Infrared Cooling in an Anharmonic Cascade Framework: 2-Cyanoindene, the Smallest Cyano-PAH Identified in Taurus Molecular Cloud-1

Abstract

Infrared (IR) cooling of polycyclic aromatic hydrocarbon (PAH) molecules is a major radiative stabilization mechanism of PAHs present in space and is the origin of the aromatic infrared bands (AIBs). Here, we report an anharmonic cascade model in a master equation framework to model IR emission rates and emission spectra of energized PAHs as a function of internal energy. The underlying (simple harmonic cascade) framework for fundamental vibrations has been developed through the modeling of cooling rates of PAH cations and other carboneaous ions measured in electrostatic ion storage ring experiments performed under “molecular cloud in a box” conditions. The anharmonic extension is necessitated because cyano-PAHs, recently identified in Taurus Molecular Cloud-1 (TMC-1), exhibit strong anharmonic couplings, which make substantial contributions to the IR emission dynamics. We report an experimental mid-IR (650–3200 cm^–1) absorption spectrum of 2-cyanoindene (2CNI), which is the smallest cyano-PAH that has been identified in TMC-1 and model its IR cooling rates and emission properties. The mid-IR absorption spectrum is reasonably described by anharmonic calculations at the B3LYP/N07D level of theory that include resonance polyad matrices, although the CN-stretch mode frequency continues to be difficult to describe. The anharmonic cascade framework can be readily applied to other neutral or charged PAHs and is also readily extended to include competing processes, such as recurrent fluorescence and isomerization.