Abstract
The photoemission properties of photocathodes affect the ultimate performance of electron accelerator based facilities and the cost. Several major properties of titanium nitride (TiN) irradiated by ultraviolet femtosecond laser pulses were studied. Specifically, the quantum efficiency and intrinsic emittance were measured, and the response time was estimated indirectly. For photon energies higher than the work function, quantum efficiency values of $7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}--5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ were obtained. The measured intrinsic emittance was $0.54\text{ }\text{ }\ensuremath{\mu}\mathrm{m}/\mathrm{mm}$ for the typical wavelength of 266 nm (4.66 eV). When the photon energy was lower than the work function, two-photon photoemission appeared, and the intrinsic emittance increased with the laser intensity. At photon energies of higher than and slightly lower than the work function, the intrinsic emittance ranged from 0.28 to $0.60\text{ }\text{ }\ensuremath{\mu}\mathrm{m}/\mathrm{mm}$ and tended toward a minimum constant of $0.28\text{ }\text{ }\ensuremath{\mu}\mathrm{m}/\mathrm{mm}$ if only single-photon photoemission was considered. The response time of TiN was proven to be close to that of gold by comparing the transverse expansion of electron beams under space charge force for the irradiated photon energy higher than the work function. The autocorrelation measurement of the photoemission of TiN and gold were performed, and a comparison of these results indicated that the two-photon photoemission of TiN excited by a photon energy slightly lower than the work function was prompt.
Highlights
Photoemission based electron sources are widely used in cutting-edge applications of electron accelerators, such as ultrafast electron diffraction (UED) [1,2], x-ray free electron lasers (FEL) [3], and inverse Compton scattering based x-ray sources [4]
When the photon energy was lower than the work function, two-photon photoemission appeared, and the intrinsic emittance increased with the laser intensity
The autocorrelation measurement of the photoemission of titanium nitride (TiN) and gold were performed, and a comparison of these results indicated that the two-photon photoemission of TiN excited by a photon energy slightly lower than the work function was prompt
Summary
Photoemission based electron sources are widely used in cutting-edge applications of electron accelerators, such as ultrafast electron diffraction (UED) [1,2], x-ray free electron lasers (FEL) [3], and inverse Compton scattering based x-ray sources [4]. As a crucial component of the electron source, the properties of the photocathode affect the quality of the electron beam and the ultimate performance and the cost of these facilities [4,5,6]. The main properties of a photocathode are the response spectrum, quantum efficiency, intrinsic emittance, response time, operational lifetime, vacuum requirement, etc., [7]. An ideal photocathode usually has high quantum efficiency, small intrinsic emittance, fast response time, and a long operational lifetime [8,9]. There is no photocathode that meets all these criteria simultaneously, and the selection of the material depends mainly on the required
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