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Context. The absorption bands of the interstellar medium (ISM) in the optical and near-infrared regions called the diffuse interstellar bands (DIBs) have been known for almost a century, yet their origins remain largely unknown. Knowledge of molecular carriers of DIBs would allow for a much better understanding of the chemistry and physics of the ISM. Polycyclic aromatic hydrocarbons (PAHs) and, among them, polyacenes have been suggested as promising candidates for carriers of DIBs. Aims. In this paper, we report on the spectroscopy of heptacene (Hep), the polyacene molecule consisting of seven aromatic rings in a linear arrangement, in its cationic and anionic forms (Hep+/−). The performed spectroscopic studies made it possible to accurately determine the Hep+/− absorption band positions and to conduct a direct comparison of laboratory and observational spectra. Methods. We utilized helium-tagging action spectroscopy to measure the spectra of Hep+/− in a wide spectral range of 3000–13 000 Å. In most cases, the spectra obtained by this method can be directly compared with the observational spectra. By analyzing the spectral shift as a function of the number of attached helium atoms, we obtained precise estimates of the gas-phase band positions. Quantum-chemical computations were used to support and interpret the findings. Matrix isolation spectroscopy provided information on the spectral properties of neutral Hep and extended the spectral range for Hep+. Results. We found several absorption bands characterized by a rather large full width at half maximum in the spectra of Hep+/−. The two most intense bands were found at 4714 ± 5 Å and 12 250 ± 12 Å for Hep+ and at 4673 ± 14 Å and 11326 ± 4 Å for Hep−. We did not find any good match between laboratory and observational spectra. In particular, the intrinsic width of the absorption bands of Hep+/− is much higher than that of most observed DIBs. Conclusions. The non-detection of Hep+/− in the observational spectra excludes the bottom-up formation route for polyacenes in the ISM. Larger polyacene molecules could still be considered as potential carriers of DIBs in the case of an efficient top-down formation route. All currently measured polyacene ions exhibit relatively broad absorption bands. Therefore, additional spectroscopy studies of neutral polyacenes and larger polyacene ions as well as the study of possible top-down formation routes are suggested.

Abstract Physisorption on planar or curved graphitic surfaces or aromatic rings has been investigated by various research groups, but in these studies, the substrate was usually strictly rigid. Here, we report a combined experimental and theoretical study of helium adsorption on cationic hexaphenylbenzene (HPB), a propeller-shaped molecule. The orientation of its propeller blades is known to be sensitive to the environment, with substantial differences between the molecule in the gas phase and in the crystalline solid. Mass spectra of He$$_{n}$$ n HPB$$^{+}$$ + , synthesized in helium nanodroplets, indicate enhanced stability for ions containing $$n = 2, 4, 14, 28, 42, 44$$ n = 2 , 4 , 14 , 28 , 42 , 44 , or 46 helium atoms. Path-integral molecular dynamics simulations reveal a significant dependence of the dissociation energy on the details of the HPB geometry. Good agreement between the experimental data and calculated dissociation energies is obtained, provided that the symmetry of HPB$$^{+}$$ + is reduced from $$D_{6}$$ D 6 to $$D_{2}$$ D 2 , such a lower symmetry being suggested from quantum chemical calculations as arising upon electron removal. Graphic Abstract

Context. Insight into the conditions that drive the physics and chemistry in interstellar clouds is gained from determining the abundance and charge state of their components. Aims. We propose an evaluation of the C60:C60+ ratio in diffuse and translucent interstellar clouds that exploits electronic absorption bands so as not to rely on ambiguous IR emission measurements. Methods. The ratio is determined by analyzing archival spectra and literature data. Information on the cation population is obtained from published characteristics of the main diffuse interstellar bands attributed to C60+ and absorption cross sections already reported for the vibronic bands of the cation. The population of neutral molecules is described in terms of upper limit because the relevant vibronic bands of C60 are not brought out by observations. We revise the oscillator strengths reported for C60 and measure the spectrum of the molecule isolated in Ne ice to complete them. Results. We scale down the oscillator strengths for absorption bands of C60 and find an upper limit of approximately 1.3 for the C60:C60+ ratio. Conclusions. We conclude that the fraction of neutral molecules in the buckminsterfullerene population of diffuse and translucent interstellar clouds may be notable despite the non-detection of the expected vibronic bands. More certainty will require improved laboratory data and observations., 13 pages, 7 figures, to be published in Astronomy & Astrophysics

The adsorption of up to ∼100 helium atoms on cations of the planar polycyclic aromatic hydrocarbons (PAHs) anthracene, phenanthrene, fluoranthene, and pyrene was studied by combining helium nanodroplet mass spectrometry with classical and quantum computational methods. Recorded time-of-flight mass spectra reveal a unique set of structural features in the ion abundance as a function of the number of attached helium atoms for each of the investigated PAHs. Path-integral molecular dynamics simulations were used with a polarizable potential to determine the underlying adsorption patterns of helium around the studied PAH cations and in good general agreement with the experimental data. The calculated structures of the helium-PAH complexes indicate that the arrangement of adsorbed helium atoms is highly sensitive toward the structure of the solvated PAH cation. Closures of the first solvation shell around the studied PAH cations are suggested to lie between 29 and 37 adsorbed helium atoms depending on the specific PAH cation. Helium atoms are found to preferentially adsorb on these PAHs following the 3×3 commensurate pattern common for graphitic surfaces, in contrast to larger carbonaceous molecules like corannulene, coronene, and fullerenes that exhibit a 1 × 1 commensurate phase.

