Absorption spectrum

An absorption spectrum is a spectrum of absorption lines or bands produced when light from a hot source, which itself produces a continuous spectrum, passes through a cooler gas. The absorption spectrum of a material shows the fraction of incident electromagnetic radiation that is absorbed by the material over a range of frequencies. The absorption spectrum is primarily determined by the atomic and molecular composition of the material. An absorption spectrum is, in a sense, the opposite of an emission spectrum. Radiation is more likely to be absorbed at frequencies that correspond to the energy difference between two quantum mechanical states of the molecules. The absorption due to a transition between two states is called an absorption line, and a spectrum is typically composed of many lines.

Each chemical element has absorption lines at several specific wavelengths corresponding to the differences between the energy levels of its orbitals. The frequencies at which absorption lines occur and their relative intensities depend primarily on the electronic and molecular structure of the sample. The frequencies will also depend on the interactions between molecules in the sample, the crystal structure in solids, and various environmental factors (e.g., temperature, pressure, electromagnetic field). The lines will also have a width and shape that is primarily determined by the spectral density or density of states of the system. For example, an object that absorbs blue, green, and yellow light will appear red when viewed under white light. Absorption spectra can therefore be used to identify the elements present in a gas or liquid. This method is used to infer the presence of elements in stars and other gaseous objects that cannot be measured directly.

How an absorption spectrum is formed

Atoms and molecules can change states when they absorb certain amounts of energy. Atomic states are defined by the arrangement of electrons in atomic orbitals. An electron in one orbital can be excited to a more energetic orbital by absorbing exactly one photon of energy equal to the energy difference between the two orbitals.

Molecular states are defined by the vibrational and rotational modes of the molecule. These vibrational and rotational modes are quantized, similar to atomic orbitals, and may be excited by the absorption of single photons.

In both the atomic and molecular cases, the excited states do not persist: after a random amount of time, the atoms and molecules return to their original, lower energy state. In atoms, the excited electron returns to a lower orbital and emits a photon. In molecules, the vibrational or rotational mode decays, also emitting a photon.

When this decay occurs, the resulting photon is not necessarily emitted in the same direction as the original photon. The most common angle of this has been shown to be about 45 degrees from the original photon. This applies to any situation where gases lie between a light source and an observer: the observer will see gaps in the spectrum of the light corresponding to the wavelengths of the photons that were absorbed. These gaps occur despite the re-emission of photons, because the re-emitted photons are equally likely to travel in all directions, and it is statistically unlikely that they will travel along the original path to the observer. These gaps appear as black lines in an image of the spectrum.

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