Symmetry-specific electron-phonon coupling for electronic states near the Fermi energy of metallic polyaniline: resonant Raman scattering

The resonance Raman excitation profiles of the vibrational modes of the emeraldine salt form of polyaniline protonated with camphor sulfonic acid are remarkably and selectively enhanced, indicating improved structural and electronic order. We demonstrate that the electronic states near the Fermi energy (E_F) of metallic polyaniline interact with a specific optical phonon mode. The 1598 cm^-1 Raman-active vibrational mode (A_g symmetry) exhibits a distinct resonance enhancement associated with the mid-infrared (mid-IR) absorption in metallic PANI. Since the mid-IR oscillator strength results from the intraband free-carrier Drude absorption, the symmetry of the electronic wavefunctions near E_F matches the vibrational pattern of the 1598 cm^-1 normal mode. In contrast, the same mode shows no resonance enhancement with the interband absorption at 2.6 eV, implying that the symmetry of the bond relaxation of the excited state is orthogonal to the vibrational pattern of the 1598 cm^-1 mode. By contrast, the resonance Raman excitation profile of the 1626 cm^-1 mode is in strong resonance with the 2.6 eV absorption, but this mode is not as strongly in resonance with the free-carrier Drude absorption.