Double ionization electron spectra for helium

Double ionization electron spectra have now been measured for helium. Atoms exposed to a focused, strong laser field have a tendency to simultaneously lose two or more electrons more often than expected (see Physics Today, March 2000, page 9 ). Such nonsequential double ionization (NSDI) has been seen in several noble gases. Due to its simplicity, however, only He is accessible to a full theoretical treatment. Unfortunately, the experimental measurement of correlated NSDI in He has been plagued with difficulties, due to its high ionization energy and the overwhelming preponderance of single-ionization events. Now, using an electron–ion coincidence technique at Brookhaven National Laboratory, a team of researchers has succeeded in measuring the energies of the correlated electron pairs from He. In addition, the scientists performed a model calculation that, together with their spectra, supports the “rescattering” explanation wherein one electron is freed by the laser field, then accelerated back by the same oscillating electric field to knock out the other electron as well. (R. Lafon, Phys. Rev. Lett., in press.)