Inner-shell excitation and fragmentation of small acetaldehyde clusters at the oxygen K-edge

Inner-shell excitation spectra and fragmentation of acetaldehyde (AA) clusters have been studied in the oxygen K-edge region under cluster beam conditions. We identified intense production of a series of protonated clusters (AA)nH+, in the time-of-flight spectra of the AA clusters. Cluster-specific excitation spectra could be generated without contribution of free molecules by monitoring partial-ion-yield spectra of the fragments originating from the clusters. Comparison of the cluster-specific band with the molecular band revealed that the first resonance O1s→π*CO band of small AA clusters shifts to a high energy by ∼0.15 eV relative to the free molecule. The O1s→π*CO band of small clusters was compared well with the theoretical prediction derived for the most stable dimer configuration. The band-shift of the O1s→π*CO was interpreted as being due to the augmentation of HOMO–LUMO energy gap upon complexation where a hydrogen bonding interaction could be identified.