Resonance in the 1π photoemission cross-section of CO on Pt(1 1 1) measured by high harmonic radiation

Femtosecond pulses of a table-top, high-repetition rate (1 kHz) Ti:sapphire laser system are focussed into rare gases to produce high harmonic radiation in the range of 60–11 nm (hν=20–120 eV). The c(4×2)-2CO-covered Pt(1 1 1) surface is investigated by angle resolved photoelectron spectroscopy using this radiation. Both the coarse tunability in steps of 3.1 eV as well as a fine tuning within a single harmonic are utilized in the measurements. The photoemission cross-section of CO induced structures, in particular the CO 1π orbital, is investigated. For this state a narrow resonance is observed, which has not been reported so far. A resonant excitation of electrons from the occupied 3σ level into the unoccupied 2π level followed by the autoionization of the 1π CO state or Auger decay is proposed to cause this resonance.