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Simultaneous emission of two photons by an H atom in the metastable 2s level is considered as a source of a continuum in planetary nebulae. Detailed calculations show that the probability of two-photon emission is 8.23 and that the intensity of radiation emitted by this process increases markedly with increasing frequency. About 32 per cent of electron captures lead directly to the 2s level and then to two- photon emission, since collisional de-excitation proves unimportant. Transitions from the 2p to the 2s level, induced by collisions with free electrons, convert a part of the remaining La quanta into this continuous radiation, conversion occurring after a quantum of La radiation has been scattered about 1010 times, on the average, in a region of ionized H. A neutral hydrogen region may surround the H ii region. In such an H I envelope, about 1013 scatterings are required for conversion, but the density of neutral H is so much greater that most conversion by collision probably takes place there. The fraction of La so converted may vary over a wide range, depending on the physical conditions. The theory is used to predict the total emission from an ionized hydrogen gas. The Balmer jump is reduced, and the decrement of the continua shortward of the Paschen and Balmer limits is also reduced. A bluish continuum is to be expected in the region from X 6000 to X 3646. Our analysis applied to the ultraviolet observations now available results in a reduction of electron temperatures by about 2,5 per cent. The available wide-slit observations indicate the possibility that a bluish "visual continuum may exist.
Lyman et al. (Thu,) studied this question.