LAB TECHNIQUE - INTRODUCTION TO SPECTROSCOPY
Spectroscopy & Electromagnetic Energy
Spectroscopy is the study of the interaction of electromagnetic energy (sometimes called radiant energy) and matter. The electromagnetic spectrum diagram is shows the different forms of electromagnetic energy as a reference.
When molecules or atoms of a sample are exposed to a form of electromagnetic radiation, the radiation might just bounce off the particles. But in many cases, some of the radiation is absorbed by the element or molecule. The remainder of the energy of the radiation is emitted. The wavelength at which energy is absorbed gives us information about the molecules and atoms that make up the sample.
Examples of Spectroscopy
Photoelectron Spectroscopy (PES)
When x-ray radiation is shone at a substance, it has so much energy that it will remove electrons. The particular wavelength of x-ray used to remove each electron within the substance gives scientists an idea of the number of electrons in shells and how tightly they are held to the nucleus and thus, the identity of an element. This is known as photoelectron spectroscopy (PES).
When white visible light is shone on certain colorful molecules or transition metal ions, electrons in the bonds (or in the d sublevels of the transition metal ions) can be excited. Since visible light is lower energy than x-rays, the electrons are not removed from the atom, but rather the valence electrons only are excited to a different state. Since, electrons in atoms AND in molecules have quantized states, only certain wavelengths of light will be asborbed.
The other wavelengths will bounce off giving a certain “color” to the molecule or ion. Molecules with extensive alternating double and single bonds (aka delocalized electrons/conjugation/resonance), often absorb visible light. Molecules without this extensive delocalization tend to absorb wavelengths at much higher energies and therefore absorb light in the ultraviolet region, but reflect all visible light and, therefore, appear colorless to us. Using the wavelength of light absorbed within the UV or visible region of the electromagnetic spectrum to identify a molecule is known as UV-Visible spectroscopy. (UV-VIS)
In the less energetic regions of the electromagnetic spectrum (infrared radiation and microwave radiation), the radiation doesn’t have enough energy to excite electrons and certainly not enough to remove electrons. Instead, energy at certain wavelengths of infrared or microwave can be absorbed and cause a particular bond in a molecule to have a different vibrational state. Microwave ovens cook by causing water molecules to rotate. However, since the wavelength is on the order of millimeters, the water must be in objects at least that size. Ants and rice are largely unaffected by microwaves because they’re too small. We tend to think of bonds as being rigid, but in reality, the atoms and electrons have a certain amount of kinetic energy. The atoms on either side of a bond can bounce closer and further away from each other. If there are several bonds near each other in a molecule, the atoms can even scissor. See this link for an animation that illustrate this rotation & vibration: UC-Davis Introduction to Infrared Spectroscopy
Each of these vibrations has a specific energy. Therefore, if the molecule switches from one vibrational state to another state, a specific amount of energy is absorbed or released, and this is usually in the infrared region of the spectrum. Further, the amount of energy absorbed depends on the order of the bonds and the atoms in the bonds. So, chemists can measure at what wavelengths energy is absorbed and thus the region of the electromagnetic spectrum to determine what types of bonds or functional groups are present. Carbon dioxide, absorbs energy at 4 micrometers and 15 micrometers (in the infrared part of the spectrum), and does not allow these wavelengths to escape into space, effectively heating up our atmosphere. Other greenhouse gases absorb different, specific wavelengths of infrared depending on their particular molecular structure. Measuring the wavelengths that are absorbed within the infrared spectrum can help identify a compound and/or determine its molecular structure and is known as infrared spectroscopy. (IR)