TWO-ELECTRON PRISON BREAK. New experiments studying the cooperation among electrons undergoing ionization show that electrons do not act alone when intense light liberates two of them at once from helium and other rare-gas atoms. When an intense light pulse removes more than one electron from an atom, it's simplest to assume that electrons respond to the light independently of their brethren and leave one by one. However, this "independent electron model" fails by many orders of magnitude in predicting double-ionization rates of atoms.

Using the COLTRIMS "momentum microscope" for atoms and molecules (Update 373), two multi-institutional experiments in Germany at the University of Marburg and the Max Born Institute in Berlin have measured the complete 3D momentum values for singly and multiply ionized helium (Reinhard Doerner, University of Frankfurt, 011-49-69-798-24218, doerner@hsb.uni-frankfurt.de) and neon (Robert Moshammer, University of Freiburg, 011-49-761-203-5741, moshammer@physik.uni-freiburg.de). If the electrons had acted independently, and left one by one in two successive steps, then the momentum data for double ionization would look like single ionization occurring twice. But the data show otherwise, leaving only the possibility of coordinated behavior.

Going further, the authors of the neon observations suggest that their data support a cooperative-behavior scenario known as "rescattering": the laser pulse's oscillating electric field first removes one electron, then pushes electron and ion back together, and finally the electron knocks out one of its comrades. These experiments can begin to test the extensive theoretical models of strongly interacting electrons in intense light fields. (Weber et al. and Moshammer et al., Physical Review Letters, 17 January /pnu/2000/; images at Physics News Graphics; papers available from Select Articles.)