The Metastable State in Mercury Vapor

Metastable state produced in mercury vapor at low pressures by electron impacts of 4.9 volts.—The persistence of the "radiation" produced by 4.9 volt impacts in mercury vapor was measured in a four electrode tube of the type usually employed in resonance potential measurements, consisting of an equipotential cathode of simple design, an inner grid $G$ for controlling the velocity of the impacting electrons, an outer photo-electric grid $H$ and a photo-electric plate $P$. A.c. and d.c. voltages were applied to the two grids in such a way that the electrons had sufficient velocity to excite the radiation only in alternate half-cycles and that the photo-electric current to the electrometer alternated in direction simultaneously. As a result of the persistence of the radiation there resulted a decrease in the current with increasing frequency, reaching a minimum at 1800 cycles for a distance between grids of 17 mm and at 3800 cycles for a distance of 8.5 mm. These results did not vary much with pressure,.003 to.032 mm. They show a lapse of time between the excitation and the arrival of the first radiation at the plate $P$ of about 1/3600 and 1/7600 sec. respectively. A mathematical discussion shows that the diffusion of the radiation by repeated emission and re-absorption (the "imprisonment" of radiation theory) cannot account for these results. A calculation, based on the assumption that the excited atoms remain in a metastable state and carry the energy of excitation to the photo-electric surfaces and there give it up, gives results in very close agreement with the observations. The conclusion is that a metastable state is formed by the atoms excited by the 4.9 volt electron impacts and that these are the effective ones in producing the photo-electric response in the tubes. In these experiments the $2p_2$ state is the only one excited by the impacts in appreciable amount. How these results can be reconciled with the Bohr theory, according to which the $2p_2$ state is not metastable, is not clear.