Attosecond delays in molecular photoionization

Abstract

We report measurements of energy-dependent attosecond photoionization delays between the two outer-most valence shells of N2O and H2O. The combination of single-shot signal referencing with the use of different metal foils to filter the attosecond pulse train enables us to extract delays from congested spectra. Remarkably large delays up to 160 as are observed in N2O, whereas the delays in H2O are all smaller than 50 as in the photon-energy range of 20-40 eV. These results are interpreted by developing a theory of molecular photoionization delays. The long delays measured in N2O are shown to reflect the population of molecular shape resonances that trap the photoelectron for a duration of up to ∼110 as. The unstructured continua of H2O result in much smaller delays at the same photon energies. Our experimental and theoretical methods make the study of molecular attosecond photoionization dynamics accessible.Photoionization and photoelectron spectroscopies are powerful approaches to measuring the electronic structure of matter [1,2]. A complete quantum-mechanical description of photoionization, both in the time and frequency domains, requires the amplitude and phase of all dipole matrix elements, e.g. in a partial-wave expansion. Most experiments to date measure photoionization cross sections which are described by the sum of squared moduli of individual partial-wave dipole matrix elements. Cross sections thus contain no information about the partial-wave phase shifts. In contrast, photoelectron angular distributions are highly sensitive to partial-wave phase shifts [3,4] between continua associated with the same ionic state. Phase-shifts between continua associated with different ionization thresholds are not measurable in the frequency domain because the lack of spectral overlap between the corresponding photoelectrons erases the coherence required to measure such phase shifts.In this letter, we show that attosecond metrology can be employed to measure this information in molecular photoionization. Specifically, we study the effect of molecular shape resonances on the measured photoionization delays. When the combined molecular (or atomic) and centrifugal potential felt by the photoelectron displays a barrier, one or several quasi-bound states can emerge [5][6][7]. These resonances decay by tunneling through the potential barrier and often lead to a local enhancement of the photoionization cross section. Such shape resonances have so far only been measured by frequency-resolved measurements. Here, we show that attosecond metrology provides access to the time-domain manifestation of shape resonances. In the case of N 2 O, our measurements indeed reveal surprisingly large delays reaching up to 160 as in the range of 20 to 40 eV. In contrast, delays measured at the same photon energies in H 2 O all lie below 50 as in magnitude. These results are interpreted by developing a theory of molecular photoionization delays relying on accurate molecular scattering calculations. This analysis shows that the delays measured ...

Symmetry resolved atomic photoionization phase shift by attosecond coincidence metrology

“…How do the sophisticated twophoton transition elements relate to symmetry breaking in the full atomic system, including the laser field coupling in a long-range Coulomb potential, transitions involving the continuum electron, and the accurate atomic potential which is polarized by the NIR field? Additionally, our experimental methods provide a state-of-the-art approach which can investigate and manipulate symmetry-resolved photoemission dynamics on an attosecond time scale, including shape resonances dominated by high-angular-momentum continuum waves 9,10,19 , constructive and destructive interference between multiple coupled states 41 , and correlated shake-up and shake-off double ionization assisted by Auger decay 18,20 .…”

“…Both techniques have been applied to a wide range of attosecond time-resolved photoelectron emission dynamics in atoms [6][7][8] , molecules [9][10][11] and even condensed matters [12][13][14][15] . Studies have demonstrated the photoelectron emission time delays arising from different ionization shells [16][17][18] , shape resonances 9,10,19 , electron correlation 20 , orbital asymmetry 21 , spatial asymmetry (chirality) 11 , and electron delocalization 22 . These time delays are attributed to the energy dependence of the scattering phase shift during electron transitions in a short-range potential and the laser field coupling effect in the continuum 23,24 .…”