The simple answer the longer wavelenght the smaller chance for liuminescence. For the specific molecule one has to know the absorbtion spectra.

Justinas is correct. However, it may also be possible to use a UV source for the Raman, such that the Stokes shifted Raman is still higher energy than any fluorescence. However, it depends on the PAH!

Thanks for the information. I was looking at the longer wavelength laser but then ran across some interesting articles comparing UV lasers to some of the longer wavelength lasers. However the PAHs in the article were considerable smaller than the ones I typically study.

I will be visiting different Raman vendors and checking out some of my PAH samples to see which lasers work with the majority of my samples.

If you use a deep UV wavelength (UVC ) then you will probably find that the fluorescence is significantly red-shifted from the excitation line (maybe 10 nm) and that the interesting Raman scattering occurs in the gap between the excitation and fluorescence. You will also get a huge resonance enhancement in cross-section.

See for example:

Erik D. Emmons, Ashish Tripathi, Jason A. Guicheteau, Augustus W. Fountain, III, and Steven D. Christesen, “Ultraviolet Resonance Raman Spectroscopy of Explosives in Solution and the Solid State,” J. Phys. Chem. A 117, 4158-4166 (2013).

Steven D. Christesen, Jay Pendell Jones, Joseph M. Lochner, and Aaron M. Hyre, “Ultraviolet Raman Spectra and Cross-Sections of G-series Nerve Agents,” Appl. Spectrosc. 62(10) 1078-1083 (2008).

L.C. Pacheco-Londono, W. Ortiz-Rivera, O.M. Primera-Pedrozo, S.P. Hernandez-Rivera. ‘‘Vibrational Spectroscopy Standoff Detection of Explosives’’. Anal. Bioanal. Chem. 395(2), 323-335 (2009)

Augustus W. Fountain III, Steven D. Christesen, Raphael P. Moon, Jason A. Guicheteau, and Erik, D. Emmons, “Recent Advances and Remaining Challenges for the Spectroscopic Detection of Explosive Threats,” Appl. Spectrosc. 68(8) 795-811 (2014).

If you want to see an example of a narrow linewidth single peak DUV excitation laser then the following may be of interest. https://www.spiedigitallibrary.org/conference-proceedings-of-spie/9304/93040T/A-six-color-four-laser-mobile-platform-for-multi-spectral/10.1117/12.2077999.short

This is a 250 nm pulsed solid state laser with < 20 GHz linewidth.

Try FT-Raman if you have an access.

Thanks everyone. I have tried FT-Raman and it is great for these compounds. Unfortunately FT-Raman is not a good fit for my application.

The 1064 nm laser did great with the samples, but it is costly.

With larger PAHs you are always going to run into fluorescence issues... In general 514/520/532 nm works pretty well for most, 630 nm will be an option for some others... whereas up to 785 or 1064 nm might be an even better choice ... but ultimately there isn't going to be a universal solution just by choosing the wavelength... you probably want a few options on hand but then you run into different requirements for the gratings, detectors, etc. ... going to lower wavenumbers may give rise to some interesting problems; 405 nm should make a lot of them fluoresce but can actually lead to enhancements in signal since there can be resonance Raman occurring for PAHs... I'd steer clear of UV-Raman in general, unless you have a specific need... the signal can be more intense, but the photons at 248 nm (~ 5eV) are sufficient to start inducing bond cleavages, and the spectral resolution down there will start to become an issue... no fluorescence though. ... A better option is to use picosecond gated Raman... I don't know many commercial companies that do this (https://www.timegate.com/ is one)... but it isn't that difficult to build your own Raman system.