A Practical Guide Paper on Bulk and PLD Thin-Film Metals Commonly Used as Photocathodes in RF and SRF Guns
Abstract
1. Introduction
2. Highlights on Metallic Photocathodes
Cu | Mg | Y | Pb | Nb | Ba | |
---|---|---|---|---|---|---|
WF (eV) | 4.6 [74] | 3.6 [9] | 3.1 [75] | 4.2 [76] | 4.3 [76] | 2.5 [77] |
QE (at 266 nm) Bulk | 2.0 × 10−5 * [78] | 7.6 × 10−4 [79] | 3.0 × 10−3 [79] | 7 × 10−5 [80] | 3.2 × 10−6 at 248 nm [81] | 7.6 × 10−4 at 248 nm [82] 1 × 10−3 at 337 nm [83] |
Thin film | 6.0 × 10−5 [84] | 1.8 × 10−3 [79] | 3.3 × 10−4 [79] | 8.0 × 10−5 [80] | NA | NA |
τob (years) [19] τotf (months) | No limit <1 [84] | ~1 2–3 [85] | <1 3–5 ** | >1 4–6 ** | No limit 3–5 ** | <<1 <1 ** |
τd (years) | No limit | >2 | >1 | >1 | No limit | ~1 |
τr (ps) | <<1 | <<1 | <<1 | <<1 | <<1 | <<1 |
Id (nA) | 53 at 14.6 MV/m [86] | 20 at 90 MV/m [84] | NA | 240 at 14.5 MV/m [87] | NA | NA |
εb (mm × mrad) | 0.6 [88,89] 0.35 [90] | 0.4 [91] 0.5 [92] | NA | 0.8 ** [39] | 0.6 [93] | NA |
εtf (mm × mrad) | NA | 0.85 [85] | 0.76 [94] | NA | NA | NA |
Pbg (Pa) | <10−7 | <10−8 | <10−8 | <10−6 | <10−6 | <10−8 |
Cu | Mg | Y | Pb | Nb | Ba | |
---|---|---|---|---|---|---|
δ (nm) λ (nm) | 14 280 [98] | 12 266 [99] | 18 266 [99] | 9 280 [100] | 11 400 [101] | NA |
nb (bulk) | 1.16 at 450 nm 0.15 at 600 nm [102] | 0.18 at 405 nm [96] 0.37 at 589 nm [102] | NA | 1.78 700 nm [103] | 2.52 at 405 nm 2.92 at 589 nm [104] | NA |
kb (bulk) | 2.4 at 450 nm 3.29 at 600 nm [102] | 3.57 at 405 nm [96] 4.42 at 589 nm [102] | NA | 3.57 at 700 nm [103] | 2.63 at 405 nm 2.87 at 589 nm [104] | NA |
* Rb (Φ = 0) % | 55 at 450 nm 95 at 600 nm | 97 at 405 nm 93 at 589 nm | 45 at 405 nm 53 at 578 nm | 65 at 700 nm | 48 at 405 nm 50 at 589 nm | 43 at 400 nm 53 578 nm [77] |
ntf | 0.87 at 450 nm 0.186 at 600 nm [102] | 0.52 at 405 nm 0.48 at 589 nm [105] | 0.8 at 405 nm 1.3 at 578 nm [106] | 1.68 at 700 nm [102] | NA | 0.82 at 405 nm 0.88 at 578 nm [102] |
ktf | 2.20 at 450 nm 2.98 at 600 nm [102] | 2.05 at 405 nm 3.71 at 589 nm [105] | 1.6 at 405 nm 2.4 at 578 nm [106] | 3.67 at 700 nm [102] | NA | 1.07 at 405 nm 1.52 at 578 nm [105] |
* Rtf (Φ = 0) % | 58 at 450 nm 93 at 600 nm | 68 at 405 nm 88 at 589 nm | 45 at 405 nm 53 at 578 nm | 67 at 700 nm | NA | 26 at 405 nm 40 at 578 nm |
Ra (nm) | 7.5 [107] | 3 [108] | 2 [109] | 14 [110] | 9 [111] | NA |
Cu | Mg | Y | Pb | Nb | Ba | |
---|---|---|---|---|---|---|
Tm (K) | 1358 | 923 | 1799 | 601 | 2750 | 1000 |
Te (K) | 2835 | 1363 | 3203 | 2022 | 5017 | 2118 |
ρ (g/cm3) | 8.92 | 1.74 | 4.47 | 11.34 | 8.57 | 3.51 |
Electronegativity (Pauling) [112] | 1.90 | 1.31 | 1.22 | 1.80 | 1.60 | 0.89 |
Chemical reactivity | VERY LOW | MEDIUM | MEDIUM | VERY LOW | LOW | VERY HIGH |
Ei (eV) | 7.19 | 7.12 | 5.94 | 6.41 | 6.99 | 8.69 |
ΔHf (kJ/mol) | 13.3 | 8.5 | 11.4 | 4.8 | 30.0 | 7.1 |
ΔHe (kJ/mol) | 300 | 128 | 363 | 179 | 690 | 142 |
σT [W/(m × K)] [112] | 400 | 156 | 17.2 | 35.3 | 53.7 | 18.0 |
C [J/(kg × K)] | 385 | 1020 | 300 | 129 | 265 | 204 |
DT (mm2/s) | 116.0 [106] | 87.9 * | 12.8 * | 23.0 [113] | 23.6 * | 25.1 * |
ρe (nΩ × m) [112] | 16.8 | 43.9 | 596 | 208 | 152 | 332 |
Ke (MS/m) | 59.5 | 22.7 | 1.7 | 4.8 | 6.6 | 3.0 |
Ne (×1022 cm−3) | 8.5 | 8.6 | 6.0 | 13.2 | 5.6 | 3.15 |
μe (cm2/(V × s) ** | 43.8 | 1.6 | 1.8 | 2.3 | 7.5 | 5.9 |
De *** (cm2/s) | 1.2 | 0.04 | 0.05 | 0.06 | 0.19 | 0.15 |
Electronic shell structure | [Ar] 3d104s1 | [Ar] 3d104s1 | [Kr] 5s24d1 | [Kr] 5s24d1 | [Kr] 5s14d4 | [Xe] 6s2 |
3. Main Factors That Influence the Photo-Emissive Properties of Metallic Photocathodes
4. Conclusions
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
