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The rotationally resolved infrared (IR) spectrum of the He-H

The rotationally resolved infrared (IR) spectrum of the He–[Formula: see text] complex has been measured in a cryogenic ion trap experiment at a nominal temperature of 4 K. Predissociation of the stored complex has been invoked by excitation of the degenerate ν2 mode of the [Formula: see text] sub-unit using a pulsed optical parametric oscillator system. An assignment of the experimental spectrum became possible through one-to-one correlations with bands of the spectrum theoretically predicted in Paper I [Harding et al., J. Chem. Phys. 156, 144307 (2022)]. 19 bands have been assigned and analyzed, and the energy term diagram of the lower states of this floppy molecular complex has been derived from combination differences (CDs) in the experimental spectrum. Ground state combination differences (GSCDs) reveal a large part of the energy term diagram for the He–[Formula: see text] complex in its vibrational ground state, v = 0. Experimental and theoretical term energies agree within experimental accuracy for the rotational fine structure associated with the total angular momentum quantum number J and the parity e/ f as well as for the coarse spacing of the lowest K states of the complex. This favorable comparison shows that the potential energy surface (PES) calculated in Paper I is accurate. The barriers between the three equivalent global minima in this PES are relatively low and the He–[Formula: see text] complex is extremely floppy, with nearly unhindered internal rotation of the [Formula: see text] sub-unit. The resulting Coriolis interactions couple the internal and end-over-end rotation of the complex and contribute significantly to the energy terms. They are observed both in experiment and theory and are, e.g., the origin of different rotational constants for states of e and f parity. Also in this respect, experiment and theory agree very well. Despite the assignment and analysis of many bands of the extremely rich IR spectrum of He–[Formula: see text], higher levels of excitation, including the complex stretching mode, need further attention.

A cryogenic 22-pole ion trap apparatus is used in combination with a table-top pulsed IR source to probe weakly bound CH(+)-He and CH(+)-He(4) complexes by predissociation spectroscopy at 4 K. The infrared photodissociation spectra of the C–H stretching vibrations are recorded in the range of 2720–2800 cm(−1). The spectrum of CH(+)-He exhibits perpendicular transitions of a near prolate top with a band origin at 2745.9 cm(−1), and thus confirms it to have a T-shaped structure. For CH(+)-He(4), the C-H stretch along the symmetry axis of this oblate top results in parallel transitions.

Experiments within a cryogenic 22-pole ion trap have revealed an interesting reaction dynamic phenomenon, where rovibrational excitation of an ionic molecule slows down a reaction with a neutral partner. This is demonstrated for the low-temperature hydrogen abstraction reaction $\mathrm{c}\text{\ensuremath{-}}{\mathrm{C}}_{3}{{\mathrm{H}}_{2}}^{+}+{\mathrm{H}}_{2}$, where excitation of the ion into the ${\ensuremath{\nu}}_{7}$ antisymmetric C-H stretching mode decreased the reaction rate coefficient toward the products $\mathrm{c}\text{\ensuremath{-}}{\mathrm{C}}_{3}{{\mathrm{H}}_{3}}^{+}+\mathrm{H}$. Supported by high-level quantum-chemical calculations, this observation is explained by the reaction proceeding through a $\mathrm{c}\text{\ensuremath{-}}{\mathrm{C}}_{3}{{\mathrm{H}}_{2}}^{+}\ensuremath{-}{\mathrm{H}}_{2}$ collision complex in the entrance channel, in which the hydrogen molecule is loosely bound to the hydrogen atom of the $\mathrm{c}\text{\ensuremath{-}}{\mathrm{C}}_{3}{{\mathrm{H}}_{2}}^{+}$ ion. This discovery enables high-resolution vibrational action spectroscopy for $\mathrm{c}\text{\ensuremath{-}}{\mathrm{C}}_{3}{{\mathrm{H}}_{2}}^{+}$ and other molecular ions with similar reaction pathways. Moreover, a detailed kinetic model relating the extent of the observed product depletion signal to the rate coefficients of inelastic collisions reveals that rotational relaxation of the vibrationally excited ions is significantly faster than the rovibrational relaxation, allowing for a large fraction of the ions to be vibrationally excited. This result provides fundamental insight into the mechanism for an important class of chemical reactions, and is capable of probing the inelastic collisional dynamics of molecular ions.

