In this work we delve into the vibration-rotation spectrum of the propargyl H₂CCCH⁺ cation. In previous studies, guided by the analysis of the high resolution IR spectrum of the ν₁ and ν₃+ν₅ bands, W. G. D. P. Silva, D. Gupta, E. Plaar, J.L. Doménech, S. Schlemmer & O. Asvany High resolution rovibrational and rotational spectroscopy of H₂CCCH⁺, Molecular Physics, e2296613 (2023) DOI: 10.1080/00268976.2023.2296613ourteen rotational lines of this ion were measured in the laboratory, leading, in the end, to the discovery of H₂CCCH⁺ in the TMC-1 cloud. W. G. D. P. Silva, J. Cernicharo, S. Schlemmer, N. Marcelino, J.-C. Loison, M. Agúndez, D. Gupta, V. Wakelam, S. Thorwirth, C. Cabezas, B. Tercero, J. L. Doménech, R. Fuentetaja, W.-J. Kim, P. de Vicente & O. Asvany. Discovery of H₂CCCH⁺ in TMC-1. A&A, 676 (2023) L1 DOI:10.1051/0004-6361/202347174owever, given the nature of the studied transitions (all of them a-type) it was not possible to do an accurate determination of the A₀ rotational constant. In the present work, we have studied vibration-rotation transitions in two perpendicular (b-type) bands: the fundamental ν₆ (antisymmetric H-C-H stretch) and the combination band ν₃+ν₇ (C ≡ C stretch and CH₂ rock), in the 3056 -3137 cm⁻¹ region. As in the previous studies, we have carried out the experiments in a 4 K cryogenic ion trap, employing the novel Leak-Out-Spectroscopy (LOS) method. The analysis of these transitions has allowed a fifty-fold improvement on the precision of the A₀ rotational constant of H₂CCCH⁺.

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W. G. D. P. Silva, D. Gupta, E. Plaar, J.L. Doménech, S. Schlemmer & O. Asvany High resolution rovibrational and rotational spectroscopy of H₂CCCH⁺, Molecular Physics, e2296613 (2023) DOI: 10.1080/00268976.2023.2296613f W. G. D. P. Silva, J. Cernicharo, S. Schlemmer, N. Marcelino, J.-C. Loison, M. Agúndez, D. Gupta, V. Wakelam, S. Thorwirth, C. Cabezas, B. Tercero, J. L. Doménech, R. Fuentetaja, W.-J. Kim, P. de Vicente & O. Asvany. Discovery of H₂CCCH⁺ in TMC-1. A&A, 676 (2023) L1 DOI:10.1051/0004-6361/202347174H

Glycolaldehyde [HOCH₂C(O)H, GA], the primitive sugar-like molecule detected in the interstellar medium (ISM), is a potential precursor for the synthesis of complex sugars; it hence plays an important role in the formation of the chemical building blocks of life. The mechanism governing the formation of these higher-order sugars from GA under interstellar circumstances remains elusive. Radical intermediates HOCH₂CH₂O^· (1) , HOCH₂C^·HOH (2) , HOCH₂C^·O (3) , HOC^·HC(O)H (4) , and O^·CH₂C(O)H (5) derived from GA could be potential precursors for the formation of glyceraldehyde (aldose sugar), dihydroxyacetone (ketose sugar), and ethylene glycol (sugar alcohol) in dark regions of ISM. However, the spectral identification of these intermediates and their roles were little investigated. We conducted reactions involving H atoms and the cis-cis (Cc)-conformer of GA in solid p-H₂ at 3.2 K and identified IR spectra of radicals Cc-HOCH₂C^·O (3) and Cc-HOC^·HC(O)H (4) as well as closed-shell HOCHCO (6) produced via H-abstraction channels of GA. In addition, Cc-HOCH₂CH₂O^· (1) and C^·H₂OH + H₂CO were produced through the H-addition and the H-atom-induced fragmentation channels, respectively. In darkness, when only H-tunneling reactions occur, the formation of (3) is major and that of (1) is minor. In contrast, during IR irradiation to produce H atoms with higher energy, the formation of (4) and C^·H₂OH + H₂CO becomes important. We also successfully converted most Cc-GA to the second-least-energy conformer trans-trans (Tt)-GA by prolonged IR irradiation at 2827 nm and investigated H + Tt-GA; Tt-HOCH₂C^·O (3’) , Tt-HOC^·HC(O)H (4’) , HOCHCO (6) , Tt-HOCH₂CH₂O^· (1’) , and C^·H₂OH + H₂CO were observed. This work points out possible routes for the formation of higher-order sugars or related compounds involving C^·H₂OH, (1) , (3) , and (4) , but not (2) as was proposed previously or (5) . Such previously unreported rich chemistry in the reaction of H + GA, with four channels of three distinct types, indicates the multiple roles that GA might play in astronomical chemistry.

