Diversity of protonated mixed pyrene-water clusters investigated by collision induced dissociation.

Protonated mixed pyrene-water clusters, (Py)m(H2O)nH+, where m = [1-3] and n = [1-10], are generated using a cryogenic molecular cluster source. Subsequently, the mass-selected mixed clusters undergo controlled collisions with rare gases, and the resulting fragmentation mass spectra are meticulously analyzed to discern distinct fragmentation channels. Notably, protonated water cluster fragments emerge for n ≥ 3, whereas they are absent for n = 1 and 2. The experimental results are complemented by theoretical calculations of structures and energetics for (Py)(H2O)nH+ with n = [1-4]. These calculations reveal a shift in proton localization, transitioning from the pyrene molecule for n = 1 and 2 to water molecules for n ≥ 3. The results support a formation scenario wherein water molecules attach to protonated pyrene PyH+ seeds, and, by extension, to (Py)2H+ and (Py)3H+ seeds. Various isomers are identified, corresponding to potential protonation sites on the pyrene molecule. Protonated polycyclic aromatic hydrocarbons are likely to be formed in cold, dense interstellar clouds and protoplanetary disks due to the high proton affinity of these species. Our findings show that the presence of protonated PAHs in these environments could lead to the formation of water clusters and mixed carbon-water nanograins, having a potential impact on the water cycle in regions of planet formation.

Threshold collision induced dissociation of protonated water clusters.

We report threshold collision induced dissociation experiments on protonated water clusters thermalized at low temperature for sizes n = 19-23. Fragmentation cross sections are recorded as a function of the collision energy and analyzed with a statistical model. This model allows us to account for dissociation cascades and provides values for the dissociation energies of each cluster. These values, averaging around 0.47 eV, are in good agreement with theoretical predictions at various levels of theory. Furthermore, the dissociation energies show a trend for the n = 21 magic and n = 22 anti-magic numbers relative to their neighbours, which is also in agreement with theory. These results provide further evidence to resolve the disagreement between previously published experimental values. A careful quantitative treatment of cascade dissociation in this model introduces interdependence between the dissociation energies of neighboring sizes, which reduces the number of free fitting parameters and improves both reliability and uncertainties on absolute dissociation energies deduced from experiments.

Water Attachment onto Size-Selected Cationic Pyrene Clusters.

We report measurements of the attachment rates of water molecules onto mass-selected cationic pyrene clusters for size from n = 4 to 13 pyrene units and for different collision energies. Comparison of the attachment rates with the collision rates measured in collision-induced dissociation experiments provides access to the values of the sticking coefficient. The strong dependence of the attachment rates on size and collision energy is rationalized through a model in which we use a Langevin-type collision rate and adjust on experimental data the statistical dissociation rate of the water molecule from the cluster after attachment. This allows us to extrapolate our results to the conditions of isolation and long time scales encountered in astrophysical environments.

Collision-induced dissociation of protonated uracil water clusters probed by molecular dynamics simulations.

Collision-induced dissociation experiments of hydrated molecular species can provide a wealth of important information. However, they often need a theoretical support to extract chemical information. In the present article, in order to provide a detailed description of recent experimental measurements [Braud et al., J. Chem. Phys., 2019, 150, 014303], collision simulations between low-energy protonated uracil water clusters (H2O)1-7,11,12UH+ and an Ar atom were performed using a quantum mechanics/molecular mechanics formalism based on the self-consistent-charge density-functional based tight-binding method. The theoretical proportion of formed neutral vs. protonated uracil containing clusters, total fragmentation cross sections as well as the mass spectra of charged fragments are consistent with the experimental data which highlights the accuracy of the present simulations. They allow to probe which fragments are formed on the short time scale and rationalize the location of the excess proton on these fragments. We demonstrate that this latter property is highly influenced by the nature of the aggregate undergoing the collision. Analyses of the time evolution of the fragments populations and of their relative abundances demonstrate that, up to 7 water molecules, a direct dissociation mechanism occurs after collision whereas for 11 and 12 water molecules a statistical mechanism is more likely to participate. Although scarce in the literature, the present simulations appear as a useful tool to complement collision-induced dissociation experiments of hydrated molecular species.

Threshold collision induced dissociation of pyrene cluster cations.

