The existence of negative molecular ions in space has been known for over a decade. Despite the determinations of anion abundances within various regions of the interstellar medium, questions remain as to how these weakly bound systems are formed within such harsh environments. The proposed formation mechanism, radiative electron attachment (REA), in which the negative ion is formed through a collision between the neutral parent molecule and a free electron with the emission of a photon is considered too slow to explain observed abundances. REA rates may be augmented however by the electron first transitioning through a dipole-bound state (DBS) close to the threshold for attachment. The C₃N molecule possesses a permanent dipole of magnitude supercritical for the formation of DBS. In this talk, we present the results of threshold photodetachment spectroscopy of C₃N⁻. Observed excitation in the detachment cross section below threshold at room temperature is attributed to activation of the two lowest energy bending modes in the trapped ion. This excitation disappears at cryogenic temperatures leading to a very sharp threshold. High resolution scans of the threshold region resolve two features which we assign to the P and R rotational branches of a two-photon detachment process via a DBS with ¹Σ⁺ symmetry. Our group recently presented results from threshold photodetachment spectroscopy of CN⁻ in which the energy dependence of the cross section was explained in terms of a model function [Simpson et al, J. Chem. Phys. 153, 184309 (2020)]. By adapting this model to C₃N⁻ and including the resonant interaction, we are able to determine both the rotational origin of the DBS and an improved value for the electron affinity of the C₃N molecule thereby confirming the bound nature of the observed state.

Definitive identification of single molecules is a natural limit of analytic chemistry. While single molecule spectroscopy is routinely performed on select classes of molecules, a general method for identifying individual molecules - including molecules without optical transitions - is beyond the current state of the art. I will present progress from my group towards a general method for realizing infrared spectroscopy, and thus identification, on molecular ions with masses between 20 and 200 amu. While the technique presented today will have modest spectral resolution, I will also present straightforward extensions that will allow for high-resolution spectroscopy of single molecules, and chiral recognition of single molecules.

r0.5 The fine-structure components of the N=0→ 1 and N=1 → 2 transitions of CO⁺ (²Σ⁺) have been measured using the action spectroscopic technique ROSAA (Rotational State-dependent Attachment of rare gas Atoms) [1,2] in a cryogenic 22-pole ion trap. The recorded high-resolution spectra show resolved Zeeman splittings caused by the Earth's magnetic field, which are analyzed with an effective Hamiltonian approach. The ROSAA signal intensity of rotational transitions depends intrinsically on a difference in the ternary attachment rate of helium atoms to the molecular ion for the participating rotational fine-structure levels, as well as on experimental parameters such as temperature, excitation power, and helium gas pressure. Numerical simulation results of the underlying kinetics are compared to the observed CO⁺ spectra.
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[1] S. Brünken, L. Kluge, A. Stoffels, O. Asvany , S. Schlemmer, Astrophys. J. 783 (2014) L4.
[2] S. Brünken, L. Kluge, A. Stoffels, J. Pérez-ríos, S. Schlemmer, JMS, 332 (2017) 67-78.

The chemistry of matter surrounding stars depends strongly on the elements that the center star produces during its lifetime but also on the transport of these elements to the outer layers of the star. Stars like IRC+ 10216 transport large amounts of carbon into their outer layers creating a carbon/oxygen ratio 1. Thus not all C is bound into carbon monoxide and a variety of carbonaceous molecules like silicon carbides can form. Small molecules like SiC, SiC₂, SiC₃, SiC₄, Si₂C but also \upmum-sized silicon carbide dust grains have been observed around these stars, but intermediates remain unknown. In this contribution, we concentrate on Si₄C₂⁺ as an example for a possible grain seed. We present the first gas phase UV/VIS spectrum of the Si₄C₂⁺ cation obtained by photodissociation spectroscopy in the range 208–330 nm and compare the experimental results with possible ground state structures and vertical absorption spectra obtained via DFT and TD-DTF calculations.^[1]
[1]M. Förstel et al., J. Mol. Spectrosc., 2021, 377, 111427

