Emission Spectrum and Atomic Spectra

The measurement of atoms (and ions) by their interaction with electromagnetic radiation in atomic spectra is an important topic. The coloured lines of light emitted by the excited atoms form the atomic spectra. We all know about light refraction. Light either bends toward the normal or away from the normal as it passes from one medium to another. The refraction effect is primarily due to the difference in the speed of light in different mediums. The speed of light depends upon the density of the medium it travels through.

Let us understand the phenomenon of white light dispersion through a prism, and about the continuum of emissions and the spectrum of absorption.

Emission Spectrum

Electrons shift from lower energy levels to higher energy levels as energy is consumed by an atom's electrons. To return to ground states from the excited state, which is unstable, these excited electrons have to radiate energy. The spectrum of pollution is formed by the frequency of the light emitted.

• The electrons in these atoms will absorb energy as electromagnetic radiation interacts with atoms and molecules of matter, and leap to a higher energy state, losing their stability.

• They need to switch from the higher energy state to the preceding lower energy state in order to recover their equilibrium.

• These atoms and molecules emit radiation in different regions of the electromagnetic spectrum to accomplish this task.

• This radiation spectrum emitted by electrons in the excited atoms or molecules is known as the spectrum of emissions.

Absorption Spectrum

On the other hand, as energy is absorbed by electrons in the ground state to enter higher energy states, the absorption range is constituted by the frequencies of light emitted by dark bands.

• We observe that it experiences refraction twice when a beam of white light falls on a prism.

• Once from a rare medium (air) to a denser medium (glass) and again from a denser medium (glass) to a rare medium (glass) (air).

• Finally, we see a band of colors, called the continuum, which is built out of a white light beam. The color with a smaller wavelength deviates the most as we study this spectrum more closely and vice versa.

• Thus red with the longest wavelength suffers the least variance, a continuum of colors ranging from red to violet is observed.

• As violet merges into blue, blue into green and so on, this sort of spectrum is called a continuous spectrum.

However, in the gas phase, the emission spectrum of atoms does not display a constant spread of wavelengths from one color to another. Instead, the light emitted consists of a single wavelength with dark spaces occurring between them. These kinds of spectra are referred to as spectra of atoms or line spectra.

Emission Spectrum & Absorption Spectrum

In an atom, the electrons appear to be arranged in such a way that the atom's energy is as minimal as possible. The lowest energy state of the atom is the ground state of an atom. The electrons consume the energy and transfer to a higher energy level when those atoms are given energy. These electron energy levels in atoms are quantified, meaning again that the electron must travel in discrete steps rather than constantly from one energy level to another.

An atom's excited state is a state where its potential energy is greater than the state of the earth. An atom is not stable in a state of excitation. As it goes back to the ground state, in the form of electromagnetic radiation, it releases the energy it had previously accumulated.

• An absorption spectrum is like a negative photograph of a spectrum of pollution.

• Electromagnetic radiation is bombarded with a sample that absorbs radiation from certain wavelengths to observe the absorption spectrum.

Hydrogen Emission and Absorption Series

In astronomy, the spectrum of hydrogen is especially significant since much of the Universe is made of hydrogen. The processes of hydrogen emission or absorption give rise to series, which are sequences of lines corresponding to atomic transitions, each ending or starting with the same hydrogen atomic state. The Balmer Series, for example, includes transitions beginning (for absorption) or ending (for emission) with the first excited state of hydrogen, while the Lyman Series involves transitions beginning or ending with hydrogen's ground state; the adjacent picture shows the atomic transitions that create emissions from these two series.

The Balmer Series is in the visible spectrum and the Lyman Series is in the UV, owing to the specifics of hydrogen's atomic structure. Some of the transitions in the Balmer series are illustrated in the following illustration.

Molecular Spectra

Molecules can interact with electromagnetic radiation and give rise to characteristic spectra, in addition to spectra associated with atoms and ions. Because of the fundamental atomic and molecular structure, infrared wavelengths usually provide the spectra associated with molecules. Moreover, since molecules are typically fragile, molecular spectra are mainly essential in relatively cold objects, such as planetary atmospheres, very cool star surfaces, and different interstellar regions.