  A modified version of a micro scale atmospheric pressure plasma jet
(\mu-APPJ) source, so-called X-Jet, is used to study the role of plasma
generated VUV photons in the inactivation of E. coli bacteria. The plasma is
operated in He gas or a He/O2 mixture and the X-Jet modification of the jet
geometry allows effective separation of heavy reactive particles (such as O
atoms or ozone molecules) from the plasma-generated photons. The measurements
of the evolution of zone of inhibitions formed in monolayers of vegetative E.
coli bacteria, of VUV emission intensity and of positive ion spectra show that
photochemistry in the gas phase followed by photochemistry products impacting
on bacteria can result in bacterial inactivation. Interestingly, this process
is more effective than direct inactivation by VUV radiation damage. Mainly
protonated water cluster ions are detected by mass spectrometry indicating that
water impurity has to be carefully considered. The measurements indicate that
the combination of the presence of water cluster ions and O2 molecules at the
surface leads to the strongest effect. Additionally, it seem that the
interaction of VUV photons with effluent of He/O2 plasma leads to enhanced
formation of O3, which is not the case when only O2 molecules and gas
impurities at room temperature interacts with plasma generated VUV photons.
