Problems with Raman Spectrometer

[pfollansbee]

I'm currently updating an old raman spectrometer that had fallen out of alignment requiring some modifications to its design.

I have attached an image of what it currently looks like. I am using a HeNe laser which is directed into a bifurcated fiberoptic. The combined end is pointed at my sample to hit it with the laser and collect its emission. The other end is pointed at a lens which then enters a double monochromater before hitting a PMT. The PMT is hooked into a

This setup was optically aligned using an LED on a fluorescent compound. Fluorescent peaks were observed, so the machine is working fine. I am also getting plenty of excitation from the laser.

So here is my dilemma, why am I not seeing raman peaks? I am just seeing a flat line, no structure at all, when I should see... http://www.chem.uAlberta.ca/~mccreery/images/Photo raman intensity standard/polystyrene.jpg

Things that have been tried
1. Slits are WIDE open
2. Moved fiber optic away from sample to avoid reflections
3. Angled sample to omit direct reflections
4. Checked and rechecked wavenumbers for correct range

Ideas
1. Filter to remove laser light.
-I am hesitant to use this though because I don't want to block out my raman signal.

or

2. Focusing optics at the joined end of the fiber optic between it and the sample.
-This may be the issue because the fiber optic is acting like a point source rather than a beam.

So what do you guys think? Are fiber optics a bad idea for transmitting laser light? Would focusing optics do the trick? Just needs a filter?

Thanks! :D

 

[chemisttree]

Filter(s) would be my first choice.

 

[pfollansbee]

Dunno if anyone will ever read this... but I discovered what my problem was. The fiberoptics were a bad idea. They spread out the light too much for a good raman signal. I am thinking that in order to use fiberoptics for raman, it would be necessary to have a very small FO to preserve beam diameter and power concentration.

The double monochromator we have has been sufficient to block out any unwanted light from our sample, so (thankfully) a raman signal was able to be seen without the need for filters. Unfortunately, the samples I am working with are weird and fluoresce under HeNe excitation.

 

[aoner]

It's perhaps good to have a look at how your laser looks like. Maybe it has a broad excitation wavelength so what you are seeing is a reflection of the laserlight and not fluorescence. You could think about buying a line filter to remove these unwanted wavelengths.

 

[pfollansbee]

Naw, it's definitely luminescence. I tried other samples and only got raman signal. And I am operating a HeNe laser at 632.8nm. I also know where all the plasma lines are, so no issues, just a weird sample.

 

[aoner]

What kind of samples are you scanning?

 

[pfollansbee]

Lanthanides with full coordination spheres embedded in metal cyanide chains.

 

[aoner]

Do you have some more info on your setup? Is your current problem maybe that the light from the laser is too high and that the raman signal is just to weak compared to that. You could use a dichroic or beam splitter to reflect the laser light at one angle and remove it at another. If your laser power allows it you should be able to have plenty of Raman intensity.

 

[pfollansbee]

Like I said earlier. It's working now, I just need a different wavelength laser. I am currently looking at higher wavelength diode lasers because I'm interested in upconversion as well.