Comparison of double and single ionization in He by photons and by charged projectiles

Recognizing that the term ``electron correlation`` is a subject of central interest in current discussions of both photoionization and atomic collision physics, we adopt here the simplified definitions used by James McGuire in his recent article in Adv. At. Mol. Opt. Physics: (1) any effect arising from the Coulomb interaction between electrons; (2) any deviation of the electronic wave function from a product of single-particle orbitals. Within these definitions, double photoionization in He is exclusively attributable to electron correlation, since a single photon interacts with only one target electron. It is interesting to compare single and double photoionization cross sections with corresponding charged particle interaction cross sections. In the limit of high photon energies and projectile particle energies, the native expectation is that apart from kinematic factors the single ionization cross sections for particles and photons converge to the same values. Because of its specific sensitivity to electron correlation in double ionization, and because common factors drop out, it has become customary to compare the ratio of the double to single ionization cross sections rather than the raw cross sections themselves. Yet this ratio has been found experimentally to lie in the range R(h{nu}) = 4 to 5% in themore » photon energy range from 100 to 600 eV, and R (protons, antiprotons, electrons) in the range 0.2 to 0.4% in corresponding limit for high velocity charged particles. These values are more than an order of magnitude different.« less