  Energetic processing of interstellar ice mantles and planetary atmospheres
via photochemistry is a critical mechanism in the extraterrestrial synthesis of
prebiotic molecules. Photochemistry is defined as chemical processes initiated
by photon-induced electronic excitation, not involving ionization. In contrast,
photons with energies above the ionization threshold initiate radiation
chemistry (radiolysis). Vacuum-ultraviolet (6.2-12.4 eV) light may initiate
photochemistry and radiation chemistry because the threshold for producing
secondary electrons is lower in the condensed phase than in the gas phase.
Approximately half of cosmic-ray induced photons incident on interstellar ices
in star-forming regions initiate photochemistry while the rest initiate
radiation chemistry. While experimental techniques such as velocity map imaging
may be used to extract exquisite details about gas-phase photochemistry, such
detailed information cannot be obtained for condensed-phase photochemistry,
which involves greater complexity, including the production of excitons,
excimers, and exciplexes. Because a primary objective of chemistry is to
provide molecular-level mechanistic explanations for macroscopic phenomena, our
ultimate goal in this book chapter is to critically evaluate our current
understanding of the photochemistry that likely leads to the synthesis of
extraterrestrial prebiotic molecules.
