Chi-Fong Lei

Time-resolved UPS: a new experimental technique for the study of surface chemical reactions on femtosecond time-scales

Abstract Recent progress in the generation of ultrashort XUV-pulses by means of high harmonic generation provides a means to monitor the dynamics of surface chemical reactions using photoemission spectroscopy. In this paper we describe details of an experimental setup for time-resolved photoemission spectroscopy using high harmonic generation. We also present results where the different steps involved in the laser-induced change of an adsorbate–surface bond were monitored with sub-100 femtosecond time-resolution.

The laser-assisted photoelectric effect on surfaces

We present the first observation of the laser-assisted photoelectric effect in surface photoemission. Illuminating a Pt(111) sample simultaneously with 1.6 eV and 42 eV pulses, we observe ldquodressingrdquo of the EUV photoelectron spectrum.

Laser-Assisted Photoelectric Effect on Pt(111)

We observe the laser-assisted photoelectric effect on a surface for the first time. Simultaneously illuminating Pt(111) with the fundamental and 27th harmonic of a Ti:sapphire femtosecond laser results in energy modulation of the photoelectron spectrum.

Ultrashort-pulse EUV and soft x-ray sources based on high-harmonic generation : principles and applications

High harmonic generation (HHG) is a useful source of coherent light in the extreme ultraviolet (EUV) region of the spectrum. However, both the conversion efficiency and the highest achievable photon energy have in the past been limited in the past by the inability to phase-match the frequency conversion process. In this paper, we summarize recent results on the development of new techniques for phase-matching the high-harmonic conversion process. We also summarize finding from three series of experiments that make use of the coherent EUV light generated using HHG: 1) probing of acoustic dynamics in materials; 2) monitoring of chemical dynamics at surfaces using photoelectron spectroscopy; and 3) time-resolved plasma imaging.

Probing adsorbate oscillation on metal surfaces using ultrafast extreme ultraviolet pulses

We observe the oscillation of CO on a Pt (111) surface excited by ultrafast infrared pulses using time-resolved extreme ultraviolet photoemission spectroscopy.

Small‐scale Coherent EUV Light Sources from High‐Harmonic Generation

Coherent EUV light can be generated using the process of high‐harmonic upconversion of pulses from a high‐intensity femtosecond laser. Recent advances in ultrafast laser technology, combined with the development of efficient techniques for high‐harmonic generation (HHG), now make routine small‐scale, table‐top experiments using coherent EUV light. New advances, such as the use of pulse shaping and quasi‐phase matched geometries, give new flexibility in the design of coherent EUV sources.

Direct observation of surface chemistry using ultrafast EUV pulses

Summary form only given. Molecular oxygen adsorbs on a platinum [111] single crystal surface in two distinct chemisorbed phases at different temperatures (Puglia et al, 1995). At temperatures of -197/spl deg/C, molecular oxygen preferentially absorbs to the platinum surface as a superoxolike phase (O/sub 2//sup -/). However, at temperatures of -135/spl deg/C, it preferentially absorbs as a peroxo-like phase with one additional electron (O/sub 2//sup -2/). These two states have different static (equilibrium) photoelectron spectra, corresponding to the different oxidation states and different positions of the oxygen molecules on the Pt[111] surface. In our work, we use time-resolved photoemission spectroscopy to monitor changes in the chemical ground state of oxygen on Pt[111], after ultrafast excitation by an IR (800 nm) pump pulse. The pump pulse transiently heats the electrons in the metal, promoting oxygen from the superoxo (O/sub 2//sup -/) to peroxo (O/sub 2//sup 2-/) phase. This enables us,for the first time, to directly observe changes in the chemical bond character of a molecule adsorbed on a surface, with the sub-100 femtosecond time-resolution necessary to follow the complete progress of the reaction. This is in contrast to previous work, which monitored the electron distribution on surfaces (Rettenberger et al, 1996), or which followed only the first 1/2 oscillation of a vibrating molecule on a surface by monitoring a very short-lived excited state (Petek et al, 2000).