Laser based soft-x-ray pulses for photoelectron spectroscopy of surfaces

Abstract

Light pulses in the extreme ultraviolet (EUV) are generated in the photon energy range hν from 20 to 120 eV in the conversion media argon, neon, and helium by a tabletop high-repetition-rate 30-fs Ti:sapphire laser system. This high-harmonic radiation is applied in the energy range from 20 to 50 eV for photoelectron spectroscopy studies of clean and adsorbate covered Ni(111) and Pt(111) surfaces. The photoemission spectra of the clean Pt(111) surface show secondary electron emission structures that influence the cross-section analysis of CO-induced states. Taking these Pt resonances into consideration, the well-known 4σ and 5σ CO shape resonances are observed at photon energies of 37 and 28 eV. In contrast to previously published experimental data, a resonance is also observed at hν=31 eV for the CO 1π state. On the basis of theoretical approaches this signal structure is identified as an autoionization resonance, experimentally detected for the first time to our knowledge.