When is absorption spectrum produced

Kirchhoff's Law of Spectral Analysis. The same phenomena are at work in the non-visible portions of the spectrum, including the radio range. As the radiation passes through a gas, certain wavelengths are absorbed. Those same wavelengths appear in emission when the gas is observed at an angle with respect to the radiation source. Why do atoms absorb only electromagnetic energy of a particular wavelength?

And why do they emit only energy of these same wavelengths? The answers lie in quantum mechanics. The electrons in an atom may be in a number of allowed energy states. In order to jump to one of a limited number of allowed higher energy levels, the atom must gain a very specific amount of energy.

These discrete packets of energy are called photons. Thus, each spectral line corresponds to one particular transition between energy states of the atoms of a particular element. An absorption line occurs when an electron jumps from a lower energy state to a higher energy state, extracting the required photon from an outside source of energy such as the continuous spectrum of a hot, glowing object. An absorption band is a series of very closely spaced absorption lines resulting from the absorption of light by molecules.

Bands caused by titanium dioxide and carbon compounds occur in the spectra of low temperature stars see M stars , including red giants. So-called diffuse interstellar bands , most likely due to complex carbon molecules, are observed in the spectra of remote stars. Atoms and molecules may change states when they absorb specific amounts of energy. Atomic states are defined by the arrangement of electrons in atomic orbitals. An electron in some orbital may be excited to a more energetic orbital by absorbing exactly one photon which has energy equal to the energy difference of the two orbitals.

Molecular states are defined by the molecule's modes of vibration and rotation. These vibrational and rotational modes are quantized, similar to the atomic orbitals, and may be excited by absorbing single photons. In both the atomic and molecular cases, the excited states do not persist: after some random amount of time, the atoms and molecules revert back to their original, lower energy state. In atoms, the excited electron returns to a lower orbital, emitting a photon.

In molecules, the vibrational or rotational mode decays, also emitting a photon. Absorption lines are usually seen as dark lines, or lines of reduced intensity, on a continuous spectrum. This is seen in the spectra of stars , where gas mostly hydrogen in the outer layers of the star absorbs some of the light from the underlying thermal blackbody spectrum. Wavelength is measured in Angstroms, while the flux is in arbitrary units.