Photodetachment spectroscopy of cold trapped NH2- near threshold

Microsolvation of Hot Ions: Spectroscopy and Statistical Mechanics of Phenide-Water Interactions

Thermal excitation alters the spectroscopic signatures of solvated ions and affects their interactions with neighboring molecules. By analyzing the photoelectron spectra of microhydrated phenide (Ph-), the temperatures of the Ph-·H2O and Ph-·(H2O)2 clusters from a hot ion source were determined to be 560 and 520 K, respectively, vs 700 K for unsolvated Ph-. Compared to theory predictions for cold clusters, the high temperature of the environment significantly reduces the average hydration stabilization of the ions and the corresponding band shifts in their spectra. The results are discussed in terms of a statistical model that describes the energy content of the intermolecular (IM) degrees of freedom of the cluster, ⟨EIM⟩. We show that over the entire solvation energy range, the density of states associated with the IM modes of Ph-·H2O, of which there are only 6, is more than an order of magnitude greater than that associated with the 27 internal vibrations of the core anion. The results suggest that the observed cluster temperatures are not determined by the ion source but represent the intrinsic properties of the clusters. The energetics and statistical mechanics of microsolvation limit the excitation that the IM degrees of freedom can sustain without significant solvent evaporation on the timescale of the experiment. The limit is expressed as a characteristic solvation temperature (CST), which is the maximum canonical temperature of a stable cluster ensemble. Driven by evaporative cooling, the terminal cluster temperature from a hot ion source will always be close to the cluster's CST. Only if the source temperature is lower than CST will the observed cluster temperature be determined by the source conditions. An approximate rule is proposed for estimating the characteristic temperature of any cluster using the inflection point on the ⟨EIM⟩ vs T curve.

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.

Anion of Oxalyl Chloride: Structure and Spectroscopy

The structure and spectroscopy of the anion of oxalyl chloride are investigated using photoelectron imaging experiments and ab initio modeling. The photoelectron images, spectra, and angular distributions are obtained at 355 and 532 nm wavelengths. The 355 nm spectrum consists of a band assigned to a transition from the ground state of the anion to the ground state of the neutral. Its onset at ∼1.8 eV corresponds to the adiabatic electron affinity (EA) of oxalyl chloride, in agreement with the coupled-cluster calculations predicting an EA of 1.797 eV. The observed vertical detachment energy, 2.33(4) eV, is also in agreement with the theory predictions. The 532 nm spectrum additionally reveals a sharp onset near the photon-energy limit. This feature is ascribed to autodetachment via a low-energy anionic resonance. The results are discussed in the context of the substitution series, which includes glyoxal, methylglyoxal (single methyl substitution), biacetyl (double methyl substitution), and oxalyl chloride (double chlorine substitution). The EAs and anion detachment energies follow the trend: biacetyl < methylglyoxal < glyoxal ≪ oxalyl chloride. The electron-donating character of the methyl group has a destabilizing effect on the substituted anions, reducing the EA from glyoxal to methylglyoxal to biacetyl. In contrast, the strong electron-withdrawing (inductive) power of Cl lends additional stabilization to the oxalyl chloride anion, resulting in a large (∼1 eV) increase in its detachment energy compared to glyoxal.

Influence of a Supercritical Electric Dipole Moment on the Photodetachment of C_{3}N^{-}

Threshold photodetachment spectroscopy of the molecular ion C_{3}N^{-} has been performed at both 16(1) and 295(2) K in a 22-pole ion trap. The 295(2) K spectrum shows a large increase in the cross section with an onset about 200  cm^{-1} below threshold, which is explained by significant vibrational excitation of the trapped ions at room temperature. This excitation disappears at cryogenic temperatures leading to an almost steplike onset of the cross section at threshold, which cannot be adequately described with a Wigner threshold law. Instead, we show that the model developed by O'Malley for photodetachment from neutrals with large permanent dipoles [Phys. Rev. 137, A1668 (1965)PHRVAO0031-899X10.1103/PhysRev.137.A1668] fits very well to the data. A high-resolution scan of the threshold region yields additional features, which we assign to the rotational P and R branches of an electronic transition to a dipole-bound state with ^{1}Σ^{+} symmetry. This state is found 2(1)  cm^{-1} below threshold in very good agreement with a recent computational prediction. We furthermore refine the value of the electron affinity of C_{3}N to be 34 727(1)  cm^{-1}.

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.

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.

Collisional Quantum Dynamics for MgH- (1Σ+) With He as a Buffer Gas: Ionic State-Changing Reactions in Cold Traps

We present in this paper a detailed theoretical and computational analysis of the quantum inelastic dynamics involving the lower rotational levels of the MgH⁻ (X¹Σ⁺) molecular anion in collision with He atoms which provide the buffer gas in a cold trap. The interaction potential between the molecular partner and the He (¹S) gaseous atoms is obtained from accurate quantum chemical calculations at the post-Hartree-Fock level as described in this paper. The spatial features and the interaction strength of the present potential energy surface (PES) are analyzed in detail and in comparison with similar, earlier results involving the MgH⁺ (¹Σ) cation interacting with He atoms. The quantum, multichannel dynamics is then carried out using the newly obtained PES and the final inelastic rats constants, over the range of temperatures which are expected to be present in a cold ion trap experiment, are obtained to generate the multichannel kinetics of population changes observed for the molecular ion during the collisional cooling process. The rotational populations finally achieved at specific temperatures are linked to state-selective laser photo-detachment experiments to be carried out in our laboratory.All intermediate steps of the quantum modeling are also compared with the behavior of the corresponding MgH⁺ cation in the trap and the marked differences which exist between the collisional dynamics of the two systems are dicussed and explained. The feasibility of the present anion to be involved in state-selective photo-detachment experiments is fully analyzed and suggestions are made for the best performing conditions to be selected during trap experiments.

Collisional relaxation kinetics for ortho and para NH2- under photodetachment in cold ion traps

The collisional cooling of the internal rotational states of the nonlinear anion NH2- (1A1), occurring at the low temperature of a cold ion trap under helium buffer gas cooling, is examined via quantum dynamics calculations and ion decay rate measurements. The calculations employ a novel ab initio potential energy surface that describes the interaction anisotropy and range of action between the molecular anions and the neutral He atoms. The state changing integral cross sections are employed to obtain the state-to-state rate coefficients, separately for the ortho- and the para-NH2- ions. These rates are in turn used to compute the state population evolution in the trap for both species, once photodetachment by a laser is initiated in the trap. The present work shows results for the combined losses of both species after the photodetachment laser is switched on and analyzes the differences of loss kinetics between the two hyperfine isomers.