Abbreviations
EDX | Energy dispersive X-ray analysis |
ERL | Energy recovery linac |
FEL | Free-electron laser |
HV | High vacuum |
LCT | Laser cleaning treatment |
NA | Not available |
Nd:YAG | Neodinium-doped yttrium aluminum garnet |
PLD | Pulsed laser deposition |
RF | Radio frequency |
SEM | Scanning electron microscopy |
SRF | Superconductive radio frequency |
UEM/D | Ultrafast electron microscopy and diffraction |
UHV | Ultra-high vacuum |
Appendix A
Definition | Symbol |
---|---|
Boiling point | Te |
Dark current | Id |
Dark lifetime | τd |
Density | ρ |
Electrical conductivity | σe |
Electrical resistivity | ρe |
Electronegativity | χ |
Electron charge | e |
Electron diffusivity | De |
Electron mobility | μe |
Extinction coefficient for bulk | kb |
Extinction coefficient for thin films | ktf |
First ionization energy | Ei |
Latent heat of evaporation | ΔHe |
Latent heat of fusion | ΔHf |
Melting point | Tm |
Number density of free electrons | Ne |
Operational lifetime for bulk | τob |
Operational lifetime for thin film | τotf |
Optical penetration depth | δ |
Optical reflection coefficient for bulk | Rb |
Optical reflection coefficient for thin film | Rtf |
Quantum efficiency | QE |
Radiation wavelength | λ |
Recommended vacuum level in the gun system | Pbg |
Refractive index for bulk | nb |
Refractive index for thin film | ntf |
Response time | τr |
Roughness | Ra |
Thermal capacity | C |
Thermal emittance for bulk | εb |
Thermal emittance for thin film | εtf |
Thermal conductivity | σT |
Thermal diffusivity | DT |
Work function | WF |
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Mg | Before LCT | 5.0 × 10−4 | [134] |
---|---|---|---|
After 200 lpps * | 8.9 × 10−4 | [124] | |
After 300 lpps | 1.1 × 10−3 | [124] | |
After 400 lpps | 1.4 × 10−3 | [124] | |
After 500 1pps | 1.8 × 10−3 | [124] | |
Y | Before LCT | ~10−5 | [74] |
After 100 lpps | 1.2 × 10−4 | [74] | |
After 400 lpps | 2.8 × 10−4 | [74] | |
After 900 lpps | 3.3 × 10−4 | [74] | |
Pb | Before LCT | 3.0 × 10−5 | [80] |
After thousands lpps | 8.0 × 10−5 | [80] |
Mg Thin Film on Cu Back-Flange | Y Thin Film on Cu Back-Flange | Pb Thin Film on Nb Back-Flange | |
---|---|---|---|
Compatibility with a Cu RF gun | High | Medium | - |
Compatibility with a Nb SRF gun | - | - | High |
Mg Bulk Cathode with Cu-Mask | Y Bulk Cathode with Cu-Mask | Pb Bulk Cathode with Nb-Mask | |
---|---|---|---|
Compatibility with a Cu RF gun | Very high | High | - |
Compatibility with a Nb SRF gun | - | - | Very high |
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Perrone, A.; Aziz, M.R.; Gontad, F.; Vainos, N.A.; Caricato, A.P. A Practical Guide Paper on Bulk and PLD Thin-Film Metals Commonly Used as Photocathodes in RF and SRF Guns. Chemistry 2025, 7, 123. https://doi.org/10.3390/chemistry7040123
Perrone A, Aziz MR, Gontad F, Vainos NA, Caricato AP. A Practical Guide Paper on Bulk and PLD Thin-Film Metals Commonly Used as Photocathodes in RF and SRF Guns. Chemistry. 2025; 7(4):123. https://doi.org/10.3390/chemistry7040123
Chicago/Turabian StylePerrone, Alessio, Muhammad Rizwan Aziz, Francisco Gontad, Nikolaos A. Vainos, and Anna Paola Caricato. 2025. "A Practical Guide Paper on Bulk and PLD Thin-Film Metals Commonly Used as Photocathodes in RF and SRF Guns" Chemistry 7, no. 4: 123. https://doi.org/10.3390/chemistry7040123
APA StylePerrone, A., Aziz, M. R., Gontad, F., Vainos, N. A., & Caricato, A. P. (2025). A Practical Guide Paper on Bulk and PLD Thin-Film Metals Commonly Used as Photocathodes in RF and SRF Guns. Chemistry, 7(4), 123. https://doi.org/10.3390/chemistry7040123