Infrared bands of neutral gas-phase carbon clusters in a broad spectral range.

The identification of species in the interstellar medium requires precise and molecule-specific spectroscopic information in the laboratory framework, in broad spectral ranges and under conditions relevant to interstellar environments. In this work, we measure the gas-phase infrared spectra of neutral carbon clusters, CN (N = 6-11), in a molecular beam. The CN distribution is formed by photofragmentation of C60 molecules, concurrently showing a top-down formation mechanism. A broad spectral range in the infrared between 500-3200 cm-1 (20-3.125 µm) is investigated. We observe strong bands between 5 and 6 µm, in conjunction with novel features in the 3 µm region. Density functional theory calculations reveal that these short wavelength modes correspond to combination bands with significant infrared intensity. Moreover, we identify the N ≤ 10 clusters as linear, while C11 adopts a ring configuration, placing the linear-to-ring transition at N = 11 under our molecular beam conditions. The linearity of C10 is discussed based on the formation pathway from larger clusters in energetic conditions. Given the vast and very precise infrared information already been released from the James Webb Space Telescope mission, this infrared spectroscopic data set in conjunction with information on formation mechanisms is of major relevance for identifying neutral carbon clusters in astronomical environments.

The infrared absorption spectrum of phenylacetylene and its deuterated isotopologue in the mid- to far-IR.

Anharmonicity strongly influences the absorption and emission spectra of polycyclic aromatic hydrocarbon (PAH) molecules. Here, IR-UV ion-dip spectroscopy experiments together with detailed anharmonic computations reveal the presence of fundamental, overtone, as well as 2- and 3-quanta combination band transitions in the far- and mid-infrared absorption spectra of phenylacetylene and its singly deuterated isotopologue. Strong absorption features in the 400-900 cm-1 range originate from CH(D) in-plane and out-of-plane wags and bends, as well as bending motions including the C≡C and CH bonds of the acetylene substituent and the aromatic ring. For phenylacetylene, every absorption feature is assigned either directly or indirectly to a single or multiple vibrational mode(s). The measured spectrum is dense, broad, and structureless in many regions but well characterized by computations. Upon deuteration, large isotopic shifts are observed. At frequencies above 1500 cm-1 for d1-phenylacetylene, a one-to-one match is seen when comparing computations and experiments with all features assigned to combination bands and overtones. The C≡C stretch observed in phenylacetylene is not observed in d1-phenylacetylene due to a computed 40-fold drop in intensity. Overall, a careful treatment of anharmonicity that includes 2- and 3-quanta modes is found to be crucial to understand the rich details of the infrared spectrum of phenylacetylene. Based on these results, it can be expected that such an all-inclusive anharmonic treatment will also be key for unraveling the infrared spectra of PAHs in general.

Radiative cooling in silver and palladium doped gold clusters.

The emission of photons from a thermally populated electronic excited state, via the process of recurrent fluorescence, has been recognized as a prominent cooling channel in hot molecules and small metal clusters. For the latter case, however, only monometallic species have been investigated to date. An active radiative cooling channel has a stabilizing effect and can favor the size and composition specific production of selected clusters. In this work, the influence of silver and palladium doping on the radiative cooling of gold cluster cations is studied. The quenching of metastable fragmentation due to radiation of laser-excited Aun+, AgAun-1+ and PdAun-1+ (n = 11-15) clusters is investigated in a single-pass molecular beam setup. The observed high radiation rates, with values in the range from 103 to 105 s-1, are consistent with recurrent fluorescence. The rates present a pronounced odd-even staggering with higher values for the clusters with closed-shell electronic configurations. While substitution of Au with Ag does not alter the odd-even pattern with cluster size, replacing Au with Pd shifts the pattern by one atom. The experimental observations are discussed in terms of the dissociation energy of the clusters, which sets their effective temperature during photon emission, and the low-lying electronic excited states involved in the photon emission process. Linear-response time-dependent density functional theory calculations on selected species are used to illustrate the significant effect of the electronic structure on the radiation rates. For n = 14, substitution of Au with Ag lowers the energy of the lowest-energy transition in the cluster, which in addition has a higher oscillator strength, favoring radiative cooling. The opposite effect is seen in Pd doped clusters. Based on this analysis, conclusions can be drawn about the significance of radiative cooling in laser-excited alloy clusters, with a concomitant fast stabilization at high internal energy conditions.

A multidisciplinary approach to estimating wolf population size for long-term conservation.

