Photoelectron emission studies in CsBr at
(a) Schematic of the experimental arrangement in the SDT system at SSRL to study light transmission in CsBr films. (b) Correlation between the CsBr film absorption (transmitted light) and incident laser power. The observed behavior is consistent with the formation of intra-band-gap absorption sites by the UV irradiation. (c) Observed correlation between the CsBr photoelectron yield (nA/mW top trace) and the normalized light transmission (arbitrary units (A.U.) bottom trace) for several incident laser powers (mW) shown in parenthesis. The photoelectron yield increases when more light is absorbed in the film, consistent with intra-band-gap absorption site formation. The photoelectron yield data were averaged to reduce system noise.
(a) The main components of the SDT system optics and electronics. The photocathode sample is shown in the transmission mode. (b) Reflection and (c) transmission modes of operation studied in the SDT system.
(a) Focused beam, laser power, charge-coupled device integration time, 20 averages (raw data minus the dark current background). The fluorescence peaks from the optical fiber are observed. However, the peak is buried in the noise. (b) Two turn defocused beam, at laser power, integration time, and 20 times average. The fluorescence peak is observed after beam defocusing. (c) Normalized peak area vs defocus length plot. Curves B: , C: , D: , and E: .
(a) Efficiency of production of the fluorescence of signal from the intraband states of CsBr obtained in the reflection mode with the beam defocused. (b) Fluorescence efficiency vs laser power for CsBr sample after several days of laser experiments utilizing a defocused beam in reflection mode.
(a) Transmission mode data for the focused beam case. Note that the peak is small relatively to the fluorescence quartz peak. (b) Data obtained after defocusing the lens away from the focal plane at . The peak increases as the focal plane of the fluorescence signal is approached. (c) Photoelectron yield data obtained scanning the piezoelectric flexure stage of the SDT system relative to the focused laser beam at a pressure of . The photoelectron yield was recorded in an field at 120 points along each scan line. The chrome bars and small square marks on the sapphire substrate are shown in black (low photoemission yield). The area where the laser hits the CsBr sample is shown as a bright spot. Intensity profiles along two perpendicular directions are also shown in the figure. (d) Corresponding data obtained after the sample were subjected to a pressure burst of .
(a) Correlation of photocurrent and fluorescence efficiency for a CsBr sample in transmission mode (TM) with time. (b) Correlation of photocurrent and amplitude of the fluorescence peak with increasing temperature at laser power in TM. (c) Model for photoemission below the band gap energy in CsBr. Electrons are photoemitted after UV absorption by the IBAS. The Cs layer which has been shown (Ref. 1) to migrate to the surface with irradiation lowers the work function and increases the photoelectron yield. The electrons can return via the Cr film located under the CsBr film. The Fermi level of Cr is at below the vacuum level and is shown to be within the IBAS.
(a) Photoelectron emission spectra from the valence band of CsBr obtained at incident energy from the SSRL storage ring. (b) Enlarged region from (a) indicating the region inside the energy gap. The Fermi level of a test Au sample is shown for calibration purposes. The fluorescence band coincides with the electron states observed by photoelectron emission after white beam activation of the CsBr sample. The experimental results are consistent with the proposed model.
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