Protonated polycyclic aromatic hydrocarbons (H⁺PAH) have been reported to have infrared (IR) bands at wavenumbers near those of unidentified infrared (UIR) emission bands from interstellar objects. However, recording IR spectra of H⁺PAH in laboratories is challenging. Two spectral methods have been employed previously to yield IR spectra of H⁺PAH. One employs IR multiphoton dissociation (IRMPD) of H⁺PAH, but the bands are broad and red-shifted. ¹ Another measures the single-photon IR photodissociation action spectrum of cold H⁺PAH tagged with a weakly bound ligand, such as Ar, but application of this method to large PAH is difficult.² A new method for investigating IR spectra of H⁺PAH and their neutral counterparts was developed using electron bombardment during p-H₂ matrix deposition. With this technique, we have recorded IR absorption spectra of protonated forms of benzene (C₆H₇⁺), naphthalene (1- and 2-C₁₀H₉⁺), pyrene (1-C₁₆H₁₁⁺), coronene (1-C₂₄H₁₃⁺), and their neutrals.³ The significant superiority of the spectra thus recorded to those with the Ar-tagging and IRMPD methods is demonstrated. The narrow widths of the lines enabled us to distinguish clearly between isomers 1-C₁₀H₉⁺ and 2-C₁₀H₉⁺; 2-C₁₀H₉⁺ was unstable and converted to 1-C₁₀H₉⁺ in less than 30 min. A survey of these experimental results shows that three major lines in the 7-9 μm region are red-shifted from 7.19, 7.45, and 8.13 μm of 1-C₁₆H₁₁⁺ to 7.37, 7.53, and 8.21 μm of 1-C₂₄H₁₃⁺, showing the direction towards the UIR bands near 7.6, 7.8, and 8.6 μm. In contrast, the line at 11.5 μm for 1-C₁₆H₁₁⁺ is blue-shifted to 11.4 μm for 1-C₂₄H₁₃⁺, showing the direction toward the UIR band near 11.2 μm. Other examples will be presented if time permits.
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¹O. Dopfer, PAHs and the Universe, 46, 103 (2011). ²A. M. Ricks, G. E. Douberly, M. A. Duncan, Astrophys. J. 702, 301 (2009). ³M. Bahou, Y.-J.  Wu, Y.-P. Lee, J. Chem. Phys. 136, 154304 (2012); M. Bahou, Y.-J. Wu, Y.-P. Lee, Phys. Chem. Chem. Phys. 15, 1907 (2013); M. Bahou, Y.-J.  Wu, Y.-P. Lee, J. Phys. Chem. Lett. 4, 1989 (2013); M. Bahou, Y.-J.  Wu, Y.-P. Lee, Angew. Chem. Int. Ed. 53, 1021 (2014).

Naphthalene (Np) and its methylated derivatives (1-Me-Np and 2-Me-Np) are prototype molecules for spectroscopists as first members of the polycyclic aromatic hydrocarbons (PAHs) family. High resolution studies are capable to explore the details of the internal rotation of the methyl group. Although this was achieved in neutral PAHs¹, the task is not the same in cations. Me-Np cations have been probed by resonance-enhanced multiphoton dissociation², showing only very broad and unresolved spectra, while absorption in argon matrix revealed more resolved vibronic bands³. The electronic absorption gas phase spectra of 1-Me-Np⁺ and 2-Me-Np⁺ were measured using an Ar-tagging technique. In both cases, a band system was observed in the visible range and assigned to the D₂← D₀ transition. The 1-Me-Np⁺ absorption bands revealed a red shift of 808 cm⁻¹, relative to Np⁺ (14 906 cm⁻¹)⁴ , while for 2-Me-Np⁺ a blue shift of 226 cm⁻¹ was found. A short vibrational progression was also observed. Moreover, insights into the internal rotation motion of the CH₃ were inferred, although intrinsic broadening due to intramolecular relaxation was present. These measurements were supported by detailed quantum chemical calculations that allowed exploration of the potential energy curves, along with a complete simulation of the harmonic FC factors using the cumulant Gaussian fluctuations formalism, extended to include the internal rotation.
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¹see for instance Baba et al, J.Phys.Chem.A, 2009, 113, 2366 ²Dunbar et al, J. Am. Chem. Soc. 1976, 98, 7994-7999 and J.Phys.Chem. 1985, 89, 3617 ³Andrews et al, J.Phys.Chem. 1982, 86, 2916 ⁴Pino et al, J. Chem. Phys. 1999, 111, 7337-7347

