I have observed in case of SERS of an important drug molecule on Ag NPs, in 514 nm laser excitation the higher wavenumber side has high intensity but on changing the laser excitation to 633 nm, the lower wavenumber peaks are enhanced and became almost equal to the higher wavenumber side. In 514 nm laser, lower wavenumber peaks were not even visible, but in 633 nm they all have cropped up. Mostly high wavenumber peaks belong to in plane stretching frequencies and lower ones to those of out of plane vibrations. This may be due to change in orientation w.r.t to plasmonic dipole. Now, my query is:
Has anyone ever come across any such report?
Has anyone ever come across a paper describing plasmonic dipole intercoupling leading to a low energy plasmon?
Have you measured the experimental extinction curve of the AgNPs? Because the SERS effect mainly arises from the plasmon resonances of the nanostructures, it is typically wavelength dependent. A particular SERS substrate will therefore exhibit good enhancement in a limited excitation wavelength range. Probably the SERS substrate you are using shows highest enhancement around 630 nm. Therefore when exciting at 512, the Raman peaks at higher wavelengths are more enhanced while exciting at 633 nm are the peaks at lower wavelengths that are more enhanced. There are several papers showing this effect (eg. J. Phys. Chem. B 2005, 109, 11279-11285; Nanoscale, 2012, 4, 1606)
Thank you for your reply Anna. But my doubt about the exact phenomenon still persists. When we are talking of Raman shift intensity increase that generally means there exists an energy overlap between the dipole of that mode of vibration and the plasmonic dipole. Intensity increase may indicate to the orientation of the molecule on the plasmonic surface. We have observed wavelength dependent change in intensity in lower part of raman spectra and higher part of raman spectra (x axis = wavenumber). How is it possible that a molecule show indications for two orientations just by changing the excitation wavelength? A probable idea may be resonance between excitation radiation and coupled plasmonic dipole. Is there any report seen earlier?
You collect a spectra from a spot much bigger than the nanoscale features therefore you are getting an average of all the orientations of the molecule possible on the substrate. The underlying geometry of the nanoscale template will influence the raman signal. If you have got a well ordered substrate, it will increase the intensity but it will not lead to introduction of new bands which you are seeing due to change of excitation wavelength. The paper suggested by Anna points to that effect.
Thank you Aurelio and Mukul for your reply.
Yes, the instrument was perfectly calibrated.