Non-linear broadening observed in He at moderate densities

The pressure broadening of the He line 21P - 21S at lambda = 2.06 μm is reported, for liquid nitrogen temperature and for densities up to 3 x 1018 atoms/cm3. Surprisingly the broadening curve is not linear in this density range although collisions are considered as binary collisions according to the usual criteria. The effect of inelastic collisions is suggested to explain the data. (orig.)