Astrochemical reactions on the surfaces of dust grains, for instance, are thought to be responsible for the formation of complex organic molecules, which are of potential importance for the origin of life. In the situation where the chemical composition of dust surfaces is not precisely known, knowledge of the fundamental reaction properties gains significance. Here we describe an experimental technique that can be used to measure the energy released in reactions involving of a single pair of reactants. These data can be directly compared with the results of quantum chemical computations leading to unequivocal conclusions regarding the reaction pathways and the presence of energy barriers. It allows the prediction of the outcomes of astrochemical surface reactions with higher accuracy compared with that achieved based on gas-phase studies. However, for the highest accuracy, some understanding of the catalytic influence of specific surfaces on the reactions is required. The method was applied to study the reactions of C atoms with H2, O2 and C2H2. The formation of HCH, CO + O and triplet cyclic-C3H2 products has been revealed, correspondingly. By conducting chemical reactions involving a single pair of reactants within helium nanodroplets, Henning and Krasnokutski have measured the energetics of simple reactions relevant to astrochemistry. This approach allows the reaction pathways of surface reactions to be predicted with more accuracy than before.

A century of unsuccessful attempts to identify the neutral ethylenedione molecule combined with the results of quantum-chemical computations resulted in the conclusion on the instability of this molecule. In this article, we demonstrate that although the lowest energy isomer of ethylenedione with linear geometry is indeed unstable, a higher energy three-membered cyclic isomer can be stabilized, and at low temperature has a life-time longer than one millisecond. In our study, the ethylenedione C2O2 molecule was synthesized in the low-temperature reaction CO2 + C → C2O2 inside liquid helium nanodroplets. To study the reaction, a newly developed calorimetric technique was applied. Single pairs of reactants were incorporated into tiny helium droplets having a temperature of 0.37 K. The reaction energy was transferred to liquid helium stabilizing an intermediate gas-phase reaction product. The energy transfer also resulted in the evaporation of helium atoms. Therefore, the change of the helium droplets’ size allowed precise calorimetry on a molecular scale.

Helium droplets represent a cold inert matrix, free of walls with outstanding properties to grow complexes and clusters at conditions that are perfect to simulate cold and dense regions of the interstellar medium. At sub-Kelvin temperatures, barrierless reactions triggered by radicals or ions have been observed and studied by optical spectroscopy and mass spectrometry. The present review summarizes developments of experimental techniques and methods and recent results they enabled.

The solid-state reaction C + H$_2$O $\rightarrow$ H$_2$CO was studied experimentally following the codeposition of C atoms and H$_2$O molecules at low temperatures. In spite of the reaction barrier and absence of energetic triggering, the reaction proceeds fast on the experimental timescale pointing to its quantum tunneling mechanism. This route to formaldehyde shows a new "non-energetic" pathway to complex organic and prebiotic molecules in astrophysical environments. Energetic processing of the produced ice by UV irradiation leads mainly to the destruction of H$_2$CO and the formation of CO$_2$ challenging the role of energetic processing in the synthesis of complex organic molecules under astrophysically relevant conditions., Comment: to appear in APJ

The feasibility of contemporary gas-grain astrochemical models depends on the availability of accurate kinetics data, in particular, for surface processes. We study the sensitivity of gas-grain chemical models to the energy barrier Ea of the important surface reaction between some of the most abundant species: C and H2 (surface C + surface H2 = surface CH2). We used the gas-grain code ALCHEMIC to model the time-dependent chemical evolution over a 2D grid of densities (nH: 10^{3} - 10^{12}cm^{-3}) and temperatures (T: 10 - 300 K), assuming UV-dark (Av = 20 mag) and partly UV-irradiated (Av = 3 mag) conditions that are typical of the dense interstellar medium. We considered two values for the energy barrier of the surface reaction, Ea = 2500 K (as originally implemented in the networks) and Ea = 0 K (as measured in the laboratory and computed by quantum chemistry simulations). We find that if the C + H2 = CH2 surface reaction is barrierless, a more rapid conversion of the surface carbon atoms into methane ice occurs. Overproduction of the CHn hydrocarbon ices affects the surface formation of more complex hydrocarbons, cyanides and nitriles, and CS-bearing species at low temperatures < 10-15 K. The surface hydrogenation of CO and hence the synthesis of complex (organic) molecules become affected as well. As a result, important species whose abundances may change by more than a factor of two at 1 Myr include atomic carbon, small mono-carbonic (C1) and di-carbonic (C2) hydrocarbons, CO2, CN, HCN, HNC, HNCO, CS, H2CO, H2CS, CH2CO, and CH3OH (in either gas and/or ice). The abundances of key species, CO, H2O, and N2 as well as O, HCO+, N2H+, NH3, NO, and most of the S-bearing molecules, remain almost unaffected. Further accurate laboratory measurements and quantum chemical calculations of the surface reaction barriers will be crucial to improve the accuracy of astrochemical models., Submitted to Astronomy & Astrophysics (A&A), 9 pages

The biological molecules delivered to Earth on the board of meteorites and comets were called one of the possible causes of the origin of life. Therefore, our understanding of the routes of formation of biomolecules in space should shed the light on the possibility of the existence of habitable extrasolar planets. The large abundance of organic molecules is found in the space regions with the lowest temperature. Different routes of the organics formation in these areas were suggested. In this article, we demonstrate that complex organic molecules same as important biological molecules can be formed due to the reaction of C atoms with the mantels of molecular ices covering refractory dust grains present in the interstellar medium (ISM). Having four valence electrons, C atoms act as glue joining simple non-organic molecules and converting them into organic matter. The formation of many molecules is barrierless and thus can happen at low temperature. The barrierless reaction C + NH3 + CO -> NH2CHCO attracts particular interest. The product of this reaction is an isomer of the central residue of a peptide chain and expected to be efficiently formed in the translucent molecular clouds. The polymerization of these molecules leads to the formation of proteins that according to some theories are life's first molecules. Considering a high abundance of atomic carbon in the ISM, we expect a high efficiency of the formation of a large variety of different organic molecules, and show why the amount of organic material formed by condensation of atomic carbon may be underestimated., Submitted to Low Temperature Physics

