Velocity-tunable beam of continuously decelerated polar molecules for cold ion-molecule reaction studies

Utilizing Quantum Cascade Lasers for Ultranarrow Velocity Resolution and Quantum-State Selectivity in Molecular Beam Scattering and Spectroscopy

We demonstrate the capability of a narrow linewidth quantum cascade laser (QCL) to selectively excite a very narrow velocity range of nitric oxide (σ ≤ 7(3) m/s) with a pure ro-vibrational quantum state. By implementing a counter-propagating geometry, the molecules are selectively excited according to the Doppler shift of the ro-vibrational transition frequency such that the velocity width associated with the excited molecules depends only on the QCL linewidth. We demonstrate a velocity distribution limited by the effective linewidth of our free-running QCL (Γ = 3.2 MHz). Our development provides a cost-effective, flexible approach to resolve quantum-state selective chemical dynamics with excellent velocity resolution in a wide variety of molecules with infrared-active transitions. This technique has been formulated to provide ultrahigh collisional energy resolution in molecular beams to delineate final quantum-state product pairs in studies of molecular collisions.

Reactions of Acetonitrile with Trapped, Translationally Cold Acetylene Cations

The reaction of the acetylene cation (C₂H₂⁺) with acetonitrile (CH₃CN) is measured in a linear Paul ion trap coupled to a time-of-flight mass spectrometer. C₂H₂⁺ and CH₃CN are both noted for their astrochemical abundance and predicted relevance for understanding prebiotic chemistry. The observed primary products are c-C₃H₃⁺, C₃H₄⁺, and C₂NH₃⁺. The latter two products react with excess CH₃CN to form the secondary product C₂NH₄⁺, protonated acetonitrile. The molecular formula of these ionic products can be verified with the aid of isotope substitution via deuteration of the reactants. Primary product reaction pathways and thermodynamics are investigated with quantum chemical calculations and demonstrate exothermic pathways to two isomers of C₂NH₃⁺, two isomers of C₃H₄⁺, and the cyclopropenyl cation c-C₃H₃⁺. This study deepens our understanding of the dynamics and products of a pertinent ion-molecule reaction between two astrochemically abundant molecules in conditions that mimic those of the interstellar medium.