Despite the large number of known diffuse interstellar bands (DIB), their carriers remain vastly unknown; only the buckminsterfullerene cation C₆₀⁺ has been confirmed as the carrier of five DIB based on its electronic absorption spectrum. Large polycyclic aromatic hydrocarbons (PAH) and their derivatives are considered particularly promising candidates; however, laboratory spectra of these species suitable for comparison to astronomical observations are rarely available. Because solid para-H₂ is soft and provides a more homogeneous environment, it is expected to induce small and consistent matrix shifts on band positions relative to those in the gaseous phase. Over the past years, we have studied the dispersed fluorescence and fluorescence excitation spectra of several PAH and PANH containing up to 42 carbon atoms isolated in solid para-H₂ for which gas-phase and rare-gas matrix isolation spectra have been reported. For these, we found consistently small red-shifts, 40 to 110 cm⁻¹, induced by the para-H₂ environment. We present the dispersed fluorescence and fluorescence excitation spectra of ovalene (C₃₂H₁₄) which has been considered as a potential DIB carrier. Our spectra are generally consistent with previously reported data, indicating a rather small red shift of  ∼ 90 cm⁻¹relative to the gas phase. Amirav et al.Amirav et al. J. Chem. Phys. 1981, 74, 3745.ssigned their spectra of jet-cooled C₃₂H₁₄ to the S₁–S₀ transition with an origin band at 21449 cm⁻¹; however, our experimental data clearly shows that the origin was misassigned and should be at 20124 cm⁻¹. Furthermore, a comparison to simulated spectra obtained from Franck-Condon Herzberg-Teller calculations suggests that the observed spectra are instead associated with the S₂ state. From our experiments, we successfully located the previously unreported S₁–S₀ transition of C₃₂H₁₄ with its origin band at  ∼ 19430 cm⁻¹. Consistent with theoretical predictions, the S₁ state of C₃₂H₁₄ is considerably shorter lived than the S₂ state, with fluorescence lifetimes of  ∼ 13 ns and  ∼ 1.68 μs, respectively. Vibronic assignments are provided for the first time. Amirav et al. J. Chem. Phys. 1981, 74, 3745.a

l0pt Figure The diffuse interstellar bands (DIBs) represent an enigma in astronomical spectroscopy, as, despite intense efforts, only five out of more than 500 observed DIBs have been assigned, the sole responsible molecule being C₆₀⁺. Carbon chains are considered as potential DIB carriers. Several carbon chain molecules have been detected in space through microwave transitions including oxygen-containing carbon chains such as HC₇O,M. Cordiner, S. Charnley, Z. Kisiel, B. McGuire, and Y.-J. Kuan, Astrophys. J. 850, 187 (2017). nd HC₃O⁺.J. Cernicharo, N. Marcelino, M. Agúndez, Y. Endo, C. Cabezas, C. Bermúdez, B. Tercero, and P. De Vicente, Astron. Astrophys. 642, L17 (2020).o learn more about these species, we have used a custom-built instrument to measure electronic spectra of charged oxygen-containing carbon chains through photodissociation action spectroscopy. The target species, including OC_nO⁺, HC_nO⁺, and H₂C_nO⁺ clusters were generated through ion-molecule reactions between carbon cluster cations and CO, and were cooled in a cryogenically cooled ion trap prior to spectroscopic interrogation. Notably, low resolution electronic spectra of HC₆O⁺, HC₈O⁺, and HC₁₀O⁺ exhibit strong vibronic transitions whose peak wavelengths match catalogued DIBs (within 1 Å). However, higher resolution spectra for HC₆O⁺ and HC₈O⁺ show that the measured bands are broader than the DIBs, with the broadening possibly associated with the photodissociation technique used to obtain the spectra.

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M. Cordiner, S. Charnley, Z. Kisiel, B. McGuire, and Y.-J. Kuan, Astrophys. J. 850, 187 (2017). a J. Cernicharo, N. Marcelino, M. Agúndez, Y. Endo, C. Cabezas, C. Bermúdez, B. Tercero, and P. De Vicente, Astron. Astrophys. 642, L17 (2020).T

