Thermally assisted photoemission effect on ${\mathrm{CeB}}_6$ and ${\mathrm{LaB}}_6$ for application as photocathodes

The thermally assisted photoemission (TAPE) effect was investigated for the hexaboride thermionic cathodes (${\mathrm{LaB}}_6$ and ${\mathrm{CeB}}_6$). It was found that the quantum efficiency of these cathodes can be increased by raising the cathode temperature along with the thermionic emission. In addition both materials can emit a measurable photoemission current by being irradiated with a laser having a photon energy below the work function at sufficiently high cathode temperatures. Our measurements indicate that this process is linear. These results show the significance of the TAPE effect. An additional effect of cathode heating is surface cleaning. This effect was detected as the hysteresis of the quantum efficiency which can be observed by heating up and cooling down the cathode. This effect slightly changes the quantum efficiency but not as much as the TAPE effect.

Figure 1

Energy diagram schematic of thermally assisted photoemission (TAPE).

Figure 2

Experimental setup. (a) Schematic view. (b) Photographic view.

Figure 3

Equivalent circuit of measurement setup.

Figure 4

Thermionic emission from ${\mathrm{LaB}}_6$ and ${\mathrm{CeB}}_6$.

Figure 5

Photocurrent of ${\mathrm{LaB}}_6$ by excitation with 355 nm at a cathode temperature of 1400 K.

Figure 6

Photoemission properties of ${\mathrm{LaB}}_6$ and ${\mathrm{CeB}}_6$ irradiated by 355 nm.

Figure 7

Results of relative quantum efficiency measurement of ${\mathrm{LaB}}_6$ cathode being irradiated at three different laser wavelengths, 266, 355 and 532 nm, measured by heating up the cathode.

Figure 8

Calculation of cathode temperature rise by incident laser pulse energy. Cathode material ${\mathrm{LaB}}_6$ at 1500 K. Laser wavelength 532 nm.

Figure 9

Thermionic and photocurrent for different cathode temperatures: (a) ${\mathrm{CeB}}_6$ for excitation by laser of 355 nm at $12\mu \mathrm{J}$ and of 532 nm at $\sim 84\mu \mathrm{J}$; (b) ${\mathrm{LaB}}_6$ for excitation by laser of 355 nm at $\sim 13\mu \mathrm{J}$ and of 532 nm at $\sim 32\mu \mathrm{J}$.

Figure 10

Results of relative quantum efficiency measurement of ${\mathrm{CeB}}_6$ cathode being irradiated at three different laser wavelengths, 266, 355 and 532 nm, measured by heating up the cathode.

Figure 11

Dependence of the peak photoemission current on laser pulse energy at 532 nm. (a) ${\mathrm{LaB}}_6$ for the cathode temperature of around 1340 K. (b) ${\mathrm{CeB}}_6$ for the cathode temperature of around 1800 K. For (a) and (b) the laser illuminated area was 1.72 mm in diameter.

Figure 12

Relative QE of ${\mathrm{LaB}}_6$ which is obtained with 266, 355 and 532 nm laser irradiation as the function of the cathode temperature during heating up and cooling down processes.

Figure 13

Relative QE of ${\mathrm{CeB}}_6$ which is obtained with 266, 355 and 532 nm laser irradiation as the function of the cathode temperature during heating up and cooling down processes.