We report threshold collision induced dissociation experiments on cationic pyrene clusters, for sizes n = 2-6. Fragmentation cross sections are recorded as a function of the collision energy and analyzed with a statistical model. This model can account for the dissociation cascades and provides values for the dissociation energies. These values, of the order of 0.7 eV-1 eV, are in excellent agreement with those previously derived from thermal evaporation. They confirm the charge resonance stability enhancement predicted by theoretical calculations. In addition, remarkable agreement is obtained with theoretical predictions for the two smaller sizes n = 2 and 3. For the larger sizes, the agreement remains good, although the theoretical values obtained for the most stable structures are systematically higher by 0.2 eV. This offset could be attributed to approximations in the calculations. Still, there is an indication in the results of an incomplete description of the role of isomerization and/or direct dissociation upon collisions. Finally, by-product clusters containing dehydrogenated species are found to dissociate at energies comparable to the non-dehydrogenated ones, which shows no evidence for covalent bonds within the clusters.

Thermal evaporation of pyrene clusters.

This work presents a study of the thermal evaporation and stability of pyrene (C16H10)n clusters. Thermal evaporation rates of positively charged mass-selected clusters are measured for sizes in the range n = 3-40 pyrene units. The experimental setup consists of a gas aggregation source, a thermalization chamber, and a time of flight mass spectrometer. A microcanonical Phase Space Theory (PST) simulation is used to determine the dissociation energies of pyrene clusters by fitting the experimental breakdown curves. Calculations using the Density Functional based Tight Binding combined with a Configuration Interaction (CI-DFTB) model and a hierarchical optimization scheme are also performed in the range n = 2-7 to determine the harmonic frequencies and a theoretical estimation of the dissociation energies. The frequencies are used in the calculations of the density of states needed in the PST simulations, assuming an extrapolation scheme for clusters larger than 7 units. Using the PST model with a minimal set of adjustable parameters, we obtain good fits of the experimental breakdown curves over the full studied size range. The approximations inherent to the PST simulation and the influence of the used parameters are carefully estimated. The derived dissociation energies show significant variations over the studied size range. Compared with neutral clusters, significantly higher values of the dissociation energies are obtained for the smaller sizes and attributed to charge resonance in line with CI-DFTB calculations.

Size-dependent proton localization in hydrated uracil clusters: A joint experimental and theoretical study.

A collision-induced dissociation study of hydrated protonated uracil (H2O)n=1-15UH+ clusters is reported. The mass-selected clusters collide with water molecules and rare gases at a controlled center of mass collision energy. From these measurements, absolute fragmentation cross sections and branching ratios are extracted as a function of the uracil hydration. For small clusters, up to n = 4, we observe that only neutral water molecules are evaporated upon collisions, whereas, for larger clusters, neutral uracil is also evaporated: this transition in the nature of the evaporation products is interpreted considering the lowest-energy isomers of each species that are obtained from a combination of density-functional based tight-binding and MP2 calculations. The simulations show that in (H2O)1-4UH+ the proton is located on the uracil molecule or on a water molecule strongly bound to uracil whereas, in larger clusters, the proton is bound to water molecules far from uracil. This correlation between the structure of the low-energy isomers and the experimental fragmentation channel suggests that dissociation may occur in a very short time after collisions so that energy has not enough time to be redistributed among all degrees of freedom and the ground-state geometry of the parent cluster partly determines the nature of the favored fragmentation channels. Of course, thermal dissociations originating from long lived, thus thermalized, collision complexes cannot be ruled out but they are not expected to play the major role since the experimental results can be satisfactorily accounted for by assuming that the fragmentation processes are mainly impulsive.

Theoretical investigation of the solid-liquid phase transition in protonated water clusters.

Protonated water clusters have received a lot of attention as they offer tools to bridge the gap between molecular and bulk scales of water. However, their properties are still not fully understood and deserve further theoretical and experimental investigations. In this work, we simulate the caloric curves of protonated water clusters (H2O)nH+ (n = 20-23). These curves, which have recently been measured experimentally, are characteristic of the phase changes occurring in the aggregates with respect to temperature. The present simulations are achieved by combining parallel-tempering molecular dynamics and the self-consistent-charge density-functional based tight-binding approach and are focused on a restricted size range around (H2O)21H+ which presents singular properties. The shape of the experimental caloric curves and their size dependence are satisfactorily reproduced by the simulations which allows us to further provide a description of the phase transition in terms of structural modifications, dynamics of water molecules and proton mobility. Similar to the experiments, we observe that (H2O)21H+ exhibits a sharper phase transition than the neighbouring size clusters, which can be traced back to both structural and dynamic peculiarities.

