This paper reports the first observation and measurement of two-body ion-neutral association at low temperatures and densities. The rate at 13 K for C${\mathrm{H}}_3^{+}$ + ${\mathrm{H}}_2$ → C${\mathrm{H}}_5^{+}$ + $h\nu$ has been measured to be (1.8 ± 0.3) × ${10}^{-13\mathrm{}}$ ${\mathrm{cm}}^3$/s for a normal (3 to 1) mix of ortho and para hydrogen. The reaction conditions were such that the process could only have proceeded by radiative stabilization of the collision complex.

• Received 17 November 1983

DOI:https://doi.org/10.1103/PhysRevLett.52.902