Molecular size effect on the site-specific fragmentation of N and O K shell excited CH3OCOCN and CH3OCOCH2CN molecules

The effect of molecular size on the site-specific fragmentation was investi gated by exciting the N and O K shells of methyl cyanoformate (CH3OCOCN) an d methyl cyanoacetate (CH3OCOCH2CN). The fragmentation patterns were essent ially identical in the N and O core excitations of CH3OCOCN. Site and state dependent fragmentation was clearly observed in the core hole CH3OCOCH2CN: The fragment ions mainly observed at the O K edge excitation were CH3+, HC O+, CH2CN+, CH3OCO+, and COCH2CN+. At the N(1s) excitation the large CH3OCO + and COCH2CN+ fragments were extremely depressed, and alternatively the sm all CH3+, HCO+, and CH2CN+ ions were dominant. Especially at the pi*(CN)<-- N(1s) resonance transition the N+ and CO+ fragment ions were exclusively pr oduced with yields of 40 and 15%, respectively. The total photoabsorption c ross sections were measured in the N and O K regions and the observed peaks were tentatively assigned. Reflectron type time-of-flight mass spectrometr y was employed to attain the mass separation M/Delta M greater than or equa l to 100 at M/e approximate to 100 for the core excited molecules ejecting fragments with considerable kinetic energies. (C) 2000 Elsevier Science B.V . All rights reserved.