Coupling laboratory FTIR data with PROCODA kinetic modeling for the radiolysis of frozen formic acid using swift ions

Abstract

Formic acid (HCOOH) is one of the simplest carboxylic acids detected in interstellar and circumstellar environments, often associated with prebiotic organic chemistry. In this work, we combine laboratory data from the radiolysis of pure HCOOH ice at 15 K by 267 MeV ⁵⁶Fe²²⁺ ions with kinetic simulations performed using the PROCODA code. This numerical model solves a system of coupled differential equations describing 1631 reactions among 73 molecular and radical species, allowing the determination of effective reaction rate coefficients (ERCs), equilibrium abundances, and desorption yields. The best-fit model successfully reproduces experimental infrared data and predicts the formation of non-observed intermediates such as HOCO, CH₂OO, and H₂CO. The results indicate that CO, CO₂, and H₂O are the dominant radiolysis products, while HCOOH acts primarily as a transient radical source rather than a net precursor of complex organics. Hydrogen abstraction and radical recombination were identified as the main mechanisms governing molecular evolution, with HOCO and OH radicals playing key intermediate roles. The calculated desorption yield (1.5 × 10⁵ molecules per ion) and effective rate constants confirm the efficiency of heavy ion sputtering in cold ices. These findings provide quantitative insight into the radiation-driven chemistry of formic acid and offer constraints for astrochemical models of dense clouds, protostellar disks, and icy moons.