  We present a theoretical study of the measurements of photoinduced force
microscopy (PiFM) for composite molecular systems. Using the discrete dipole
approximation, we calculate the self-consistent response electric field of the
entire sample including the PiFM tip, substrate, and composite molecules. We
demonstrate a higher sensitivity for the PiFM measurement on resonant molecules
than by the previously obtained tip-sample distance dependency $z^{-4}$ owing
of the multifold enhancement of the field between the localized electric field
induced at the tip-substrate nanogap and the molecular polarization. The
enhanced localized electric field induced at the tip-substrate nanogap in PiFM
allows high-resolution observation of the forbidden optical electronic
transition in dimer molecules. We investigated the wavelength dependence of
PiFM for dimer molecules and obtained images at incident light wavelengths
corresponding to allowed and forbidden transitions. We reveal that these PiFM
images drastically change with the frequency-dependent spatial structures of
the localized electric field vectors and resolve different types of
nanoparticles beyond the resolution for the optically allowed transitions. This
study demonstrates that PiFM provides multifaceted information based on
microscopic interactions between nanomaterials and light.
