Enhancement of laser-induced field emission in tungsten due to a metastable d band

A possible theory explaining the experimentally observed large enhancement of the field emission current from a tungsten needle exposed to a laser beam is presented. Calculations based on the density functional theory indicate a large density of 5d states and less dense 6p states above the Fermi level. The incident laser beam of 2.5 eV pumps the electrons to an unoccupied 6p band, some of which then de-excite to 5d states at lower energies, forming metastable states since the radiative decay from 5d states to the ground state 6s band is forbidden. On the other hand, the tunneling time from the metastable state is shorter than that from the ground state because of a smaller surface barrier, which may lead to the observed enhancement of the emitted current.