Femtosecond photoelectron and photoion spectrometer with vacuum ultraviolet probe pulses

We describe a femtosecond photoelectron and photoion spectroscopy setup using a simple, novel way to produce a vacuum ultraviolet (VUV) probe pulse. We isolate the femtosecond VUV pulse from a high harmonic continuum by using the bandpass characteristics of a thin indium (In) metal filter. The filter transmits the 9th harmonic of the 800 nm laser corresponding to a wavelength of 89 nm (hν = 14 eV). The 9th harmonic is obtained with high conversion efficiencies and has sufficient photon energy to access the complete set of valence electron levels in most molecules. Compared to using grating monochromators, the filter strategy does not change the length of VUV pulse. The setup also allows for direct comparison of VUV single-photon probe with 800 nm multi-photon probe without influencing the delay of excitation and probe pulse or the beam geometry. We use a magnetic bottle spectrometer with high collection efficiency for electrons, serving at the same time as a time of flight spectrometer for ions. Characterization measurements on Xe reveal the spectral width of the 9th harmonic to be 190 ± 60 meV and a photon flux of ∼1 ×107 photons/pulse after spectral filtering. As a first application, we investigate the S1 excitation of perylene using time-resolved ion spectra obtained with multiphoton probing and time-resolved electron spectra from VUV single-photon probing. The time resolution extracted from cross-correlation measurements is 65 ± 10 fs for both probing schemes and the pulse duration of the 9th harmonic is found to be 35 ± 8 fs.