Photon Factory Activity Report 2005 #23 Part B (2006)
Atomic and Molecular Science

20A/2003G018

Double-to-Single Photoionization Ratios of Mg and Ca Atoms
near the Double Ionization Limit
Tetsutaro OSAWA1, Yuko TOHYAMA1, Ryohei KOBAYASHI1, Satoshi OBARA2,Yoshiro AZUMA2, and Tetsuo NAGATA1
1
Dept. of Phys., Meisei University, Hodokubo 2-1-1, Hino-shi, Tokyo 191-8506, Japan2
Photon Factory, IMSS, KEK, Tsukuba, Ibaraki 305-0801, Japan
Introduction
The double photoionization of valence-shell elec- tronsof an atom near its double ionization limit is due to theelectron correlation among the electrons and provides
useful means for investigating the electron correlation [1,2]. We have measured the cross-section ratios σ2+/σ+ forMg and Ca atoms in the photon energy range from thedouble ionization threshold Eth (= 22.7 eV for Mg and17.9 eV for Ca) to just below the lowest excited state of
the np subvalence electron Eex (= 57.5.eV for Mg and 30
eV for Ca).

potential of Mn+. The fraction F was 0.012 at 17.5 eV and0.0026 at 24 eV, decreasing with E.
Results and conclusionThe results obtained for Mg and Ca atoms are shown in
Figures below. In the case of Mg, the data points in the28.5 ~ 43 eV region are lacking. Although thesecond-order light is very weak in this region, it was notpossible to remove its effect sufficiently with eq.(2)mainly because of very strong and very complex np
photoabsorption spectrum.

Experiment and data analysis
The experiment was carried out at Beam-line BL- 20A.
This beam line is equipped with a 3m normal incidence
monochromator and provides SR in the range 15~50 eV.
We used a time-of-flight mass spectrometer combinedwith an atomic beam oven. One of difficulties in themeasurements in such low energy regions is the existenceof second-order light. This effect was eliminated byreferring to the TOF spectra recorded in the 2Eth to 2Eex
regions.
When there is no second-order-light, the doubleto-single photoionization ratio σ2+/σ+ at photon energy E
is simply given by

σ 2+ ( E ) N 2+ ( E )=,σ + (E )N + ( E)(1)

where N2+(E) and N+(E) are the numbers of doublycharged and singly-charged ions counted at photon energy
E, respectively, after corrected for the difference incounting efficiencies. When the second- order light (2E)
is mixed in the light of energy E, the ratio must be
calculated using

σ 2+ ( E ) N 2+ ( E ) − F ⋅ N 2+ (2 E )).
=σ + (E )N + ( E)(2)

The counts N2+(E) in this case includes the doublycharged ions due to the second order light. The quantity F
is the fraction of the second-order light mixing in the lightat E. The fraction F was deter- mined previously using
Mn+ (= He+, Ca2+, Sr2+, and Kr2+) ions from the intensity
ratio Mn+(E)/Mn+(2E) with E being below the appearance

Experimental double-to-single photoionization ratio forHe atom has been reported to be 4.2 % around 180 eV, thepeak of the ratio curve. Although the experimental
uncertainties in the present study are considerably large, it
is certain that the ratios in Mg and Ca atoms exceed 10%
at the maxima of their ratio curves.
References[1] A. S. Kheifets and I. Bray: Phys. Rev. A 54 R995
(1996).
[2] R. Wehlits et al.: Phys. Rev. A 71 012707 (2005).

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