Measurement of cross sections for collisions of the second kind in helium

We have measured the cross sections for collisions of the second kind between helium atoms in the n 1 P ( n = 3,4) state and in the normal state. As a consequence of the collision the atoms are transferred from the n 1 P to the n 1 D, n 3 D or n 3 F state. The n 1 P states were excited selectively by optical pumping. The collision cross sections for the transitions 3 1 P -> 3 1 D, 3 1 P -> 3 3 D, 4 1 P -> 4 1 D, 4 1 P -> 4 3 D and 4 1 P -> 4 3 F were measured as 3.2 × 10 -16 , 2.2 × 10 -19 , 2.8 × 10 -16 , 2.2 × 10 -16 and 9.5 × 10 -17 cm 2 , respectively. Thus they are smaller by several orders of magnitude than those reported previously.