Rotational Spectroscopy of a Triatomic Molecular Anion

Kinetic and dynamic studies of the NH2+ + H2 reaction on a high-level ab initio potential energy surface

The dynamics and kinetics of the NH2+ + H2 reaction are investigated on a newly developed ab initio potential energy surface using the quasi-classical trajectory method.

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.

The necessity of periodic boundary conditions for the accurate calculation of crystalline terahertz spectra

Terahertz vibrational spectroscopy has emerged as a powerful spectroscopic technique, providing valuable information regarding long-range interactions - and associated collective dynamics - occurring in solids. However, the terahertz sciences are relatively nascent, and there have been significant advances over the last several decades that have profoundly influenced the interpretation and assignment of experimental terahertz spectra. Specifically, because there do not exist any functional group or material-specific terahertz transitions, it is not possible to interpret experimental spectra without additional analysis, specifically, computational simulations. Over the years simulations utilizing periodic boundary conditions have proven to be most successful for reproducing experimental terahertz dynamics, due to the ability of the calculations to accurately take long-range forces into account. On the other hand, there are numerous reports in the literature that utilize gas phase cluster geometries, to varying levels of apparent success. This perspective will provide a concise introduction into the terahertz sciences, specifically terahertz spectroscopy, followed by an evaluation of gas phase and periodic simulations for the assignment of crystalline terahertz spectra, highlighting potential pitfalls and good practice for future endeavors.

Study on the kinetics and dynamics of the H2 + NH2- reaction on a high-level ab initio potential energy surface

Gas-phase ion-molecule reactions play major roles in many fields of chemistry and physics. The reaction of an amino radical anion with a hydrogen molecule is one of the simplest proton transfer reactions involving anions. A globally accurate full-dimensional potential energy surface (PES) for the NH₂^- + H₂ reaction is developed by the fundamental invariant-neural network method, resulting in a root mean square error of 0.116 kcal mol^-1. Quasi-classical trajectory calculations are then carried out on the newly developed PES to give integral cross sections, differential cross sections and thermal rate coefficients. This reaction has two reaction channels, proton transfer and hydrogen exchange. The reactivity of the proton transfer channel is about one or two orders of magnitude stronger than that of the hydrogen exchange channel in the energy range studied. Vibrational excitation of H₂ promotes the proton transfer reaction, while fundamental excitation of each vibrational mode of NH₂^- has a negligible effect. In addition, the theoretical rate coefficients of the proton transfer reaction on the PES show inverse temperature dependence from 150 to 750 K, in accordance with the available experimental results.

A sub-4 Kelvin radio frequency linear multipole wire trap

A linear cryogenic 16-pole wire ion trap has been developed and constructed for cryogenic ion spectroscopy at temperatures below 4 K. The trap is temperature-variable, can be operated with different buffer gases, and offers large optical access perpendicular to the ion beam direction. The housing geometry enables temperature measurement during radio frequency operation. The effective trapping potential of the wire-based radio frequency trap is described and compared to conventional multipole ion trap designs. Furthermore, time-of-flight mass spectra of multiple helium tagged protonated glycine ions that are extracted from the trap are presented, which prove very low ion temperatures and suitable conditions for sensitive spectroscopy.

Threshold photodetachment spectroscopy of the astrochemical anion CN. J Chem Phys 2020;153:184309. [PMID: 33187436 DOI: 10.1063/5.0029841]

Threshold photodetachment spectroscopy has been performed on the molecular anion CN^- at both 16(1) K and 295(2) K in a 22-pole ion trap and at 295(2) K from a pulsed ion beam. The spectra show a typical energy dependence of the detachment cross section yielding a determination of the electron affinity of CN to greater precision than has previously been known at 31 163(16) cm^-1 [3.864(2) eV]. Allowed s-wave detachment is observed for CN^-, but the dependence of the photodetachment cross section near the threshold is perturbed by the long-range interaction between the permanent dipole moment of CN and the outgoing electron. Furthermore, we observe a temperature dependence of the cross section near the threshold, which we attribute to a reduction of the effective permanent dipole due to higher rotational excitation at higher temperatures.