The efficiency of dust formation in a variety of environments is an ongoing topic for discussions, especially if it comes to dust formation in the interstellar medium. Although this possibility is discussed in a wide range of numerical studies, experiments on the formation of dust at low densities and temperatures are mostly lacking. This contribution summarizes the main findings of our low-temperature condensation experiments including the formation of silica, complex silicates with pyroxene and olivine stoichiometry, and of carbonaceous refractory materials. Atomic and molecular species to be expected as products of supernovae shock fronts were produced by laser ablation of silicates and graphite. These species were deposited together with a rare gas on cold substrates representing the surfaces of surviving dust grains in the interstellar medium. After characterizing the precursor species, the rare gas matrix was annealed to induce diffusion and reactions between the initial components. We found the production of amorphous and homogeneous silica and magnesium iron silicates at temperatures of about 12 K in a barrierless reaction as monitored by infrared spectroscopy. The 10 μm band of the low-temperature siliceous condensates shows a striking similarity to the 10 μm band of interstellar silicates. Carbonaceous atoms and molecules can also react without a barrier and form an amorphous or hydrogenated amorphous carbon material. The refractory condensate has properties comparable to fullerene-like carbon grains formed at high temperatures.

We have studied the reactions of iron atoms and clusters with oxygen, acetylene, and water molecules in superfluid He droplets at T = 0.37 K. For all systems, the formation of weakly bound adducts was found, but the insertion reaction of iron into existing molecular bonds could not be observed. The formation of FeOH2 and FeC2H2 complexes was evidenced by mass spectrometry. However, it was found that the reaction of iron atoms with oxygen molecules under similar conditions leads to the stabilization of an intermediate reaction product, the weakly bound linear FeOO adduct, which undergoes complete dissociation upon electron impact ionization. All reactions observed are not expected to proceed in the gas phase. The R2PI spectrum of the y5D4 0 ← a5D4 atomic transition of Fe solvated in helium droplets was recorded. A relatively small blue shift of ∼ 120 cm(-1) with respect to the gas phase position was found.

Several unidentified infrared emission bands (UIBs) have been assigned to neutral C60 molecules present in circumstellar and interstellar environments. However, due to the similarity of the infrared (IR) spectra of C60 in the solid state and in the gas phase, as of yet there is no consensus on the aggregation state of C60. In this article, we show that even strong covalent chemical bonding might have very little influence on the IR spectrum of C60, and that therefore such chemically bonded C60 could be the carrier of the same UIBs. It would best explain observations like the missing emission from C60 ions and a large variation of relative band intensities between different sources. We demonstrate that such a chemically bonded C60 can be produced by the co-condensation of C atoms together with C60 molecules, which leads to the formation of a three-dimensional C60 polymer film. Such polymerized C60 molecules cannot easily desorb, while their spectral properties in the visible and

Threshold ionization spectra of nozzle-jet cooled tetrakis(dimethylamino)ethylene (TDAE) were measured with high-resolution electron and laser ionization techniques, and Raman spectra of the molecule at room temperature and under liquid nitrogen were recorded with laser excitation. The TDAE ion signal shows a gradual increase at the onset of ionization, and the upper bounds of the adiabatic ionization potential (IP) measured from the electron ionization and laser ionization are 5.3 ± 0.2 and 5.20 ± 0.05 eV, respectively. In combination with the experimental measurements, density functional theory calculations were used to predict the adiabatic and vertical IPs and vibrational frequencies. The predicted adiabatic IP (5.2 eV) and C C stretching frequency (1622 cm−1) are in excellent agreement with the measured values. The adiabatic IP is about 0.6 eV lower than the vertical IP (5.8 eV). The large difference between the adiabatic and vertical IPs arise from the significant geometry change upon ionization and is consistent with the experimental observation of the slowly rising ion signal at the ionization onset of the molecule in both the electron ionization and photoionization experiments. Raman spectroscopy of TDAE at room temperature and at liquid nitrogen (Raman under nitrogen, RUN) is reported in an attempt at examining higher energy conformers.

Resonant two-photon ionization (R2PI) spectroscopy has been applied to investigate the solvation of Al atoms in helium droplets. The R2PI spectra reveal vibrational progressions that can be attributed to Al-He(n) vibrations. It is found that small helium droplets have very little chance to pick up an aluminum atom after collision. However, the pick-up probability increases with the size of the helium droplets. The absorption band that is measured by monitoring the ions on the mass of the Al dimer is found to be very little shifted with respect to the Al monomer band (∼400 cm(-1)). However, using the same laser wavelength, we were unable to detect any Al(n) photoion with n larger than two.

The life cycle of the population of interstellar polycyclic aromatic hydrocarbon (PAH) molecules depends partly on the photostability of the individual species. We have studied the dissociative photoionization of two ethynyl-substituted PAH species, namely, 9-ethynylphenanthrene and 1-ethynylpyrene. Their adiabatic ionization energy and the appearance energy of fragment ions have been measured with the photoelectron photoion coincidence (PEPICO) spectroscopy technique. The adiabatic ionization energy has been found at 7.84 +/- 0.02 eV for 9-ethynylphenanthrene and at 7.41 +/- 0.02 eV for 1-ethynylpyrene. These values are similar to those determined for the corresponding non-substituted PAH molecules phenanthrene and pyrene. The appearance energy of the fragment ion indicative of the loss of a H atom following photoionization is also similar for either ethynyl-substituted PAH molecule and its non-substituted counterpart. The measurements are used to estimate the critical energy for the loss of a H atom by the PAH cations and the stability of ethynyl-substituted PAH molecules upon photoionization. We conclude that these PAH derivatives are as photostable as the non-substituted species in HI regions. If present in the interstellar medium, they may play an important role in the growth of interstellar PAH molecules., Comment: Accepted for publication in Astrophys. J. on 1 August 2015, 19 pages, 6 figures, 1 table

