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I'm reading a paper in which I found this line:

The other modes have relatively small contributions for the displacements of the heavy atoms and can be considered as spectator modes for the internal conversion

I cannot find a definition or explanation of term “spectator mode”. What is it exactly?

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    $\begingroup$ I suppose they may have invented it. I would assume that they took the term from spectator ions. $\endgroup$ Jun 29, 2016 at 14:11
  • $\begingroup$ There is no such term, in that you won't find it in the dictionaries. This is just a combination of two words, and if you know the meaning of each word, then you know what do they mean when put together. $\endgroup$ Jun 29, 2016 at 14:25
  • $\begingroup$ Without the paper its hard to follow this, also the English is not that clear, 'for the displacements' or ' towards the displacements', but it implies that these modes are not the promoting mode(s) in the radiationless conversion, i.e they don't take part, hence 'spectator'. $\endgroup$
    – porphyrin
    Jul 1, 2016 at 17:48

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The paper in question is Ab Initio Trajectory Surface-Hopping Study on Ultrafast Deactivation Process of Thiophene. From a quick Google Scholar search that returns 187 hits, it is clear that the expression “spectator mode” for a harmonic vibration mode is not a standard terminology, but is nonetheless used by plenty of authors.

As to the meaning, I think you got it from the everyday meaning of “spectator”: they are vibration modes that do not participate in the phenomenon of interest, i.e. that are not coupled to the chemical or physical process studied (which can be a chemical reaction, an adsorption process or a photophysical process).

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