  This paper describes a near-UV/VIS study of a pyrene:H_2O interstellar ice
analogue at 10 K using optical absorption spectroscopy. A new experimental
approach makes it possible to irradiate the sample with vacuum ultraviolet
(VUV) light (7-10.5 eV) while simultaneously recording spectra in the 240-1000
nm range with subsecond time resolution. Both spectroscopic and dynamic
information on VUV processed ices are obtained in this way. This provides a
powerful tool to follow, in-situ and in real time, the photophysical and
photochemical processes induced by VUV irradiation of a polycyclic aromatic
hydrocarbon containing inter- and circumstellar ice analogue. Results on the
VUV photolysis of a prototype sample - strongly diluted pyrene in H_2O ice -
are presented. In addition to the pyrene cation (Py+), other products -
hydroxypyrene (PyOH), possibly hydroxypyrene cation (PyOH+), and
pyrene/pyrenolate anion (Py-/PyO-) - are observed. It is found that the charge
remains localized in the ice, also after the VUV irradiation is stopped. The
astrochemical implications and observational constraints are discussed.