The wolf (Canis lupus) is among the most controversial of wildlife species. Abundance estimates are required to inform public debate and policy decisions, but obtaining them at biologically relevant scales is challenging. We developed a system for comprehensive population estimation across the Italian alpine region (100,000 km2 ), involving 1513 trained operators representing 160 institutions. This extensive network allowed for coordinated genetic sample collection and landscape-level spatial capture-recapture analyses that transcended administrative boundaries to produce the first estimates of key parameters for wolf population status assessment. Wolf abundance was estimated at 952 individuals (95% credible interval 816-1120) and 135 reproductive units (i.e., packs) (95% credible interval 112-165). We also estimated that mature individuals accounted for 33-45% of the entire population. The monitoring effort was spatially estimated thereby overcoming an important limitation of citizen science data. This is an important approach for promoting wolf-human coexistence based on wolf abundance monitoring and an endorsement of large-scale harmonized conservation practices.

Una estrategia multidisciplinaria para la estimación del tamaño poblacional de los lobos para la conservación a largo plazo Resumen El lobo (Canis lupus) está entre las especies de fauna más controversiales. Se requieren estimaciones de abundancia para informar al debate público y las decisiones políticas, pero es un reto obtenerlos en escalas con relevancia biológica. Desarrollamos un sistema para la estimación completa de la población en la región alpina de Italia (100,000 km2 ), con la participación de 1,513 operadores entrenados que representan a 160 instituciones. Esta red extensa permitió una colecta coordinada de muestras genéticas y análisis de captura-recaptura espacial que trascendieron las fronteras administrativas para así producir las primeras estimaciones de los parámetros clave para la evaluación del estado de la población de los lobos. Se estimó la abundancia en 952 individuos (95% intervalo de confianza 816-1120) y 135 unidades reproductivas (es decir, manadas) (95% intervalo de confianza 112-165). También estimamos que los individuos maduros representaban el 33-45% de toda la población. El esfuerzo de monitoreo se estimó espacialmente, por lo que sobrepasó una limitación importante de la ciencia ciudadana. Esta estrategia es importante para promover la coexistencia entre lobos y humanos con base en el monitoreo de la abundancia y el apoyo a las prácticas armonizadas de conservación a gran escala.

Fragmentation channels of non-fullerene cationic carbon clusters.

The unimolecular fragmentation channels of highly excited small cationic carbon clusters have been measured with a time-of-flight mass spectrometer after photofragmentation. The dominant channel is loss of the neutral trimer, for all CN+N = 10-27 clusters except for N = 11, 12 which decay by monomer emission, and C25+ which shows competing loss of C2 and C3. The results permit to quantify the role of the rotational entropy in the competition between monomer and trimer decays with the help of energies calculated with density functional theory.

The spin magnetic order of Con+ (n ≤ 5) clusters.

The magnetism of transition metal clusters has been for decades a complicated puzzle, with experimental results disagreeing with calculations performed within the density functional theory formalism. In this work, we provide a key to this puzzle by investigating the lowest-energy spin states of cobalt cluster, Con+ (n ≤ 5), using CASSCF/NEVPT2 calculations with very large active spaces. The geometries as well as the spin configurations adopted by the clusters in their ground-state are known from experiments, making Con+ clusters an ideal model system for theoretical investigation. Here, using the experimentally known geometries determined by far-infrared spectroscopy as inputs, we calculated the lowest-energy spin configurations of the clusters, revealing that the CASSCF/NEVPT2 formalism correctly predicts the preferred electronic configuration of the clusters known experimentally. This is in contrast to the widely used density functional theory, with results that depend on the selected exchange-correlation functional. The reasons for the failure of density functional theory, in opposition to CASSCF/NEVPT2, are discussed, providing a solid framework for investigating other transition metal and transition metal oxide clusters.

Influence of oxidation on the magnetism of small Co oxide clusters probed by Stern-Gerlach deflection.

We report on the magnetic properties of small neutral suboxide ConOm (n = 5-18 and m = 0-10, m ≤ n) clusters produced by laser vaporisation and gas aggregation. Their magnetism is probed experimentally by means of Stern-Gerlach magnetic deflection. The results imply that the cobalt atoms couple ferromagnetically not only in pure Con clusters, as known from previous investigations, but also in their oxidized counterparts. It was found that the magnetic moment per cobalt atom is mostly enhanced in the oxide clusters with respect to the pure cobalt clusters and generally increases with the oxygen content in the studied composition range. The spin magnetism of selected clusters is also investigated by density functional theory (DFT) calculations. The computations allow to attribute the effect of oxidation on the magnetic response of the ConOm clusters to electron transfer from the cobalt 3d and 4s valence orbitals to oxygen. The cobalt 3d levels preferentially donate electrons of minority spin, but both spin orientations are involved in the transfer of cobalt 4s electrons.

