FIG. 13.

Temperature dependence of calculated and experimentally assessed collision rate coefficients in He. Ab-initio calculated rates are plotted for excitation transfer (dashed lines) and total (solid lines) rates of collisions for the 2³S state (red curves, labeled ‘S’) and the 2³P state (black curves, ‘P’). Experimental k_ME data (symbols, see legend) are derived from published values of linewidths or ME cross sections in ³He (see text). Three calculated rate coefficients slowly decrease with decreasing temperature and thermal velocities, but the rate coefficients k_ME for ME collisions (the dashed red line and symbols) abruptly decrease below room temperature. This is attributed to a weak repulsive barrier at large distance in the He^*-He interaction potential. Adapted from Vrinceanu and Sadeghpour (2010, Fig. 3) for the calculated curves.