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an object of higher temperature radiate infrared rays as a way to decrease the temperature. how a molecule produce a electromagnetic wave? in atoms electromagnetic radiation is caused by electrons. what is responsible in molecules?

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  • $\begingroup$ Electrons are in molecules too. $\endgroup$
    – Jon Custer
    Commented Jan 17, 2017 at 16:32
  • $\begingroup$ Are you specifically asking about infrared, and from isolated molecules? If so this is normally the result of vibrational transitions. If you're interested in solids, or visible light from isolated molecules then the mechanisms are different. $\endgroup$ Commented Jan 17, 2017 at 16:41

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in atoms electromagnetic radiation is caused by electrons. what is responsible in molecules?

There exist atomic and molecular orbitals of the electrons composing atoms and molecules.

This means that the charge distribution around an atom or a molecule in space is uniform only for some quantum numbers. Otherwise there is a shape to the charge distribution of the electrons, allowing positive charge regions from the positive charge of the nucleus. The positive and negative charge regions cause the attraction that pairs (or clusters) of atoms have and build into a molecule. Electrons in molecules also have spatial charge distributions and this is what creates the attraction for bonding.

This bonding has extra degrees of freedom of rotation and vibration, and the electrons are in energy levels that are almost continuous, thus can have small transitions of energy leading to infrared radiation.

The positive and negative field regions will also give rise to radiation in a gas, when molecules bounce off each other.

So different models are used to predict the radiation, which ultimately is the black body radiation, depending on the type of matter under study, gas, or fluid or solid.

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As it was correctly noted by others, molecules consist of atoms, and the radiation can be emitted as transitions between the atomic orbitals. Molecules also have other degrees of freedom, related to the rotational and vibrational motion of atoms within a molecule, their frequency being usually in infrared or even radio range.

It is worth noting that not every transition between two energy levels may result in emission of electromagnetic waves: the transition should necessarily have a non-zero matrix element of a dipolar or magnetic moment, so that it couples to the EM field (although more complex types of coupling, e.g., quadrupole coupling, are also possible). Thus, vibrational and rotational modes are usually not active themselves in the EM spectrum, but modify the electronic transitions, by adding satellite lines: $$\hbar\omega_{optical} \rightarrow \hbar\omega_{optical}\pm n\hbar\Omega_{vibrational/rotational}.$$

Finally, it is necessary to mention organic dyes - a special class of organic molecules where complex re-arrangement of electronic structure is possible. This makes these molecules fluorescent and widely used in laser technology.

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