Photofield-emission spectroscopy of bulk electronic states of tungsten

A novel apparatus for the study of photofield emission has made it possible to observe for what is believed to be the first time departures from free-electron behavior in the total-energy distributions in photofield emission due to transitions between bulk electronic states. For plane-polarized light normally incident on the (100), (110), (111), (211), and (510) crystal planes of tungsten, the distributions are found to depart from free-electron behavior at those energies where there is a rapid variation in the product of the initial and final densities of bulk states along the axis normal to the emission plane. Emission by umklapp processes is also observed. The shapes of the distributions and their polarization dependences are consistent with those expected for nondirect bulk photoexcitation, whereas no clear evidence is found for direct transitions in the bulk. Some possible reasons for this are discussed. It is concluded that photofield emission with light at normal incidence can be used to investigate the spectrum of bulk electronic states between the Fermi level and the vacuum level.