Doping Platinum with Germanium: An Effective Way to Mitigate the CO Poisoning.

The vulnerability towards CO poisoning is a major drawback affecting the efficiency and long-term performance of platinum catalysts in fuel cells. In the present work, by a combination of density functional theory calculations and mass spectrometry experiments, we test and explain the promotional effect of Ge on Pt catalysts with higher resistance to deactivation via CO poisoning. A thorough exploration of the configurational space of gas-phase Ptn + and GePtn-1 + (n=5-9) clusters using global minima search techniques and the subsequent electronic structure analysis reveals that germanium doping reduces the binding strength between Pt and CO by hindering the 2π-back-donation. Importantly, the clusters remain catalytically active towards H2 dissociation. The ability of Ge to weaken the Pt-CO interaction was confirmed by mass spectrometry experiments. Ge can be a promising alloying agent to tune the selectivity and improve the durability of Pt particles, thus opening the way to novel catalytic alternatives for fuel cells.

Computing gold cluster energies with density functional theory: the importance of correlation.

Energies calculated with density functional theory depend critically on the choice of the exchange-correlation functional. In this work, we use measured dissociation energies of Aun+ (n = 5-17) clusters as benchmark data to test two very different functionals for calculating total energies in these clusters; the simpler (and fast) PBE and the evolved (and expensive) B2PLYP double-hybrid functionals. PBE consistently gives poor agreement with the experimental results. In contrast, the B2PLYP functional, which implicitly includes electron correlation by performing a perturbative second-order correction, significantly improves the agreement of the calculations, at the cost of much more demanding computations. The better performance of the double-hybrid functional is ascribed to the longer range of the interatomic potential.

Argon tagging of doubly transition metal doped aluminum clusters: The importance of electronic shielding.

The interaction of argon with doubly transition metal doped aluminum clusters, AlnTM2 + (n = 1-18, TM = V, Nb, Co, Rh), is studied experimentally in the gas phase via mass spectrometry. Density functional theory calculations on selected sizes are used to understand the argon affinity of the clusters, which differ depending on the transition metal dopant. The analysis is focused on two pairs of consecutive sizes: Al6,7V2 + and Al4,5Rh2 +, the largest of each pair showing a low affinity toward Ar. Another remarkable observation is a pronounced drop in reactivity at n = 14, independent of the dopant element. Analysis of the cluster orbitals shows that this feature is not a consequence of cage formation but is electronic in nature. The mass spectra demonstrate a high similarity between the size-dependent reactivity of the clusters with Ar and H2. Orbital interactions provide an intuitive link between the two and further establish the importance of precursor states in the reactions of the clusters with hydrogen.

Argon Adsorption on Cationic Gold Clusters Aun+ (n ≤ 20).

The interaction of Aun+ (n ≤ 20) clusters with Ar is investigated by combining mass spectrometric experiments and density functional theory calculations. We show that the inert Ar atom forms relatively strong bonds with Aun+. The strength of the bond strongly varies with the cluster size and is governed by a fine interplay between geometry and electronic structure. The chemical bond between Aun+ and Ar involves electron transfer from Ar to Au, and a stronger interaction is found when the Au adsorption site has a higher positive partial charge, which depends on the cluster geometry. Au15+ is a peculiar cluster size, which stands out for its much stronger interaction with Ar than its neighbors, signaled by a higher abundance in mass spectra and a larger Ar adsorption energy. This is shown to be a consequence of a low-coordinated Au adsorption site in Au15+, which possesses a large positive partial charge.

Water Splitting by C60 -Supported Vanadium Single Atoms.

