  We have studied the fundamental rotational relaxation and excitation
collision of OH- J=0 <-> 1 with helium at different collision energies. Using
state-selected photodetachment in a cryogenic ion trap, the collisional
excitation of the first excited rotational state of OH- has been investigated
and absolute inelastic collision rate coefficients have been extracted for
collision temperatures between 20 and 35 K. The rates are compared with
accurate quantum scattering calculations for three different potential energy
surfaces. Good agreement is found within the experimental accuracy, but the
experimental trend of increasing collision rates with temperature is only in
part reflected in the calculations.

Atomic Physics;Chemical Physics