Considerable matrix shift in the electronic transitions of helium-solvated cesium dimer cation Cs2He

We investigate the photodissociation of helium-solvated cesium dimer cations using action spectroscopy and quantum chemical calculations. The spectrum of Cs2He+ shows three distinct absorption bands into both bound and dissociative states. Upon solvation with further helium atoms, considerable shifts of the absorption bands are observed, exceeding 0.1 eV (850 cm-1) already for Cs2He10+, along with significant broadening. The shifts are highly sensitive to the character of the excited state. Our calculations show that helium atoms adsorb on the ends of Cs2+. The shifts are particularly pronounced if the excited state orbitals extend to the area occupied by the helium atoms. In this case, Pauli repulsion leads to a deformation of the excited state orbitals, resulting in the observed blue shift of the transition. Since the position of the weakly bound helium atoms is ill defined, Pauli repulsion also explains the broadening.

Modeling Ionic Reactions at Interstellar Temperatures: The Case of NH2- + H2 ⇔ NH3 + H. J Phys Chem A 2019;123:9905-9918. [PMID: 31633351 DOI: 10.1021/acs.jpca.9b07317]

Structural features and enthalpy details are presented for the title reactions, both for the exothermic (forward) path to NH₃ formation and for the endothermic (reverse) reaction to NH₂^- formation. Both pathways have relevance for the nitrogen chemistry in the interstellar medium (ISM). They are also helpful to document the possible role of H^- in molecular clouds at temperatures well below room temperature. The structural calculations are carried out using different ab initio methods and are further employed to obtain the reaction rates down to the interstellar temperatures detected in earlier experiments. The reaction rates are obtained from the computed minimum energy path (MEP) using the variational transition-state theory (VTST) approach. The results indicate very good accord with experiment results at room temperature, while measured low temperature data down to 8 K are well described using an appropriately modified VTST approach. This is done by employing a temperature-dependent scaling, from room temperature conditions down to the lower ISM temperatures, which acknowledges the noncanonical behavior of the fast, barrierless exothermic reaction. The reasons for this behavior and the need for improving on the VTST method when used away from room temperatures are discussed.

Modeling state-selective photodetachment in cold ion traps: Rotational state "crowding" in small anions

Using accurate ab initio calculations of the interaction forces, we employ a quantum mechanical description of the collisional state-changing processes that occur in a cold ion trap with He as a buffer gas. We generate the corresponding inelastic rates for rotational transitions involving three simple molecular anions OH^-(¹Σ), MgH^-(¹Σ), and C₂H^-(¹Σ) colliding with the helium atoms of the trap. We show that the rotational constants of these molecular anions are such that within the low-temperature regimes of a cold ion trap (up to about 50 K), a different proportion of molecular states are significantly populated when loading helium as a buffer gas in the trap. By varying the trap operating conditions, population equilibrium at the relevant range of temperatures is reached within different time scales. In the modeling of the photodetachment experiments, we analyze the effects of varying the chosen values for photodetachment rates as well as the laser photon fluxes. Additionally, the changing of the collision dynamics under different buffer gas densities is examined and the best operating conditions, for the different anions, for yielding higher populations of specific rotational states within the ion traps are extracted. The present modeling thus illustrates possible preparation of the trap conditions for carrying out more efficiently state-selected experiments with the trapped anions.

The FELion cryogenic ion trap beam line at the FELIX free-electron laser laboratory: infrared signatures of primary alcohol cations

The FELion beamline – a cryogenic 22-pole trap for vibrational spectroscopy of molecular ions at the FELIX Laboratory.

Double Resonance Rotational Spectroscopy of Weakly Bound Ionic Complexes: The Case of Floppy CH_{3}^{+}-He

A novel rotational spectroscopy method applicable to ions stored in cold traps is presented. In a double resonance scheme, rotational excitation is followed by vibrational excitation into a dissociative resonance. Its general applicability is demonstrated for the CH_{3}^{+}-He complex, which undergoes predissociation through its C-H stretching modes ν_{1} and ν_{3}. High resolution rotational transitions are recorded for this symmetric top, and small unexpected splittings are resolved for K=1. Advantages and potential future applications of this new approach are discussed.

Photodetachment spectroscopy of cold trapped NH2- near threshold

We have studied photodetachment of the amidogen anion NH2- as a function of photon energy near the detachment threshold. The detachment spectrum is obtained over the energy range of 6190-6355 cm^-1 from the loss rate of the anions from a cryogenic radiofrequency multipole ion trap. By modeling all accessible rotational state-to-state photodetachment transitions, we can assign rotational state-specific thresholds to the measured spectrum. In this way, we have determined the electron affinity of NH₂ to be 6224 ± 1 cm^-1.