Attachment of Water and Alcohol Molecules onto Water and Alcohol Clusters.

Absolute attachment cross sections of single molecules M (M = water, ethanol, or methanol) onto positively charged mass-selected clusters XnH(+) (X = water, ethanol, or methanol) were measured for cluster sizes ranging from tens to hundreds of molecules and center-of-mass collision energies varying from 0.1 to ∼1 eV. The attachment cross sections, which converge as expected toward geometrical cross sections at large cluster sizes, are systematically and noticeably lower than geometrical cross sections at small sizes. Attachment cross sections depend barely on the nature of the reactants. Homogeneous attachment reactions XnH(+) + X → Xn+1H(+) can be accounted for by a dynamical collisional model, in which the intermolecular interactions between the target cluster and the impinging molecule can be neglected. Dynamical arguments account satisfactorily for size and energy dependences of attachment cross sections and also for their variation from one element to another. It is thus suggested that either the attachment probabilities are likely to be more governed by the capacity of clusters to absorb collision energy rather than by cluster/molecule intermolecular interactions, or it indicates that the strength of these interactions does not differ noticeably among the hydrogen-bonded systems investigated. However, for inhomogeneous reactions of the form XnH(+) + Y → XnYH(+) (X, Y = water, ethanol, methanol), although the global size dependences are qualitatively reproduced, the variations of attachment cross sections with the nature on the impinging molecule are not satisfactorily accounted for within the simple empirical model proposed for homogeneous reactions.

Experimental nanocalorimetry of protonated and deprotonated water clusters.

An experimental nanocalorimetric study of mass selected protonated (H2O)nH(+) and deprotonated (H2O)n-1OH(-) water clusters is reported in the size range n = 20-118. Water cluster's heat capacities exhibit a change of slope at size dependent temperatures varying from 90 to 140 K, which is ascribed to phase or structural transition. For both anionic and cationic species, these transition temperatures strongly vary at small sizes, with higher amplitude for protonated than for deprotonated clusters, and change more smoothly above roughly n ≈ 35. There is a correlation between bonding energies and transition temperatures, which is split in two components for protonated clusters while only one component is observed for deprotonated clusters. These features are tentatively interpreted in terms of structural properties of water clusters.

Attachment cross-sections of protonated and deprotonated water clusters.

Attachment cross-sections of water molecules onto size selected protonated (H(2)O)(n)H(+) and deprotonated (H(2)O)(n - 1)OH(-) water clusters have been measured in the size range n = 30-140 for 10 eV kinetic energy of the clusters in the laboratory frame. Within our experimental accuracy, the attachment cross-sections are found to have the same magnitude and size dependence for both species. It is shown that electrostatic interactions are likely to play a role even for the largest sizes investigated.

Heat capacities of mass selected deprotonated water clusters.

Heat capacities of mass selected deprotonated water clusters (H(2)O)(n-1)OH(-) have been measured in the size range n = 48-118, as a function of temperature. We have found that they undergo a melting-like transition in the range 110-130 K. The transition temperature is size dependent with a strong correlation with the dissociation energy around the shell closure at n = 55.

Fragmentation cross sections of protonated water clusters.

We have measured fragmentation cross sections of protonated water cluster cations (H(2)O)(n=30-50)H(+) by collision with water molecules. The clusters have well-defined sizes and internal energies. The collision energy has been varied from 0.5 to 300 eV. We also performed the same measurements on deuterated water clusters (D(2)O)(n=5-45)D(+) colliding with deuterated water molecules. The main fragmentation channel is shown to be a sequential thermal evaporation of single molecules following an initial transfer of relative kinetic energy into internal energy of the cluster. Unexpectedly, that initial transfer is very low on average, of the order of 1% of collision energy. We evaluate that for direct collisions (i.e., within the hard sphere radius), the probability for observing no fragmentation at all is more than 35%, independently of cluster size and collision energy, over our range of study. Such an effect is well known at higher energies, where it is attributed to electronic effects, but has been reported only in a theoretical study of the collision of helium atoms with sodium clusters in that energy range, where only vibrational excitation occurs.

Attachment cross sections of protonated water clusters.

