H₃⁺ is the simplest polyatomic molecule, and its rovibrational energy levels provide valuable benchmarks for ab initio theorists. Calculations of the H₃⁺ potential energy surface which take into account effects beyond the Born-Oppenheimer approximation can predict rovibrational transitions from low lying states with an accuracy of 0.001 cm⁻¹,L. G. Diniz, J. R. Mohallem, A. Alijah, M. Pavanello, L. Adamowicz, O. L. Polyansky, and J. Tennyson, Phys. Rev. A, 88, 032406 (2013).nd agreement is on the order of 0.01-0.1 cm⁻¹ for transitions from higher levels. As the accuracy of theoretical methods begins to rival experimental uncertainties, new measurements are needed to benchmark future ab initio calculations. In order to provide accurate experimentally determined energy levels, a survey of rovibrational transitions of H₃⁺ has been collected using the sub-Doppler technique Noise-Immune Cavity-Enhanced Optical Heterodyne Velocity Modulation Spectroscopy (NICE-OHVMS).B. M. Siller, M. W. Porambo, A. A. Mills, and B. J. McCall, Opt. Express, 19,24822-7 (2011).n total, we have measured 56 transitions in the ν₂←0 fundamental band,J. N. Hodges, A. J. Perry, P. A. Jenkins II, B. M. Siller, and B. J. McCall, J. Phys. Chem., 139, 164201, (2013).A. J. Perry, J. N. Hodges, C. R. Markus, G. S. Kocheril, and B. J. McCall, J. Mol. Spectrosc., 317,71-73, (2015). 17 transitions in the 2ν₂²←ν₂ hot band, and 7 transitions in the 2ν₂²←0 overtone band with approximately 4 MHz uncertainty. For most transitions, this was an improvement by a factor of 40 or more. Combination differences were used to calculate ground state rotational levels relative to the lowest ortho and para states. A fit of the ground vibrational state to an effective Hamiltonian was used to determine energy levels relative to the forbidden (0,0) rotational state. Overall, 18 absolute energy levels were determined with uncertainties of approximately 0.0003 cm⁻¹ (10 MHz). In addition, frequencies of forbidden rotational transitions were predicted, including a possible astrophysical maser.J. H. Black, Faraday Discussions, 109, 257-266 (1998).html:<hr /><h3>Footnotes: L. G. Diniz, J. R. Mohallem, A. Alijah, M. Pavanello, L. Adamowicz, O. L. Polyansky, and J. Tennyson, Phys. Rev. A, 88, 032406 (2013).a B. M. Siller, M. W. Porambo, A. A. Mills, and B. J. McCall, Opt. Express, 19,24822-7 (2011).I J. N. Hodges, A. J. Perry, P. A. Jenkins II, B. M. Siller, and B. J. McCall, J. Phys. Chem., 139, 164201, (2013).

---

Footnotes:

J. H. Black, Faraday Discussions, 109, 257-266 (1998).

The pure rotational spectrum of the elusive CN⁺ cation has been observed for the first time using the cryogenic ion trap apparatus Coltrap by applying an action spectroscopy scheme. For the ¹²C¹⁴N⁺ species, the three lowest rotational transitions have been observed each of which exhibits hyperfine structure from the presence of the ¹⁴N nucleus. The rare ¹²C¹⁵N⁺ isotopolog has been studied up to the J=4−3 transition. The observations conclusively confirm CN⁺ to occupy a ¹Σ⁺ electronic ground state. Given the ubiquity of the CN radical in space, CN⁺ is an appealing candidate for future radio astronomical searches.

r0pt Figure Previous studies of the cyanomethyl radical CH₂CN⁻ have observed sharp resonances in the photoelectron spectrum of the anion, that occur due to the existence of a dipole bound state (DBS).M. L. Weichman, J. B. Kim, and D. M. Neumark, J. Chem. Phys. 140, 104305 (2014)J. Lyle, O. Wedig, S. Gulania, A. Krylov, and R. Mabbs, J. Chem. Phys. 147, 234309, (2017) At specific detachment wavelengths, the anion may be excited to the DBS, which subsequently autodetaches. Autodetachment transitions may be readily identified in the anion photoelectron spectrum by both their sensitivity to laser wavelength, and the differing relationship between photon and electron energy. This study investigates CH₂CN⁻ using high-resolution photoelectron imaging. Spectra were recorded at a range of wavelengths from 355−780 nm, with resonances observed at hv = 662, 722, 725, and 767 nm. These measurements resolve the K-rotational structure of the molecule, allowing for the exact rotational transitions involved in the autodetachment to be identified. The spectra are further complicated by the presence of vibronic coupling between the neutral ground ²B₁ and excited ²A₁ states, which may be identified from changes in the anisotropy of the electron distribution.

