Threshold Effects in the Dissociative Recombination Cross Section of HD$^+$
M. Lange$^a$, J. Levin$^a$, U. Hechtfischer$^a$, L. Knoll$^a$, D. Schwalm$^a$, R.
Wester$^a$, A. Wolf$^a$, X. Urbain$^c$, and D. Zajfman$^b$ 
$^a$ Max-Planck-Institut f\"ur Kernphysik and 
Physikalisches Institut der Universit\"at Heidelberg, D--69029 Heidelberg, Germany
$^b$ Department of Particle Physics, Weizmann Institute of Science, Rehovot, 76100, Israel
$^c$ D\'epartment de Physique, Universit\'e Catholique de Louvain, Chemin du
Cyclotron 2, B1348, Louvain-la-Neuve, Belgium
Thresholds are observed in the dissociative recombination rate coefficient of
vibrationally cold HD$^+$ in the energy range where new channels H(1s)
+ D($n$) (or D(1s) + H($n$)) with $n>2$ open.  Also, the ion pair
production is observed as a sink to the neutral production rate.
These thresholds demonstrate that the dissociative recombination
cannot be described as a two step process.