engleski

How slow should be ultrafast measurements to reveal features of quasipartiles excited in surface bands: From optically induced transient excitonic precursor states to relaxed band states of image potential

Ultrafast pump-probe spectroscopies, like the time-resolved two-photon photoemission (TR 2PPE), have greatly contributed to our understanding of the band structure and real time domain electron dynamics at surfaces [1]. In the past two decades the surface-state and image-potential bands on metal surfaces have served as paradigms in the exploration and determination of decoherence, energies and decay of quasiparticles (electrons and holes) excited in electronic states of reduced dimensionality [2]. Utilization of femtosecond pump and probe laser pulses with short delays have opened up new possibilities in the studies of extreme ultrafast electron dynamics at surfaces but have also posed new questions on the interpretation of characteristics of unoccupied intermediate quasiparticle states probed in these experiments. We first outline how due to the retarded character of electronic surface response the intermediate electronic states in TR 2PPE from metal surfaces [typically Cu(111) and Ag(111)] begin to evolve from transient coherent excitonic states over into the stationary states of image potential bands in which the propagation of excited electrons can be interpreted within the quasiparticle framework. Concurrently, the evolution of quasiparticles excited in surface bands itself strongly depends on the interaction with the environment which acts to destroy their initial coherence. Here we demonstrate how the two major decoherence and dissipation mechanisms, viz. the interactions of quasiparticles with the dynamical surface electronic response [3] and with defects [4] may affect the state energies, decays and overall lifespans of quasiparticles excited into image potential bands. From this we assess optimum temporal intervals for the determination of these quantities by TR 2PPE spectroscopy as well as for the modelling of measured 2PPE photoelectron yield by standard methods like the optical Bloch equations [5]. \vspace*{;5mm}; \noindent$^*$ Corresponding author. e-mail: branko@ifs.hr\\[5mm] [1] H. Ueba and B. Gumhalter, Prog. Surf. Sci. {;\bf 82};, 193 (2007). \\[1pt] [2] F. El-Shaer and B. Gumhalter, Phys. Rev. Lett. {;\bf 93};, 236804 (2004)\\[1pt] [3] P. Lazi\'{;c};, V.M. Silkin, E.V. Chulkov, P.M. Echenique and B. Gumhalter, Phys. Rev. Lett. {;\bf 97};, 086801 (2006) ; P. Lazi\'{;c};, V.M. Silkin, E.V. Chulkov, P.M. Echenique and B. Gumhalter, Phys. Rev. B {;\bf 76};, 045420 (2007)\\[1pt] [4] B. Gumhalter, A. \v{;S};iber, H. Buljan, and Th. Fauster, Phys. Rev. B {;\bf 78};, 155410 (2008).\\[1pt] [5] P. Lazi\'{;c};, D. Aumiler and B. Gumhalter, Surf. Sci. {;\bf 603};, 1571 (2009).\\[1pt]

electrons; photons; surfaces; interfaces; optical properties

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