  Surface-enhanced infrared absorption (SEIRA) spectroscopy exploits the field
enhancement near nanophotonic structures for highly sensitive characterization
of (bio)molecules. The vibrational signature observed in SEIRA spectra is
typically interpreted as field-enhanced molecular absorption. Here we study
molecular vibrations in the near field of single antennas and show that the
vibrational signature can be equally well explained by field-enhanced molecular
scattering. Although the infrared scattering cross section of molecules is
negligible compared to their absorption cross section, the interference between
the molecular-scattered field and the incident field enhances the spectral
signature caused by molecular vibrational scattering by 10 orders of magnitude,
thus becoming as large as that of field-enhanced molecular absorption. We
provide experimental evidence that field-enhanced molecular scattering can be
measured, scales in intensity with the fourth power of the local field
enhancement and fully explains the vibrational signature in SEIRA spectra in
both magnitude and line shape. Our work may open new paths for developing
highly sensitive SEIRA sensors that exploit the presented scattering concept.