Polycyclic aromatic hydrocarbon (PAH) clusters have been proposed as candidates for evaporating very small grains that are revealed by their mid-IR emission at the surface of UV-irradiated clouds in interstellar space¹. This suggestion is a motivation for further characterization of the properties of these clusters in particular when they are ionized. We have used a molecular beam coupled to the photoelectron-photoion coincidence spectrometer DELICIOUS II/ III ² at the VUV beamline DESIRS of the synchrotron SOLEIL to characterize the electronic properties of cationic coronene (C₂₄H₁₂) and pyrene (C₁₆H₁₀) clusters up to the pentamer and heptamer, respectively. These experimental results are analysed in the light of electronic structure calculations. Simulations of the properties of ionized PAH clusters are faced with the difficulty of describing charge delocalization in these large systems. We will show that recent developments combining a Density Functional Tight Binding method with Configuration Interaction scheme³ is successful in simulating the ionization potential, which gives strong confidence into the predicted structures for these PAH clusters. We will also present current effort to study charge transfer states by performing complementary measurements with the PIRENEA ion trap set-up.
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¹M. Rapacioli, C. Joblin and P. Boissel Astron. & Astrophys. 429 (2005), 193-204. ²G. Garcia, H. Soldi-Lose and L. Nahon Rev. Sci. Instrum. 80 (2009), 023102; G. Garcia, B. Cunha de Miranda, M. Tia, S. Daly, L. Nahon, Rev. Sci. Instrum. 84 (2013), 053112 ³M. Rapacioli, A.  Simon, L. Dontot and F. Spiegelman Phys. Status Solidi B 249 (2) (2012), 245-258; L. Dontot, M. Rapacioli and F. Spiegelman (2014) submitted

Protonated polycyclic hydrocarbons have been added to the list of suggested carriers of the diffuse interstellar absorptions. To test this proposition requires laboratory spectra measured under interstellar conditions, in particular with the rotational and vibrational degrees of freedom equilibrated to low temperatures. This has been achieved for protonated pyrene with absorption bands in the visible, using an ion trap and collisional cooling to ≈  15 K. A two-photon excitation-dissociation scheme was employed to record the (1) ¹A′ ← X ¹A′ electronic spectrum on around 10⁵ ions per duty cycle. The origin band of the absorption spectrum of this relatively large polycyclic aromatic species with 27 atoms is located at 4858.86 Å. Two further comparably intense spectral features are present at 4834.48 and 4809.32 Å. This is one of the largest protonated aromatics studied in the gas phase and compared to astonomical observations; however, it is not a carrier of known diffuse interstellar bands.

Figure Azabenzenes are derivatives of benzene containing between one and six nitrogen atoms. Protonated azabenzenes are the fundamental building blocks of many biomolecules, charge-transfer dyes, ionic liquids and fluorescent tags. However, despite their ubiquity, there exists limited spectroscopic data that reveals the structure, behaviour and stability of these systems in their excited states. For the case of pyridinium (C₅H₅N-H⁺), the simplest azabenzene, the electronic spectroscopy is complicated by short excited state lifetimes, efficient non-radiative deactivation methods and limited fluorescence. Ultraviolet (UV) photodissociation (PD) action spectroscopy¹ provides new insight into the spectroscopic details, excited state behaviour and photodissociation processes of a series of protonated azabenzenes including pyridinium, diazeniums and their substituted derivatives. The room-temperature UV PD action spectra, often exhibiting vibronic detail,^b will be presented alongside PD mass spectra and the kinetic data from structurally-diagnostic ion-molecule reaction kinetics. Analysis of the spectra, with the aid of quantum chemical calculations, reveal that many azabenzenes prefer a non-planar excited state geometry reminiscent of the structures encountered in 'channel 3'-like deactivation of aromatics. The normal modes active in this isomerization contribute largely to the spectroscopy of the N-pyridinium ion as they build upon totally-symmetric vibronic transitions leading to repeating sets of closely-spaced spectral features.
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¹Hansen, C.S. et al.; J. Am. Soc. Mass Spectrom. 24:932-940 (2013) ^bHansen, C.S. et al.; J. Phys. Chem. A 117:10839-10846 (2013)