Water splitting is an important source of hydrogen, a promising future carrier for clean and renewable energy. A detailed understanding of the mechanisms of water splitting, catalyzed by supported metal atoms or nanoparticles, is essential to improve the design of efficient catalysts. Here, we report an infrared spectroscopic study of such a water splitting process, assisted by a C60 supported vanadium atom, C60 V+ +H2 O→C60 VO+ +H2 . We probe both the entrance channel complex C60 V+ (H2 O) and the end product C60 VO+ , and observe the formation of H2 as a result from resonant infrared absorption. Density functional theory calculations exploring the detailed reaction pathway reveal that a quintet-to-triplet spin crossing facilitates the water splitting reaction by C60 -supported V+ , whereas this reaction is kinetically hindered on the isolated V+ ion by a high energy barrier. The C60 support has an important role in lowering the reaction barrier with more than 70 kJ mol-1 due to a large orbital overlap of one water hydrogen atom with one carbon atom of the C60 support. This fundamental insight in the water splitting reaction by a C60 -supported single vanadium atom showcases the importance of supports in single atom catalysts by modifying the reaction potential energy surface.

The structures of cationic gold clusters probed by far-infrared spectroscopy.

Determining the precise structures of small gold clusters is an essential step towards understanding their chemical and physical properties. Due to the relativistic nature of gold, its clusters remain planar (2D) up to appreciable sizes. Ion mobility experiments have suggested that positively charged gold clusters adopt three-dimensional (3D) structures from n = 8 onward. Computations predict, depending on the level of theory, 2D or 3D structures as putative energy-minimum for n = 8. In this work, far-infrared multiple photon dissociation spectroscopy, using Ar as tagging element, is combined with density-functional theory calculations to determine the structures of Aun+ (n≤ 9) clusters formed by laser ablation. While the Au frameworks in Au6Arm+ and Au7Arm+ complexes are confirmed to be planar and that in Au9Arm+ three-dimensional, we demonstrate the coexistence of 3D and planar Au8Arm+ (m = 1-3) isomers. Thus, it is revealed that at finite temperatures, the formal 2D to 3D transition takes place at n = 8 but is not sharp.

Stability of cationic silver doped gold clusters and the subshell-closed electronic configuration of AgAu14.

Silver doping is a valuable route to modulate the structural, electronic, and optical properties of gold clusters. We combine photofragmentation experiments with density functional theory calculations to investigate the relative stability of cationic Ag doped Au clusters, AgAuN-1 + (N ≤ 40). The mass spectra of the clusters after photofragmentation reveal marked drops in the intensity of AgAu8 +, AgAu14 +, and AgAu34 +, indicating a higher relative stability of these sizes. This is confirmed by the calculated AgAuN-1 + (N ≤ 17) dissociation energies peaking for AgAu6 +, AgAu8 +, and AgAu14 +. While the stability of AgAu6 + and AgAu8 + can be explained by the accepted electronic shell model for metal clusters, density of states analysis shows that the geometry plays an important role in the higher relative stability of AgAu14 +. For this size, there is a degeneracy lifting of the 1D shell, which opens a relatively large HOMO-LUMO gap with a subshell-closed 1S21P41P21D6 electronic configuration.

Tuning the Reactivity of Small Metal Clusters by Heteroatom Doping.

The reactivity of small metallic clusters, nanoparticles composed of a countable number of atoms (typically up to ∼100 atoms), has attracted much attention due to the fascinating properties these objects possess toward a variety of molecules. Cluster reactivity often is significantly different from the homologous bulk, with gold as prototypical example. Bulk gold is the noblest of all metals, whereas small gold clusters react with carbon monoxide, molecular oxygen, and hydrocarbons, among others. Furthermore, cluster reactivity is strongly size and composition dependent, allowing a wide range of tuning possibilities. The study of cluster reactivity usually follows two routes of investigation. In the first, research aims for fundamental understanding of mechanisms, mainly driven by curiosity. One consequence of the inherent small size of a cluster is that atoms can arrange themselves very differently from the crystallographic structure of the homologous bulk. In addition, quantum confinement effects dominate the electronic structure of a cluster with atom-like electronic shells instead of the electronic bands in bulk. These features result in a very rich and size-dependent interaction of a cluster with small molecules, governed by a fine interplay between the geometry and the electronic structure of the system. An alternative research approach uses the investigation of chemical reactions of isolated small clusters in the gas phase as model systems for the reactions taking place in more complex systems. This offers several advantages compared to more conventional methods and techniques used to study such complex systems. First, clusters can be produced under well-defined conditions, with control over size, composition, and charge state. Second, clusters in the gas phase solely interact with the molecule(s) chosen by the researcher, since contaminations are limited by the high vacuum conditions of the experiments. Third, due to the small number of atoms involved, detailed quantum chemical calculations can be performed on the systems under investigation. Thus, even though gas phase clusters differ significantly in size and in environmental conditions from those encountered, for example, in industrial catalysis, they can be used to unravel the complicated nature of a metal-molecule chemical bonding process. In this Account, both routes of investigation are discussed. The nature of the interaction between small gas phase clusters with diverse molecules is described, stressing the broader relevance of these studies. Particular emphasis is given to the effect of heteroatom doping. By adding a different element to a cluster, its geometric and electronic structure is modified, thereby altering its reactivity. Specifically, the effect of varying size and composition of doped gold, platinum, and aluminum clusters on their reactivity toward diverse molecules, relevant for catalytic applications, is discussed. Most studies presented here combine experiments based on mass spectrometric techniques with density functional theory calculations, allowing a deep understanding of the reaction mechanisms at a molecular level.