The carbon chain ions HC$_3$O$^+$ and HC$_3$S$^+$ - longer variants of the famous "X-ogen" line carrier HCO$^+$ - have been observed for the first time using two cryogenic 22-pole ion trap apparatus (FELion, Coltrap) and two different light sources: the Free Electron Laser for Infrared eXperiments (FELIX), which was operated between 500 and 2500 cm$^{-1}$ and an optical parametric oscillator operating near 3200 cm$^{-1}$; signals from both experiments were detected by infrared predissociation action spectroscopy. The majority of vibrational fundamentals were observed for both ions and their vibrational wavenumbers compare very favorably with results from high-level anharmonic force field calculations performed here at the coupled-cluster level of theory. As the action spectroscopic scheme probes the Ne-tagged weakly bound variants, Ne$-$HC$_3$O$^+$ and Ne$-$HC$_3$S$^+$, corresponding calculations of these systems were also performed. Differences in the structures and molecular force fields between the bare ions and their Ne-tagged complexes are found to be very small., Comment: Accepted for publication in Mol. Phys

The ν 1 fundamental vibrational mode of the C5S carbon-sulfur chain has been observed at high-spectral resolution in the gas phase for the first time. C5S was produced using laser ablation of carbon-sulfur targets and observed in a free-jet expansion using quantum cascade laser spectroscopy. The ν 1 mode is found centered at 2142.3072(1) cm−1, blueshifted by some 4 cm−1 from the ν 4 vibrational fundamental of the C7 pure carbon cluster which is also observed under the same experimental conditions. The present study is complemented by high-level CCSD(T) calculations revealing very good agreement between calculated and experimental molecular parameters.

High-resolution infrared studies of laser ablation products from carbon-selenium targets have revealed a new vibrational band at 2057 cm−1 that is identified as the ν 3 vibrational fundamental of the SeC3Se cluster. Because of the rich isotopic composition of selenium and the heavy nuclear masses involved, the vibrational band shows a relatively compact and complex structure despite the simple linear geometric arrangement. Overall, rotational-vibrational lines of six isotopologues could be assigned and fitted permitting the derivation of an accurate selenium-carbon bond length. Spectroscopic analysis has been greatly supported by high-level quantum-chemical calculations of the molecular structure and the harmonic and anharmonic force fields performed at the CCSD(T) level of theory. Scalar-relativistic effects on the molecular structure were also considered but found of little importance.

5 pags., 5 figs., 2 tabs., A cryogenic 22-pole ion trap apparatus is used in combination with a table-top pulsed IR source to probe weakly bound CH-He and CH-He complexes by predissociation spectroscopy at 4 K. The infrared photodissociation spectra of the C–H stretching vibrations are recorded in the range of 2720–2800 cm. The spectrum of CH-He exhibits perpendicular transitions of a near prolate top with a band origin at 2745.9 cm, and thus confirms it to have a T-shaped structure. For CH-He, the C-H stretch along the symmetry axis of this oblate top results in parallel transitions., This work has been supported via Collaborative Research Centre 956, sub-project B2, funded by the Deutsche Forschungsgemein- schaft (DFG, project ID 184018867) as well as DFG SCHL 341/15–1 (‘‘Cologne Center for Terahertz Spectroscopy”) and AS 319/2–2. JLD acknowledges partial financial support from the Agencia Estatal de Investigación (AEI) through grant FIS2016- 77726-C3-1-P and from the European Research Council through grant agreement ERC-2013-SyG-610256-NANOCOSMOS.

The ν 7 antisymmetric C–H stretching fundamental of c- C 3 H 2 + has been characterized in a cryogenic 22-pole ion trap by a novel type of action spectroscopy, in which the rovibrational excitation of c- C 3 H 2 + is detected as a slowing down of the low-temperature reaction c- C 3 H 2 + + H2 → C 3 H 3 + + H. Ninety-one rovibrational transitions with partly resolved fine structure doublets were measured in high resolution. Supported by high-level quantum chemical calculations, spectroscopic parameters were determined by fitting the observed lines with an effective Hamiltonian for an asymmetric rotor in a doublet electronic ground state, X ˜ A 1 2 , yielding a band origin at 3113.6400(3) cm−1. Based on these spectroscopic parameters, the rotational spectrum of this astronomically important molecule is predicted.

