Infrared (IR) spectroscopy based on vacuum-ultraviolet (VUV) one-photon ionization detection is carried out for bare acetone and its dimer by use of tunable coherent VUV light. This is the first application of tunable VUV light to IR spectroscopy of a size-selected cluster, which has no suitable chromophore for conventional detection schemes. The advantage of this spectroscopic scheme is demonstrated. The observed IR spectrum of acetone dimer indicates formation of a C-2h symmetric structure, where the acetone molecules stack with an antiparallel orientation of the carbonyl groups. (C) 2009 Elsevier B.V. All rights reserved.