Toward Detection of FeH+ in the Interstellar Medium: Infrared Multiple Photon Dissociation Spectroscopy of Ar2FeH. J Phys Chem Lett 2022;13:5867-5872. [PMID: 35728268 PMCID: PMC9251756 DOI: 10.1021/acs.jpclett.2c01511]

Symmetry reduction induced by argon tagging gives access to low-lying excited states of FeH+ in the overtone region of the Fe-H stretching mode

Iron is the most abundant transition metal in the interstellar medium (ISM), and is thought to be involved in a variety of astrochemical processes. Here, we present the infrared multiple photon dissociation (IRMPD) spectra of Ar1,2FeH+ and their deuterated isotopologues in the region of 2240-14 000 cm-1. The Fe-H overtone stretching mode in ArFeH+ and Ar2FeH+ is observed at 3636 ± 28 cm-1 and 3659 ± 13 cm-1, respectively. Deuteration shifts these bands to 2618 ± 31 cm-1 and 2650 ± 14 cm-1 in ArFeD+ and Ar2FeD+, respectively. Additionally, the spectra of Ar2FeH+ and Ar2FeD+ feature broad transitions at ∼2200-4000 cm-1 and ∼4500-6500 cm-1. We assign these bands to electronic transitions from the thermally populated X5A2/X'5A1 ground state manifold into the A'5B2 and B5A1 states, which we model with multi-reference quantum chemical calculations including spin-orbit coupling. The calculations show that these transitions are symmetry forbidden in FeH+ and in the equilibrium geometry of ArFeH+/ArFeD+, while the zero-point oscillation of the bending mode of the triatomic molecule leads to some oscillator strength. Upon addition of the second argon atom, the transitions become weakly allowed in the equilibrium geometry of Ar2FeH+/Ar2FeD+ due to symmetry reduction from C∞v to C2v.

Iron Complexes as Potential Carriers of Diffuse Interstellar Bands: The Photodissociation Spectrum of Fe+(H2O) at Optical Wavelengths

The fullerene ion C60+ is the only carrier of diffuse interstellar bands (DIBs) identified so far. Transition-metal compounds feature electronic transitions in the visible and near-infrared regions, making them potential DIB carriers. Since iron is the most abundant transition metal in the cosmos, we here test this idea with Fe+(H2O). Laboratory spectra were obtained by photodissociation spectroscopy at 80 K. Spectra were modeled with the reflection principle. A high-resolution spectrum of the DIB standard star HD 183143 served as an observational reference. Two broad bands were observed from 4120 to 6800 Å. The 4120-4800 Å band has sharp features emerging from the background, which have the width of DIBs but do not match the band positions of the reference spectrum. Calculations show that the spectrum arises from a d-d transition at the iron center. While no match was found for Fe+(H2O) with known DIBs, the observation of structured bands with line widths typical for DIBs shows that small molecules or molecular ions containing iron are promising candidates for DIB carriers.

Dynamics of the Oxygen Atom Transfer Reaction between Carbon Dioxide and the Tantalum Cation

The understanding of fundamental atomic-level processes often requires well-defined model systems. The oxygen atom transfer from CO₂ to a transition metal cation in the gas phase presents such a model system. We investigate the reaction of Ta⁺ + CO₂ for which the formation of TaO⁺ is highly efficient and attributed to multistate reactivity. Here, we study the atomistic dynamics of the oxygen atom transfer reaction by recording experimental energy and angle differential cross sections by crossed beam velocity map imaging supported by ab initio quantum chemical calculations. Product ion velocity distributions are dominated by signatures for indirect dynamics, despite the reaction being highly exothermic. Product kinetic energy distributions show little dependence on additional collision energy even with only four atoms involved, which hints at dynamical trapping behind a submerged barrier.