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

  • Merkt F
  • Osterwalder A
  • Seiler R
 et al. 
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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.

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  • F. Merkt

  • A. Osterwalder

  • R. Seiler

  • R. Signorell

  • H. Palm

  • H. Schmutz

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