# Do homonuclear diatomic molecules interact with infrared light?

A molecule will appear on IR spectra if its dipole moment changes during vibration. Due to symmetry, the dipole moment of homonuclear diatomic molecules ($\mathrm{H_2, N_2, O_2, F_2, \dots}$) is zero.

Does it mean that homonuclear diatomic molecules do not interact at all with infrared light? Not even elastic absorption and re-emission?

## 2 Answers

The quote you mention is for electric dipole transitions, which are not allowed for homonuclear diatomics. These transitions are for absorption or emission of one photon

You are correct to question absorption and re-emission; this is Raleigh scattering if the photon does not change wavelength (energy). The photon can change energy, though, in Raman scattering. The combined absorption and emssion process is generally very much weaker than allowed emission or absorption of one photon. Modern experiments generally observe Raleigh / Raman scattering with use of lasers, mostly in the visible I think, but no reason that it should not work in the IR as well. One final point is that in Raleight scattering the wavelength of light can change a little bit due to momentum transfer, but the internal energy of the molecule stays the same.

Final point is that some transitions that are not allowed as electric dipole transitions are allowed are electric quadrupole transitions or magnetic dipole transitions, for example. These processes are significantly weaker (less probable) than electric dipole transitions.

The answer by Tom is correct.

I would like to add a few more points about scattering phenomena. Using Raman spectroscopy rotation and vibration of these homo-nuclear molecules are easily probed. (Classically, scattering is explained by induced dipole moment; while Quantum-mechanical treatment is based on the Perturbation theory).

However, Raman signal is extremely weak compared to Rayleigh scattering.

PS- If IR laser is used then vibrational resonance will happen and vibrational-Raman signal will be enhanced significantly. If microwave laser is used then rotational resonance will happen and rotational-Raman signal will be enhanced significantly.