Resonance enhanced laser ionisation mass spectrometry of four aromatic molecules

Multiphoton ionisation and fragmentation of aniline, benzene, N,N-dimethyl and 2,4-dimethyl aniline has been studied by laser ionisation mass spectrometry under collision free conditions. All four molecules undergo efficient resonance-enhanced two-photon ionisation (R2PI) with relatively low laser intensities at λ = 266 nm producing the parent ion almost exclusively. At higher intensities, high order processes compete producing extensive fragmentation. At 266 nm, all the major fragment ions are produced by R3PI. For aniline excited at 294 nm, energetic considerations suggest R4P1 formation of fragments with differences in fragmentation between 266 and 294 nm reflecting the differing orders and energies above threshold of the competing processes. Comparison of R2P1 efficiencies in aniline and benzene shows that the cross sections for ionisation of the resonant intermediate ₁B² excited state in both molecules are approximately equal and independent of wavelength in the range 250–300 nm.