The attachment of water molecules onto size selected protonated water clusters has been experimentally investigated. Absolute attachment cross sections are measured as a function of cluster size, collision energy, and initial cluster temperature. Although thermal evaporation is ruled out in our experiment, attachment cross sections become significantly smaller than hard sphere cross sections as the collision energy increases. This feature is attributed to a transition from adiabatic to nonadiabatic regime. It is shown to be due to a dynamical effect: as the collision duration becomes shorter than the typical time required for collision energy redistribution into clusters internal energy, the attachment probability is reduced. We relate this typical time to the period of the main surface vibrational mode excited by the collisions. This hypothesis is further supported by results obtained with deuterated water clusters.

Two-step melting of Na(41)(+).

The heat capacity of the mass selected Na(41) (+) cluster has been measured using a differential nanocalorimetry method. A two-peak structure appears in the heat capacity curve of Na(41) (+), whereas Schmidt and co-workers [M. Schmidt, J. Donges, Th. Hippler, and H. Haberland, Phys. Rev. Lett. 90, 103401 (2003)] observed, within their experimental accuracy, a smooth caloric curve. They concluded from the absence of any structure that there is a second order melting transition in Na(41) (+) with no particular feature such as premelting. The observed difference with the latter results is attributed to the better accuracy of our method owing to its differential character. The two structures in the heat capacity are ascribed to melting and premelting of Na(41) (+). The peak at lower temperature is likely due to an anti-Mackay to Mackay solid-solid transition.

Sticking properties of water clusters.

Absolute attachment cross sections of single water molecules onto mass-selected protonated water clusters have been measured in the 30-200 size range and for collision energies down to approximately 50 meV. The major surprise is that the attachment cross sections are always smaller or equal to the hard sphere cross section. The attractive interaction between water molecules and charged water nanodroplets does not enhance the sticking probability as expected. The attachment probability is reduced due to dynamical processes on a short time scale; that is, inelastic scattering occurs when the duration of the collision becomes smaller than the period of the O..O..O surface bending motion. A simple analytical expression is derived that gives the sticking probability of a water molecule onto a water nanodroplet in a vapor as a function of the vapor temperature and the size of the droplet.

Growth and melting of droplets in cold vapors.

A model has been developed to investigate the growth of droplets in a supersaturated cold vapor taking into account their possible solid-liquid phase transition. It is shown that the solid-liquid phase transition is nontrivially coupled, through the energy released in attachment, to the nucleation process. The model is based on the one developed by J. Feder, K. C. Russell, J. Lothe, and G. M. Pound [Adv. Phys. 15, 111 (1966)], where the nucleation process is described as a thermal diffusion motion in a two-dimensional field of force given by the derivatives of a free-energy surface. The additional dimension accounts for droplets internal energy. The solid-liquid phase transition is introduced through a bimodal internal energy distribution in a Gaussian approximation derived from small clusters physics. The coupling between nucleation and melting results in specific nonequilibrium thermodynamical properties, exemplified in the case of water droplets. Analyzing the free-energy landscapes gives an insight into the nucleation dynamics. This landscape can be complex but generally exhibits two paths: the first one can generally be ascribed to the solid state, while the other to the liquid state. Especially at high supersaturation, the growth in the liquid state is often favored, which is not unexpected since in a supersaturated vapor the droplets can stand higher internal energy than at equilibrium. From a given critical temperature that is noticeably lower than the bulk melting temperature, nucleation may end in very large liquid droplets. These features can be qualitatively generalized to systems other than water.

A novel experimental method for the measurement of the caloric curves of clusters.

A novel experimental scheme has been developed in order to measure the heat capacity of mass selected clusters. It is based on controlled sticking of atoms on clusters. This allows one to construct the caloric curve, thus determining the melting temperature and the latent heat of fusion in the case of first-order phase transitions. This method is model-free. It is transferable to many systems since the energy is brought to clusters through sticking collisions. As an example, it has been applied to Na(90) (+) and Na(140) (+). Our results are in good agreement with previous measurements.

Experimental determination of nucleation scaling law for small charged particles.

We investigated the nucleation process at the molecular level. Controlled sticking of individual atoms onto mass selected clusters over a wide mass range has been carried out for the first time. We measured the absolute unimolecular nucleation cross sections of cationic sodium clusters Na{n}{+} in the range n=25-200 at several collision energies. The widely used hard sphere approximation clearly fails for small sizes: not only should vapor-to-liquid nucleation theories be modified, but also, through the microreversibility principle, cluster decay rate statistical models.