Exciton-Induced Photoemission from BaO near 80°K

This paper reports a new photoelectric study of BaO at 80°K. Special attention was devoted to the spectral region near $h\nu =4\mathrm{}$ ev, where Zollweg and Jahoda have found four peaks in the optical absorption. When BaO contains electron donor states lying less than 2 ev below the vacuum, these peaks show up clearly at 80°K in the spectral distribution of the photoelectric yield. The effect appears similar in character to exciton-induced photoemission from alkali halides. In BaO, the phenomenon furnishes a particularly sensitive means of studying the weak absorption peak at 3.8 ev, which can give rise to photoemission at photon energies as low as 3.3 ev. This is probably the source of the unexpected slow electrons previously observed by Philipp in energy distributions. Electron donor states giving rise to photoelectric thresholds as small as 0.8 ev were seen during the present studies.