Adsorption and electronic structure of submonolayer Ca on Ag(111)

By frequency-time spectroscopy, microscopy, and electronic structure theory, we investigate the low coverage adsorption and electronic structure of Ca on an Ag(111) surface. The chemisorption of alkaline-earth Ca atoms starting from their closed 4s² shell electronic configuration differs from that of the open ns¹ shell alkali-metal atoms because they retain some $4s$ electron charge and can involve hybridization with low-lying unoccupied $3d$ orbitals. Spectroscopic investigation of the unoccupied electronic structure of Ca on Ag(111) by three-photon photoemission (3PP) reveals the electronic states with dominant 3d-orbital character, as confirmed by electronic structure theory. Coverage-specific spectral features enable attribution to two distinct chemisorption phases at submonolayer coverages, which are confirmed at the atomic scale by scanning tunneling microscopy and spectroscopy measurements. Furthermore, time-resolved 3PP spectroscopy reveals a resonant two-photon transition between occupied (~-0.3 eV) and unoccupied (~2.7 eV) states with dominant s- and d-orbital character. Interferometric time-resolved measurements probing this resonance reveal a dephasing time of ~21 fs and population lifetime of ~52 fs for the transiently populated d-like state at ~2.7 eV above EF. Contrary to alkali-metal atom chemisorption, the characteristic development of the chemisorption and electronic structure of Ca reveals the importance of Ca adsorbate and Ag substrate interactions that cause a low-coverage atomic condensation into compact islands. Discovery of the long-lived d-character state informs on the catalytic and optoelectronic applications of the alkaline-earth element Ca.