---

Footnotes:

M. L. Weichman, J. B. Kim, and D. M. Neumark, J. Chem. Phys. 140, 104305 (2014)

---

Footnotes:

Mid-infrared spectra of protonated methylamine, CH₃NH₃⁺, and ethylamine, C₂H₅NH₃⁺, have been recorded using the FELion ion trap connected to the Free Electron Laser for Infrared eXperiments (FELIX; Radboud University, Nijmegen, The Netherlands) employing infrared photodissociation of the corresponding neon-clusters. In addition, the pure rotational spectrum of CH₃NH₃⁺ has been observed for the first time. Rotational transitions were observed in the frequency region between 80 and 240 GHz in the Coltrap apparatus using the method of state-selective He-attachment. In contrast to methylamine which features a complex CH₃-internal-rotation/NH₂-inversion spectrum, its protonated variant CH₃NH₃⁺ exhibits the spectrum of a simple symmetric rotor in its ground vibrational state.

Ion-neutral reactions often occur rapidly at Langevin rate constants of  ∼ 10⁻⁹ cm³s⁻¹, even at low temperatures, which is why they are able to drive the chemistry of interstellar space. The reaction C₃H₂⁺ + H₂→ C₃H₃⁺ + H does not follow this trend, and has been known to occur at a much slower rate of  ∼ 2×10⁻¹³ cm³s⁻¹.I. Savi\'c and D. Gerlich, Phys. Chem. Chem. Phys., 7, 1026-1035, (2005).t was recently observed that when C₃H₂⁺ ions were resonantly excited by mid-IR light in a multipole ion trap held at 8 K, they were less likely to react with H₂ to form C₃H₃⁺. Here, possible mechanisms behind this inhibited reaction process are discussed. Additionally, this effect can be utilized as a new form of action spectroscopy by counting the number of formed product ions as a function of the excitation wavelength, which was used here to measure 91 rovibrational transitions in the ν₇ asymmetric C-H stretch band of the cyclic C₃H₂⁺ isomer. The new frequencies were used to determine accurate molecular constants that are compared to high-level ab initio calculations.

---

Footnotes:

I. Savi\'c and D. Gerlich, Phys. Chem. Chem. Phys., 7, 1026-1035, (2005).I

HO₄⁺ has been proposed as an astrochemical sink of oxygen but has never been observed.Xavier, G. D.; Bernal-Uruchurtu, M. I.; Hernandez-Lamoneda, R., J. Chem. Phys. 2014, 141, 5, 081101.olecular cations of HO₄⁺ and DO₄⁺ are produced in a supersonic expansion. They are mass-selected, and their infrared photodissociation spectra are measured with the aid of argon-tagging. Previous theoretical studies have modeled these systems as proton-bound dimers of molecular oxygen.Xavier, F. G. D.; Hernandez-Lamoneda, R., PCCP 2015, 17, 16023-16032.everal conformers were located on the quintet, triplet, and singlet surfaces, differing in energy by, at most, a few thousand wavenumbers; the singlet and triplet conformers have pronounced multiconfigurational character. Our HO₄⁺ is formed in a relatively hot environment, and similar experiments have been shown capable of producing multiple conformers in low-lying electronic states.Wagner, J. P.; McDonald, D. C.; Duncan, M. A., Angewandte Chemie-International Edition 2018, 57, 5081-5085.one of the predicted HO₄⁺ isomers can be ruled out a priori based on energetic arguments. We interpret our spectra with second-order vibrational perturbation theory with resonances (VPT2+K) using quartic force fields based on an economical combination of single- and multi-reference theories. The VPT2+K simulations include the effect of electrical anharmonicity; this is particularly important for transitions involving the shared-proton stretch. Previously unidentified chain structures (H-O-O-O-O) that exist on the singlet and triplet surfaces likely represent the global minima.

