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Spectroscopy is the interaction between radiation and matter. Specifically, this topic focuses on the interaction between radiation and electrons in matter.

In a Bohr atomic model, electrons orbit the nucleus in stable energy levels. There are multiple energy levels in an atom, the further away an orbit is from the nucleus, the greater the associated energy level it has.

Atomic electrons naturally occupy the lowest energy state known as the ground state. Higher energy states are called excited states.

Electronic Excitation

electronic excitation

Electronic excitation refers to the process when electrons in their ground state absorb energy to transition to a higher energy state. The energy that is to be absorbed can be in any form e.g. electromagnetic radiation, heat, electrical energy etc.

Excitation only occurs if the energy absorbed exactly matches the energy difference between the initial energy state and the final energy state.

If the energy provided does not exactly match this energy difference, it is does not absorbed by the electron.

Electronic Relaxation

Electronic relaxation

Excited electrons do not remain in their excited energy states permanently. When electrons return from their excited state to the ground state, energy is released in the form of electromagnetic radiation (EMR).

During this process, electrons will release the same amount of energy as they absorbed (law of conservation of energy). Since atoms have multiple excitation states, there may be multiple electronic transitions, causing the emitted electromagnetic radiation to vary in energy and thus have different frequencies and wavelengths.

Absorption and Emission Spectrum

Absorption Spectrum

White light consists of visible light of all wavelengths. When white light is dispersed through a glass prism, it forms a continuous spectrum.

When the same source of light is passed through atoms, ground state electrons can absorb specific amounts of energy to transition to higher energy states.

Different energy of light corresponds to different frequencies/wavelengths. Therefore, absorption of light during electronic excitation causes there to be absent frequencies/wavelengths of light in the otherwise continuous spectrum. These absent lines are called absorption lines. This type of spectrum is called an absorption spectrum.

For atoms of a particular element, there are usually more than one absorption line present because a ground state electron can be excited to different excited energy states by absorbing different amounts of energy (in the form of light).

Emission Spectrum

In a sample of hot gas where electrons are in their excited energy states due to the provision of heat as an energy source, these electrons can return to their ground states and release energy in the form of EMR.

The amount of energy released produces EMR of specific frequencies/wavelengths because it equals the difference between energy states. This produces an emission spectrum where only certain frequencies/wavelengths of light are present on a black background. These streaks of light are called emission lines.

Elements have unique emission and absorption spectra.

The atomic structure varies among elements which means the energy states of orbits differ among elements. The energy states found in a hydrogen atom would be different to those found in a helium atom. As a result, the amount of energy absorbed during electronic excitation and released during electronic relaxation also differs among atoms of different elements.
Consequently, no two elements will produce identical absorption and emission spectra. The absorption or emission spectrum would be unique for an element, and thus, can be used to identify elements.