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Difference Between Emission and Absorption Spectra

Emission vs Absorption Spectra | Absorption Spectrum vs Emission Spectrum

Light and other forms of electromagnetic radiations are very useful, and widely used in analytical chemistry. The interaction of radiation and matter is the subject of the science called spectroscopy. Molecules or atoms can absorb energy or release energy. These energies are studied in spectroscopy. There are different spectrophotometers to measure different types of electromagnetic radiations such as IR, UV, visible, X-ray, microwave, radio frequency, etc.

Emission Spectra

When a sample is given, we can obtain information about the sample depending on its interaction with the radiation. First, the sample is stimulated by applying energy in the form of heat, electrical energy, light, particles, or a chemical reaction. Before applying energy, the molecules in the sample are in a lower energy state, which we call the ground state. After applying external energy, some of the molecules will undergo a transition to a higher energy state called the excited state. This excited state species is unstable; therefore, trying to emit energy and come back to the ground state. This emitted radiation is plotted as a function of frequency or wavelength, and it is then called an emission spectra. Each element emits specific radiation depending on the energy gap between the ground state and the excited state. Therefore, this can be used to identify the chemical species.

Absorption Spectra

An absorption spectrum is a plot of absorbance versus wavelength. Other than the wave length absorbance can also be plotted against frequency or wave number. Absorption spectra can be of two types as atomic absorption spectra and molecular absorption spectra. When a beam of polychromatic UV or visible radiation passes through atoms in the gas phase, only some of the frequencies are absorbed by the atoms. Absorbed frequency differs for different atoms. When the transmitted radiation is recorded, the spectrum consists of a number of very narrow absorption lines. In atoms, these absorption spectra are seen as a result of electronic transitions. In molecules, other than the electronic transitions, vibration and rotational transitions are also possible. So the absorption spectrum is quite complex, and the molecule absorbs UV, IR and visible radiation types.

What is the difference between Absorption spectra Vs emission spectra?

• When an atom or molecule excites, it absorbs a certain energy in the electromagnetic radiation; therefore, that wavelength will be absent in the recorded absorption spectrum.

• When the species come back to the ground state from the excited state, the absorbed radiation is emitted, and it is recorded. This type of spectrum is called an emission spectrum.

• In simple terms, absorption spectra records the wavelengths absorbed by the material, whereas emission spectra records wavelengths emitted by materials, which have been stimulated by energy before.

• Compared to the continuous visible spectrum, both emission and absorption spectra are line spectra because they only contain certain wavelengths.

• In an emission spectrum there’ll be only few colored bands in a dark back ground. But in an absorption spectrum there’ll be few dark bands within the continuous spectrum. The dark bands in the absorption spectrum and the colored bands in the emitted spectrum of the same element are similar.


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