Non-covalent Interactions and Charge Transfer between Propene and Neutral Yttrium-Doped and Pure Gold Clusters.

The dopant and size-dependent propene adsorption on neutral gold (Aun ) and yttrium-doped gold (Aun-1 Y) clusters in the n=5-15 size range are investigated, combining mass spectrometry and gas phase reactions in a low-pressure collision cell and density functional theory calculations. The adsorption energies, extracted from the experimental data using an RRKM analysis, show a similar size dependence as the quantum chemical results and are in the range of ≈0.6-1.2 eV. Yttrium doping significantly alters the propene adsorption energies for n=5, 12 and 13. Chemical bonding and energy decomposition analysis showed that there is no covalent bond between the cluster and propene, and that charge transfer and other non-covalent interactions are dominant. The natural charges, Wiberg bond indices, and the importance of charge transfer all support an electron donation/back-donation mechanism for the adsorption. Yttrium plays a significant role not only in the propene binding energy, but also in the chemical bonding in the cluster-propene adduct. Propene preferentially binds to yttrium in small clusters (n<10), and to a gold atom at larger sizes. Besides charge transfer, relaxation also plays an important role, illustrating the non-local effect of the yttrium dopant. It is shown that the frontier molecular orbitals of the clusters determine the chemical bonding, in line with the molecular-like electronic structure of metal clusters.

Stability of small cationic platinum clusters.

The relative stability of small cationic platinum clusters is investigated by photofragmentation experiments. Mass spectra show a smooth intensity distribution except for a local intensity minimum at Pt5+, revealing enhanced stability of the platinum tetramer Pt4+. The possibility that radiative cooling competes with statistical fragmentation after photoexcitation is examined and it is shown that clusters in the N = 3-8 size range do not radiate on the time scale of the experiment. In the absence of radiative cooling, the mass spectra of photofragmented clusters can be well explained by dissociation energies computed using density functional theory. The large calculated HOMO-LUMO gap for Pt4+ (∼1.2 eV) is attributed to its highly symmetric structure and provides an explanation for the surprisingly low reactivity of this cluster in different gas-phase reactions.

Hydrogen Chemisorption on Singly Vanadium-Doped Aluminum Clusters.

The effect of vanadium doping on the hydrogen adsorption capacity of aluminum clusters (Aln+ , n=2-18) is studied experimentally by mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy. We find that vanadium doping enhances the reactivity of the clusters towards hydrogen, albeit in a size-dependent way. IRMPD spectra, which provide a fingerprint of the hydrogen binding geometry, show that H2 dissociates upon adsorption. Density functional theory (DFT) calculations for the smaller Aln V+ (n=2-8,10) clusters are in good agreement with the observed reactivity pattern and underline the importance of activation barriers in the chemisorption process. Orbital analysis shows that the activation barriers are due to an unfavorable overlap between cluster and hydrogen orbitals.

Effects of Charge Transfer on the Adsorption of CO on Small Molybdenum-Doped Platinum Clusters.

The interaction of carbon monoxide with platinum alloy nanoparticles is an important problem in the context of fuel cell catalysis. In this work, molybdenum-doped platinum clusters have been studied in the gas phase to obtain a better understanding of the fundamental nature of the Pt-CO interaction in the presence of a dopant atom. For this purpose, Ptn+ and MoPtn-1+ (n=3-7) clusters were studied by combined mass spectrometry and density functional theory calculations, making it possible to investigate the effects of molybdenum doping on the reactivity of platinum clusters with CO. In addition, IR photodissociation spectroscopy was used to measure the stretching frequency of CO molecules adsorbed on Ptn+ and MoPtn-1+ (n=3-14), allowing an investigation of dopant-induced charge redistribution within the clusters. This electronic charge transfer is correlated with the observed changes in reactivity.