Direct time-domain observation of attosecond final-state lifetimes in photoemission from solids

Abstract

Attosecond spectroscopic techniques have made it possible to measure differences in transport times for photoelectrons from localized core levels and delocalized valence bands in solids. We report the application of attosecond pulse trains to directly and unambiguously measure the difference in lifetimes between photoelectrons born into free electron-like states and those excited into unoccupied excited states in the band structure of nickel (111). An enormous increase in lifetime of 212 ± 30 attoseconds occurs when the final state coincides with a short-lived excited state. Moreover, a strong dependence of this lifetime on emission angle is directly related to the final-state band dispersion as a function of electron transverse momentum. This finding underscores the importance of the material band structure in determining photoelectron lifetimes and corresponding electron escape depths.

Distinguishing attosecond electron–electron scattering and screening in transition metals

“…The associated time delay can be clearly seen in the experimentally measured interferograms of Cu(111) as an obvious phase shift in the oscillations of the RABBITT quantum interferences (Fig. 3B), which interestingly is absent in Ni(111) for free-electron final states (33). We note that we can exclude the possibility that the finite photoelectron lifetime in this energy range in Cu(111) is caused by another final-state resonance because we did not observe any photoelectron yield enhancement in this energy range (Fig.…”

“…The photoelectron spectrum is then collected using a hemispherical photoelectron analyzer (Specs Phoibos 100). Note that it has already been shown that RABBITT and attosecond-streaking yield the same temporal information about the photoemission process (31), whereas ARPES adds significant advantages of band specificity (33). Moreover, by simultaneously measuring two photoelectron wavepackets from different initial states excited by the same harmonic orders we can eliminate the influence of the HHG phase (28).…”

“…We used attosecond-ARPES to measure a photoelectron lifetime of ∼210 as, which was measured for a final state that coincides with an unoccupied excited state in the band structure of Ni (32,33). We also showed that the photoelectron lifetime sensitively depends on the band dispersion of the material (i.e., the photoelectron emission angle).…”

“…High harmonic generation (HHG) provides attosecond pulses and pulse trains that are perfectly synchronized to the driving laser and that are ideal for probing the fastest coupled charge and spin dynamics in atoms, molecules, and materials (23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33). To date, two approaches have been used to probe attosecond electron dynamics in matter through photoemission, taking advantage of laser-assisted photoemission sidebands (24,25).…”

Observation of ionization enhancement in two-color circularly polarized laser fields

“…In the simplest semi-classical picture of HHG, the electron can return to the parent ion with high kinetic energy and then any excess energy greater than the ionization potential can then be emitted as a high-harmonic photon. When the HHG process is properly phase matched, a bright coherent beam of extreme ultraviolet (EUV) or soft X-ray light is generated [6][7][8][9][10], which can be used to uncover coupled dynamics in materials with femtosecond-to-attosecond temporal resolution [11][12][13][14][15], and can also be used for high-resolution imaging [16][17][18][19]. Alternatively, if the electron does not recombine upon re-encountering the ion it may rescatter from the ion, encoding information about the sub-ångström and sub-femtosecond structure of the scattering potential into the photoelectron momentum distribution [20][21][22].…”