Thin films of calcium were evaporated under ultrahigh vacuum conditions (pressures less than 5 × 10−10 Torr) onto quartz substrates to a thickness of 20 nm. The photoelectric yield and hence the work function of the layers were determined as a function of thickness. For film thicknesses greater than 10 nm, the work function was constant with a value of 2.87 ± 0.06 eV. The work function suows minima at film thicknesses of about 2.5 nm and 5.5 nm. Results from films thicker from 5.5 are a good fit to the Fowler function; for thinner films, the results suggest the existence of a second photoelectric threshold with a corresponding work function ϕs. A theory based on the energy states of the surface electrons is proposed to explain both this second threshold and the ir absorption of thin calcium films.
L. Gaudart and R. Rivoira, "Propriétés photoélectriques des couches minces de calcium," Appl. Opt. 10, 2336-2343 (1971)