We present measurements of the quantum detection efficiency (QDE) of rubidium bromide opaque photocathodes over the 44–1560-Å wavelength range. We achieved QDEs of >60% at λ = 68 Å, and >40% at λ ≈ 920 Å, for RbBr photocathode layers applied to the surface of microchannel plates (MCPs). The photoelectric threshold is observed at λ ≈ 1560 Å, and there is a broad (≈100-Å) QDE minimum centered at λ ≈ 775 Å which correlates with 2× the band gap energy for RbBr. The QDE is characterized by four peaks centered at λ ≈ 68 Å, λ ≈ 400 Å, λ ≈ 600 Å, and ≈1050 Å. The QDE peaks at λ ≈ 400 Å, ≈600 Å, and ≈1050 Å correspond with emission of 3, 2, and 1 photoelectrons, respectively. The QDE at the λ ≈ 68-Å peak is associated with a d-f resonant absorption feature of RbBr. QDE contributions of the photocathode material inside the channels, and on the interchannel web, have been determined. Measurements of the angular variation of the QDE from 0° to 35° to the channel axis are also presented. We describe a simple QDE model and show that its predictions are in accord with the QDE measurements. Preliminary assessment of the stability of RbBr indicates that no QDE degradation occurs after limited exposure (20 h) to air at low humidity (<30%). Examination of the photocathode structure with an electron microscope reveals a rough surface with a scale of the order of 0.5 μm.
© 1990 Optical Society of America
Original Manuscript: December 15, 1989
Published: November 1, 1990
Oswald H. W. Siegmund and Geoffrey A. Gaines, "Extreme ultraviolet quantum detection efficiency of rubidium bromide opaque photocathodes," Appl. Opt. 29, 4677-4685 (1990)