Articles you may be interested inPhoton-driven charge transfer and Herzberg-Teller vibronic coupling mechanism in surface-enhanced Raman scattering of p-aminothiophenol adsorbed on coinage metal surfaces: A density functional theory study A charge-transfer surface enhanced Raman scattering model from time-dependent density functional theory calculations on a Ag 10 -pyridine complex Surface enhanced Raman scattering from a single molecule adsorbed on a metal particle aggregate: A theoretical study Complete analysis of the surface-enhanced Raman scattering of pyrazine on the silver electrode on the basis of a resonant charge transfer mechanism involving three states Two-dimensional localization of adsorbate/substrate charge-transfer excited states of molecules adsorbed on metal surfaces Surfaced-enhanced Ramon scattering ͑SERS͒ spectra of pyrazine are analyzed on the basis of the properties of the electronic states of the metal-adsorbate surface complex. Ab initio CIS calculations have been carried out for the Ag 2 -pyrazine complex, which have enabled us to find two excited singlets, namely CT 0 ; 1 B 1 and CT 1 ; 1 A 2 , with properties quite similar to those of the pyrazine radical anion in its electronic 2 B 3u and 2 A u states, respectively, and with energies falling in the range of the exciting photons usually employed in Raman spectroscopy. SERS spectra of pyrazine are compatible with a resonance Raman enhancement mechanism involving electronic transitions between the ground state S 0 ; 1 A 1 and both CT levels of the surface complex.

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The potential energy surfaces of the ground and valence excited states of both 3H-diazirine and diazomethane have been studied computationally by mean of the CASSCF method in conjunction with the cc-pVTZ basis set. The energies of the critical points found on such surfaces have been recomputed at the CASPT2/cc-pVTZ level. Additionally, ab initio direct dynamic trajectory calculations have been carried out on the S(1) and S(2) surfaces, starting each trajectory run at the region dominated by the conformational molecular rearrangement of diazomethane. It is found that both isomers are interconnected along a C(s)() reaction coordinate on each potential surface. Radiationless deactivation of the corresponding S(1) state of each isomer occurs through the same point on the surface, an S(1)/S(0) conical intersection. Thereafter, the system has enough energy to surmount the barrier which leads to dissociation products (CH(2) + N(2)) on S(0) state. Therefore, photoexcitation to S(1) state of either diazirine of diazomethane produces methylene in its lower singlet state on a very short time scale (ca. 100 fs). Furthermore, both isomers can generate excited singlet carbene when they are excited onto the S(2) surface; in this case, they lose the activation energy passing through another common S(2)/S(1) conical intersection and then proceed to dissociation into carbene and N(2) on the S(1) surface. For the special case of methylene, it rapidly experiences deexcitation to S(0) state.

A general model concerning the role of the electrode potential on the charge-transfer enhancement mechanism of SERS is proposed, justifying the existence of a parameter β which reduces the effectiveness of the electrode potential in order to shift the excited charge-transfer level. This explains the observed deviation of β from the unity, and allows for relating the intensity/electrode-potential (SERS−CT) profiles of similar adsorbates with the respective electron affinities. The SERS of pyridine, pyrazine and methylpyrazines have been recorded on silver, being found that the electrode potential in the maximum of the SERS−CT profiles shifts toward more negative values as the number of methylsubstituents attached to the pyrazine ring increases. This result has been explained on the basis of the relative stability of the radical anions given that a correlation between those shifts and the calculated ab initio vertical transition energies between the neutral molecules and their respective radical anions can be established. This confirms the participation of a resonant charge-transfer mechanism in the SERS as well as the usefulness of this technique to gain insight into the electronic properties of the doublet states of the here studied molecules.

This communication reports, for the first time, the dependence of the SERS intensities under resonant CT conditions (SERS-CT) on the electrode potential. SERS-CT intensities have been estimated from the properties of S(0)-CT(i) transitions ranging between 200-1200 nm of selected [Ag(n)-pyridine](q) and [Ag(n)-pyrazine](q) complexes.

Surface-enhanced Raman spectra (SERS) of pyrimidine recorded on a silver electrode have been analyzed on the basis of a resonant Raman (RR) process involving photoexcited charge transfer (CT) states of the metal-adsorbate surface complex. The main feature of the SERS of benzene and azine derivatives is the enhancement of the totally symmetric ring stretching mode 8a due to Franck-Condon contributions related to the CT transition. Although this behavior is observed in the SERS of pyrimidine, its spectrum is also characterized by the strong enhancement of the nontotally symmetric mode 8b. This peculiar feature can be explained only by the redistribution of the Franck-Condon factors between the 8ab pair of vibrations originated by the descent in symmetry occurring when pyrimidine is bonded to silver nanoclusters. This conclusion is a new evidence of the main role of the RR-CT enhancement mechanism in the SERS of aromatic molecules and shows once again the usefulness of the methodology developed by our group in order to analyze these complex spectra.

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The analysis of photoinduced charge transfer (CT) processes in a particular SERS spectrum is not a trivial task since each system must be studied separately and no universal selection rules can be proposed for this kind of enhancement mechanism. Despite this, we found out in previous studies that the SERS-CT spectra of benzene-like molecules are mainly characterized by the strong enhancement of the totally symmetric 8a ring-stretching vibration and we have used this as a propensity rule to recognize the presence of resonant CT processes in the SERS of these adsorbates. However, it is necessary to cautiously consider the relevant symmetry of the system under study before applying this simplified rule. When the molecule shows a low symmetry (or the relevant symmetry is reduced by the adsorption on the metal as it was demonstrated in the case of pyrimidine), the selective intensification of the 8a mode can be disguised by the redistribution of the CT enhancement with another close vibration in wavenumber, namely the 8b mode, usually a non-totally symmetric fundamental. 3-Methylpyridine is an illustrative example of this given that similar SERS enhancement of both 8a and 8b modes can be quantitatively explained through Franck−Condon factors without having to resort to Herzberg−Teller contributions.