All pages in this lab

1. Laser Induced Fluorescence and Raman Scattering
2. Hologram Development Procedures
3. Laser Startup Instructions
4. Optics and the Spectrometer
5. Troubleshooting

Absorption

First of all

Are you getting anything at all? Put your hand in front of the slit to block the light and see if this makes any difference. Try this on a very small scale on the strip chart recorder (i.e. 60 mV) If not, then most likely something is either turned off or disconnected. Check all your power switches and connections.

Note: just because the power supply to the PMT is turned on does not mean that it is outputting the power that you have set. Check the display to make sure it is actually working. You must switch on BOTH switches on the front panel.

Lamp/Optics

• Is the filament of the lamp focused on the spectrometer slit or reasonably close to it? It should be simple to do this by placing one 6 cm focal length lens between the lamp and the iodine tube.
• Are the spectrometer slits, iodine tube, lamp, and lens all approximately the same height from the table?

The Photomultiplier Tube

• Is the same tube connected to the power supply that is on, the spectrometer, and the strip chart recorder?
• Try turning up the voltage to the PMT. It should be somewhere between 800 and 1300 volts to get a good spectrum.
• = STOP = The staff Will Do This for You! Make sure that the tube is secure in its case; if the core of the tube is facing the wrong direction, you will not get enough signal! See DON or Professor to remove the tube from the back of the spectrometer and look through the window. You should see a bunch of thin, curved pieces of metal. If you don't, it may be facing the wrong direction. Loosen the screw at the edge of the case and turn it until you see these curves of metal. Then tighten the screw to secure it. (Check everything else before you remove the PMT from the back of the spectrometer because doing so may misalign the spectrometer slits!!)

The Spectrometer

• Is the spectrometer set to "high blaze"? And, is it set in the visible region? The highest signal should be observed around 530-550 nm. If you are not getting a signal, search in this region.
• Are the slits (entrance and exit) aligned? Both slits should be horizontal--that is, parallel to the table. If the slits are misaligned, you will have bad resolution but you should still see some signal. If you think that this is the problem that you have, ask for help aligning the slits with the HeNe laser.
• Are the entrance and exit slits the same size? (10 microns is good for absorption.) Again, this would give you resolution problems but would not ruin your signal altogether.

Fluorescence

Laser/Optics

• Is the laser on and reaching at least 0.8 Watts power?
• Is the laser beam passing through the iodine cell? You should be able to see a green line through the window in the cell.
• Is the window in the cell facing the spectrometer?
• Is the laser line focused on the spectrometer slit? You should be able to see a bright green line in good focus either with a piece of white paper in front of the slit or on the slit with white paper taped inside of it.
• Are the spectrometer slits, iodine tube, and lens all approximately the same height from the table?

The Photomultiplier Tube

• Is the same tube connected to the power supply that is on, the spectrometer, and the strip chart recorder?
• Try turning up the voltage to the PMT.
• Make sure that the PMT is secure in its case (See absorption section.)

The Spectrometer

• Is the spectrometer set to "high blaze"? And, is it set near the laser line? You will have to turn the wavelength knob to find the laser line because the monochromator is not perfectly calibrated. Scan through a few nanometers on each side of 514.5 nm.
• Are the slits (entrance and exit) aligned? Both slits should be horizontal--that is, parallel to the table. If the slits are misaligned, you will have bad resolution but you should still see some signal. If you think that this is the problem that you have, ask for help aligning the slits with the HeNe laser.
• Are the entrance and exit slits the same size? (50 microns is good for fluorescence.) Again, this would give you resolution problems but would not ruin your signal altogether.

Raman Scattering

Laser/Optics

• Is the laser on and reaching at least 1.0 Watt power?
• Is the laser beam passing through the Raman cell? You should be able to see a green line through the window in the cell.
• Is the laser line focused on the spectrometer slit? You should be able to see a faint green line in good focus either with a piece of white paper in front of the slit or on the slit with white paper taped inside of it if you cover the whole apparatus with the wooden shields and black cloth.
• Are the spectrometer slits, laser line, and lenses all approximately the same height from the table?
• Are the slits, Raman cell, and lenses all centered on the optical axis? Use a meter stick to align their centers side-to-side. Also, make sure that the optical axis is perpendicular to the Raman cell axis for the best signal. The spectrometer should be about 9.5 cm from the edge of the optical bench. (Match up line drawn on the front of the spectrometer to one drawn on the optical bench.)
• Are the lenses approximately 10 cm from the Raman cell and the spectrometer slit respectively? Are the flat sides of the lenses facing the Raman cell and the spectrometer respectively?
• Try covering the sides of the Raman cell window with black tape. You could be getting lots of laser light scattered off of metal surfaces which can also change its wavelength. If this is happening, the laser line will spread out and hide the Raman lines.

The Photomultiplier Tube

• Is the same tube connected to the power supply that is on, the spectrometer, and the strip chart recorder?
• Try turning up the voltage to the PMT. It should be around 1100 V. If it gets much above 1200 V, you will get too much noise to distinguish the lines.
• Make sure that the PMT is secure in its case (See absorption section.)

The Spectrometer

• Is the spectrometer set to "low blaze"? And, is it set near the laser line? You will have to turn the wavelength knob to find the laser line because the monochromator is not perfectly calibrated. Scan through a few nanometers on each side of 514.5 nm.
• Are the slits (entrance and exit) aligned? Both slits should be horizontal--that is, parallel to the table. If the slits are misaligned, you will have bad resolution. The laser line will spread out and hide the Raman lines. If you think that this is the problem that you have, ask for help aligning the slits with the HeNe laser.
• Are the entrance and exit slits the same size? (50 microns is good for Raman scattering.) This would give you resolution problems, which is crucial in Raman scattering.

Sign and turn this into the 111-Lab Staff and complete your laser Quiz