1P015

Extending electron momentum spectroscopy to study the double
ionization of water(IMRAM, Tohoku University) !D. B. Jones, M. Yamazaki, N. Watanabe, M, Takahashi[Introduction] Water (H2O) is a major constituent in living organisms and makes up a vast proportionof the Earths surface. An understanding of its structure and interactions are therefore important inunderstanding its behaviour in biological systems as well as our own and other planetary atmospheres.
Double ionization (DI) processes are of considerable interest to many fields due to the governing roleof electron-electron correlations in a wide range of mechanisms. However in the case of water, whilst
substantial knowledge has been gained regarding single ionization processes and the resultant cations
produced, the structure and behaviour of the dication is considerably less well understood [1].
The binary (e,2e) or electron momentum spectroscopy (EMS) technique has long been established
as a sensitive probe of molecular structure [2,3]. This technique, which has traditionally been
employed to study single ionization mechanisms, has recently been extended to consider the double
ionization of helium through an (e,3-1e) method [4,5]. Here an electron impact double ionization
experiment is performed with the energy and momentum of two fast outgoing electrons being detectedin coincidence. Through the conservation of energy and momentum this (e,3-1e) method is sensitive tothe energy and momentum remaining with the dication and undetected electron. This (e,3-1e)
experimental method has been identified as an intermediate method lying between EMS and full (e,3e)
momentum spectroscopy experiments, which suffer from extremely low count rates as they require
three outgoing electrons to be detected in coincidence. In the absence of two-step processes, theexperimental (e,3-1e) cross section is directly proportional to the projection of the ground state on thecontinuum Coulomb state averaged over initial molecular rotations and outgoing electron directions[6].
[Experimental] We have performed an (e,3-1e) experiment to examine the double ionization of water.
The experiment has been conducted in the symmetric non-coplanar EMS geometry in an apparatus
that has been described previously [7]. Here an incident electron energy of 2055 eV is used, whilst thetwo fast outgoing electrons are detected in coincidence after undergoing energy and momentum
dispersion in a spherical analyser having a mean pass energy of 1000 eV. This allows the measured
spectra to be obtained over the binding energy range of 0-100 eV. The present experimental techniqueyields the simultaneous detection of both the traditional (e,2e) events and the double ionization(e,3-1e) events. It is noted that this allows the intensity of double ionization events to be directlycompared against the intensity of the outer-valence (e,2e) events.
[Results and Discussions] A representative example of the observed binding energy spectra summed
over the measured azimuthal angles is presented in figure 1. Here it is noted that whilst the double
ionization onset is 34 eV [1], the onset of DI is some what masked by a breakdown of the molecularorbital picture for the 2a1 orbital [8]. However, we highlight the spectral feature observed at ~57 eVthat is prominent above the continuum and is associated with the opening of additional doubleionization channels.

Figure 1: A representative example of the binding energy spectrum of water containing traditional (e,2e) and(e,3-1e) double ionization events.

To our knowledge, this experiment represents the first time that (e,3-1e) momentum spectroscopy for amolecular target has been explicitly considered. The present experimental results therefore represent
an important test of theoretical models of double ionization, which have most recently been used tocalculate the full (e,3e) five fold differential cross section under a different set of kinematicalconditions [9]. The experimental double ionization onsets are also directly comparable to thosepredicted theoretically.
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