Matrix isolation spectroscopy was first developed in Pimentel's group during the 1950’s to facilitate spectroscopic studies of transient species.¹ Cryogenic matrices of condensed rare gases provide an inert chemical environment with facile energy dissipation and are transparent at all wavelengths longer than vacuum UV, making them ideal for studying labile and reactive species such as radicals, weakly bound complexes, and ions. Since frozen rare gases are poor electrolytes, studies of ions require near-equal populations of anions and cations in order to stabilize the number densities required for spectroscopic experiments. Many techniques for generation of ions for using in matrix isolation studies satisfy this criterion intrinsically, however when ion beams generated in external sources are deposited, the counter-ions typically arise via secondary processes that are at best loosely controlled.² It has long been recognized that it would be desirable to stabilize deposition of mass-selected ions generated in an external source using simultaneous co-deposition of a beam of counter-ions, however previous attempts to achieve this have been reported as unsuccessful.³ The Moore group at Lehigh has demonstrated successful experiments of this type, using mass-selected anions generated from a metal cluster source, co-deposited with a balanced current of cations generated in a separate electron ionization source. This talk will focus on the details of the technique, and present some results from proof-of-concept studies on anionic copper carbonyl complexes formed in argon matrices following co-deposition of Cu⁻ with Ar⁺ or Kr⁺.
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¹Whittle et al., J. Chem. Phys. 22, p.1943 (1954); Becker et al., J. Chem. Phys. 25, p.224 (1956). ²Godbout et al., J. Chem. Phys. 96, p.2892 (1996). ³Sabo et al., Appl. Spectrosc. 45, p. 535 (1991).

Using matrix isolation FTIR, we have observed the formation of anionic copper carbonyl complexes [Cu(CO)_n]⁻ (n=1-3) following co-deposition of Cu⁻ and counter-cations (Ar⁺ or Kr⁺) into argon matrices doped with CO.¹ The infrared bands have been previously assigned in argon matrix studies employing laser ablation, however they were quite weak compared to the bands for the corresponding neutral species.² In the current study, when the deposition is carried out in fully darkened conditions at 10 K with high CO concentrations (1-2%), only the bands for the anionic complexes are observed initially via FTIR. However, upon mild irradiation with broadband visible light, the anionic bands are rapidly depleted, with concomitant appearance of bands corresponding to neutral copper carbonyl complexes. This photo-triggered neutralization is attributed to photodetachment of electrons from the anions, which then "flow" through the solid argon matrix to recombine in the matrix with non-adjacent trapping sites. This mechanism is supported by the appearance of a new band near 1515 cm⁻¹, assigned to the (CO)₂⁻ species in argon.³ The wavelength dependence of the photodetachment will be discussed in detail, although preliminary indications are that the thresholds for the copper carbonyls, which are normally in the infrared,⁴ are shifted into the visible region of the spectrum in argon matrices. This likely occurs because the conduction band of solid argon is known to lie about 1 eV above the vacuum level,⁵ and thus the electron must have at least this much energy in order to escape into the matrix and find a trapping site.
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¹Ryan M. Ludwig and David T. Moore, J. Chem. Phys. 139, 244202 (2013). ²Zhou, M.; Andrews, L., J. Chem. Phys. 111, 4548 (1999). ³Thompson, W.E.; Jacox, M.E.; J. Chem. Phys. 91, 735 (1991). ⁴Stanzel, J. et al.; Collect. Czech. Chem. Comm. 72, 1 (2007). ⁵Harbich, W. et al.; Phys. Rev. B. 76, 104306 (2007).

