IR double resonance experiments with size selected clusters for identification of isomers

Infrared photodissociation spectra of (CH₃OH) _n clusters (n=2, 3 and 6) and the mixed dimer C₂H₄ · CH₃COCH₃ are presented. The clusters are generated in a supersonic jet expansion and size selected by scattering from a helium atomic beam combined with mass spectrometric detection. Continuous CO₂-lasers are used to vibrationally excite the molecules in the cluster leading to rapid dissociation of the complex. Various dissociation peaks that are found in single-laser dissociation spectra can be assigned unambigously in a pump-probe experiment with two lasers to either different isomers (acetone-ethene dimer) or splitted lines of one isomer (methanol hexamer). For size distributions, the method is able to select contributions of single masses which is demonstrated for mixtures of methanol dimers and trimers.