International audienceWe photoionize nitrogen molecules with a train of extreme ultraviolet attosecond pulses together with a weak infrared field. We measure the phase of the two-color two-photon ionization transition ͑molecular phase͒ for different states of the ion. We observe a 0.9␲ shift for the electrons produced in the ionization channels leading to the X 2 ⌺ g + , vЈ = 1, and vЈ = 2 states. We relate this phase shift to the presence of a complex resonance in the continuum. By providing both a high spectral and temporal resolution, this general approach gives access to the evolution of extremely short-lived states, which is often not accessible otherwise. DOI: 10.1103/PhysRevA.80.011404 PACS number͑s͒: 33.80.Eh, 33.60.ϩq, 42.65.Ky, 82.53.Kp Ionization of atoms and molecules by absorption of ul-trashort extreme ultraviolet ͑xuv͒ radiation provides rich structural information on the considered species. The ioniza-tion process releases an electron wave packet, which can be described as a coherent superposition of partial waves. The relative contributions and phases of the partial waves can be extracted from photoelectron angular distributions at a given energy ͓1͔. However, the temporal structure of the ejected wave packet, which is imposed by the phase relation between different energy components, is not accessible with such experiments. To access this phase, one needs to couple two energy components of the electron wave packet and record the resulting interference. This can be achieved by absorption of high-order harmonics of an infrared laser pulse in the presence of the fundamental field. An intense laser pulse propagating in a gas jet produces coherent xuv radiation constituted of odd harmonics ͑2q +1͒␻ 0 of the fundamental frequency ␻ 0. These harmonics are all approximately phase locked with the fundamental and form an attosecond pulse train ͑APT͒ ͓2͔. In photoionization experiments with high harmonics, the photoelectron spectrum exhibits equidistant lines resulting from single-photon ionization ͓Fig. 1͑a͔͒. If an additional laser field with frequency ␻ 0 is added, two-photon ionization can occur: absorption of a harmonic photon accompanied by either absorption or stimulated emission of one photon ␻ 0. New lines ͑sidebands͒ appear in the spectrum, in between the harmonics ͓Fig. 1͑a͔͒. Since two coherent quantum paths lead to the same sideband, interferences occur. They are observed in an oscillation of the sideband amplitude as the delay ␶ between the probe ͑ir͒ and harmonic fields is scanned ͓2,3͔. This is the basis of the reconstruction of attosecond beating by interference of two-photon transitions ͑RABBITT͒ technique. The phase of the oscillation is determined by the phase difference between consecutive harmonics ͑phase locking͒ and by additional phase characteristics of the ionization process. The same process can be described in the time domain. The APT creates a train of attosecond electron wave packets. The additional laser field acts as an optical gate on the electrons , which can be used to re...

Timing the release in sequential double ionization

“…By giving this definition we intend to avoid confusion with other times describing the ionization process, such as the tunnelling delay time 2 , the electron release time 12 or the delay in photoemission 13 , and stress the fact that the semi-classical ionization time is only meaningful within the framework of the semi-classical model. Figure 2 describes the semi-classical ionization time measurement using the attoclock 1,2 .…”

Attosecond chronoscopy of photoemission

“…They include the attosecond streaking technique Hentschel et al, 2001;Itatani et al, 2002;Kienberger et al, 2004;Yakovlev et al, 2005;Sansone et al, 2006), RABBIT ("reconstruction of attosecond harmonic beating by interference of two-photon transitions"; Paul et al, 2001;Véniard et al, 1996;Toma and Muller, 2002;Muller, 2002;Haessler et al, 2009;Klünder et al, 2011). The roles of pump and probe are reversed in attosecond transient absorption (ATA; Goulielmakis et al, 2010;Wang et al, 2010;Gaarde et al, 2011;Holler et al, 2011;Santra et al, 2011;Chen et al, 2012Chen et al, , 2013Pabst et al, 2012;Gallmann et al, 2013;Ott et al, 2013Ott et al, , 2014Beck et al, 2015) where the IR pulse creates the wavepacket while the modulation of the absorption of the attosecond XUV pulse probes the time evolution of the electronically exited system.…”

Full Characterization of a Molecular Cooper Minimum Using High-Harmonic Spectroscopy

“…Here, we present the first molecular CM phase measurement, derived from measurements of the XUV group delay (GD), for the case of the highest occupied molecular orbital of methyl chloride (CH 3 Cl), which is analogous to the 3p orbital of argon. As we show, the CH 3 Cl CM was only clearly revealed through the combined analysis of XUV spectral intensity and phase.…”

“…In recent years, many of these techniques were extended to small molecular systems in high-harmonic generation (HHG) [2] and high-harmonic spectroscopy (HHS) [3,4]. As the field moves toward the investigation of dynamics in more complex targets, it becomes important to determine the applicability of techniques developed for atoms to the molecular case, and when found lacking, to update these methods to properly resolve dynamics.…”