ELECTRONIC STRUCTURE AND PHOTOEXCITATION DYNAMICS OF CHEMISORBED ALKALI ATOMS INDUCED RESONANCES

We investigate the electronic structure and photoexcitation dynamics of alkali atoms (Rb and Cs) chemisorbed on transition Ru(0001) and noble metal Cu(111) surfaces by angle and time- resolved multi-photon photoemission (mPP) spectroscopy. Although the electronic structure of alkali atoms on noble surfaces has been studied, the development of mPP methods, combined with wavelength tunable femtosecond laser excitation, provides more incisive tools for exploration of alkali chemisorption induced electronic resonances, mapping of electronic wavefunctions, and probing of electron relaxation dynamics. On Ru(0001), three-photon photoemission (3PP) spectroscopic features due to the σ- and π-resonances arising from the ns and np states of free alkali atoms are observed at ~2 and ~1 eV below the vacuum level in the zero coverage limit, respectively. As the alkali coverage is increased to 0.02 monolayer, the resonances are stabilized by formation of a surface dipole layer and form dispersive bands with nearly free-electron mass. Density functional theory calculations confirm the band formation through substrate-mediated interaction involving hybridization with the unoccupied d-bands. Time-resolved measurements provide the experimental measurements of phase and population decay in the 3PP process via σ- and π-resonances; simulations by solving the four-energy level optical Bloch equations quantify the phase and population relaxation times. By contrast, on Cu(111) we observe clear signatures of Cs and Rb alkali atom-localized electronic states in 3PP spectra. The angular distributions reflect the non-dispersive σ and π symmetries of the alkali atom localized states. Due to the high dispersion of Shockley surface state (SS) of Cu(111), the resonant two-photon transition is driven from SS to π-resonance under visible light. Time-resolved measurements and corresponding Fourier transforms (FT) with respect to time describe the phase and population relaxation dynamics. In the case of the σ-resonance with ħν=1.92 eV, the interferometric measurements contain extra frequency components at fractions of the laser frequency, which we attribute to multielectron (ME) dynamics. Two-dimensional electronic spectroscopy shows that the photoexcitation creates coherent polarization components outside of the excitation laser bandwidth, through Coulomb interaction induced decay of an electron excited from the SS to a two-photon virtual state decaying into one electron in the σ-resonance and the other excited from SS to the Fermi level. (3/6/2017)