Iodine oxides (I_xO_y) are atmospherically important due to their high reactivity with ozone. The I⁻ anion has been detected in the atmosphere, along with IO₂⁻ and IO₃⁻ however, IO⁻ is yet to be directly detected despite being a key intermediate to their formation.Frege C. et al., Atmospheric Chemistry and Physics, 2017 / Koenig T. K. et al., Proceedings of the National Academy of Sciences, 2020 / He X.-C. et al., Science, 2021 recent gas phase studyBhujel M. et al., Physical Chemistry and Chemical Physics, 2020onfirms I⁻ will react with ozone to form IO⁻ with subsequent step-wise ozone reactions to form IO₂⁻ then IO₃⁻. The aim of the present study is to investigate the photodepletion of IO⁻ within the visible range of 650 - 450 nm (15385 - 22222 cm⁻¹) via photodetachment and photodissociation spectroscopy. Gas phase spectra are obtained at room temperature by coupling a tuneable OPO laser with a linear ion-trap mass spectrometer. A bound excited state is present in this visible range, which will either undergo autodetachment via electron loss (IO^• + e⁻) or undergo photodissociation (I⁻ + O(³P)). Depending on the vibrational energy level of the excited state, either photodissociation or photodetachment is favoured. Investigation of the potential energy surface using the multireference configuration interaction (MRCI) method shows that within the investigated energy range IO⁻ will excite from the X¹Σ⁺ ground state and populate the 1¹Π excited state and, in the Franck-Condon region, this is close to the curve crossing that mediates the photodissociation and close to the electron affinity for electron detachment. From this study IO⁻ has been shown to photodeplete at visible wavelengths and, with previous studies showing that IO⁻ will react with ozone to form higher order iodine oxides, may together contribute to the low abundance of IO⁻ in the atmosphere and therefore why it has eluded detection.

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Frege C. et al., Atmospheric Chemistry and Physics, 2017 / Koenig T. K. et al., Proceedings of the National Academy of Sciences, 2020 / He X.-C. et al., Science, 2021A Bhujel M. et al., Physical Chemistry and Chemical Physics, 2020c

The nitrous oxide cation (N₂O⁺) is an important intermediate in the upper atmosphereG. Chambaud, H. Gritli, P. Rosmus, H. J. Werner, and P. J. Knowles, Mol. Phys. 98, 1793 (2000) The photodissociation spectra of N₂O⁺ have been measured in the UV range using the new STARGATE instrument (Spectroscopy of Transient Anions and Radicals by Gated and Accelerated Time-of-flight Experiment) developped in UCLouvain. This talk will present the rovibronic analysis of the òΣ⁺(002)←~X²Π(000), òΣ⁺(101)←~X²Π(000) and òΣ⁺(003)← ~X²Π(000) bands measured at 550 K in the 30500-32500 cm⁻¹ range. A global vibronic fit has been performed including these bands, Q-branch head of overtones, combination bands and data from other studiesM. Gharaibeh and D. Clouthier, J. Chem. Phys. 136, 044318 (2012).C. E. Fellows and M. Vervloet, Chem. Phys. 264, 203 (2001).. The Renner-Teller effect involving the ~X²Π and òΣ⁺ states is taken into account in the global fit procedure. The improvement of the description of the vibronic energy level will be discussed.

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G. Chambaud, H. Gritli, P. Rosmus, H. J. Werner, and P. J. Knowles, Mol. Phys. 98, 1793 (2000). M. Gharaibeh and D. Clouthier, J. Chem. Phys. 136, 044318 (2012).

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A cryogenic 22-pole ion trap apparatus is used in combination with a table-top pulsed IR source to probe weakly bound CH⁺-He and CH⁺-He₄ complexes by predissociation spectroscopy at 4 K. The infrared photodissociation spectra of the C–H stretching vibrations are recorded in the range of 2720–2800 cm⁻¹. The spectrum of CH⁺-He exhibits perpendicular transitions of a near prolate top with a band origin at 2745.9 cm⁻¹, and thus confirms it to have a T-shaped structure. For CH⁺-He₄, the C-H stretch along the symmetry axis of this oblate top results in parallel transitions.

Cobalt cation-water complexes are generated in a pulsed supersonic expansion by laser vaporization. Complexes are mass-selected in a time-of-flight mass spectrometer and their spectra are measured using infrared laser photodissociation of the rare gas tagged complexes. The spectrum of Co⁺(H₂O) was measured with He, Ne, and Ar₃ tags. These spectra reveal that the water remains intact in the metal cation-water complex. The Ar₃ tag is found to cause a small blue shift of the O-H stretches relative to that observed with helium tagging. Rotational structure observed for Co⁺(H₂O) and Co⁺(D₂O) with helium tagging reveal perturbations to the rotational constants introduced by the motion of the helium in the ground vibrational state. Nuclear spin statistics were found to maintain a room temperature population of states which indicates an antisymmetric ground electronic state. Spin-rotation coupling was observed to vary between isotopologues and differed from values obtained in untagged measurements.

