WHAT IS THE EMISSION SPECTRUM? (WITH EXAMPLES) - PHYSICAL - 2024

The emission spectrum is the spectrum of wavelengths of light emitted by atoms and molecules when making a transition between two energy states. The white light or visible light striking a prism breaks down into different colors with specific wavelengths for each color. The pattern of colors that is obtained is the visible radiation spectrum of light called the emission spectrum.

Atoms, molecules, and substances also have an emission spectrum due to the emission of light when they absorb the right amount of energy from outside to transit between two energy states. By passing this light through a prism, it breaks down into spectral colored lines with different wavelengths specific to each element.

The importance of the emission spectrum is that it allows determining the composition of unknown substances and astronomical objects through the analysis of their spectral lines using emission spectroscopy techniques.

The following explains what the emission spectrum consists of and how it is interpreted, some examples are mentioned and the differences that exist between the emission spectrum and the absorption spectrum.

What is an emission spectrum?

The atoms of an element or a substance have electrons and protons that are held together by the electromagnetic force of attraction. According to the Bohr model the electrons are arranged in such a way that the energy of the atom is the lowest possible. This energy energy level is called the ground state of the atom.

When the atoms acquire energy from the outside, the electrons move towards a higher energy level and the atom changes its ground state to an excited state.

In the excited state, the residence time of the electron is very short (≈ 10-8 s) (1), the atom is unstable and returns to the ground state, passing through intermediate energy levels, if it is the case.

Figure 1. a) Emission of a photon due to the transition of the atom between the excitation energy level and the fundamental energy level. b) emission of photons due to the transition of the atom between intermediate energy levels.

In the process of transition from an excited state to a ground state, the atom emits a photon of light with energy equal to the difference in energy between the two states, being directly proportional to the frequency and inversely proportional to its wavelength λ.

The emitted photon is shown as a bright line, called the spectral line (2), and the spectral energy distribution of the collection of emitted photons at the atom's transitions is the emission spectrum.

Interpretation of the emission spectrum

Some of the atom's transitions are caused by an increase in temperature or by the presence of other external sources of energy such as a beam of light, a stream of electrons, or a chemical reaction.

If a gas such as hydrogen is placed in a chamber at low pressure and an electric current is passed through the chamber, the gas will emit light with its own color that differentiates it from other gases.

By passing the emitted light through a prism, instead of obtaining a rainbow of light, discrete units are obtained in the form of colored lines with specific wavelengths, which carry discrete amounts of energy.

The lines of the emission spectrum are unique in each element and their use from the spectroscopy technique allows to determine the elemental composition of an unknown substance as well as the composition of astronomical objects, by analyzing the wavelengths of the emitted photons. during the transition of the atom.

Difference between emission spectrum and absorption spectrum.

In absorption and emission processes the atom has transitions between two energy states but it is in absorption that it gains energy from outside and reaches the state of excitation.

The spectral line of emission is opposite to the continuous spectrum of white light. In the first, the spectral distribution is observed in the form of bright lines and in the second, a continuous band of colors is observed.

If a beam of white light hits a gas such as hydrogen, enclosed in a chamber at low pressure, only a portion of the light will be absorbed by the gas and the rest will be transmitted.

When transmitted light passes through a prism it breaks down into spectral lines, each with a different wavelength, forming the absorption spectrum of the gas.

The absorption spectrum is totally opposite to the emission spectrum and it is also specific for each element. When comparing both spectra of the same element, it is observed that the emission spectral lines are the ones that are missing in the absorption spectrum (Figure 2).

Figure 2. a) Emission spectrum and b) Absorption spectrum (Author: Stkl. Source:

Examples of emission spectra of chemical elements

a) The spectral lines of the hydrogen atom, in the visible region of the spectrum, are a red line of 656.3 nm, a light blue of 486.1nm, a dark blue of 434nm and a very faint violet of 410nm. These wavelengths are obtained from the Balmer - Rydberg equation in its modern version (3).

is the wave number of the spectral line

is Rydberg's constant (109666.56 cm-1)

is the highest energy level

is the highest energy level

Figure 3. Emission spectrum of hydrogen (Author: Adrignola. Source: commons.wikimedia.org

b) The emission spectrum of helium has two series of main lines, one in the visible region and the other near the ultraviolet. Peterson (4) used the Bohr model to calculate a series of helium emission lines in the visible portion of the spectrum, as a result of several simultaneous transitions of two electrons to the n = 5 state, and obtained values ​​of the wavelength consistent with experimental results. The wavelengths that were obtained are 468.8nm, 450.1nm, 426.3nm, 418.4nm, 412.2nm, 371.9nm.

c) The emission spectrum of sodium has two very bright lines of 589nm and 589.6nm called D lines (5). The other lines are much weaker than these and, for practical purposes, all the sodium light is considered to come from the D lines.

References

1. Measurement of lifetimes of excited states of the hydrogen atom. VA Ankudinov, SV Bobashev, and EP Andreev. 1, 1965, Soviet Physics JETP, Vol. 21, pp. 26-32.
2. Demtröder, W. Laser Spectroscopy 1. Kaiserslautern: Springer, 2014.
3. DKRai, SN Thakur and. Atom, laser and spectroscopy. New Delhi: Phi Learning, 2010.
4. Bohr Revisited: Model andespectral lines of helium. Peterson, C. 5, 2016, Journal of young investigators, Vol. 30, pp. 32-35.
5. Journal of chemical Education. JR Appling, FJ Yonke, RA Edgington, and S. Jacobs. 3, 1993, Vol. 70, pp. 250-251.