Infrared, or IR, spectroscopy is one type of vibrational spectroscopy, which, as you might have guessed, is a spectroscopic technique where molecular vibrations are analyzed. To fully understand IR spectroscopy, you must first understand the principles of simple harmonic motion.
Imagine two spheres, or masses, connected with a spring. In case you are confused, what you are imagining should look roughly like this.
This is what is known as a simple harmonic oscillator. Once set into motion, the sphere will oscillate, or vibrate back and forth on the spring, at a certain frequency depending on the masses of the spheres and the stiffness of the spring. A sphere with a small mass is lighter and easier to move around than one with a large mass. Therefore, smaller masses oscillate at a higher frequency than larger masses. A very stiff spring, like a bedspring, is hard to deform and quickly returns to it's original shape when the deforming force is removed. On the other hand, a weak spring is easily deformed and takes much longer to return to it's shape. Therefore, a stiffer spring will oscillate at a higher frequency than a weak one. A chemical bond between two atoms can be thought of as a simple harmonic oscillator. The bond is the spring, and the two atoms, or groups of atoms, connected by the bond are the masses. Every atom has a different mass, and single, double and triple all have different stiffnesses, and therefore each combination of atoms and bonds has its own characteristic harmonic frequency. You can learn more about these vibrations and frequencies here.
When an object is vibrating at a certain frequency and encounters another vibration of exactly the same frequency, the oscillator will absorb that energy. Take a guitar string for example. If you were to pluck the G-string and set it "a-vibratin'," it would make that beautiful "G" sound. If you then plucked the D-string while holding it at the fifth fret, it would also make the "G" sound, but if you looked at the strings closely, you would see that not only would the D-string be vibrating, but the G-string would also be vibrating because some of energy from the vibrating D-string was transferred to the G-string, making it vibrate too. This is also true of vibrating molecules, except plucking a G-string won't affect the vibrations of chemical bonds.
At any temperature above absolute zero, all the "eensy-weensy" little simple harmonic oscillators that make up any molecule are vibrating vigorously. Infrared light just happens to be in the same frequency range as a vibrating molecule. So, if you hit a vibrating molecule with some IR light, it will absorb those frequencies in the light which exactly match the frequencies of the different harmonic oscillators that make up that molecule. When this light is absorbed, the little oscillators in the molecule will continue to vibrate at the same frequency, but since they have absorbed the energy of the light, they will have a larger amplitude of vibration. This means that the "springs" will stretch further than before the light was absorbed. The remaining light which was not absorbed by any of the oscillators in the molecule is transmitted through the sample to a detector, and a computer will analyze the transmitted light and determine what frequencies were absorbed.
In the past, it was only possible to get good data by hitting the molecule with only one frequency of IR light at a time. This took a very long time because there are a lot of frequencies and to get a good spectrum, many scans must be taken. But now, thanks to the amazing Fourier Transform Algorithm, you can hit the molecule with every frequency of IR light at once, and get a perfect spectrum in only a fraction of the time! WOW! In case you are curious, here is that amazing Fourier Transform Algorithm.
IR spectroscopy is a very simple analytical technique, and Leslie, who you will notice is clad in all the proper safety attire, including the worlds dirtiest lab coat, is happy to show you how easy it really is. First, you need to put the material to be analyzed in some form that can be put into the infrared spectrometer. This is usually accomplished by casting a film on a sodium chloride (table salt) salt plate, or by grinding the material up with potassium bromide, KBr, and making a pellet out of it. These salts are used because they are invisible to IR light. There are other ways to make a sample, but these are the most common when dealing with polymers. Next, you place the sample into the spectrometer where Leslie is so kind to point out, close the lid, wait a few seconds for the sample chamber to purge of carbon dioxide, press the "SCAN" button on the computer, and VOILA, in less than a minute you have an IR spectrum.