The coupling of ion mobility with mass spectrometry (IM-MS) allows for the selection of isolated, gas-phase molecular ions with defined mass/charge as well as geometrical size/charge ratios. When coupled to cryogenic ion traps, those molecular ions can be tagged with atoms or molecules to allow for action spectroscopy of very well defined cold species, and several instruments to do so have been developed in laboratories worldwide. We recently upgraded our drift tube IM-MS instrument that is coupled to the Fritz Haber Institute FEL to perform IR multiple photon dissociation spectroscopy with a cryogenic ion trap to allow for messenger atom/molecule action spectroscopy. The cryogenic ion trap itself implements a novel concept by having a segmented trap consisting of a high and a low pressure region. Ions can be injected cw or at a high repetition rate into the high pressure region, portions of the ions can then be transferred at a variable rate to the low pressure region from which they are ejected to the laser interaction region at a rate, matched to the laser repetition rate and independent from the injection rate into the trap. First results on the IR spectroscopy of biological molecules will be presented.

In cryogenic ion vibrational predissociation (CIVP) spectroscopy, the tag's influence on the spectrum is an important consideration. Whereas for small ions, several studies show that the tag effects can be significant, these effects are less understood for large ions, or for large numbers of tags. Nevertheless, common assumptions are that i) if the investigated molecular ion is large enough, the perturbations arising from the tag are small, and ii) the more weakly bound the tag is, the less it perturbs the CIVP spectrum. Under these assumptions, CIVP spectra effectively represent infrared (IR) absorption spectra of the free molecular ion. In the Chen group, we have recently started studying IR spectra of large ions in the gas phase.[1] To understand the complicated IR fingerprints of those systems, we employ several theoretical methods. Density functional theory is a common tool for calculating IR spectra. An alternative approach is to calculate the IR spectrum by the Fourier transform of the dipole moment autocorrelation function from a classical trajectory,[2] e.g., from Born-Oppenheimer molecular dynamics (BOMD). In contrast to the harmonic approximation, BOMD includes vibrational couplings and conformational sampling at a given temperature. Having observed unexpected splittings in otherwise unproblematic CIVP spectra of some tagged ions, we calculated IR spectra by BOMD simulations with the semi-empirical GFN-xTB methods.[3,4] Our simulations indicate that mobility among the more weakly bound tags leads to the surprise splittings. We compared the behavior of two tags commonly used in CIVP spectroscopy (H₂ and N₂) with a large pyridinium cation. Our experimental results surprisingly show that the more weakly bound tag can, under the appropriate circumstances, perturb the CIVP spectra more than the more strongly bound tag, by not just shifting, but also splitting the observed bands. References: [1] A. Tsybizova, L. Fritsche, V. Gorbachev, L. Miloglyadova, and P. Chen, J. Chem. Phys. 151, 234304 (2019). [2] D.W. Noid, M.L. Koszykowski, and R.A. Marcus, The Journal of Chemical Physics 67, 404 (1977). [3] S. Grimme, C. Bannwarth, and P. Shushkov, J. Chem. Theory Comput. 13, 1989 (2017). [4] C. Bannwarth, S. Ehlert, and S. Grimme, J. Chem. Theory Comput. 15, 1652 (2019).

Known for their stable structural and thermal properties, diamondoids and their radical cations are viable candidates as carriers for diffuse interstellar bands.¹ While previous diamondoid research has mainly focused on neutral molecules and their derivatives, little is known about their radical cations, which may form in interstellar environments by ionizing radiation.² We report the first experimental optical spectrum of the simplest diamondoid cation, the adamantane radical cation (C₁₀H₁₆⁺), obtained via electronic photodissociation spectroscopy at 5 K between 310–1000 nm. The optical spectrum reveals a broad peak between 420–850 nm, assigned to the D₂(²E) ← D₀(²A₁) transition. This feature exhibits no vibrational structure, despite an experimental temperature below 20 K, due to lifetime broadening and/or Franck-Condon congestion. A second band system originating at 345 nm does reveal a vibrational progression and is attributed to the overlapping D₅(²A₁)/D₆(²E) ← D₀(²A₁) transitions split by the Jahn-Teller effect. Comparison of the spectrum with known diffuse interstellar bands suggests that C₁₀H₁₆⁺ is not likely to be a carrier. However, the strong absorption features in the UV to near IR show promise in the investigation of higher order diamondoid cations as potential candidates.³

