Accurate Frequency Determination of Vibration-Rotation and Rotational Transitions of SiH. Astrophys J 2017;849:60. [PMID: 29142330 PMCID: PMC5683345 DOI: 10.3847/1538-4357/aa8fca]

Electronic and vibrational spectroscopies of aromatic clusters with He in a supersonic jet: The case of neutral and cationic phenol-Hen (n = 1 and 2)

Van der Waals clusters composed of He and aromatic molecules provide fundamental information about intermolecular interactions in weakly bound systems. In this study, phenol-helium clusters (PhOH-Hen with n ≤ 2) are characterized for the first time by UV and IR spectroscopies. The S1 ← S0 origin and ionization energy both show small but additive shifts, suggesting π-bound structures of these clusters, a conclusion supported by rotational contour analyses of the S1 origin bands. The OH stretching vibrations of the PhOH moiety in the clusters match with those of bare PhOH in both the S0 and D0 states, illustrating the negligible perturbation of the He atoms on the molecular vibration. Matrix shifts induced by He attachment are discussed based on the observed band positions with the help of complementary quantum chemical calculations. For comparison, the UV and ionization spectra of PhOH-Ne are reported as well.

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.

Characterization of the low-lying electronic states of tin monohydride cation including the spin-orbit coupling effect: A theoretical perspective

High-level ab initio computations have been performed on SnH⁺. The potential energy curves and spectroscopic constants of the low-lying Λ-S electronic states, as well as their associated Ω states, are derived at the icMRCI + Q level employing basis sets of quintuple-ζ quality. The transition dipole moments, Einstein coefficients, radiative lifetimes and Franck-Condon factors of three spin-forbidden transition bands ( [Formula: see text] , [Formula: see text] and [Formula: see text] ) are determined. Comparisons between our predictions and available experimental results indicate reasonable agreement. The spin-orbit coupling effect has been proved to affect these low-lying electronic states significantly.

High-resolution double resonance action spectroscopy in ion traps: vibrational and rotational fingerprints of CH2NH2. Phys Chem Chem Phys 2019;21:26406-26412. [PMID: 31793941 DOI: 10.1039/c9cp05487a]

By applying various action spectroscopic techniques in a 4 K cryogenic ion trap instrument, protonated methanimine, CH2NH2+, has been investigated by high-resolution rovibrational and pure rotational spectroscopy for the first time. In total, 39 rovibrational transitions within the fundamental band of the ν2 symmetric C-H stretch were measured around 3026 cm-1, which were used to predict pure rotational transition frequencies of CH2NH2+ in the ground vibrational state. Based on these predictions, nine rotational transitions were observed between 109 and 283 GHz using a novel double resonance method, which significantly improved the sensitivity of the rotational measurements. This double resonance method consists of rotational excitation followed by vibrational excitation, which is finally detected as a dip in the number of CH2NH2+-He complexes formed in the 4 K He bath of the trap. The new measurements and the derived predictions of pure rotational transitions will enable the first radio-astronomical search for CH2NH2+.

Double resonance rotational spectroscopy of He-HCO. Phys Chem Chem Phys 2019;21:3440-3445. [PMID: 30191208 DOI: 10.1039/c8cp04532a]

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.

Accurate Rotational Rest Frequencies for Ammonium Ion Isotopologues

We report rest frequencies for rotational transitions of the deuterated ammonium isotopologues NH3D+,NH 2 D 2 + and NHD D 3 + , measured in a cryogenic ion trap machine. For the symmetric tops NH3D+ andNHD 3 + one and three transitions are detected, respectively, and five transitions are detected for the asymmetric topNH 2 D 2 + . While the lowest frequency transition of NH3D+ was already known in the laboratory and space, this work enables the future radio astronomical detection of the two other isotopologues.

High-resolution infrared spectroscopy of O2H+ in a cryogenic ion trap

The protonated oxygen molecule, O₂H⁺, and its helium complex, He-O₂H⁺, have been investigated by vibrational action spectroscopy in a cryogenic 22-pole ion trap. For the He-O₂H⁺ complex, the frequencies of three vibrational bands have been determined by predissociation spectroscopy. The elusive O₂H⁺ has been characterized for the first time by high-resolution rovibrational spectroscopy via its ν₁ OH-stretching band. Thirty-eight rovibrational fine structure transitions with partly resolved hyperfine satellites were measured (56 resolved lines in total). Spectroscopic parameters were determined by fitting the observed lines with an effective Hamiltonian for an asymmetric rotor in a triplet electronic ground state, X̃³A^'', yielding a band origin at 3016.73 cm^-1. Based on these spectroscopic parameters, the rotational spectrum is predicted, but not yet detected.

First Laboratory Detection of Vibration-Rotation Transitions of 12CH+ and 13CH+ and Improved Measurement of their Rotational Transition Frequencies

The long-searched C-H stretches of the fundamental ions CH+ and 13CH+ have been observed for the first time in the laboratory. For this, the state-dependent attachment of He atoms to these ions at cryogenic temperatures has been exploited to obtain high-resolution rovibrational data. In addition, the lowest rotational transitions of CH+, 13CH+ and CD+ have been revisited and their rest frequency values improved substantially.