Using matrix isolation FTIR, we have observed the formation of anionic copper carbonyl complexes [Cu(CO)_n]⁻ (n=1-3) following co-deposition of Cu⁻ and counter-cations (Ar⁺ or Kr⁺) into argon matrices doped with CO.¹ When the deposition is carried out at 20 K, weak bands corresponding to the neutral copper carbonyl complexes Cu(CO)_n (n=1-3) are also observed, and these grow in steadily as the matrix is annealed up to 30 K. This is in contrast to what is observed at 10 K (c.f. ISMS 2014 abstract #P631), where no appreciable neutral bands are observed, and indicates that some neutralization occurs during the formation of the complexes in the 20 K matrix. In addition, sharp peaks not previously observed grow in around the anionic bands upon annealing to 30 K; this is somewhat odd, since annealing typically simplifies the spectra of matrix samples as kinetically trapped metastable species relax to more stable forms. In this case, higher-resolution (0.125 cm⁻¹) spectra reveal considerable new fine structure, with 5 and 20 peaks appearing in the regions of the mono- and tricarbonyl anions, respectively, each of which nominally has but a single IR-active CO-stretching mode. These new features are tentatively assigned (at least in part) to electric-field-induced splitting arising from long-range interactions with cationic species in the matrix. A second anomalous feature of these spectra is that, upon photodetachment, several new bands are observed in the region of the neutral copper carbonyl species. Upon annealing these bands then disappear, with concomitant growth of the expected neutral bands. This behavior raises the exciting possibility that these transient bands represent metastable "vertical detachment products", where the neutral species has been kinetically trapped by the matrix in the geometry of the anion. Evidence supporting this interpretation will be presented.
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¹Ryan M. Ludwig and David T. Moore, J. Chem. Phys. 139, 244202 (2013).

Neutral and anionic clusters containing carbon monoxide and carbon dioxide molecules were observed in FTIR matrix isolation spectroscopy experiments following co-deposition of Cu⁻ and Ar⁺ in argon matrices doped with moderate amounts (0.1-1%) of CO₂ and/or CO. Following deposition at 10 K with 0.5% CO₂, peaks for CuCO₂⁻ and Cu(CO₂)₂⁻ were observed, along with a small peak for CO₂⁻ and several bands assigned to neutral (CO₂)_x clusters. Upon annealing to 20 K, a new peak appears at 1857 cm⁻¹, which is assigned to the C₂O₄⁻ ion, based on previous work in neon matrices.¹ When CO is added as a dopant (0.5 %) to the matrix gas mix, formation of the anionic copper CO₂ complexes is suppressed, and neutral CO-CO₂ complexes are observed in the spectra, as are bands corresponding to C₂O₃⁻, based on previous studies.² Interestingly, the copper carbonyl bands typically observed for equivalent deposition conditions in the absence of CO₂ are strongly suppressed in these spectra. The implication is that complexation with the CO₂ molecules, which are far more abundant in the matrix, inhibits the CO molecules from "finding" the Cu⁻ centers during the matrix formation process. (c.f. ISMS 2014 abstract #P616)
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¹Zhou, M.F.; Andrews, L.; J. Chem. Phys. 110, 6820 (1999). ²Zhou, M.F.; et al., J. Chem. Phys. 112, 7089 (2000).

Using extended coupled cluster methods well beyond fc-CCSD(T), spectroscopic properties of several molecular anions of astrochemical interest have been calculated. Excellent agreement with MW data is observed for CN⁻, CCH⁻, C₃N⁻, and C₄H⁻ and accurate equilibrium structures are presented for all four species. The results for CCH⁻ are superior to recent theoretical results of Huang and Lee¹ and confirm the quality of our earlier predictions.² The new calculations predict ν₁ = 3209.3 cm⁻¹, ν₂ (band origin) = 510.0 cm⁻¹, and ν₃ = 1804.4 cm⁻¹, estimated errors not exceeding 1 cm⁻¹.
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¹X. Huang, T. J. Lee J. Chem. Phys. 2009, 131, 104301. ²M. Mladenovi\'c, P. Botschwina, P. Sebald, S. Carter, Theor. Chem. Acc. 1998, 100, 134.