The condensation of complex silicates with pyroxene and olivine composition at conditions prevailing in molecular clouds has been experimentally studied. For this purpose, molecular species comprising refractory elements were forced to accrete on cold substrates representing the cold surfaces of surviving dust grains in the interstellar medium. The efficient formation of amorphous and homogeneous magnesium iron silicates at temperatures of about 12 K has been monitored by IR spectroscopy. The gaseous precursors of such condensation processes in the interstellar medium are formed by erosion of dust grains in supernova shock waves. In the laboratory, we have evaporated glassy silicate dust analogs and embedded the released species in neon ice matrices that have been studied spectroscopically to identify the molecular precursors of the condensing solid silicates. A sound coincidence between the 10 micron band of the interstellar silicates and the 10 micron band of the low-temperature siliceous condensates can be noted., Comment: 12 pages, 7 figures, Faraday Discussions meeting 168, submitted on 12 Feb 2014, accepted on 13 Feb 2014

Carbon soot has been prepared by laser-induced pyrolysis of a mixture of ethylene (C2H4) and benzene (C6H6) vapor. The soluble part of the carbonaceous powder has been separated from its insoluble counterpart by soxhlet extraction in toluene. Several techniques were applied to obtain information on the composition of the extract. These included UV/visible and IR spectroscopy in solid and liquid phase, gas chromatography combined with mass spectrometry, gas-phase laser spectroscopy in a supersonic jet, and matrix spectroscopy in helium droplets, the latter being also combined with mass spectrometry. The analysis revealed that the carbonaceous powder contained various polycyclic aromatic hydrocarbons (PAHs). The highest concentration was found for the three-membered catacondensed PAHs, phenanthrene and anthracene. The results are discussed in view of the possible role of these molecules as interstellar dust components.

The electronic spectroscopy of polycyclic aromatic hydrocarbons (PAHs) is of particular interest for astrophysicists since these molecules are suggested as possible carriers of the diffuse interstellar bands. Here we report on our study of the S 1 ←S 0 transition of 2,3-benzofluorene (or benzo[b]fluorene) in the gas phase and in He droplets using cavity ring-down spectroscopy in a supersonic jet expansion and molecular beam depletion spectroscopy, respectively. The same techniques were also employed to identify the PAHs contained in the carbonaceous powder which was produced by CO 2 laser pyrolysis of a mixture of ethylene and benzene vapor. Phenanthrene and anthracene were unambiguously confirmed as components of the as-synthesized soot.

The reaction of carbon atoms with benzene has been investigated in liquid helium droplets at T = 0.37 K. We found an addition of the carbon atom to form an initial intermediate complex followed by a ring opening and the formation of a seven-membered ring. In contrast to a previous gas phase study, the reaction is frozen after these steps and the loss of hydrogen does not occur. A calorimetric technique was applied to monitor the energy balance of the reaction. It was found that more than 267 kJ mol(-1) were released in this reaction. This estimation is in line with quantum chemical calculations of the formation energy of a seven-membered carbon ring. It is suggested that reactions of this kind could be responsible for the low abundance of small polycyclic aromatic hydrocarbon molecules in the interstellar medium. We also found the formation of weakly bonded water-carbon adducts, in which the carbon atom is linked to the oxygen atom of the water molecule with a binding energy of about 33.4 kJ mol(-1).

In a series of experiments devoted to the study of polycyclic aromatic hydrocarbons for astrophysical applications, the S(2)

The conformational structures of 2-deoxyuridine (dU) were investigated using Fourier transform infrared (FTIR) matrix isolation spectroscopy. For the first time the FTIR spectra of dU in Ar matrices were obtained in the range 4000-200 cm(-1). The stabilities of conformers were estimated by the methods HF/3-21G (p), HF/6-31G (d,p) and MP2/6-31G (d,p). Ab initio calculations of the infrared spectra were performed by the methods HF/3-21G (p) and HF/6-31G (d,p). The actual occupancy of conformational isomers in matrix samples was determined. It was shown that anti-conformers of dU are dominant. The ribose rings of the main anti-conformers dU _a0, dU _a1 are in the C2'-endo conformation, but the ribose rings of minor anti-conformers dU_a2, dU_a3 have the C3'-endo conformation, stabilized by intramolecular hydrogen bonds O3'H...O5' and O5'H...O3', accordingly. Syn-conformers of dU are stabilized by the intramolecular hydrogen bond O5'H...O2 and the dominant conformation of the ribose ring is C2'-endo.

Two different types of experiments were performed. In the first experiment, we studied the low-temperature condensation of vaporized graphite inside bulk liquid helium, while in the second experiment, we studied the condensation of single carbon atoms together with H$_2$, H$_2$O, and CO molecules inside helium nanodroplets. The condensation of vaporized graphite leads to the formation of partially graphitized carbon, which indicates high temperatures, supposedly higher than 1000{\deg}C, during condensation. Possible underlying processes responsible for the instant rise in temperature during condensation are discussed. This suggests that such processes cause the presence of partially graphitized carbon dust formed by low-temperature condensation in the diffuse interstellar medium. Alternatively, in the denser regions of the ISM, the condensation of carbon atoms together with the most abundant interstellar molecules (H$_2$, H$_2$O, and CO), leads to the formation of complex organic molecules (COMs) and finally organic polymers. Water molecules were found not to be involved directly in the reaction network leading to the formation of COMs. It was proposed that COMs are formed via the addition of carbon atoms to H$_2$ and CO molecules $(C + H_2 \rightarrow HCH, HCH + CO \rightarrow OCCH_2)$. Due to the involvement of molecular hydrogen, the formation of COMs by carbon addition reactions should be more efficient at high extinctions compared with the previously proposed reaction scheme with atomic hydrogen., Comment: 24 pages, 6 figures

