Wavelength-dependent photofragmentation of pyrazine.

Ultraviolet (UV) photon-induced dissociation of larger molecules is a rich source of smaller hydrocarbons and nitrogen-bearing molecules in the interstellar medium. This top-down mechanism is of utmost importance in photodissociation regions (PDRs), where the intensities reach orders of magnitude higher than the ambient interstellar radiation field. In this work, we employed two distinct UV radiation fields, i.e., 266 nm and 355 nm, to investigate the photo-fragmentation of pyrazine, a biologically and astrochemically relevant nitrogen heterocycle in the gas phase. Measurements performed across varying laser intensities reveal a significant wavelength-dependent difference in cationic fragmentation patterns. Nitrogen-bearing fragment cations and hydrocarbons are identified as key products across both wavelength regimes. However, the photofragmentation under 355 nm irradiation produces several previously unreported fragmentation channels such as [Formula: see text], [Formula: see text],[Formula: see text]. Interestingly, the direct fragmentation of pyrazine cannot account for the observed [Formula: see text] ion through simple bond cleavage. We proposed that pyrazine undergoes photoisomerization upon 355 nm laser irradiation before dissociation. All the fragmentation channels are discussed in detail, providing essential experimental constraints for astrochemical models of UV-driven molecular destruction in PDRs and planetary atmospheres.