Surface-Enhanced Raman Scattering Due to Charge-Transfer Resonances : A Time-Dependent Density Functional Theory Study of Ag13-4-Mercaptopyridine

• We have used time-dependent density functional theory in conjunction with the CAM-B3LYP functional and MWB28/aug-cc-pVDZ basis set to determine non-, near-, and on-resonance Raman spectra for a complex formed by 4-mercaptopyridine (4-Mpy) binding with a Ag13 cluster via the thiolate Ag-S bond. Geometry optimizations of the Ag13-4-Mpy complex showed an on-top structure directly bound to one Ag atom with the ring of the molecule almost flat with respect to two Ag atoms of the complex. The corresponding B3LYP/MWB28/aug-cc-pVDZ geometry is also an on-top structure directly bound to one Ag atom, but the molecule is directed away from the surface. The near-resonance Raman calculations were carried out in the infinite lifetime approximation, while the on-resonant Raman excitation profiles were calculated with the complex polarization propagator (CPP) approach, introducing a half width at half-maximum spectral broadening of 0.2 eV. Calculation of the UV-vis spectra of the isolated 4-Mpy and of the Ag13-4-Mpy complex showed that binding shifts the spectra from deep in the UV to the visible region. Calculation of the near-resonance Raman spectra of the two structures of the complex at 410 (3.025 eV) and 425 nm (2.918 eV) showed a strong enhancement. A very large variation across vibrational modes by a factor of at least 103 was found for both the static chemical enhancement and charge-transfer (CT) enhancement mechanisms. This large variation in enhancement factor indicates that B-term Herzberg-Teller scattering is occurring because inactive or very low intensity modes in the static spectra of the molecule are much stronger in both the static and near-resonance spectra of the complex. From the excitation profile using the CPP method, an overall surface enhancement on the order 103 or higher was found for individual modes on excitation into a CT excited state. 

Ämnesord

NATURVETENSKAP  -- Kemi -- Teoretisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Theoretical Chemistry (hsv//eng)

Nyckelord

Atoms

Bins

Charge transfer

Excited states

Molecules

Raman scattering

Resonance

Surface scattering

Chemical enhancements

Complex polarization propagator

Enhancement mechanism

Geometry optimization

Half width at half maximums

Resonance Raman spectra

Surface enhanced Raman Scattering (SERS)

Time dependent density functional theory

Density functional theory

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