Recording emission Spectra

3. Recording emission Spectra

The Editor

Research grade FTIR is usually have an emission port (although this may be occupied by an FT Raman accessory if you are lucky enough to have one). Removing some sort of plug or cover and usually swinging away a mirror inside can access the port . However, there is a snag – you need a collimated beam as you will see in Figure 1. Bill suggested replacing the source by the emitting sample but this is usually very difficult to do. Illuminating the interferometer through the emission port is optically identical to replacing the source. Where do you go from here?

Figure 1.

If you are wealthy enough to have an emission accessory or can beg, steal of borrow one to try the technique all is well, but most of you just want to try the method to see if there is anything in it for you. So just as most of us paint our houses because we cannot afford to have someone do it for us. I suggest a little bit of “do-it-yourself”. To make a cheap, functional emission accessory – see Figure 2 – you need two mirrors, ideally M, they should be an off-axis paraboloid but a front surface aluminised shaving mirror quality spherical will do just fine. M2 is plane and must again be front surface aluminised. Window glass quality is fine†.

Figure 2.

† Cheap optics are not difficult to buy but if you have trouble contact Comar Instruments on +44 (0)1223 245470, ask for Mr Marsh. Either of the two gentlemen of this name will sort you out with advice and mirrors and not charge an arm and a leg.

To support the mirror and sample, I recommend you find a piece of steel (3mm thick is fine but thicker is no problem). Most workshops can help with this. Find a lab jack and for safety screw the bottom to a wooden board and the top to the steel sheet. If you find the latter is beyond you use contact adhesive. Now – engineering suppliers sell magnetic bases for “dial test indicators” and “marking out” accessories. I draw one in Figure 3. Buy two of them and mount the mirrors as I indicate. The whole set-up shouldn’t cost more than $100.

Figure 3.

If your instrument has a laser spot on axis lining up the accessory is easy but if not you will have to be a little cleverer. To do so – place M1 by eye, similarly M2. Cover the instrument detector, switch on the source and put a strip of “magic tape” or tracing paper at the normal transmission sample point. Operate the instrument as a transmission machine and mark the patch of light on the tape or paper with a pencil. Using a small torch bulb find a position where the light from the bulb goes through your home built accessory and the interferometer and illuminates centre of the mark on the white screen. Now adjust all the mirrors to get a good optical path and note the position of the bulb filament. This will be your sample point. Uncover the detector, put a beaker containing hot water at the sample point and carefully tweak the mirrors to give you maximum energy. You are now ready to proceed except for one little problem – moist air. The beam splitter in your instrument will suffer if it is exposed to the atmosphere . However, a few minutes exposure will cause no damage. Find some thin polythene sheet and some masking tape and cover your accessory carefully so that it is as sealed as you can make it. Now purge your instrument and the accessory will be purged as well.

In the tropics be VERY careful. IF the atmosphere is hot and humid and you do not have air conditioning DO NOT open the interferometer at all. 


What to do if you have no emission part or the one you have is occupied? There is an alternative method which will fit into the transmission sample area. As you will see in Figure 4, the beam passing through an interferometer goes forwards and backwards i.e. 50% passes through the device and 50% returns to the source. Although not as efficient as replacing the IR source by your sample, you can exploit this property of interferometers.

Figure 4.

In Figure 5 I show you how. The emitted radiation passes into the interferometer and the returned radiation goes to the detector.

Figure 5.

Obviously, half the exit J-stop is used to illuminate and half to view the interferometer. There are two problems with the method and both are experimental and simple to solve.

a) Mirror M2 must not cast a shadow due to its thickness so it must have a bevelled edge and

b) The mirror has to be very accurately positioned and angled.

The problem is that interferometer have a real entrance J-stop (piece of metal with a hole in it) but a virtual exit J-stop i.e. the exit J-stop is the reverse projected image of the detector – not a great help when you are trying to set up a do-it-yourself accessory. Proceed as follows.

Set the sample stand at the focus of the instrument. Put a strip of magic tape or tracing paper over the cell holder as suggested above and carefully mark with a pen, the position of the illuminated patch. Using a piece of steel as a base as above and magnetic bases, move the mirror M2 until it cuts ½ of the patch. Illuminate M1 with the light from the interferometer , adjust M1and locate the emission sample point.

Remember – before you make a measurement, turn off the normal IR source. You will find the measurements are not difficult to make especially if a cooled detector is used and the results can be very interesting indeed.

Good Luck

REF: P.J. Hendra. Internet J. Vib. Spec.[] 5, 2, 3 (2001)