A very high abundance of atomic carbon in the interstellar medium (ISM), and the high reactivity of these species toward different hydrocarbon molecules including benzene, raise questions regarding the stability of polycyclic aromatic hydrocarbon (PAH) molecules in space. To test the efficiency of destruction of PAH molecules via reactions with atomic carbon, we performed a set of laboratory and computational studies of the reactions of naphthalene, anthracene, and coronene molecules with carbon atoms in the ground state. The reactions were investigated in liquid helium droplets at T = 0.37 K and by quantum chemical computations. Our studies suggest that all small and all large catacondensed PAHs react barrierlessly with atomic carbon, and therefore should be efficiently destroyed by such reactions in a broad temperature range. At the same time, large compact pericondensed PAHs should be more inert toward such a reaction. In addition, taking into account their higher photostability, much higher abundances of pericondensed PAHs should be expected in various astrophysical environments. The barrierless reactions between carbon atoms and small PAHs also suggest that, in the ISM, these reactions could lead to the bottom-up formation of PAH molecules.

M2O2 (M = Sc, Y, and La) were synthesized in a pulsed laser-vaporization molecular beam source and studied by mass-analyzed threshold ionization (MATI) spectroscopy and ab initio calculations. Adiabatic ionization energies (AIEs) and several vibrational frequencies were measured accurately for the first time from the MATI spectra. Six possible structural isomers of M2O2 were considered in the calculations and the three converged structures were used in the spectral analysis. A planar cyclic structure in D2h point group was predicted to be the most stable one by the theory and observed by the experiment. The cyclic structure is formed by joining two MO2 fragments together through two shared oxygen atoms. In forming the ground state clusters, each metal atom loses two (n - 1)d electrons and as a result, has only one ns electron in the metal-based valence orbital. The ground electronic state of Sc2O2 is (1)A(g), and those of Y2O2 and La2O2 are (3)B(1u). Ionization of both (1)A(g) and (3)B(1u) neutral states yields the (2)A(g) ion state by removing one of the two ns electrons, and the resultant ion has a similar geometry to the neutral cluster. The AIEs of the clusters are 5.5752 (6), 5.2639 (6), 4.5795 (6) eV for M = Sc, Y, and La, respectively. The vibrational frequencies of the observed modes, including O-M and M-M stretches, are in the range of 200-800 cm(-1).

Employing molecular beam depletion spectroscopy and Fourier transform infrared matrix spectroscopy, respectively, we have studied the O–H stretching vibrations of the glycine conformers I, II, and III. The glycine molecules were either deposited into large liquid helium clusters HeN, N=11000, THe=0.4 K) or trapped in various rare gas matrices (Ne, Ar, Kr) at temperatures below 12 K. By extrapolating the experimental data plotted as a function of the square root of the critical temperature of the matrix material, the positions of the gas phase absorption bands were estimated to be 3585±2 cm−1 (conformer I), 3295±3 cm−1 (conformer II), and 3580±5 cm−1 (conformer III). The experimental results are compared with recent ab initio calculations.

The reactions of carbon atoms with dihydrogen have been investigated in liquid helium droplets at $T$ = 0.37 K. A calorimetric technique was applied to monitor the energy released in the reaction. The barrierless reaction between a single carbon atom and a single dihydrogen molecule was detected. Reactions between dihydrogen clusters and carbon atoms have been studied by high-resolution mass spectrometry. The formation of hydrocarbon cations of the type C$_m$H$_n^+$, with $m$ = 1-4 and $n$ = 1-15 was observed. With enhanced concentration of dihydrogen, the mass spectra demonstrated the main "magic" peak assigned to the CH$_5^+$ cation. A simple formation pathway and the high stability of this cation suggest its high abundance in the interstellar medium.

M(3)O(4) (M = Sc, Y, and La) were produced in a pulsed laser-vaporization molecular beam source and studied by mass-analyzed threshold ionization (MATI) spectroscopy and electronic structure calculations. Adiabatic ionization energies (AIEs) of the neutral clusters and vibrational frequencies of the cations were measured accurately for the first time from the MATI spectra. Five possible structural isomers of M(3)O(4) were considered in the calculations and spectral analysis. A cage-like structure in C(3v) point group was identified as the most stable one. The structure is formed by fusing three M(2)O(2) fragments together, each sharing two O-M bonds with others. The ground electronic state of the neutral clusters is (2)A(1) with the unpaired electron being largely a metal-based s character. Ionization of the (2)A(1) state yields a (1)A(1) ion state in a similar geometry to the neutral cluster. The AIEs of the clusters are 4.4556 (6), 4.0586(6), and 3.4750(6) eV for M = Sc, Y, and La, respectively. The observed vibrational modes of the cations include metal-oxygen stretching, metal triangle breathing, and oxygen-metal-oxygen rocking in the frequency range of 200-800 cm(-1).

Low-temperature reaction rates are important ingredients for astrophysical reaction networks modeling the formation of interstellar matter in molecular clouds. Unfortunately, such data is difficult to obtain by experimental means. In an attempt to study low-temperature reactions of astrophysical interest, we have investigated relevant reactions at ultralow temperature in liquid helium droplets. Being prepared by supersonic expansion of helium gas at high pressure through a nozzle into a vacuum, large helium clusters in the form of liquid droplets constitute nano-sized reaction vessels for the study of chemical reactions at ultra-low temperature. If the normal isotope 4He is used, the helium droplets are superfluid and characterized by a constant temperature of 0.37 K. Here we present results obtained for Mg, Al, and Si reacting with O2. Mass spectrometry was employed to characterize the reaction products. As it may be difficult to distinguish between reactions occurring in the helium droplets before they ...

