The spectral resolution in Pulsed-Field Ionization Zero-Kinetic Energy (PFI-ZEKE) photoelectron spectroscopy is related to the state selectivity in the ionization process of the Rydberg states. The selectivity is determined by the applied electric field pulse sequence. Hollenstein et al. a used discrete electric field pulses with increasing field strength in combination with a preceding field pulse of opposite polarity. By using such field pulse sequences with the smallest possible field step size (i.e., approximately 9 mV/cm), a spectral resolution of 0.06 cm −1 could be achieved. To improve the resolution further Harper et al. b recently suggested replacing the sequence of field steps by a linearly increasing field, as used earlier by Reiser et al. c , in combination with a prepulse of opposite polarity and obtained promising results on the PFI-ZEKE photoelectron spectrum of NO and CO 2 . Using a home-built narrow-bandwidth long-pulse laser system (pulse lengths up to 50 ns) in combination with a field pulse ramp b,c , we explore the resolution limit of this approach. To avoid overlap of spectral lines, we chose an atomic system, Ar, as test system and recorded PFI-ZEKE photoelectron spectra of transitions from the metastable states (3p) 5 (4sto the (3p) 5 2 P 3/2,1/2 states of Ar + . This system also offers the advantage of a precisely known ionization energy d with which the ionization energy determined with the new method can be compared.

The photoionisation of the rare-gas atoms Rg = Ar, Kr and Xe is investigated at the Rg+ […](ns)2(np)5 2P3/2 ← Rg[…](ns)2(np)5((n + 1)s)13P2 photoionisation threshold (n = 3, 4 and 5 for Ar, Kr and Xe) using pulsed-ramped-field-ionisation zero-kinetic-energy (PRFI-ZEKE) photoelectron spectroscopy.