In various astronomical environments such as the interstellar medium or (exo)planetary atmospheres, an interplay of bottom-up growth and top-down destruction processes of (polycyclic) aromatic hydrocarbons (PAHs) takes place. To get more insight into their interplay, we disentangle the fragmentation and formation processes that take place upon dissociative ionization of benzonitrile. Experiments were performed in the cryogenic ion trap tandem-mass spectrometer FELion coupled to the widely tunable infrared free-electron lasers at the FELIX LaboratoryJusko, P. et al., Faraday Discuss. 245 (2019) 221-244. Ionic fragmentation products of benzonitrile upon HCN/HNC loss, [C₆H₄]⁺, are structurally characterized using infrared action spectroscopy and assigned to the isomeric o-benzyne, m-benzyne and ethynyl-methylene-cyclopropene radical cations Rap, D.B. et al., Faraday Discuss. 245 (2023) 172-202. They were subsequently used as reactants for low-temperature bottom-up ion–molecule reactions with acetylene. By combining kinetics and infrared action spectroscopy, we reveal exothermic pathways to various (polycyclic) aromatic molecules, including the pentalene and phenylacetylene radical cations. We determine the reaction rate coefficients and unambiguously assign the structures of the reaction products. The data is supplemented by potential energy surface calculations and the analysis of non-covalent interactions. This study shows the unexpected formation of a linked four- and six-membered ring structure (phenylcyclobutadiene radical cation) with molecular formula C₁₀H₈⁺, and not the commonly observed isomer naphthalene⁺ Rap, D.B. et al., Phys. Chem. Chem. Phys 26 (2024) 7296-7307. All observed reactions proceed via radiative association processes and are relevant for the chemistry in (cold) astrochemical environments.

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Jusko, P. et al., Faraday Discuss. 245 (2019) 221-244.. Rap, D.B. et al., Faraday Discuss. 245 (2023) 172-202.. Rap, D.B. et al., Phys. Chem. Chem. Phys 26 (2024) 7296-7307..

The spectroscopy of HCN⁺ is of astronomical and fundamental interest. The HCN⁺ cation has two low lying electronic states-a X̃²Π ground state and an excited òΣ⁺ state-which are mixed by spin-orbit and vibronic coupling.Tarroni R, Mitrushenkov A, Palmieri P, and Carter S. 2001, Energy levels of HCN⁺ and DCN⁺ in the vibronically coupled X̃²Π and òΣ⁺ states. J. Chem. Phys., 22;115(24):11200-12.lthough these states have been observed previously by photoionising neutral HCN,Wiedmann, R.T. and White, M.G., 1995, Vibronic coupling in the X̃²Π and òΣ⁺ states of HCN⁺. J. Chem. Phys., 102(13), pp.5141-5151.as-phase transitions between cationic electronic states of HCN⁺ have never been reported. This is predominantly because the aggressive reactivity of HCN⁺ obstructs spectroscopic investigation in supersonic expansions and discharges. Furthermore, the low energy of the transition (approx 3250 cm⁻¹) complicates action spectroscopic techniques like infrared multiple photo dissociation or messenger tagging. To overcome these obstacles and measure the X̃²Π to òΣ⁺ transition of HCN⁺, we employ the novel leak-out spectroscopy technique (LOS).Schmid, P.C., Asvany, O., Salomon, T., Thorwirth, S. and Schlemmer, S., 2022, Leak-out spectroscopy, a universal method of action spectroscopy in cold ion traps. J. Phys. Chem. A, 126(43), pp.8111-8117.he spectrum exhibits rotational, fine, and hyperfine structure that precisely describe the nature of these electronic states. This is the first reported electronic spectrum using the LOS method. Future measurements might demonstrate whether LOS is well suited to measuring electronic spectra.

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Footnotes:

Tarroni R, Mitrushenkov A, Palmieri P, and Carter S. 2001, Energy levels of HCN⁺ and DCN⁺ in the vibronically coupled X̃²Π and òΣ⁺ states. J. Chem. Phys., 22;115(24):11200-12.A Wiedmann, R.T. and White, M.G., 1995, Vibronic coupling in the X̃²Π and òΣ⁺ states of HCN⁺. J. Chem. Phys., 102(13), pp.5141-5151.g Schmid, P.C., Asvany, O., Salomon, T., Thorwirth, S. and Schlemmer, S., 2022, Leak-out spectroscopy, a universal method of action spectroscopy in cold ion traps. J. Phys. Chem. A, 126(43), pp.8111-8117.T