The doping of He droplets by Al atoms and their reactions with H(2)O and O(2) at T = 0.37 K was investigated. It was found that at high doping concentrations, the incorporated Al atoms do not aggregate to form clusters. They rather remain as separated atoms inside of the He droplets. Mass spectrometry and the recently developed depletion method have been applied to study the reactions. It was found that single Al atoms react with single O(2) molecules. The dominant product of this reaction occurring inside of the He droplets is AlO(2). The reaction between Al and O(2) clusters has also been detected. The Al clusters react with single H(2)O molecules or clusters. While single Al atoms react with H(2)O clusters, no reaction of single Al atoms with a single water molecule was found.

The reaction between Si and O 2 was studied in liquid He droplets at low temperature (T = 0.37 K) by monitoring the energy release during the reaction. Additionally, the reactions of Si atoms and clusters with the oxidation agents H 2 O and O 2 have been studied by mass spectrometry. It was found that Si atoms react fast with O 2 molecules. On the other hand, Si atoms and clusters do not react with H 2 O molecules. The energy released during the chemical reaction leads to the ejection of the products from small He droplets. In contrast, large He droplets (N He > 20000) are capable of keeping part of the reaction products in their interior. The observation of SiO 2 products with the mass spectrometer reveals that the He droplet can stabilize intermediate products in the exit channel.

The reaction of magnesium atoms and clusters with oxygen molecules has been investigated in liquid helium droplets at T = 0.37 K. The energy released during the reaction predominantly leads to the ejection of the reaction products out of the He droplets. The product molecules Mg(2)O, Mg(2)O(2), Mg(3)O, Mg(3)O(2), and Mg(4)O(2) were detected by mass spectrometry. With more than one Mg atom picked up per He droplet, chemiluminescence (CL) light could be detected. This CL results from electronically and vibrationally excited reaction products (Mg(n)O(2), nor = 2) which left the He droplets. For large He droplets with diameter d13 nm, the intensity of the CL light decreases linearly with increasing He droplet diameter. No evidence for chemical reaction was found for d50 nm. A considerable time delay between chemical reaction and light emission was observed. It was found that the chemical reaction is unexpectedly fast and has a first-order reaction rate larger than 5 x 10(4) s(-1).

Li complexes of adenine, uracil, and thymine were produced by laser vaporization of rods made of Li and nucleobase powders in a metal-cluster beam source and studied by pulsed-field-ionization zero-electron-kinetic-energy (ZEKE) spectroscopy and density functional theory calculations. The ZEKE measurements determined the adiabatic ionization energies of the three neutral complexes and frequencies of several vibrational modes for the metal-adenine and -uracil ions. The measured spectra were compared with spectral simulations to determine the preferred metal binding sites. For adenine, the most stable structure is formed by Li/Li(+) bidentately binding to both the N7 atom of the imidazole ring and the NH(2) group of the pyrimidine ring. For uracil and thymine, the ideal site for Li/Li(+) coordination is the O4 atom. Although it has only a small effect on the geometries of uracil and thymine, lithium coordination forces the rotation of the NH(2) group out of the adenine plane. The adiabatic ionization energies of the three complexes follow the trend of uracil (33910+/-5 cm(-1))>thymine (33386+/-5 cm(-1))>adenine (32240+/-5 cm(-1)), whereas their metal-ligand bond dissociation energies are about the same, (92-97) +/-6 kJ mol(-1). For all three complexes, the neutral bond energies are smaller than those of the corresponding ions due to a weaker electrostatic interaction and stronger electron repulsion.

Metal-pyridine (metal = Li, Ca, and Sc) complexes are produced in laser-vaporization molecular beams and studied by pulsed-field-ionization zero-electron-kinetic-energy (ZEKE) spectroscopy and theoretical calculations. Both sigma and pi structures are considered for the three complexes by theory, and preferred structures are determined by the combination of the ZEKE spectra and calculations. The Li and Ca complexes prefer a sigma bonding mode, whereas the Sc complex favors a pi mode. Adiabatic ionization energies and metal-ligand vibrational frequencies are determined from the ZEKE spectra. Metal-ligand bond dissociation energies of the neutral complexes are obtained from a thermodynamic cycle. The ionization energies follow the trend of Li-pyridine (32,460 cm(-1))Ca-pyridine (39,043 cm(-1))Sc-pyridine (42,816 cm(-1)), whereas the bond energies are in the order of Ca-pyridine (27.0 kJ mol(-1))Li-pyridine (49.1 kJ mol(-1))Sc-pyridine (110.6 kJ mol(-1)). The different bonding modes between the main group metals and transition element are discussed in terms of Sc 3d orbital involvement. The bond energy differences between the Li and Ca metals are explained by the number of valence s electrons and the size of the metal atoms.

Al-uracil (Al-C 4 H 4 N 2 O 2 ) was synthesized in a laser-vaporization supersonic molecular beam source and studied with pulsed field ionization-zero electron kinetic energy (ZEKE) photoelectron spectroscopy and density functional theory (DFT). The DFT calculations predicted several low-energy Al-uracil isomers with Al binding to the diketo, keto-enol, and dienol tautomers of uracil. The ZEKE spectroscopic measurements of Al-uracil determined the ionization energy of 43 064(5) cm -1 [or 5.340(6) eV] and a vibrational mode of 51 cm -1 for the neutral complex and several vibrational modes of 51, 303, 614, and 739 cm -1 for the ionized species. Combination of the ZEEK spectrum with the DFT and Franck-Condon factor calculations determined the preferred isomeric structure and electronic states of the Al-uracil complex. This isomer is formed by Al binding to the 04 atom of the diketo tautomer of uracil and has a planar C s symmetry. The ground electronic states of the neutral and ionized species are 2A" and 1 A', respectively. The 2A" neutral state has a slightly shorter Al-O4 distance than the 1 A' ion state. However, the 1 A' ion state has stronger metal-ligand binding compared to the 2A" state. The increased Al-04 distance from the 2 A" state to the 1 A' state is attributed to the loss of the π binding interaction between Al and 04 in the singlet ion state, whereas the increased metal-ligand binding strength is due to the additional charge-dipole interaction in the ion that surpasses the loss of the π orbital interaction.

