Electromagnetic (EM) radiation consists of oscillating electric and magnetic fields that travel in space as waves. As shown in the figure below, these fields are perpendicular to each other and oscillate perpendicular to the direction of propagation. The propagating wave allows the transfer of energy in a vacuum at a rate of 299,792,458 meters per second (usually rounded to 3x10 8 ms -1 ).
According to the theory of wave-particle duality, electromagnetic radiation can act as both waves and particles. Particles of light called photons are massless and have no charge. Each photon has an energy E, given by:
where h = 6.626x10 -34 Js (joule seconds) is Planck's constant and v is the frequency of oscillation of the synchronized electric and magnetic fields.
Photons can also be characterized by their wavelength λ, where λ is the distance between successive peaks, given by:
Different wavelengths of light correspond to different regions of the electromagnetic (EM) spectrum, with wavelengths around 400-750 nm corresponding to the visible spectrum. In the visible spectrum, different wavelengths are observed as different colors, with 400 nm corresponding to violet light and 750 nm corresponding to red light.
Spectroscopy generally deals with light in the ultraviolet (UV), visible, and near-infrared (NIR) regions of the electromagnetic spectrum. The spectrum covers the UV range from 10 nm to 400 nm and the NIR light from 750 nm to 1,400 nm.
The UV range can be further divided into several subcategories. Closer to the visible light region - and undoubtedly the most famous - are the UVA (315-400 nm) and UVB (280-315 nm), which are able to penetrate the Earth's ozone layer and reach its surface. UVC (100-280 nm) is almost completely absorbed by the Earth's atmosphere and only a very small amount reaches the Earth's surface. Ultraviolet radiation with a wavelength of 10-200 nm can only travel in a vacuum and is therefore known as vacuum ultraviolet or VUV.
Spectroscopy can also be performed at wavelengths outside this range; for example, in the X-ray or microwave regions.
Electron shells and orbitals
In atoms, electrons exist in electron orbitals within electron shells. According to Aufbau's principle, electrons need to start with the lowest energy orbitals and fill them in order of increasing energy. The number of electrons in each atom depends on the atomic number of the element involved, i.e. the further down the periodic table an element is, the more electrons it has. For example, a carbon atom with atomic number 6 has 6 electrons.
The lowest energy state of an atom or molecule is the state in which all occupied shells are filled; each orbital in each occupied shell has two electrons, and all high-energy shells are empty. Therefore, in order to reach the lowest energy state, atoms with partially occupied shells will form bonds with other atoms to form compounds with fully occupied shells.
All atoms and compounds have their own electronic structure, which results from the electron energy levels allowed within the material. It is this electronic structure that is studied using spectroscopy.
Absorption and Emission of Photons
When a material absorbs a photon, an electron is lifted from a lower energy level to a higher energy level, for example from its ground state E0 to its first excited state E1. Instead, photons are emitted when electrons relax from a higher energy level to a lower energy level. In both cases, the wavelength Eph of the photon will be related to the energy gap between the two energy levels according to E=hc/λ (see electromagnetic radiation). These are called electronic transitions, and the energy gap between the two energy levels usually corresponds to a photon in the ultraviolet or visible region of the electromagnetic spectrum.
Absorption and emission of photons also occurs as electrons transition between vibrational and rotational states. Vibrational transitions in molecules occur due to bending and stretching of bonds, and these transitions involve photons in the near-infrared and mid-infrared parts of the electromagnetic spectrum. In this case, the transition will occur within a single electronic state. Rotational transitions involve photons in the far infrared (lower energy) and are due to quantized changes in the molecular angular momentum. Rotational transitions occur within the same vibrational state (except rotational vibrational transitions).
