The mode of depopulation of the 2 35 level of helium in a very dilute radiation field is examined. The probability per second of transitions to the ground state from the 2 35 level by two-photon emissions is estimated to be 2.2 X 10- and transitions via superelastic collisions are found to compete with two-photon emission in nebulae of densities of 10 electrons/cm3 or higher. Transitions to the ground state from the 2 35 level are found to be more important than radiative excitations and ionizations if the dilution factor of the radiation is less than 10- . Under such conditions the populations of the triplet terms are independent of the properties of the exciting star. The population parameters, bj, defined by Menzel (1937), are given for eighteen of the low triplet terms for electron temperatures from 8000 to 200000 K. The importance of inelastic and exchange collisions is estimated. Observational evidence from planetary nebulae is found to support satisfactorily the theoretical prediction of the ratio of the strengths of He I 4471 and 5876.
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John S. Mathis (1957) studied this question.