Lets say we have a substrate without any background noise which is coated plasmonic nanomaterials (Au or Ag N.P) just like the schematic image below. Since In Raman measurements 3 lasers with different wavelengths (532nm/633nm/780nm) can be used, based on absorption wavelength of both organic analyte and plasmonic nanoparticles, Does absorption of Light by N.P can contribute to the SERS enhancement? which of the following conditions is ideal for SERS measurement and can give us better peak enhancement ?
Condition 1: wavelength of laser should be close to absorption wavelength of Analyte
Condition 2: wavelength of laser should be close to absorption wavelength of Plasmonic Nanoparticles
Condition 3: it is better that both analyte and N.P have similar absorption wavelength
Condition 4: There is no overlap of absorption wavelength of analyte and N.P
Yes, irradiating with the plasma frequency of the used enhancing material should increase the effect.
If you additionally hit the absorption frequency of the analyte, you should get resonance Raman which would also enhance the effect, but that's of course a rare coincidence.
Be careful to know the photophysical properties of your "organic analyte" well. Fluorescence is a far more probable process than the stokes Raman scattering for many organic molecules and it will potentially drown out any meaningful SERS data.
Higher wavelength lasers (785 nm) are often chosen for SERS for this reason, particularly avoiding being on resonance with the analyte of interest.
Ultimately it depends entirely on the analyte you are interested in, but you should be able to get good SERS data from simply choosing a wavelength that has resonance with your plasmonic material and not your analyte at all.
For better SERC enhancement, condition 2 is a good option. However analyte absorption should not be very far away. However, for recording Raman spectra one should be careful in selecting the wavelength to avoid over lap with fluorescence spectra of analyte.