Time-Resolved 2PPE Study on Alkali/Noble Metal System

 

Fig. 1 The experimental set-up includes a Ti:Sapphire femtosecond laser source, many optical and electronical instruments such as a β-Barium-Borate (BBO) crystal, Mach-Zehnder Interferometer (MZI) and oscilloscope and an Ultra-High Vacuum (UHV) chamber.

Fig. 2 Our Ultra-High Vacuum (UHV) chamber.

Fig. 3 The “pump” pulse of energy hν=3.1 eV excites an electron from its initial state E_i to a virtual intermediate state, then the “probe” pulse of the same energy further excites it to the final state E_f. If the energy of E_f is higher than the vacuum energy (or work function, Φ) of the metal, the electron is ejected with a kinetic energy E_kin=E_f-E_vac. The following scheme displays the analogy between 1PPE and 2PPE:

Simulation of 2PPE From The Bulk sp-bands of Ag(111):

Our goal is to understand how 2PPE occurs by simulating our experimental data with a simple model. Miller et al. have demonstrated a quantitative study of 1PPE spectrum and developed a model to fit the experimental data within the Fermi's Golden Rule (FGR) approximation by including realistic bulk wave functions and accounting for the bulk-surface interference. A similar modeling has not been attempted for 2PPE.

With the help of Fermi’s Golden Rule (FGR) and Optical Bloch Equations (OBE) we reach the following conclusion:

Photoemission Studies on Adsorption of Alkali Metals on Noble Metals:

We aim to understand the nature of the processes taking place between the occupied and unoccupied states of alkali atom covered noble metal surfaces in response to excitation by a near-UV light. Since we studied the photoexcitation dynamics and simulated the spectrum of a bare Ag(111) surface before, we will now attempt to simulate the spectrum of adsorbate-covered surfaces. We employ Ag(111) and Cu(111) as substrates and alkali atoms Li, Na, K, Rb, Cs and alkali earth atom Ba as adsorbates.