Single and double photoionization of lithium
M.-T. Huang,
1,
* R. Wehlitz,
1
Y. Azuma,
1
L. Pibida,
2
I. A. Sellin,
2
J. W. Cooper,
3
M. Koide,
4
H. Ishijima,
5
and T. Nagata
5
1
Photon Factory, Institute of Materials Structure Science, KEK, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan
2
Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996-1200
3
Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742-2431
4
University of Electro-communications, Chofu, Tokyo 182-8585, Japan
5
Meisei University, Hodokubo, Hino, Tokyo 191-8605, Japan
Received 5 October 1998; revised manuscript received 28 December 1998
The photoion Li
2+
/Li
+
production cross section ratio of ground-state atomic lithium has been measured for
photon energies ranging from 80 to 424 eV. The absolute cross sections for the Li
2+
and Li
+
yield are also
derived. In this energy region, the Li
2+
/Li
+
ratio reaches a plateau of about 1.0% before reaching a maximum
of about 4.5%, then decreases slowly. Good agreement is found between the measured total photoionization
cross sections of lithium and theoretical calculations. The Li
2+
/Li
+
ratio is also compared to the He
2+
/He
+
ratio from excited He(1 s 2 s ) for photon energies up to 70 eV above threshold. The branching ratio of Li
2+
to
total Li ion production is also compared to the single-ionization cross section of electron impact on Li
+
ions.
S1050-29479903605-7
PACS numbers: 32.80.Fb, 32.80.Hd
I. INTRODUCTION
During the past decade, double photoionization or, the
ratio of double to single photoionization of helium has been
under intensive investigation, both experimentally and theo-
retically 1–10. The reason is that, since the photoelectric
operator is a single-particle operator, the simultaneous de-
tachment of two electrons by a single photon stems purely
from electron correlation effects, which cannot be accounted
for by the independent electron approximation. This makes
double photoionization an ideal test case for our understand-
ing of electron correlation effects. Although discrepancies
between experiments and theories still exist, agreement has
greatly improved over the years.
Lithium is the simplest open-shell atom, and the simplest
atom that exhibits intershell electron correlation. In this
three-electron system, there are fundamentally different pro-
cesses involved in photoionization which do not find any
analogue in a closed-shell two-electron system like helium.
Examples for such processes are the intershell electron cor-
relation, the overlap with resonant-photoexcitation of hollow
lithium 12,13, two step processes for double ionization, and
direct triple photoionization 14. Moreover, lithium has an
optically active electron that can be excited easily by laser
light. This allows investigations on the electron correlation
for different initial state configurations. Hence, the system
provides a still richer testing ground for the understanding of
electron-correlation effects.
The main purpose of this paper is to present the photoion
Li
2 +
/Li
+
production cross section ratio of ground-state
atomic lithium, as well as the absolute production cross sec-
tions for Li
2 +
and Li
+
yields, for photoionization as a func-
tion of photon energy.
II. EXPERIMENTAL METHOD
The data presented in this paper were acquired with the
same lithium oven and ion-time-of-flight TOF apparatus
described previously 11–13. So, the experimental methods
used will be described only briefly.
The experiments were done at the 2.5-GeV electron stor-
age ring of the Photon Factory, KEK, utilizing the extreme-
ultraviolet XUV bending-magnet beam-line BL3B 15,
and for some of the higher energy data points, the undulator
beam-line BL16B. A photoion TOF analyzer viewed perpen-
dicularly the Li atomic beam target effusing from an oven
source. The interaction region was defined by the intersec-
tion of the incident monochromatized XUV radiation beam
from the third orthogonal direction and the lithium vapor.
Partial charge-state ion-yield spectra were acquired in the
pulsed-field-TOF-extraction-mode with gated data acquisi-
tion. Background corrections derived from equal TOF-
spectrum regions adjacent to and in-between the peaks cor-
responding to the singly and doubly charged states of
6
Li
and
7
Li were made by subtraction, with error propagation
being carried forward by standard statistical methods. Silicon
and carbon foils were used upstream from the target region
to filter out the contributions from higher-order light and
stray zeroth order light. The voltages across the cheveroned
double MCP stack were set to be -4 kV for entrance and
-2 kV for exit, and the CFD threshold was set very low at
about 35 mV to ensure that there was no discrimination be-
tween ‘‘1+’’ and ‘‘2+’’ ions. This was confirmed by mea-
suring the Li
2 +
to Li
+
ratio as a function of MCP voltages
and CFD threshold settings. In the interest of achieving high
count rates and thereby good statistics, a modest resolution
of 0.5 eV was used.
III. DATA ANALYSIS
The Li
2 +
to Li
+
ratios were obtained by directly taking
the ratio of the integrated and background-corrected Li
2 +
*Electronic address: mth@mail.kek.jp
PHYSICAL REVIEW A MAY 1999 VOLUME 59, NUMBER 5
PRA 59 1050-2947/99/595/33975/$15.00 3397 ©1999 The American Physical Society