Spectroscopy and dynamics of excited states in maleimide and N-methyl maleimide: Ionic projection and <i>ab initio</i> calculations

Abstract

The state that is responsible for the strong one-photon absorption around 200 nm in the vapor absorption spectrum of maleimide and N-methyl maleimide has been investigated using excited-state photoelectron spectroscopy in combination with ab initio calculations. The projection of the wave function of the excited state on the ionic manifold done in this way reveals multiple, vibrationally resolved, ionization pathways to ground- and excited states of the radical cation, which provide direct evidence for electronic couplings with other, lower-lying states. From a comparison of the experimental intensity distribution over the ionic vibrational states with ab initio calculated Franck–Condon factors, we are able to elucidate the role of the various electronically excited states in the ionization process. The experiments also provide the first determination of adiabatic ionization energies in the two molecules. For maleimide values of 10.330 and 10.903 eV are found for D0 and D1, respectively; for N-methyl maleimide D0 is found at 9.897 or, in an alternative interpretation of the spectrum, at 9.676 eV. Calculations and experiment demonstrate that in this molecule the ground ionic state changes its character with respect to maleimide from a lone pair to a π orbital ionization.