sample abstract

A New Scaling Law for Double-Photoionization Processes
J. B. Bluett1, S. B. Whitfield2, and R. Wehlitz1
SRC, UW-Madison, 3731 Schneider Dr., Stoughton, WI-53589
Dept. of Physics and Astronomy, UW-Eau Claire, Eau Claire, WI 54702
We have measured the double-to-single photoionization ratio of atomic lithium and
beryllium using the ion time-of-flight method. The photon beam of the PGM undulator beam line
ionizes the vaporized target atoms. The ions created are extracted by a pulsed electric field across
the interaction region and are detected by a Zstack of micro-channel plates [1].
In the figure shown above, we compare the
double-to-single photoionization ratios of He[2],
Li[3-5], H2[6], and Be[7]. Besides lining up the
respective double-ionization thresholds, we have
chosen an energy axis in units of the energy
difference between the corresponding double and
single ionization thresholds. For the photon
energies considered here, both ejected electrons
originate from the same shell in the case of He
and Be. Therefore, only one single-ionization
threshold is of relevance, and ΔE can be
unambiguously calculated.
However, the lowest double-ionization
Figure 1: Double-to-single photoionization
process in Li is an intershell process that involves
ratio of He [2] (asterisk), Li [3-5] (open circles),
H2 (open squares) [6], and Be (full circles) [7].
a 1s and a 2s electron. In this case, we have used
the 1s 1S ionization threshold, because the 2s
photoionization contributes only marginally to the total photoionization cross-section above the
double-ionization threshold.
In the case of H2 we have used the lowest single-ionization threshold H2+ X 2Σg (15.4eV).
While the double-photoionization threshold for a vertical transition is 51.1eV, the data of [6]
show that the threshold is at a slightly lower energy. Here, we have used an energy of 50eV.
After dividing the He, Li, Be and H2 ratios with a suitable “correlation factor” Cr, all data line up
very well and show surprisingly the same energy dependence.
This work is supported by NSF Grant No. 9987638. The SRC is operated under Grant No.
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[3] M.-T. Huang, R. Wehlitz, Y. Azuma, L. Pibida, I.A. Sellin, J.W. Cooper, M. Koide, H. Ishijima, and
T. Nagata, Phys. Rev. A 59, 3397 (1999).
[4] R. Wehlitz, J.B. Bluett, and S.B. Whitfield, Phys. Rev. A 66, 012701 (2002).
[5] R. Wehlitz, J.B. Bluett, and S.B. Whitfield, Phys. Rev. Lett., in press (2002).
[6] G. Dujardin, M.J. Besnard, L. Hellner, and Y. Malinovitch, Phys. Rev. A 35, 5012 (1987).
[7] R. Wehlitz and S.B. Whitfield, J. Phys. B 34, L719 (2001).