Atmospheric cluster-ions are the smallest components of the aerosol spectrum, comprising a charged molecule surrounded by ligands. Optical instruments for measuring larger aerosol particles are well-known, but the standard approach for ion detection is to use sensitive electrical instrumentation. However, optical detection of ions in the atmosphere seems possible in principle, after laboratory measurements showing that ions absorb in the infra-red at wavelengths of 9.15 and 12.3 mm. In this paper we report atmospheric results from a filter radiometer tuned to 9.15 mm, co-located with Geiger counters to detect ionisation. We show that in clear sky, ionisation from high-energy radiation (identified by simultaneous triggering of both Geiger counters) is associated with a statistically significant increase in filter radiometer signal.