Search for nuclear excitation by laser-driven electron motion.

Abstract

It has been proposed that a nucleus may be excited by first exciting the atom's electrons with UV photons. The incident photons couple to the electrons, which would then couple via a virtual photon to the nucleus. As a test case, experiments with $^{235}\mathrm{U}$ have been performed. A pulsed infrared laser produces an atomic vapor of $^{235}\mathrm{U}$ which is then bombarded by a high-brightness UV laser beam. The resulting ions are collected. The first excited nuclear state of $^{235}\mathrm{U}$ has a 26-min half-life and decays by internal conversion, resulting in emission of an atomic electron. These conversion electrons are detected by a channel electron multiplier. An upper limit of 4.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ has been obtained for the probability of exciting the nucleus of a $^{235}\mathrm{U}$ atom that is in the 248-nm UV beam for 700 fs at an irradiance in the range of 1.0\ifmmode\times\else\texttimes\fi{}${10}^{15}$ to 2.5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ W/${\mathrm{cm}}^{2}$.