Diffuse interstellar bands (DIB), electronic absorption bands in the visible to near IR, have drawn much attention since their first discovery in 1922. Polycyclic aromatic nitrogen heterocycles (PANH) and their cationic, protonated, and hydrogenated derivatives have been proposed to contribute to the unidentified infrared bands (UIR). Thus, they might also be promising candidates as the DIB carriers. However, laboratory spectra of these species suitable for comparison with astronomical observations are scarce. Taking advantages of the unique properties of solid para-hydrogen (p-H₂) as a matrix host, we recorded the fluorescence excitation and dispersed fluorescence spectra of N-protonated isoquinoline (iso-C₉H₇NH⁺). To analyse our experimental results and assign the observed features to individual vibronic transitions, we performed Franck-Condon Hertzberg-Teller simulations according to optimized geometries and scaled harmonic frequencies obtained from (TD-)DFT calculations. The dispersed fluorescence spectrum was recorded over the range 350–420 nm upon excitation at 349.8 nm; it consisted of a progression with spacing of  ∼ 500 cm⁻¹. We monitored fluorescence as a function of excitation wavelength in region 320–370 nm to record the excitation spectrum. The 0₀⁰ band of the S₁–S₀ transition of iso-C₉H₇NH⁺ was located at 28097 cm⁻¹; this indicated a blue shift of  ∼ 54 cm⁻¹with respect to the gas-phase valueC. S. Hansen, S. J. Blanksby and A. J. Trevitt, Phys. Chem. Chem. Phys., 17, 25882–25890(2015).^,G. Féraud, L. Domenianni, E. Marceca, C. Dedonder−Lardeux and C. Jouvet, J. Phys. Chem. A, 121, 2580–2587(2017).ue to the p−H₂ environment In addition, we observed the fluorescence of at least three isomers of hydrogenated isoquinoline (iso−HC₉H₇N) in the range 510–620 nm and measured fluorescence lifetimes for each isomer. We tentatively located the 0_0^0 bands of their S_1–S_0 G. Féraud, L. Domenianni, E. Marceca, C. Dedonder-Lardeux and C. Jouvet, J. Phys. Chem. A, 121, 2580–2587(2017).d

In this work, we focused on the vibrational absorption spectra of protonated and hydrogenated phenanthrene (C₁₄H₁₀) isolated in solid para-hydrogen (p-H₂). p-H₂ flowed through and mixed with phenanthrene vapor before deposition onto the nickel-plated Cu substrate at 3.2 K. During deposition, we used an electron gun to bombard the mixture of p-H₂ and C₁₄H₁₀ to generate H⁺C₁₄H₁₀ and HC₁₄H₁₀. Electron bombardment of H₂ produced H₃⁺ and H; the former can readily transfer its proton to C₁₄H₁₀ to form H⁺C₁₄H₁₀, whereas the reaction of the latter with C₁₄H₁₀ and neutralization of H⁺C₁₄H₁₀ generated HC₁₄H₁₀. Protonated species are expected to decrease in darkness over time through the neutralization process with trapped electrons to produce hydrogenated species. By observing the decay and the increase of infrared absorption lines, we could unambiguously classify features associated with protonated or hydrogenated species after the matrix was maintained in darkness for an extended period. C₁₄H₁₀ has a total of seven protonation or hydrogenation sites, including two for the carbons on the fused ring. Among isomers of H⁺C₁₄H₁₀ or HC₁₄H₁₀, spectral assignments were achieved according to the behaviour upon secondary irradiation and a comparison of experimental results with vibrational wavenumbers and IR intensities predicted with the B3LYP/6-311++G(d,p) method. For H⁺C₁₄H₁₀, secondary irradiation was performed at 619, 544, 524, and 463 nm. The absorption lines of H⁺C₁₄H₁₀ could be divided into 4 groups and assigned to 10-, 1-, 3-, and 4-H⁺C₁₄H₁₀, respectively. In the case of HC₁₄H₁₀, secondary irradiation at 423, 380, 315, and 223 nm helped to identify 1-, 4-, 3-, 10-, and 2-HC₁₄H₁₀; only hydrogenation at the carbon on the fused ring was not observed. Nearly all feasible protonated and hydrogenated PAH were produced and identified in one experiment; all spectra are new.

Due to their relative stability, nitrile molecules are commonly detected in the interstellar medium and other regions of space. Many nitriles are considered prebiotic molecules and are of significance in astrobiology. We have recorded rotationally resolved spectra of acetonitrile (CH₃CN) in the region of 3,000 to 3,600 nm using a continuous wave optical parametric oscillator (CW-OPO). The wavelength of the CW-OPO is measured by a wavemeter and calibrated using the CH₄ transitions in the CH-stretch region and the transmission fringes of an etalon. Observed vibrational bands are assigned based on vibrational frequencies and intensities calculated ab initio. A series of anharmonicity calculations have been made to shed light on the effect of the cyano group. Spectroscopic simulation of the rotational structure yielded the excited state molecular constants, while those of the ground state are fixed to previously reported values determined in fitting the microwave spectrum of the molecule. Doppler-free saturation absorption spectrum of the ν₅=1 band of acetonitrile has been recorded. Spectroscopic measurement of acrylonitrile (C₂CHCN) is in progress.