We report on laboratory simulation experiments mimicking the chemical processing of model atmospheres of exoplanets containing C3 and C4 hydrocarbons at moderate temperatures of 400 K upon interaction of catalytic surfaces of micrometeoroids. By utilizing an ultrasonic levitator device and heating singly levitated particles under simulated microgravity conditions, Raman spectroscopy is utilized as a non-invasive tool to probe on line and in situ the conversion of C3 and C4 hydrocarbons to refractory carbonaceous matter on the surfaces of levitated particles. Secondary Ion Mass Spectrometry and electron microscopic imaging were also conducted to gain further insight into the elementary composition and structures of the refractories formed. Our results provide compelling evidence that in the presence of a catalytic surface, which can be supplied in the form of micrometeoroids and atmospheric dust particles, hydrocarbon gases present in the atmospheres of exoplanets can be converted to refractory, carbon-rich carbonaceous matter of mainly graphitic structure with a carbon content of at least 90% at elevated temperatures. This finding might explain the low methane to carbon monoxide (CH4–CO) ratio in the hot Neptune GJ 436b, where the abundant methane photochemically converts to higher order hydrocarbons and ultimately to refractory graphite-like carbon in the presence of a silicon surface.

Polycyclic aromatic hydrocarbons (PAHs) are believed to be responsible for the Diffuse Interstellar Bands (DIBs), a set of absorption bands observed in Interstellar Space. To provide further information on this issue, we have started a series of experiments devoted to the electronic spectroscopy of some neutral PAH molecules. In order to overcome the difficulty in achieving densities high enough for spectroscopic studies, we have embedded the PAHs in liquid helium droplets which are considered to represent an ideal nanomatrix with very small interaction between the trapped molecule and host. Here we report on first results obtained for the two molecules anthracene (C14H10) and tetracene (C18H12). Using both laser‐induced fluorescence (LIF) and molecular beam depletion (MBD) spectroscopy, we have characterized the first electronic transition (S1 ← S0) of these species when they are solvated in liquid helium clusters.

The Fourier transform infrared spectra of 2`-deoxyribonucleoside — thymidine (dT) in low-temperature Ar matrices are obtained in the range 4000–1300 cm–¹. It is determined that anti-conformers of thymidine are dominant. The ribose rings of the main anti-conformers dT_a0, dT_a1 are in the C2`-endo conformation, but the ribose rings of minor anti-conformers dT_a2, dT_a3 have the C3`-endo conformation, stabilized by intramolecular hydrogen bonds O3`H…O5` and O5`H…O3`, respectively. The main syn-conformer dT_s2 is stabilized by the intramolecular hydrogen bond O5`H…O2 and has C2`-endo conformation of the ribose ring.

M2O2 (M = Sc, Y, and La) were synthesized in a pulsed laser-vaporization molecular beam source and studied by mass-analyzed threshold ionization (MATI) spectroscopy and ab initio calculations. Adiabatic ionization energies (AIEs) and several vibrational frequencies were measured accurately for the first time from the MATI spectra. Six possible structural isomers of M2O2 were considered in the calculations and the three converged structures were used in the spectral analysis. A planar cyclic structure in D2h point group was predicted to be the most stable one by the theory and observed by the experiment. The cyclic structure is formed by joining two MO2 fragments together through two shared oxygen atoms. In forming the ground state clusters, each metal atom loses two (n − 1)d electrons and as a result, has only one ns electron in the metal-based valence orbital. The ground electronic state of Sc2O2 is 1Ag, and those of Y2O2 and La2O2 are 3B1u. Ionization of both 1Ag and 3B1u neutral states yields the 2Ag ion state by removing one of the two ns electrons, and the resultant ion has a similar geometry to the neutral cluster. The AIEs of the clusters are 5.5752 (6), 5.2639 (6), 4.5795 (6) eV for M = Sc, Y, and La, respectively. The vibrational frequencies of the observed modes, including O-M and M-M stretches, are in the range of 200–800 cm−1.

The formation of grains in the interstellar medium, i.e., at low temperature, has been proposed as a possibility to solve the lifetime problem of cosmic dust. This process lacks a firm experimental basis, which is the goal of this study. We have investigated the condensation of SiO molecules at low temperature using neon matrix and helium droplet isolation techniques. The energies of SiO polymerization reactions have been determined experimentally with a calorimetric method and theoretically with calculations based on the density functional theory. The combined experimental and theoretical values have revealed the formation of cyclic (SiO) {sub k} (k = 2-3) clusters inside helium droplets at T = 0.37 K. Therefore, the oligomerization of SiO molecules is found to be barrierless and is expected to be fast in the low-temperature environment of the interstellar medium on the surface of dust grains. The incorporation of numerous SiO molecules in helium droplets leads to the formation of nanoscale amorphous SiO grains. Similarly, the annealing and evaporation of SiO-doped Ne matrices lead to the formation of solid amorphous SiO on the substrate. The structure and composition of the grains were determined by infrared absorption spectroscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Ourmore » results support the hypothesis that interstellar silicates can be formed in the low-temperature regions of the interstellar medium by accretion through barrierless reactions.« less

