Effect of electron excitation in nonsequential double ionization by intense laser fields**Project supported by the National Natural Science Foundation of China (Grant Nos. 61475168, 11674231, and 61575124), Natural Science Foundation of Shanghai, China (Grant No. 15ZR1430600), and Shanghai Gaofeng & Gaoyuan Project for University Academic Program Development, China.

Abstract

We study the double ionization process of atoms in intense laser fields. The momentum distributions of the correlated electrons are calculated. Contrary to the general expectation, we show an increasing proportion of the electrons ionized via excitation with the increasing laser intensity. These electrons generally have small energy thus they concentratedly distribute on the central region of the momentum diagram. Consequently, the central part of the momentum diagram becomes more notable in higher intensity laser fields. Further study suggests that this phenomenon is general in double ionization.