Photo field emission spectroscopy of Γ-P AND Γ-〈013〉 bands of tungsten

Photo field emission currents from clean (013) and clean and barium covered (111) regions of a tungsten tip have been measured in a field emission microscope using two wavelengths of laser radiation and phase-sensitive detection. The photo field emission current-voltage characteristics show various slopes and shoulders. These shoulders are considered as correlated with optical transitions between energy bands of tungsten. The final and initial energies are evaluated from the excitation energy and the known lowering of the barrier maximum. The obtained results are in fairly good agreement with the band structure of tungsten calculated by Christensen and Feuerbacher 1]. Except direct transitions which are prevailing also some indirect transitions were found. The slopes of the photocurrent characteristics are discussed.