Lanthanum dimer (La(2)) was studied by mass-analyzed threshold ionization (MATI) spectroscopy and a series of multi-configuration ab initio calculations. The MATI spectrum exhibits three band systems originating from ionization of the neutral ground electronic state, and each system shows vibrational frequencies of the neutral molecule and singly charged cation. The three ionization processes are La(2)(+) (a(2)∑(g)(+)) ← La(2) (X(1)∑(g)(+)), La(2)(+) (b(2)Π(3/2, u)) ← La(2) (X(1)∑(g)(+)), and La(2)(+) (b(2)Π(1/2, u)) ← La(2) (X(1)∑(g)(+)), with the ionization energies of 39,046, 40,314, and 40,864 cm(-1), respectively. The vibrational frequency of the X(1)Σ(g)(+) state is 207 cm(-1), and those of the a(2)Σ(g)(+), b(2)Π(3/2, u) and b(2)Π(1/2, u) are 235.7, 242.2, and 240 cm(-1). While X(1)Σ(g)(+) is the ground state of the neutral molecule, a(2)Σ(g (+) and b(2)Π(u) are calculated to be the excited states of the cation. The spin-orbit splitting in the b(2)Π(u) ion is 550 cm(-1). An X(4)Σ(g)(-) state of La(2)(+) was predicted by theory, but not observed by the experiment. The determination of a singlet ground state of La(2) shows that lanthanum behaves differently from scandium and yttrium.

Polycyclic aromatic hydrocarbons are model systems for studying the mechanisms of lithium storage in carbonaceous materials. In this work, Li complexes of naphthalene, pyrene, perylene, and coronene were synthesized in a supersonic metal-cluster beam source and studied by zero-electron-kinetic-energy (ZEKE) electron spectroscopy and density functional theory calculations. The adiabatic ionization energies of the neutral complexes and frequencies of up to nine vibrational modes in the singly charged cations were determined from the ZEKE spectra. The metal-ligand bond energies of the neutral complexes were obtained from a thermodynamic cycle. Preferred Li∕Li(+) binding sites with the aromatic molecules were determined by comparing the measured spectra with theoretical calculations. Li and Li(+) prefer the ring-over binding to the benzene ring with a higher π-electron content and aromaticity. Although the ionization energies of the Li complexes show no clear correlation with the size of the aromatic molecules, the metal-ligand bond energies increase with the extension of the π-electron network up to perylene, then decrease from perylene to coronene. The trends in the ionization and metal-ligand bond dissociation energies of the complexes are discussed in terms of the orbital energies, local quadrupole moments, and polarizabilities of the free ligands and the charge transfer between the metal atom and aromatic molecules.

Scandium (Sc) complexes of fluorobenzene (C(6)H(5)F), toluene (C(6)H(5)CH(3)), phenol (C(6)H(5)OH), and benzonitrile (C(6)H(5)CN) are produced in a laser-vaporization molecular beam source. These complexes are studied with pulsed-field-ionization zero-electron-kinetic-energy (ZEKE) spectroscopy and density functional theory calculations. Adiabatic ionization energies and low-frequency metal-ligand and ligand-based vibrational modes are measured from the ZEKE spectra. Metal binding sites and strengths and electronic states are obtained by comparing the ZEKE spectra with the theoretical calculations. The ionization energies of Sc-C(6)H(5)X (X = F, CH(3), OH, and CN) follow the trend of CNFOHCH(3), whereas the bond energies are in the order of CNCH(3) approximately OHF. The metal-ligand stretching frequency of Sc(+)-C(6)H(5)CN is nearly twice as those of the other three complexes. All neutral complexes are in low-spin doublet ground states and singly-charged cations are in singlet states. The preferred Sc binding site in these complexes are the phenyl ring with X = F, CH(3), and OH and the nitrile group with CN. For the phenol complex, two rotational conformers are identified in different OH orientations.

Al-thymine (Al-C(4)H(3)N(2)O(2)CH(3)) is produced by laser vaporization of a rod made of Al and thymine powders in a molecular beam and studied by single-photon pulsed-field ionization-zero electron kinetic energy (ZEKE) photoelectron and IR-UV resonant two-photon ionization spectroscopy and density functional theory calculations. The ZEKE experiment determines the adiabatic ionization energy of the neutral complex and 22 vibrational modes for the corresponding ion with frequencies below 2000 cm(-1). The IR-UV photoionization experiment measures two N-H and three C-H stretches for the neutral species. The theoretical calculations predict a number of low-energy isomers with Al binding to single oxygen or adjacent oxygen and nitrogen atoms of thymine. Among these isomers, the structure with Al binding to the O4 atom of the diketo tautomer is predicted to be the most stable one by the theory and is probed by both ZEKE and IR-UV measurements. This work presents the first application of the IR-UV resonant ionization to metal-organic molecule systems. Like ZEKE spectroscopy, the IR-UV photoionization technique is sensitive for identifying isomeric structures of metal association complexes.

The study of the S1(1A1)--S0(1A1) transition of benzo[g,h,i]perylene (BghiP, C22H12) in supersonic jets and solid rare gas matrices is reported. In the jet-cooled spectrum, the origin band position is located at 25,027.1+/-0.2 cm-1, the assignment being supported by the analysis of vibrational shifts and rotational band contours. Except for the origin band, which is weak, all bands are attributed to the fundamental excitation of nontotally symmetric b1 vibrational modes of S1. The intensity pattern is interpreted as a consequence of the weak oscillator strength of the electronic transition combined with intensity-borrowing through vibronic interaction between the S1(1A1) and S2(1B1) states. The spectra of the S1(1A1)--S0(1A1) and S2(1B1)--S0(1A1) transitions have also been measured for BghiP in solid neon and argon matrices. The comparison of the redshifts determined for either transition reveals that the polarizability of BghiP is larger in its S2 than in its S1 state. Bandwidths of 2.7 cm-1 measured in supersonic jets, which provide conditions relevant for astrophysics, are similar to those of most diffuse interstellar bands. The electronic transitions of BghiP are found to lie outside the ranges covered by present databases. From the comparison between experimental spectra and theoretical computations, it is concluded that the accuracy of empirical and ab initio approaches in predicting electronic energies is still not sufficient to identify astrophysically interesting candidates for spectroscopic laboratory studies.