Improved tandem mass spectrometer coupled to a laser vaporization cluster ion source

Photochemical properties of a potential interstellar dust precursor: the electronic spectrum of Si3O2. Phys Chem Chem Phys 2023. [PMID: 37365971 DOI: 10.1039/d3cp02693k] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Silicon oxide compounds are considered as precursors for silicon-based interstellar dust grains which consist mainly of silica and silicates. Knowledge of their geometric, electronic, optical, and photochemical properties provides crucial input for astrochemical models describing the evolution of dust grains. Herein, we report the optical spectrum of mass-selected Si₃O₂⁺ cations recorded in the 234-709 nm range by means of electronic photodissociation (EPD) in a quadrupole/time-of-flight tandem mass spectrometer coupled to a laser vaporization source. The EPD spectrum is observed predominantly in the lowest-energy fragmentation channel corresponding to Si₂O⁺ (loss of SiO), while the higher-energy Si⁺ channel (loss of Si₂O₂) provides only a minor contribution. The EPD spectrum exhibits two weaker unresolved bands A and B near 26 490 and 34 250 cm^-1 (377.5 and 292 nm) and a strong transition C with a band origin at 36 914 cm^-1 (270.9 nm) which shows vibrational fine structure. Analysis of the EPD spectrum is guided by complementary time-dependent density functional theory (TD-DFT) calculations at the UCAM-B3LYP/cc-pVTZ and UB3LYP/cc-pVTZ levels to determine structures, energies, electronic spectra, and fragmentation energies of the lowest-energy isomers. The cyclic global minimum structure with C_2v symmetry determined previously by infrared spectroscopy can explain the EPD spectrum well, with assignments of bands A-C to transitions from the ²A₁ ground electronic state (D₀) into the 4th, 9th, and 11th excited doublet states (D_4,9,11), respectively. The vibronic fine structure of band C is analyzed by Franck-Condon simulations, which confirm the isomer assignment. Significantly, the presented EPD spectrum of Si₃O₂⁺ corresponds to the first optical spectrum of any polyatomic Si_nO_m⁺ cation.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

The Electronic Spectrum of Si2. J Phys Chem Lett 2022;13:7624-7628. [PMID: 35951547 DOI: 10.1021/acs.jpclett.2c02200]

The optical spectrum of Si₂⁺ is presented. The two electronic band systems observed near 430 and 270 nm correspond to the two lowest optically allowed transitions of Si₂⁺ assigned to ⁴Σ_u^-(I) ← X⁴Σ_g^- and ⁴Σ_u^-(II) ← X⁴Σ_g^-. The spectra were measured via photodissociation spectroscopy of mass-selected ions at the level of vibrational resolution, and the determined spectroscopic constants provide detailed information about the geometric and electronic structure, establishing molecular constants of this fundamental diatomic cation that enable astrophysical detection on, for example, hot rocky super-Earth-like exoplanets.

Near-Infrared Spectrum of the First Excited State of Au2. Chemistry 2021;27:15074-15079. [PMID: 34423877 PMCID: PMC8596823 DOI: 10.1002/chem.202102542]

Au₂⁺ is a simple but crucial model system for understanding the diverse catalytic activity of gold. While the Au₂⁺ ground state (X²Σ_g⁺) is understood reasonably well from mass spectrometry and computations, no spectroscopic information is available for its first excited state (A²Σ_u⁺). Herein, we present the vibrationally resolved electronic spectrum of this state for cold Ar‐tagged Au₂⁺ cations. This exceptionally low‐lying and well isolated A²Σ_(u)⁺←X²Σ_(g)⁺ transition occurs in the near‐infrared range. The observed band origin (5738 cm⁻¹, 1742.9 nm, 0.711 eV) and harmonic Au−Au and Au−Ar stretch frequencies (201 and 133 cm⁻¹) agree surprisingly well with those predicted by standard time‐dependent density functional theory calculations. The linearly bonded Ar tag has little impact on either the geometric or electronic structure of Au₂⁺, because the Au₂⁺⋅⋅⋅Ar bond (∼0.4 eV) is much weaker than the Au−Au bond (∼2 eV). As a result of 6 s←5d excitation of an electron from the antibonding σ_u^* orbital (HOMO‐1) into the bonding σ_g orbital (SOMO), the Au−Au bond contracts substantially (by 0.1 Å).

Atomic Cluster Au_{10}^{+} Is a Strong Broadband Midinfrared Chromophore

We report an intense broadband midinfrared absorption band in the Au_{10}^{+} cluster in a region in which only molecular vibrations would normally be expected. Observed in the infrared multiple photon dissociation spectra of Au_{10}Ar^{+}, Au_{10}(N_{2}O)^{+}, and Au_{10}(OCS)^{+}, the smooth feature stretches 700-3400  cm^{-1} (λ=14-2.9  μm). Calculations confirm unusually low-energy allowed electronic excitations consistent with the observed spectra. In Au_{10}(OCS)^{+}, IR absorption throughout the band drives OCS decomposition resulting in CO loss, providing an alternative method of bond activation or breaking.

The Optical Spectrum of Au2. Angew Chem Int Ed Engl 2020;59:21403-21408. [PMID: 32888257 PMCID: PMC7756737 DOI: 10.1002/anie.202011337]

The electronic structure of the Au2 + cation is essential for understanding its catalytic activity. We present the optical spectrum of mass-selected Au2 + measured via photodissociation spectroscopy. Two vibrationally resolved band systems are observed in the 290-450 nm range (at ca. 440 and ca. 325 nm), which both exhibit rather irregular structure indicative of strong vibronic and spin-orbit coupling. The experimental spectra are compared to high-level quantum-chemical calculations at the CASSCF-MRCI level including spin-orbit coupling. The results demonstrate that the understanding of the electronic structure of this simple, seemingly H2 + -like diatomic molecular ion strictly requires multireference and relativistic treatment including spin-orbit effects. The calculations reveal that multiple electronic states contribute to each respective band system. It is shown that popular DFT methods completely fail to describe the complex vibronic pattern of this fundamental diatomic cation.