An infrared signal is generated due to fluctuations in the permanent dipole moment, but can an infrared signal be induced in a molecule due to a change in the quadrupole moment?


1 Answer 1


Yes, but the signals are of the order $10^{6}$ times weaker than the same for dipole transitions. This is why it is very hard to observe them.

You may want to have a read of this paper.

  • $\begingroup$ So there would realistically be no IR signal from, say, methane because it doesn't have a permanent dipole moment? $\endgroup$ Jan 14, 2022 at 2:54
  • $\begingroup$ +1 Thanks for the link. What is the fundamental reason why the signals are so much weaker than for dipole transitions? $\endgroup$ Jan 14, 2022 at 3:01
  • $\begingroup$ Is it simply due to the dipole moment contributing so much more to the overall energy of the molecule? $\endgroup$ Jan 18, 2022 at 3:30

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