Absolute single-palticle detection efficiencies of a microchannel plate (MCP) detector were determined by photo-ionization of atoms or molecules. The ionized heavy particles were detected in coincidence with the resulting electrons. This method allows the determination of detection efficiencies without a knowledge of the absolute event rates. We obtained the detection efficiencies for the ions H' , D;, H D ' , Ht, Dt and Ar' in the impact energy range 0-5 keV. The detection ekiencies saturate above 3 keV and reach generally 41%. This value seems to reflect mainly the physically effective area of the MCP.

We repolt integral cross sections for the collision processes between H(D) atoms and Hz(D2) molecules in the energy range from -hold to 170 eV leading to charged collision products. We observe not only charge exchange and target or projectile ionization such as H + HI + Ht H: and H t Hz + H t e t H : but also associative ionization, H + Hz --L H : t e, and heavy panicle exchange, D t Hz + HD+ + H -. Typical absolute cross section values are belween c d and IO-'' cm'.