Frequencies of high-J rotational lines of HCO⁺ and DCO⁺ have been measured precisely by using an Evenson-type tunable far-infrared spectrometer in Toyama. The spectrometer (sometimes called TuFIR in short) is based on synthesizing terahertz radiation from two mid-infrared CO₂ laser lines and one microwave source. The HCO⁺ ions are produced by discharging a CO, H₂ (or D₂), and Ar mixture in an extended negative glow discharge cell cooled with liquid nitrogen. The low-J rotational lines have been extensively studied. Information on the higher-J levels have been obtained by observing the vibration-rotation bands. More recently high precision infrared measurements have been made by observing the Lamb dips.¹ We focus our efforts to measuring the high-J rotational lines. Currently we have observed the lines J+1 ← J (J=7-19, 21) for HCO⁺, and J+1 ← J (J=11, 13-14, 16-24) for DCO⁺. An additional isotopologue, H¹³CO⁺ is now under investigation. From the analysis of the intensity of each rotational line, we estimate the rotational temperature to be as low as 120K. Apparently, due to this low temperature, it seems to be difficult to extend the measurements to yet higher-J lines.
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¹B. M. Siller, J. N. Hodges, A. J. Perry, and B. J. McCall, J. Phys. Chem. A. 117, 10034(2013)

Molecular ions are involved in the chemistry of many interesting systems, such as the atmosphere, combustion reactions, and the interstellar medium. Challenging aspects of studying molecular ions spectroscopically include producing ions in enough abundance and, for large or fluxional ions, overcoming the problem of quantum dilution at high vibrational and rotational temperature. Furthermore, highly precise transition frequencies are needed to answer many questions involving molecular ions, such as the presence of specific candidate ions in the interstellar medium. To address these challenges, we have constructed a mid-infrared spectrometer that uses a difference frequency generation (DFG) light source to probe cooled molecular ions produced in a continuous supersonic expansion discharge source. The cooling of the ions achieved through supersonic expansion mitigates the problem of quantum dilution. High sensitivity to detect the 10¹² ions per cm³ produced is accomplished through the use of noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) as a detection technique. Finally, an optical frequency comb is used to measure the transition frequencies of molecular ions precisely. This talk will present the current status of the instrument and preliminary studies to optimize and characterize its performance. Initial studies of room temperature methane allowed us to verify the use of NICE-OHMS for inferring rotational temperature of a molecular sample through Boltzmann plot analysis. Spectroscopy of H₃⁺ and HN₂⁺ extended this temperature verification to molecular ions. Future work on H₂CO⁺, with the goal of determining its rovibrational transitions to a precision on the order of 1 MHz to aid in astronomical detection, will also be presented.

The in-house fourth-age quantum chemical code GENIUSH is used for the variational determination of rotational-vibrational energy levels corresponding to reduced- and full-dimensional models of H₅⁺, a molecular ion exhibiting several strongly coupled large-amplitude motions and strong coupling between its vibrations and rotations. The quantum chemical computations are supplemented with simplified modeling efforts, including one- and two-dimensional exactly solvable models. These models help to understand the peculiar rovibrational energy-level structure computed for H₅⁺ and its deuterated isotopologues. It is shown that while a 1D "active torsion" model provides proper rovibrational levels when compared to the full, 9D treatment, models excluding the torsion have limited physical significance due to the extremely strong coupling between the torsion and one of the rotations. The role the choice of the internal coordinates and the embedding of the rotational axes play in obtaining converged rovibrational results is discussed in detail. The structure of the rovibrational energy levels of H₅⁺ proves that it is useful to view this ion as a prototypical astructural molecule: the rotational and vibrational level spacings are of the same order of magnitude and the level structure drastically deviates from that computed via perturbed rigid-rotor and harmonic-oscillator models.

I will describe progress in the first-principles calculations of "line-list" ro-vibrational spectra of H₅⁺, D₅⁺ and HOCO and low-resolution spectrum of H₇⁺, along with insights into the internal motions of these species. The calculations make use of full-dimensional ab initio potential and dipole moment surfaces and the code "MULTIMODE".