Light and Molecules

Mario Barbatti's Research Group

Inner-shell excitations

Inner-shell spectroscopy

Inner-shell spectroscopy provides a singular way to study the structure of molecules. While visible and ultraviolet spectra strongly depend on the overall structure of the molecular system, the orbitals involved in the inner-shell excitation processes are very localized and strongly resemble the orbitals of isolated atoms. This leads to relatively simple spectra, which have well-defined regions concerning each different element present in the system.

If a molecule has two or more atoms of the same element in non-equivalent positions, changes in the energy spectrum and other molecular properties are expected and, in principle, they may be mapped to establish a relation connecting the different molecular environment to the measured property.

In collaboration with experimentalists from the McMaster University and researchers from the Federal University of Rio de Janeiro, we investigated the inner-shell excitation induced by electron impact of the molecules listed below.

Molecule Excited shells Ref.
CH≡CH (acetylene) C 1s Ref
CH2=CH2 (ethylene) C 1s Ref
N2O (nitrous oxide) N 1s, O 1s Ref
CO2 (carbon dioxide) O 1s Ref
C4H6 (butadiene) C 1s Ref
CS2 (carbon disulfide) S 2p, C 1s Ref Ref

 

Experimental and computed generalized oscillator strength for the the S 2p → 3πu* excitation of CS2.
Experimental and computed generalized oscillator strength (GOS) for the S 2p → 3πu* excitation of CS2.

As illustrated in the figure above, the theoretical results can reproduce the main features of the experiments, which make them a valuable tool for the interpretation of this kind of spectroscopy.

The calculation of the electronic wavefunctions of inner-shell-excited molecules imposes a great challenge and demands complex ab initio methodologies, such as Generalized Valence Bond.

Young-type interference in inner-shell excitations

Experimentalists have often observed oscillations in the GOS profiles as a function of transferred momentum. We showed (Ref) with a simple analytical model that these oscillations are analogous to the interference patterns in Young’s double-slit experiment.

The figure below shows that the GOS oscillations have the same origin as Young oscillations. This implies that the scattered electron behaves as a wave during its interaction with the molecule, as it does when it is propagated through a double slit.

Computed GOS (solid line) compared to the oscillatory function of the Young interference (dotted line) for butadiene.
Computed GOS (solid line) compared to the oscillatory function of the Young interference (dotted line) for butadiene.