High-resolution infrared action spectroscopy of the fundamental vibrational band of CN. J Mol Spectrosc 2020;374:111375. [PMID: 33162609 PMCID: PMC7116308 DOI: 10.1016/j.jms.2020.111375]

(Noble Gas)n -NC+ Molecular Ions in Noble Gas Matrices: Matrix Infrared Spectra and Electronic Structure Calculations

An investigation of pulsed‐laser‐ablated Zn, Cd and Hg metal atom reactions with HCN under excess argon during co‐deposition with laser‐ablated Hg atoms from a dental amalgam target also provided Hg emissions capable of photoionization of the CN photo‐dissociation product. A new band at 1933.4 cm⁻¹ in the region of the CN and CN⁺ gas‐phase fundamental absorptions that appeared upon annealing the matrix to 20 K after sample deposition, and disappeared upon UV photolysis is assigned to (Ar)_nCN⁺, our key finding. It is not possible to determine the n coefficient exactly, but structure calculations suggest that one, two, three or four argon atoms can solvate the CN⁺ cation in an argon matrix with C−N absorptions calculated (B3LYP) to be between 2317.2 and 2319.8 cm⁻¹. Similar bands were observed in solid krypton at 1920.5, in solid xenon at 1935.4 and in solid neon at 1947.8 cm⁻¹. H¹³CN reagent gave an 1892.3 absorption with shift instead, and a 12/13 isotopic frequency ratio–nearly the same as found for ¹³CN⁺ itself in the gas phase and in the argon matrix. The CN⁺ molecular ion serves as a useful infrared probe to examine Ng clusters. The following ion reactions are believed to occur here: the first step upon sample deposition is assisted by a focused pulsed YAG laser, and the second step occurs on sample annealing: (Ar)₂⁺+CN→Ar+CN⁺→(Ar)_nCN⁺.

Rotational action spectroscopy of trapped molecular ions

Rotational action spectroscopy is an experimental method in which rotational spectra of molecules, typically in the microwave to sub-mm-wave domain of the electromagnetic spectrum (∼1-1000 GHz), are recorded by action spectroscopy. Action spectroscopy means that the spectrum is recorded not by detecting the absorption of light by the molecules, but by the action of the light on the molecules, e.g., photon-induced dissociation of a chemical bond, a photon-triggered reaction, or photodetachment of an electron. Typically, such experiments are performed on molecular ions, which can be well controlled and mass-selected by guiding and storage techniques. Though coming with many advantages, the application of action schemes to rotational spectroscopy was hampered for a long time by the small energy content of a corresponding photon. Therefore, the first rotational action spectroscopic methods emerged only about one decade ago. Today, there exists a toolbox full of different rotational action spectroscopic schemes which are summarized in this review.

Vibrational quenching of CN- in collisions with He and Ar

The vibrational quenching cross sections and corresponding low-temperature rate constants for the ν = 1 and ν = 2 states of CN^-(¹Σ⁺) colliding with He and Ar atoms have been computed ab initio using new three-dimensional potential energy surfaces. Little work has been carried out so far on low-energy vibrationally inelastic collisions for anions with neutral atoms. The cross sections and rates calculated at energies and temperatures relevant for both ion traps and astrochemical modeling are found by the present calculations to be even smaller than those of the similar C₂ ^-/He and C₂ ^-/Ar systems, which are in turn of the order of those existing for the collisions involving neutral diatom-atom systems. The implications of our finding in the present case mainly focus on the possible role of small computed rate constants in the dynamics of molecular cooling and the evolution of astrochemical modeling networks.