Cryogenic radiofrequency ion traps have become a versatile tool to study the spectroscopy and state-selected collision dynamics of molecular ions. Different types of action spectroscopy have been developed to obtain a precise and sensitive spectroscopic signature. In this talk I will give an introduction to molecular ion spectroscopy in multipole traps. Then I will present recent experimental and theoretical investigations from our group on photodetachment spectroscopy and state-selected collisions of cold OH⁻ anions colliding with helium and hydrogen. Based on these results we performed high resolution terahertz spectroscopy on the two lowest rotational transitions of OD⁻. Work is in progress to extend the rotational spectroscopy to polyatomic molecular anions.

The innovative application of the ion-trap technique by Wester and coworkers has yielded definitive experimental values of photodetachment cross sections for the atomic oxygen radical anion (O^•−) [Hlavenka et al., J. Chem. Phys. 130, 061105 (2009)]. In the present study, equation-of-motion coupled-cluster (EOM-CC) calculations have been performed to derive theoretical values of photodetachment cross sections for the negative ions of atoms in the first two periods of the periodic table as well as of those which belong to the alkali metal and halogen groups. Two methods have been employed to derive the cross sections. One involves the Dyson orbitals obtained from EOM-CC calculations and plane wave functions for the detached electron in the transition dipole moment integrals. The other method utilizes the moment theory following EOM-CC calculations of transition dipole moments for a large number of pseudo-states. The cross sections so evaluated for O^•− match the experimental values very well. Generally good agreement has been found between the theoretical and experimental values of the cross sections for the atoms in the first two periods, while the present calculations cast some doubt on reported experimental values for some atoms beyond the second period. Substantial relativistic effects on the cross section have been observed for heavy elements in the alkali metal and halogen groups.

We present the electronic spectrum of Ru(II)-tris(2,2’-bipyridine), Ru(bpy)₃²⁺, measured by photodissociation spectroscopy of mass selected Ru(bpy)₃²⁺·N₂ ions prepared in a cryogenic quadrupole ion trap. The spectrum is composed of several metal-to-ligand charge transfer (MLCT) transitions, as well as metal centered bands and ligand centered ππ^* states. We observe several partially resolved electronic transitions in the MLCT band. We discuss the results in the framework of time-dependent density functional theory.

Tris(bpy)-metal complexes (bpy = 2,2'-bipyridine) and their derivatives are important systems in metal-organic chemistry. While tris(bpy)-ruthenium, Ru(bpy)₃²⁺, has been extensively studied, less attention has been paid to analogous complexes involving first row transition metals. Here we report the electronic spectra of a series of dicationic tris(bpy) chelates with different transition metals, measured by photodisscociation spectroscopy of cryogenically prepared ions. We focus our attention on the π-π* transitions in the UV region of the spectrum.

The PIRENEA set-up combines an ion cyclotron resonance cell mass spectrometer with cryogenic cooling in order to study the physical and chemical properties of polycyclic aromatic hydrocarbons (PAHs) of astrophysical interest. In space, PAHs are submitted to UV photons that lead to their dissociation. It is therefore of interest to study fragmentation pathways and search for species that might be good interstellar candidates because of their stabilityJ. Montillaud, C. Joblin, D. Toublanc, Astron. & Astrophys. 552 (2013), id.A15 Electronic spectroscopy can bring major insights into the structure of species formed by photofragmentation. This is also a way to identify new species in space as recently illustrated in the case of C₆₀⁺E.K. Campbell, M. Holz, D. Gerlich, and J.P. Maier, Nature 523 (2015), 322-323 In PIRENEA, the trapped ions are not cold enough, and thus we cannot use complexation with rare gas in order to record spectroscopy, as was nicely performed in the work by Campbell et al. on C₆₀⁺. We are therefore using the dissociation of the trapped ions themselves instead, which requires in general a multiple photon schemeF. Useli-Bacchitta, A. Bonnamy, G. Malloci, et al., Chem. Phys. 371 (2010), 16-23; J. Zhen, A.  Bonnamy, G. Mulas, C. Joblin, Mol. Astrophys. 2 (2015), 12-17 This leads to non-linear effects that affect the measured spectrum. We are working on improving this scheme in the specific case of the photofragment obtained by H-loss from 1-methylpyrene cation (CH₃-C₁₆H₉⁺). A recent theoretical study has shown that a rearrangement can occur from 1-pyrenemethylium cation (CH₂-C₁₆H₉⁺) to a system containing a seven membered ring (tropylium like pyrene system)M. Rapacioli, A. Simon, C.C.M. Marshall, et al., J. Phys. Chem. A 119 (2015), 12845-12854 This study also reports the calculated electronic spectra of both isomers, which are specific enough to distinguish them, and as a function of temperature. We will present experiments that have been performed to study the photophysics of these ions using the PIRENEA set-up and a two-laser scheme for the action spectroscopy. AcknowledgmentsEuropean Research Council grant ERC-2013-SyG, Grant Agreement n. 610256 NANOCOSMOS.html:<hr /><h3>Footnotes: J. Montillaud, C. Joblin, D. Toublanc, Astron. & Astrophys. 552 (2013), id.A15. E.K. Campbell, M. Holz, D. Gerlich, and J.P. Maier, Nature 523 (2015), 322-323. F. Useli-Bacchitta, A. Bonnamy, G. Malloci, et al., Chem. Phys. 371 (2010), 16-23; J. Zhen, A.  Bonnamy, G. Mulas, C. Joblin, Mol. Astrophys. 2 (2015), 12-17. M. Rapacioli, A. Simon, C.C.M. Marshall, et al., J. Phys. Chem. A 119 (2015), 12845-12854. European Research Council grant ERC-2013-SyG, Grant Agreement n. 610256 NANOCOSMOS.

