Phase-resolved attosecond near-threshold photoionization of molecular nitrogen

We 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 retrieve the temporal profile of the electron wave packets ͓4,5͔. This temporal structure is set by the temporal shape of the APT but also by the photoion-ization process. Thus, RABBITT measurements with a well-characterized APT give access to the spectral phase of the photoionization ͓6,7͔, i.e., the temporal dynamics of photo-ionization. Recently Cavalieri et al. ͓8͔ reported a time-resolved measurement of photoionization of a solid target by a single attosecond pulse. Conceptually this is close to RABBITT ͓4,5͔ but using of an APT rather than a single pulse has major advantages: ͑i͒ the production of APT is much less demanding; ͑ii͒ the spectrum of APT is a comb of narrow harmonics that can be used to identify different photoioniza-tion channels; ͑iii͒ the intensity of the ir beam must be of Ϸ10 11 W cm −2 for RABBITT and about 10 13 W cm −2 with single pulses ͓9͔, which can perturb the system. Here we study the photoionization of nitrogen molecules with an APT and characterize the outgoing electron wave packets using the RABBITT technique. We probe the region just above the ionization threshold of N 2 , which is spectro-scopically very rich ͓10–12͔. We show that the " complex resonance " ͑at 72.3 nm͒ ͓10,12,13͔ induces a Ϸ␲ phase change in the molecular phase. This effect strongly depends/The Integrated Initiative of European Laser Research Infrastructures I