High Rydberg states of argon: Stark effect and field-ionization properties

The Rydberg states with principal quantum number n greater than or equal to 27 located below the P-2(3/2) ground state of the Ar+ ion have been studied by pulsed field ionization following single-photon excitation out of the (3p)(6) S-1(0) ground state of Ar. The 0.1 cm(-1) linewidth of the tunable extreme ultraviolet (XUV) laser source used enabled high-resolution measurement of the Stark effect over a wide range of principal quantum numbers and electric field strengths. Particular attention was given to the ionization of high Rydberg states induced by DC and pulsed electric fields. The lowering Delta IP (expressed in cm(-1)) of the ionization threshold by DC electric fields is accurately described by Delta IP = ((5.99 +/- 0.13)root F) when the electric field strength F is expressed in V cm(-1), a result that is in good agreement with predictions of the classical saddle-point model for field ionization. The field-ionization threshold is very sharp: its width decreases from 0.7 to 0.2 cm(-1) when the DC field strength is reduced from 580 to 50 V cm(-1). Apart from the Stark states located in a very narrow energy range around the saddle-point energy in the V(r) = -1/r-Fz potential which are found to ionize very slowly, all Stark states located below the saddle-point energy have lifetimes exceeding several microseconds, whereas those located beyond the saddle-point energy ionize within less than 20 ns. The very slow field ionization that is observed in a narrow range of energies around the classical saddle point can be used to obtain high state selectivity in the pulsed field ionization. The pulsed field-ionization behaviour observed in argon suggests that the Delta IP = 4 root F rule that is now commonly assumed in the analysis of pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) spectra to describe the low-wavenumber onset of a line relative to the position of the corresponding field-free ionization threshold must be used with caution.