The ground state of He–HCO+ is investigated using a recently developed double resonance technique, consisting of a rotational transition followed by a vibrational transition into a dissociative state. In order to derive precise predictions for the rotational states, the high resolution infrared predissociation spectroscopy of the v1 C–H stretching mode is revisited. Eleven pure rotational transitions are measured via the double resonance method. A least squares fit of these transitions to a standard linear rotor Hamiltonian reveals that the semirigid rotor model cannot fully describe the loosely bound He–HCO+ complex. The novel double resonance technique is compared with other action spectroscopic schemes, and some potential future applications are presented.

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.

The fundamental vibrational bending mode ν 2 of disulfur monoxide, S2O, and the associated hot band 2 ν 2 - ν 2 have been observed at high spectral resolution for the first time at the SOLEIL synchrotron facility using Fourier-transform far-infrared spectroscopy. This transient species has been produced in a radio-frequency discharge by flowing SO2 over elemental sulfur. The spectroscopic analysis has been performed using the newly developed Automated Spectral Assignment Procedure (ASAP) which has enabled the accurate determination of more than 3500 energy levels of the v 2 = 1 and 2 vibrational states. The procedure provides a fast and convenient way to analyze large data sets in a straightforward manner, if one of the two vibrational states involved in the transition is accurately known from prior work. In addition to the high-resolution synchrotron study, pure rotational spectra of S2O in the v 2 = 1 and 2 vibrational states were observed in the frequency range 250–500 GHz by absorption spectroscopy in a long-path absorption cell. From these combined measurements, extensive molecular parameter sets have been determined, including full sets of sextic and two octic centrifugal distortion terms. Highly accurate band centers (to better than 10 - 5 cm−1) have been derived for both vibrational bands.

The fundamental S–S stretching mode ν 3 of disulfur monoxide, S 2 O, located at 679 cm −1 , has been investigated using Fourier-transform far-infrared spectroscopy at the SOLEIL synchrotron facility. A spectroscopic analysis has been performed using an Automated Spectral Assignment Procedure (ASAP) which permits accurate determination of more than 2000 energy levels from ν 3 . In addition, the ν 3 + ν 2 − ν 2 hot band was observed for the first time and some 500 corresponding energy levels were assigned. The high-resolution synchrotron study was complemented with pure rotational spectra of S 2 O in the ( v 1 , v 2 , v 3 ) = (0, 0, 1) vibrational state observed in the frequency range from 250 to 280 GHz using a long-path absorption cell. From these combined measurements, extensive molecular parameter sets have been determined and precise band centers have been derived for both vibrational bands.

In the course of a 5 μm high-resolution infrared study of laser ablation products from carbon–sulfur targets, the ν1 vibrational mode region of linear C3S has been studied continuously from 2046 to 2065 cm−1. Besides the prominent vibrational fundamental, the region was found to feature the ν1+ν5−ν5, ν1+ν4−ν4, and even ν1+ν3−ν3 hot bands, the latter two of which were observed for the first time. Owing to the high signal-to-noise ratio obtained, the ν1 mode of C334S could also be observed in natural abundance for the first time at high spectral resolution in the infrared. At 2061 cm−1, hidden inside the R−branch of the C3S ν1 fundamental mode, a weak new band is observed which exhibits very tight line spacing and stems from a heavy both carbon and sulfur containing carrier. On the basis of high-level quantum-chemical calculations of selected carbon–sulfur chains and other carbon-rich cumulenes, this feature is attributed to the ν5 vibrational fundamental of linear SC7S, which stands for the first gas-phase spectroscopic detection of this long cumulenic chain.

A novel method of spectroscopy in ion traps termed leak-out spectroscopy (LOS) is presented. Here, mass selected, cold ions are excited by an infrared laser. In a subsequent collision with a neutral buffer gas particle their internal energy is then transferred to kinetic energy. As a result, these ions leak out from the ion trap and are detected. The LOS scheme is generally applicable, very sensitive and close to background free when operated at low temperature. The potential of this method is demonstrated and characterized here for the first time by recording the rotationally resolved spectrum of the C-H stretching vibration $\nu_1$ of linear C$_3$H$^+$. Besides performing high-resolution spectroscopy, this method opens up the way for analyzing the composition of trap content, e.g., determining isomer ratios, by selectively expelling isomers or other isobaric ions from the trap. Likewise, LOS can be used to prepare clean samples of structural and nuclear spin isomers., Comment: This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Journal of Physical Chemistry A, \copyright, after peer review