[1] T. Henning and F. Salama 1998, Science, 282, 2204. [2] C. W. Bauschlicher Jr., Y. Liu, A. Ricca, A. L. Mattioda, L. J. Allamandola, 2007, ApJ, 671, 458. [3] P. B. Crandall, D. Müller, M. Förstel, J. Leroux, O. Dopfer, 2020, ApJL, 900, L20.

l0pt Figure London dispersion, the attractive part of the van der Waals potential, is an omnipresent force in molecules. However, even a cursory glance at presumably driven by the London dispersion two systems from referencesMuller, P. Pure and Applied Chemistry. 1994, 66, 1077.^, Lyttle, M. H.; Streitweiser, A. Jr.; Kluttzt, R. Q. J. Am. Chem. Soc. 1981, 103, 3232.gTsybizova‡, A.; Fritsche‡, L.; Gorbachev‡, V.; Miloglyadova. L.; Chen, P. J. Chem. Phys. 2019, 151, 234304.O

Gas-phase hydrogen-bonded clusters are treated as microscopic models of hydrogen bond networks. We have been investigating the temperature effect on microscopic hydrogen bond structures. In our previous study, we measured ultraviolet photodissociation (UVPD) spectra of hydrated phenol cations [PhOH(H₂O)₅]⁺ trapped in our temperature-variable ion trap. We revealed temperature dependence of relative populations between two isomers having distinct hydrogen bond structures.H. Ishikawa, I. Kurusu, R. Yagi, R. Kato, Y. Kasahara, J. Phys. Chem. Lett. 8, 2641 (2017).t is known that water and methanol molecules construct different hydrogen bond networks. Thus, we measured UVPD spectra of phenol-methanol cluster cations [PhOH(MeOH)_n]⁺ (n=3, 4) in the present study. As in the case of [PhOH(H₂O)₅]⁺, isomers having ring type hydrogen bond structures are dominant in cold condition for both n = 3 and 4 cases. As the temperature elevates, populations of the chain type isomers become large in the case of the n = 3. Both bands for the ring and chain type isomers exhibit red shifts of the band positions and broadening of the band widths along the temperature elevation were observed. These changes were attributed mainly by hot bands of the intermolecular vibrational modes. In contrast, only the bands assigned as the ring isomers were observed with the temperature below 150 K. However, changes in the band profiles indicate structural changes within a ring type hydrogen bond motif.

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H. Ishikawa, I. Kurusu, R. Yagi, R. Kato, Y. Kasahara, J. Phys. Chem. Lett. 8, 2641 (2017).I

Since gas-phase hydrogen-bonded clusters are treated as microscopic models of hydrogen bond networks, numerous spectroscopic studies have been performed, so far. Structural fluctuations are one of features of hydrogen bond network. Such fluctuations correspond to isomerizations among isomers having distinct hydrogen bond structures in the cases of clusters. To investigate microscopic natures of structural fluctuations of hydrogen bond networks, we observed IR-induced isomerizations of hydrogen-bonded phenol cluster cations trapped in a cold ion trap. In a cold condition, all hydrogen-bonded phenol-methanol cluster cations [PhOH(MeOH)₃]⁺ have a ring type hydrogen bond structures. In contrast, isomers having a chain type hydrogen bond structure are dominant in a hot condition. In the present experiment, we excited the ring isomers in a cold condition by IR laser pulses. Since the energy of IR laser photon exceeded the height of isomerization barriers, isomerizations from the ring to chain isomers occurred. We successfully detected the chain type isomers in ultraviolet photodissociation spectra. Additionally, we also observed reverse isomerizations from the chain back to the ring isomers by the collisional cooling with the buffer gas. Details of the observations are presented in the paper.

High-resolution anion photoelectron spectra of cryogenically cooled NdO⁻ obtained using slow photoelectron velocity-map imaging (cryo-SEVI) are presented, providing insight into the vibronic structure of the corresponding neutral metal oxide.