3.1
Observation of the two-electron cusp in atomic collisions:
Evidence for strong electron-electron correlation
L. Sarkadi and A. Orbán
ter strongly suggests the picture of formation
of quasi-stationary low-lying two-electron continuum states around the projectile. On the
basis of this picture the observed strong energy
correlation can be explained by an angular correlation of 180◦ in the projectile-centered reference system: The correlation between the lowand high-energy emission in the laboratory system corresponds to that between backward and
forward emission in the projectile frame. This
result is consistent with the Wannier theory.
In this report we present experimental data
for a process when two electrons with velocity vectors equal to that of the projectile are
emitted from collisions. By observing the twoelectron cusp the study of the threshold phenomenon for two-electron break-up is possible.
It is a particulary interesting question whether
the outgoing charged projectile can attract the
two repulsing electrons so strongly that the
two-electron cusp is formed. If it is so, a further question arises: Are the two electrons correlated in the final state as it is predicted by
the Wannier theory [1]?
The experiments have been done at the
1 MeV VdG accelerator of ATOMKI using
our TOF spectrometer [2]. The first measurements clearly showed the formation of the
two-electron cusp and signature of the electron correlation in 200 keV He0 +He collisions
[3]. These promising results motivated us to
carry out the experiment at 100 keV beam energy where the coincidence count rate is still
reasonable but the energy resolution is better. For an acceptable data acquisition time
we improved our data acquisition and data processing system for triple coincidence measurements. In Fig. 1a we present our measured relative fourfold differential cross section (FDCS)
that shows strong electron correlation. For a
comparison, in Fig. 1b we displayed the contour plot for uncorrelated electron pair emission. These latter data were synthesized artificially, generating the energies of the electron pairs from two independent double coincidence experiments. In both figures the distributions are characterized by two ridges. In Fig.
1b the ridges are perpendicular straight lines
(E1 = E2 = 13.6 eV). As a result of the correlation, the ridges in Fig. 1a are distorted in such
a way that they have a joint straight-line section following the line E1 + E2 = 27.2 eV. This
means that the electron pairs in the vicinity of
the cusp maximum are emitted with a centerof-mass velocity equal to that of the projectile.
The two-electron emission from a moving cen-
ridge 1
18
a)
16
ridge 2
14
Energy of e2 [eV]
12
10
E1 + E2 = 27.2 eV
10
12
14
16
ridge 1
18
18
b)
16
ridge 2
14
12
10
10
12
14
16
18
Energy of e1 [eV]
Figure 1. Contour plots of FDCS as a function
of the electron energies obtained at 0◦ in 100 keV
He0 +He collisions. Part (a): Measured FDCS.
Part (b): FDCS for uncorrelated electron emission.
[1] G.H. Wannier, Phys. Rev. 90 (1953) 817.
[2] L. Sarkadi and A. Orbán, Meas. Sci. Technol. 17
(2006) 84-90.
[3] L. Sarkadi and A. Orbán, ATOMKI Annual Report (2006) 30.; L. Sarkadi and A. Orbán,
Phys. Rev. Lett. (2008) in press
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