Molecular Orbital Insight into the Near-Threshold Photoionization Cross Sections of Monocyclic Substituted Aromatic Compounds

Near-threshold
photoionization cross-section (PICS) is a key parameter
for detecting and quantifying combustion intermediates in synchrotron
vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) experiments.
In the present study, the PICSs of 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene,
1,3,5-trimethylbenzene, o-cresol, m-cresol, p-cresol, anisole, and guaiacol were measured
by PIMS. By collecting the PICSs of a series of monocyclic substituted
aromatics (with hydrocarbon, oxygen-containing and nitrogen-containing
substituents) measured in this work and those reported in literature,
the similarity and difference of the PICSs of them is discussed from
the view of molecular orbital analysis. It reveals that the frontier
HOMO and HOMO–1 of these substituted benzene compounds mainly
share the same characteristics with the degenerated HOMO of benzene.
Nevertheless, the PICSs of benzene with hydrocarbon substituents are
proportional to the photon energy after the threshold, while that
of benzene presents a “plateau” region. The PICSs of
substituted benzene where the substituents contain oxygen are more
complicated than those with hydrocarbon substituents. For nitrogen-containing
substituted benzene, cyano groups enhance the plateau characteristics
of benzene, while the characteristics of amino-substituted benzene
are similar to those of benzene with hydrocarbon substituents. The
PICS data reported in this paper provide important parameters for
SVUV-PIMS experiments. As a bridge between electronic structure and
PICS, the molecular orbital analysis reveals the source of PICS characteristics
of monocyclic substituted aromatic compounds.