On the population of the 23S level of He in planetary nebulae.

Robbins (1968) has noticed a discrepancy between the theoretical and the observed population of the level of He in nlne planetary nebulae, the theoretical value of N (2 35) being a factor of 5 to over 80 greater than the observed one. This discrepancy is mainly due to too small a value of the excitation rates of exchange collisions (2 S-2 1P), wrongly derived from the expression for the corresponding cross section as given by Burke et al. (1967). Key toords: energy level - theoretical population - observed population - - planetary nebulae