Vibrational modes of the relatively strongly bound H₂He⁺ molecular ion and its deuterated congener D₂He⁺ are investigated by low-resolution multi-photon photodissociation spectroscopy, using a combination of a 4 K cryogenic ion-trap machine and the free electron laser FELIX. The band origins obtained are fully explained by accurate variational calculations of the rovibrational states based on the three-dimensional potential energy surface of Koner et al. [Phys. Chem. Chem. Phys. 21 (2019) 24976]. Results from second-order vibrational perturbation theory, based on a linear H-H-He equilibrium structure, agree well with those of the variational calculations for energy levels up to about 1300 cm⁻¹. The low-resolution experiments corroborate the linear structure of the ions and identify the bright IR-active HH-stretch fundamental in H₂He⁺ at about 1840 cm⁻¹ and the DD-stretch fundamental in H₂He⁺ at about 1309 cm⁻¹. The calculations also reveal the H₂⁺-He bend and stretch fundamentals to be at 640 and 732 cm⁻¹ and the D₂⁺-He bend and stretch fundamentals at 478 and 641 cm⁻¹, respectively. A short outlook is given on future high-resolution experiments.

The study of ion-molecule reactions plays a vital role in cold chemistry, thus implying the need of well-controlled ion ensembles in a cold environment. The internal and external degrees of freedom of molecular ions, trapped in multipole radio frequency ion traps, can be cooled via collisions with neutral atoms [1]. Usually the coolant undergoes collisions with a thermal shield mounted on a cryostat attaining temperatures of about 4 K. To overcome this lower temperature limit two approaches have been proposed. Firstly, using a laser-cooled buffer-gas localized at the center of the ion cloud [2]. Secondly, sympathetic cooling via a second laser cooled molecular anion like C₂⁻ [3]. This, in addition to its potential role in forming carbon chain anions in the interstellar medium, makes C₂⁻ an important molecule in anion research. In this contribution we present a newly developed 16-pole cryogenic radio frequency wire trap for spectroscopic and kinetic studies of molecular anions. First results of quenching rates of the first excited vibrational level of C₂⁻ due to collisions with H₂ are presented. Furthermore, possible reaction pathways with H₂ are discussed and recent spectroscopic measurements of the second electronic excited state of C₂⁻ are presented. References: [1] R. Wester , J. Phys. B 42, 154001 (2009) [2] B. Höltkemeier et al., Phys. Rev. A 94, 062703, (2016) [3] P. Yzombard et al., Phys. Rev. Lett. 114, 213001 (2015)

CH₃⁺ ions were produced upon electron impact ionization of the He droplets doped with methane molecules.The spectra was obtained by recording the intensity of ions detected based upon upon ejection from the droplet following absorption of multiple mid infrared photonsD. Verma, E.Swetha, A.F. Vilesov, Infrared Spectroscopy of Water and Zundel cations in Helium Nanodroplets, The Journal of Physical Chemistry A 124(34) (2020) 6207-6213. Perpendicular ν₃ band of CH₃⁺ in He droplets shows three prominent peaks which are assigned to partially resolved rotational structure. The results show that CH₃⁺ retains the ability to rotate in liquid helium, however the decrement of rotational constants is larger than previously observed for neutrals. It was concluded that CH₃⁺ in helium behaves as a prolate symmetric top, whereas the free ion is reported to be an oblate top M.W. Crofton, M.F. Jagod, B.D. Rehfuss, W.A. Kreiner, T. Oka, Infrared-Spectroscopy of Carbo-Ions .III. ν₃ Band of Methyl Cation CH₃⁺, Journal of Chemical Physics 88(2)(1988) 666-678

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Footnotes:

D. Verma, E.Swetha, A.F. Vilesov, Infrared Spectroscopy of Water and Zundel cations in Helium Nanodroplets, The Journal of Physical Chemistry A 124(34) (2020) 6207-6213.. M.W. Crofton, M.F. Jagod, B.D. Rehfuss, W.A. Kreiner, T. Oka, Infrared-Spectroscopy of Carbo-Ions .III. ν₃ Band of Methyl Cation CH₃⁺, Journal of Chemical Physics 88(2)(1988) 666-678.

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. This method consists of rotational excitation followed by vibrational excitation, which is finally detected as a reduction in the number of CH₂NH₂⁺-He complexes formed in the 4 K He bath of the trap.

Following-up on our recent infrared photodissociation (IRPD) spectroscopic detection of the linear Ne-tagged HC₃O⁺ molecular ion,S. Thorwirth, M. E. Harding, O. Asvany, S. Brünken, P. Jusko, K. L. K Lee, T. Salomon, M. C. McCarthy, and S. Schlemmer 2020, Mol. Phys. 118, e1776409he C-H stretching mode ν₁ of bare HC₃O⁺ has now been studied at high spectral resolution. The spectrum was observed using a 4 K cryogenic ion trap apparatus at Cologne (COLtrap) and a cw optical parametric oscillator using the method of laser-induced inhibition of complex growth (LIICG) with helium as tagging agent. Fifteen rotational-vibrational transitions were observed, covering P(8) to R(6). The band origin is found at 3237.1 cm⁻¹ and hence blueshifted by some 5 cm⁻¹ compared to that of the HC₃O⁺−Ne weakly bound complex probed earlier in the IRPD study.