---

Footnotes:

Xavier, G. D.; Bernal-Uruchurtu, M. I.; Hernandez-Lamoneda, R., J. Chem. Phys. 2014, 141, 5, 081101.M Xavier, F. G. D.; Hernandez-Lamoneda, R., PCCP 2015, 17, 16023-16032.S Wagner, J. P.; McDonald, D. C.; Duncan, M. A., Angewandte Chemie-International Edition 2018, 57, 5081-5085.N

r0pt Figure The photoelectron spectrum of the vinoxy anion C₂H₃O⁻ is studied using velocity-imaging, with the angular distributions providing electronic-state and vibrational-mode specific characterization. A 355 nm photoelectron spectrum, together with anisotropy parameters determined for some of the stronger transitions is show in the figure. Photodetachment at longer wavelengths have also been measured. The spectroscopic analysis builds on a previous SEVISlow-Electron-Velocity-map-Imaging: Yacovitch et al. J Chem Phys 130 244309 (2009) doi:10.1063/1.3157208tudy, and a CRDSCavity-Ring-Down-Spectroscopy: Thomas et al. J Chem Phys 132 114302 (2010) doi:10.1063/1.3352976ork. Forbidden asymmetric vibrational modes may gain intensity through vibronic coupling between the neutral ground state and an excited state, of the correct symmetry, and have an anomalous anisotropy parameter. The positive X̃ transition is 9²₀ 11¹₁. In addition, the vinoxy anion has dipole-bound excited electronic states, that produce very narrow transitions. For C₂H₃O⁻ there is evidence of autodetachment from dipole-bound states of the anion, and vibronic coupling of the neutral ground state.

---

Footnotes:

Slow-Electron-Velocity-map-Imaging: Yacovitch et al. J Chem Phys 130 244309 (2009) doi:10.1063/1.3157208s Cavity-Ring-Down-Spectroscopy: Thomas et al. J Chem Phys 132 114302 (2010) doi:10.1063/1.3352976w

Transition metal ion-molecule complexes (e.g., M⁺(benzene), M⁺(furan), M⁺(methanol), etc. where M = Zn, Ag, Au) are generated in the gas phase by laser vaporization and are detected using a time-of-flight mass spectrometer. The ionization potentials of the metals are typically lower than those of many molecules, leading to the charge being localized on the metal in a cation-molecule complex. Laser excitation of the ion-molecule complexes leads to a charge transfer dissociation channel producing the molecular ion fragment. If the excitation wavelength is sufficiently high, excess kinetic energy release above the dissociation threshold can be detected using velocity map imaging. This energy release can then be used to calculate an upper bound on the metal ion-molecule bond energy.

CeC₃H₄ and CeC₃H₆ are observed in the reaction of Ce with propene in molecular beams and characterized by mass-analyzed threshold ionization (MATI) spectroscopy and relativistic quantum calculations. The MATI spectrum of each species displays two band systems, each consisting of vibronic progressions from Ce-C stretching and ligand bending excitations in the ionic states. The adiabatic ionization energies of CeC₃H₄ and CeC₃H₆ are 41035 (5) and 41868 (5) cm⁻¹, respectively. The two band systems are separated by 125 cm⁻¹for CeC₃H₄ and 60 cm⁻¹for CeC₃H₆. By comparing the splittings from the spectra with the relativistic calculations at the level of multiconfiguration quasi-degenerate second-order perturbation theory, we assign the two band systems to transitions from two spin-orbit levels of the neutral molecules.

Fluorescent proteins can incorporate a wide variety of chromophores which all lead to different absorption and emission characteristics. In the condensed phase, interactions of chromophores with their chemical environment (e.g., solvents, surrounding protein) change their photophysical properties. By examining these chromophores as mass-selected ions in vacuo, we are able to study their intrinsic photophysical properties without influence from such environments. Here we report electronic and infrared spectra of cryogenically prepared biliverdin anions. We interpret our experimental spectra through analysis of time-dependent density functional theory calculations.