Photoemission through thin dielectric coating films

Photoemission through thin coating films was studied in the scope of protection of sensitive photocathodes. The transmission of low energy $\mathrm{(\sim }$ 1 eV) electrons was measured for a large number of dielectric films (LiF, NaF, CsF, NaI, MgF${}_2$ , BaF${}_2$ , SiO, SiO${}_2$ , Al${}_2$ O${}_3$ , $n$ -C${}_{36}$ H${}_{74}$ ), evaporated in vacuum on CsI and CuI photocathodes. Some films like CsF, NaI and $n$ -C${}_{36}$ H${}_{74}$ were found to have a fairly large electron attenuation length, varying from about 20 to 100 Å at a maximum initial electron energy of 1 eV. A thin CsF layer deposited on top of CuI and Al photocathodes was found to significantly increase their quantum yield. An enhancement of the photoyield following exposure to water vapour was observed for alkali fluoride-coated photocathodes. We interpret this effect as a decrease of the electron affinity by about 0.3–0.4 eV, induced by adsorption of polarized H${}_2$ O dipoles.