HIGH RYDBERG STATES OF ARGON - STARK-EFFECT AND FIELD-IONIZATION PROPERTIES

The Rydberg states with principal quantum number n greater than or equ al to 27 located below the P-2(3/2) ground state of the Ar+ ion have b een studied by pulsed field ionization following single-photon excitat ion out of the (3p)(6) S-1(0) ground state of Ar. The 0.1 cm(-1) linew idth of the tunable extreme ultraviolet (XUV) laser source used enable d high-resolution measurement of the Stark effect over a wide range of principal quantum numbers and electric field strengths. Particular at tention 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 d escribed 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 agreeme nt with predictions of the classical saddle-point model for field ioni zation. The field-ionization threshold is very sharp: its width decrea ses 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 narr ow energy range around the saddle-point energy in the V(r) = -1/r-Fz p otential which are found to ionize very slowly, all Stark states locat ed below the saddle-point energy have lifetimes exceeding several micr oseconds, whereas those located beyond the saddle-point energy ionize within less than 20 ns. The very slow field ionization that is observe d in a narrow range of energies around the classical saddle point can be used to obtain high state selectivity in the pulsed field ionizatio n. The pulsed field-ionization behaviour observed in argon suggests th at the Delta IP = 4 root F rule that is now commonly assumed in the an alysis of pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) spe ctra to describe the low-wavenumber onset of a line relative to the po sition of the corresponding field-free ionization threshold must be us ed with caution.