We present work on a prototypical water oxidation catalyst, namely the aqua-complex [(bpy)(tpy)Ru-OH₂]²⁺ (2,2’-bpy = bipyridine, tpy = 2,2’:6’,2”-terpyridine), and its hydrated clusters [(bpy)(tpy)Ru-OH₂]²⁺ ·(H2O)_n, with n = 1 – 4. This complex is the starting species in a catalytic cycle for water oxidation. We couple electrospray ionization mass spectrometry with laser spectroscopy to circumvent challenges that arise in reactive solutions from speciation. Here, we report the electronic spectrum of [(bpy)(tpy)Ru-OH₂]²⁺ by photodissociation spectroscopy of mass selected, cryogenically prepared ions, and we examine effects of its microhydration environment on its electronic structure. In particular, we investigate the solvatochromic shift of the spectral envelope upon sequential addition of water molecules up to the tetrahydrate.

The “proline effect” is a well-known fragmentation phenomenon in mass spectrometry, in which y-fragments are produced preferentially over b-fragments during the collision induced dissociation of protonated L-proline containing peptide ions. This specific fragmentation channel is favored because of the high basicity of the secondary amine intermediate and the ring instability in alternative b_n⁺ products [ASMS 2014, 25, 1705]. In contrast, peptides containing the D-Pro stereoisomer have been shown to largely favor the production of b₄⁺ ions over y₃⁺ ions. This strongly suggests that differences in the conformational preferences between the D-Pro and L-Pro diastereomers are likely to be responsible but structural evidence has been lacking to date. Using tandem mass spectrometry and IR-UV double resonant action spectroscopy we are able to compare the 3D structures of cold [YA(D-Pro)AA+H]⁺ to [YA(L-Pro)AA+H]⁺ ions. The UV action spectra reveals two major conformers in [YA(D-Pro)AA+H]⁺ and one major conformer in [YA(L-Pro)AA+H]⁺. Clear differences in the hydrogen bonding patterns are apparent between the two conformers observed in the D-Pro specie which are both distinct from the L-Pro diastereomer. Furthermore, conformer and diastereomer specific photofragmentation patterns are observed. It is also noted that a ten-fold photofragment enhancement unique to one of the D-Pro conformers is observed upon absorption of a resonant IR photon after UV excitation. Differences in the excited state photophysics between the two D-Pro conformers suggest that vibrational excitation of S₁ turns on coupling to the dissociative –Tyr channel in one conformer, while this coupling is already present in the vibronic ground state of the other. Calculated harmonic spectra (M052X/6-31+G^*) of conformers obtained from Monte Carlo searches to the experimental spectra.

Lignins are the second most abundant naturally occurring polymer class, contributing to about 30% of the organic carbon in the biosphere. Their primary function is to provide the structural integrity of plant cell walls and have recently come under consideration as a potential source of biofuels because they have an energy content similar to coal. Herein, we employ cold ion spectroscopy (UV action and IR-UV double resonance) to unravel the spectroscopic signatures of G-type alkali metal cationized (X = Li⁺, Na⁺, K⁺) lignin tetramers connected by β-O-4 linkages. The conformation-specific spectroscopy reveals a variety of conformers, each containing distinct infrared spectra in the OH stretching region building on recent studies on the neutral and alkali metal cationized β-O-4 dimers. Based on comparisons of our infrared spectra to density functional theory [M05-2X/6-31+G*] harmonic level calculations for structures derived from a Monte Carlo conformational search, the alkali metal ion is discovered to engage in M⁺-OH-O interactions as important motifs that determine the secondary structures of these complexes. This interaction disappears in the major conformer of the K⁺ adduct, suggesting a reemergence of a neutral dimer segment as the metal binding energy decreases. Chelation of the metal cation by oxygen lone pair(s) of nearby oxygens in the β-O-4 linkage is observed to be the predominant driving force for 3D structure around the charge site, relegating OH-O H-bonds as secondary stabilizing elements.

Precision spectroscopy of molecular ions has applications in astrochemistry, quantum state controlled chemical reactions, and measurements of fundamental constants. While spectroscopy of molecular ions is challenging, we present techniques to study molecular ions co-trapped with laser-cooled atomic ions in ion traps. We recently demonstrated the measurement of the ν′ = 10 ← ν = 0 and ν′=9 ← ν = 0 overtone transitions in CaH⁺N. B. Khanyile, et. al. Nat. Commun. 6 7825 (2015). using resonant two photon dissociation. This technique is extended to the 2¹Σ← 1¹Σ electronic transition, which should be rotationally resolvable. This resolution will allow further investigation into the internal state control of CaH⁺ by techniques such as optical pumping, cryogenic cooling, and buffer gas cooling.

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Footnotes:

N. B. Khanyile, et. al. Nat. Commun. 6 7825 (2015). .