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Footnotes:

S. Thorwirth, M. E. Harding, O. Asvany, S. Brünken, P. Jusko, K. L. K Lee, T. Salomon, M. C. McCarthy, and S. Schlemmer 2020, Mol. Phys. 118, e1776409t

Nitrous Oxide is the third largest contributor to the greenhouse effect having a potency 300 times that of CO₂. Furthermore it is a metastable molecule that can be catalytically converted into harmless nitrogen and oxygen gas. Gold is an interesting candidate because small metal clusters could provide reaction pathways to enable this activation. Therefore we study the simple system Au₂N₂O⁺. In this talk we will present the measured vibrationally-resolved optical photodissociation spectrum and compare the results to quantum-chemical calculations of different isomers and bare Au₂^+[1]. [1] M. Förstel et al., Angew. Chem. Int. Ed., 2020, 123, 21587-21592.

This talk will describe single-conformation IR and UV spectroscopy of a series of single-turn and double-turn alpha helices as cryo-cooled, gas phase ions. Synthesized samples of tethered pentapeptides are known to form single-turn alpha helices in aqueous solution. When L-amino acids are used, a right-handed single-turn helix is formed while D-amino acids produce a left-handed helix. Due to the tether, these structures are remarkably stable in aqueous solution over a wide range of temperatures, pH, and denaturant. They can be concatenated, making LL, DD, LD, and DL double-turn forms. When a methylated arginine is placed at the C-terminal end of the tethered peptide, single-turn helices are the most stable structure, and are observed exclusively. The spectra in the NH stretch and amide I regions show distinct effects that depend on position along the helix, and the presence or absence of a kink due to concatenation of two opposite-handed helices. We will discuss these spectra and the prospects they offer as scaffolds for studying a wide range of interesting structural and dynamical problems.

When molecules are captured in a helium nanodroplet, the rate of cooling to an equilibrium temperature of ca. 0.4 K is faster than the rate of isomerization, resulting in “shock-freezing” that kinetically traps molecules in the local conformational minimum. This unique property enables the study of temperature-dependent conformational equilibria via infrared spectroscopy without the deleterious effects of spectral broadening at higher temperatures.D. S. Skvortsov, A. F. Vilesov, J. Chem. Phys. 2009, 130, 151101.^, C. M. Leavitt et al., J. Phys. Chem. A 2014, 118, 9692-9700.WJ. Gidden, M. T. Bowers, Eur. Phys. J. D 2002, 20, 409-419.DD. A. Thomas et al., Phys. Chem. Chem. Phys. 2020, 22, 18400-18413.

Whereas the electronic ground states of a large number of small molecular cations have been spectroscopically characterized, much less is known concerning electronically excited states, in particular because of the low densities in which molecular ions can be formed and because of the high excitation energies required. Electronically excited states of molecular ions are commonly studied using resonance-enhanced multiphoton dissociation (REMPD) [1] or isolated-core multiphoton Rydberg dissociation (ICMRD) spectroscopy [2]. These techniques rely on the fact that the excited molecular ion either predissociates rapidly or can be efficiently excited to a dissociative state by further photoabsorption. The Mg(3d_δ)Ar^{+ 2}∆ state of MgAr⁺ is an example of an electronic state that does not fulfill these conditions and cannot be studied with conventional REMPD or ICMRD. We will report on the experimental study of this state using a double-resonance spectroscopic technique we have developed. With this technique, MgAr⁺ molecules were prepared in their electronic ground state and then coupled, via an intermediate state, to both the Mg(3d_δ)Ar^{+ 2}∆ state and a predissociation continuum. In contrast to double-resonance spectroscopy involving only bound states, the presence of a predissociation continuum leads to a rich variety of spectral lineshapes, which exhibit asymmetric profiles reminiscent of Fano lineshapes. We carried out detailed simulations of these lineshapes using a quantum-optics-based effective Hamiltonian and solving the time-dependent Schrödinger equation. Agreement with experimental spectra is excellent and shows that the lineshapes are the result of quantum interferences between the different photoexcitation pathways leading to dissociation. We will discuss how the lineshapes can be controlled with external parameters such as laser pulse energies and wavenumbers in order, e.g., to facilitate spectroscopic analysis. The analysis of the rovibrational structure of the Mg(3d_δ)Ar^{+ 2}∆ electronic state will be presented, with particular emphasis on the anomalous behavior of the splitting between its two spin-orbit components. [1] P.O. Danis, T. Wyttenbach and J.P. Maier, J. Chem. Phys. 88, 3451–3455 (1988) [2] M. Génévriez, D. Wehrli and F. Merkt, Mol. Phys. 118, e1703051 (2019)

Group 13 (Al, Ga, and In)-piperidine complexes are produced in a pulsed metal-cluster beam source, observed by time-of-flight mass spectrometry, and characterized with pulsed-field-ionization zero electron kinetic energy spectroscopy. The spectrum of each complex exhibits several vibronic progressions associated with metal-ligand stretching, metal-ligand bending, or ligand-based vibrations. The Franck-Condon profile of the Al-piperidine spectrum is considerably shorter than those of Ga/In-piperidine, suggesting a larger charge effect on the structures of the heavier metal complexes. Electronic states, active vibrational modes, metal binding sites, and the conformation of the metal coordinated piperidine are identified by comparing the spectra of the three complexes and with theoretical calculations.

The McLafferty rearrangement is a well-known process in mass spectrometry. The Mclafferty rearrangement is dissociations of ionized ketones with accompanying hydrogen transfer on the gamma carbon atom to generate an enol cation. This reaction has undergone numerous mass spectrometric investigations but there has been no spectroscopic research to observe the intermediate enol cation. In this study, we have performed infrared spectroscopy of jet-cooled neutral and ionized 2-pentanone to investigate its isomerization process following photoionization. The OH stretch bands as well as CH stretch bands are observed in the spectrum of cationic 2-pentanone, while only CH stretch bands are observed for the neutral. These clearly indicate the enolization following hydrogen transfer from CH occurs in the ionized pentanone. The enolization of ionized 2-pentanone through hydrogen transfer from the gamma carbon is also supported by the theoretical reaction path search calculation.