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The Center for Bright Beams, A National Science Foundation Science and Technology Center

Heating and multi photon emission in metal photocathodes

Four lines, blue, orange, green, and red curve downward at various slopes.

Bae, Bazarov, Musumeci, Karkare, Padmore, and Maxson

Mean Transverse Energy as a function of Kinetic Energy. More in caption.

a graph showing mean transverse energy on the y-axis and hv-ø on the x-axis. A red line labeled 1o to the fifth power mj/cm-squared falls off sharply then rises. A blue line labeled 1 mj/cm-squared rises slightly, then falls and rises to nearly the same point as the red line. A black-dotted line labeled k-beta-T (300k) stays steady at the bottom of the graph.

Non-monotonic behavior of MTE with photon energy at large laser fluences.

The mean transverse energy (MTE) of photoemitted electrons limits the beam brightness in most electron beam applications. CBB researchers have investigated how intense femto-second laser pulses cause femto-second scale heating and multi-photon emission of electrons resulting in increased MTE in metal photocathodes.

At low laser fluence (red curve), the MTE is near-zero at the photoemission threshold (hnu-phi = 0), reflecting the near zero energy of the ejected electron, and rises linearly for larger laser energies. Photoemission can take place below threshold through rare two photon processes, resulting in large MTE. At high laser fluence (blue curve), two photon processes lead to high emission rates even below threshold, and thermal heating of the electrons, which occurs within picoseconds, leads to large MTE even at the photoemission threshold.


Bae et al, Journal of Applied Physics 124, 244903 (2018).