What is a “spectator mode” in this analysis of molecular vibration modes 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?
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).