Stability of solid-state formamide under Lyα irradiation

Abstract

Formamide (NH$_2$CHO) plays a pivotal role as a crucial precursor to various prebiotic molecules, including sugars and nucleobases. To gain a deeper understanding of the chemical processes involving formamide formation in astrophysical settings, it becomes imperative to refine our comprehension through astrochemical models. These models necessitate not only the inclusion of pathways for formamide formation across diverse environments, but also the elucidation of mechanisms that lead to its degradation. The primary objective of this study is to scrutinize the influence of the underlying amorphous silicate substrate and the phase of formamide ice on the kinetics of its destruction and the resulting products upon exposure to Lyalpha (121.6 nm) radiation at 16 K. To achieve this, we conducted an examination of the photodestruction of NH$_2$CHO ice, employing Fourier transform infrared spectroscopy. Our findings reveal that, while the destruction rates of amorphous formamide ice remain consistent, regardless of the presence of an underlying amorphous olivine substrate, this substrate effectively reduces the formation of NH$_3$, HNCO, and HCN within the ice following UV irradiation. On the other hand, contrary to common knowledge, crystalline formamide ice exhibits a considerably faster destruction rate, by an order of magnitude, than its amorphous counterpart under photo processing. In the interstellar medium, molecular ices often undergo phase changes depending on the environmental conditions. Our results indicate that crystalline formamide ice is more susceptible to rapid destruction than its amorphous counterpart, rendering it more elusive for detection within the lifetime of dense interstellar clouds. Furthermore, our findings emphasise the crucial significance of accounting for the influence of underlying dust grain surfaces in astrochemical models, as they have an effect on product formation during the degradation of molecular ices.