INNER-SHELL EXCITATION AND FRAGMENTATION OF NITROMETHANE CLUSTERS AT THE OXYGEN K-EDGE

Inner-shell excitation spectra and fragmentation of nitromethane (NM) clusters have been studied in the oxygen K-edge region under cluster beam conditions. Besides the protonated clusters (NM)n · H+, we identified the production of series of mixed fragment-cluster cations, [(NM)n · X+, (X = CH3, NO, NO2)] in time-of-flight spectra of the NM 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 "monomer" band revealed that the first resonance ${\rm O}1s \to \pi^*_{\rm NO}$ band of clusters shifts to a high energy by ~ 0.1 eV. The core-excitation spectrum to the antibonding state compared well with theoretical prediction. The band-shift of the first ${\rm O}1s \to \pi ^*_{\rm NO}$ was interpreted as being due to the weak hydrogen-bond interaction formed upon clustering. Fragmentation mechanism of the core-excited NM cluster...