  Surface Enhanced Infrared Absorption (SEIRA) is an experimental method where
trace amount of a compound can be detected with high sensibility. This high
detection sensibility is the result of the interaction of the molecules with a
localized plasmon, usually from a metallic nano-particle. In this study we
numerically investigate by discrete dipole approximation the origin of the
Fano-like response of the system, including the induced transparency when the
plasmon resonance and the molecular vibrational mode coincide. The detailed
analysis of the localization of the absorption show that the modification of
the absorption cross-section when the molecule is present comes from a change
of the plasmonic resonance, not from the direct molecular response which is
negligible. This sheds a new light on the SEIRA mechanism. In particular, it
demonstrates that the sensibility is associated with the influence of the
molecule on the plasmon resonance rather than with the local field enhancement
itself.
