Spectral series

The spectral series consist of a set of colored lines on a dark background, or of bright stripes separated by dark areas that emit light from all kinds of substances. Spectral series

These lines are visualized with the help of a spectrometer, an apparatus that consists of a prism or a finely divided grating, capable of separating the different components of light.

These sets of lines are called a spectrum and each substance has a characteristic spectrum, a kind of fingerprint that serves to identify its presence in the light that comes from an object. This is because each atom has its own electron configuration and allowable energy levels. Spectral series

That is why finding spectral lines is a technique widely used by astronomers to find out the composition of stars through the light they emit. In fact, everything astronomers know about stars comes from their spectra, be it emission or absorption.

The origin of the spectra Spectral series

The presence of the spectra is due to the atomic configuration. In effect, the electrons are held around the nucleus in regions called orbitals , located at certain discrete distances from it.

For example, in hydrogen, the simplest element, the orbital radii are given by 0.053 ∙ n ² nanometers, where n = 1, 2, 3, 4,…. Intermediate values ​​between these are not allowed, that is why the orbitals are said to be quantized . Also the energy state of each orbital is quantized. Spectral series

Such restrictions cause electrons to behave both as particles and also as waves, just like light. However electrons can go from one orbital to another, changing the energy state of the atom.

Electromagnetic energy absorption and emission Spectral series

For example, if an electron goes from a more internal orbital, with lower energy, to a more external and energetic one, it is necessary that it acquire the necessary electromagnetic energy, which is stored in the atom. This process is called absorption .

On the other hand, if the electron goes from an outer orbital to a more inner one, a photon is emitted in the transition, in the form of light, which is the energy corresponding to the difference in energy between the orbitals. The wavelength corresponds to this difference and is given by: Spectral series

• E is energy
• λ is the wavelength
• h is Planck’s constant
• c is the speed of light

Types of spectra Spectral series

Both absorption and emission spectra are produced, which depend on certain parameters of the object or substance, such as density and temperature. The spectrum of a thin gas is different from that of a solid at high temperature.

Continuous spectrum

Some sources emit spectra whose colored lines change smoothly and contain all colors. This is called a continuous spectrum, for example the one produced by the filament of an incandescent bulb. Spectral series

Emission spectrum

It is what certain hot substances emit and consists of a few lines of a certain wavelength. Spectral series

This type of spectrum is produced by hot, thin gases like those that fill fluorescent tubes. The aurora borealis is another example of emission that occurs in gases in the Earth’s upper atmosphere . Some clouds of interstellar gas also produce emission spectra.

Absorption spectrum

This spectrum is what is received when light from a very hot, dense object is passed through a cooler gas. In it, almost all the colors are observed, but some appear diminished and dark fringes appear in those wavelengths that are absorbed by the atoms or molecules of the gas. Spectral series

Kirchoff’s Laws of Spectroscopy

Kirchoff’s laws of spectroscopy indicate under which conditions the different spectra described above are formed:

1. The continuous spectra: they are emitted by any object at high pressure and temperature.
2. Emission spectra: are produced by a hot gas at low pressure, which emits lines in well-defined wavelengths, corresponding to the electronic transitions corresponding to each element that makes up the gas. Spectral series
3. Absorption spectra: are produced by gases at low temperatures located near sources of continuous radiation. Gas atoms or molecules absorb only certain wavelengths.

The emission spectrum of hydrogen

The emission spectrum of hydrogen is particularly important, since it is the most abundant element in the entire universe and contains a lot of important information about the stars and the Milky Way. Spectral series

The series of lines in the hydrogen spectrum were discovered by various researchers and each bears his name.

Balmer series

Hydrogen emits several lines in the visible spectrum: when the electron decays from orbital 3 to orbital 2 it emits red light, whose wavelength is 656.6 nm, and if it decays from orbital 4 to orbital 2 then it emits blue light of 486.1 nm. Spectral series

In 1885 (before Bohr proposed his theory), the Swiss mathematician and professor Johann Balmer (1825-1898) found by trial and error a formula to determine the wavelengths λ of these lines: Spectral series

Where:

• R is the Rydberg constant: 1.097 × 10 ⁷ m ^-1
• n = 3, 4, 5…., that is, n ≥ 3 (integer).

For example, for n = 3 in Balmer’s equation:

Corresponding to the red line on the right, shown in the figure above. The discovery of the Balmer series led other scientists to search for lines in the rest of the spectrum for hydrogen and other gases.

Lyman series

Note that the spectrum of hydrogen shown in the figure contains some ultraviolet lines, the two on the extreme left, whose wavelengths are 397.0 nm and 388.9. nm. Spectral series

Indeed, these ultraviolet lines correspond to the so-called Lyman series, discovered in 1906 by the physicist Theodore Lyman. Its formula is: Spectral series

Paschen series Spectral series

The Paschen series was discovered by the German physicist Friederich Paschen in 1908 and is valid for n ≥ 4, that is: n = 4, 5, 6 … Spectral series

Paschen’s lines are in the near infrared region and the final level is n = 3, that is, their values ​​occur when the electron decays from higher levels to n = 3. Since the Lyman series is in the ultraviolet, it is concludes that Balmer’s series lies between Lyman and Paschen.

Brackett Series Spectral series

This series, discovered in 1922 by Frederick Brackett, an American physicist, is located in the far infrared and consists of the spectral lines corresponding to the hydrogen transitions starting at n = 5 and continuing: Spectral series

Pfund series

The Pfund series was found in 1924 by the North American physicist August Hermann Pfund and refers to the transitions that start at n = 5, in the far infrared band:

1. Arny, T. 2017. Explorations: An introduction to Astronomy. 8th. Ed. McGraw Hill.
2. Bauer, W. 2011. Physics for Engineering and Sciences. Volume 2. Mc Graw Hill. Spectral series
3. Chang, R. 2013. Chemistry. 11th. Edition. Mc Graw Hill Education.
4. Sears, Zemansky. 2016. University Physics with Modern Physics. 14 th . Ed. Volume 2. Pearson.
5. Windows open to the universe. The different classes of spectra. Recovered from: media4.obspm.fr. Spectral series