  Strong coupling of molecules to vacuum fields has been widely reported to
lead to modified chemical properties such as reaction rates. However, some
recent attempts to reproduce infrared strong coupling results have not been
successful, suggesting that factors other than strong coupling may sometimes be
involved. Here we re-examine the first of these vacuum-modified chemistry
experiments in which changes to a molecular photoisomerisation process in the
UV-vis spectral range were attributed to strong coupling of the molecules to
visible light. We observed significant variations in photoisomerisation rates
consistent with the original work; however, we found no evidence that these
changes need to be attributed to strong coupling. Instead, we suggest that the
photoisomerisation rates involved are most strongly influenced by the
absorption of ultraviolet radiation in the cavity. Our results indicate that
care must be taken to rule out non-polaritonic effects before invoking strong
coupling to explain any changes of chemical properties arising in cavity-based
experiments.
