Photoinduced evaporation of charged clusters

Microcanonical Nucleation Theory for Anisotropic Materials Validated on Alumina Clusters

Nucleation kinetics in gas phase remains an open issue with no general model. The derivation of the reaction constants assuming a canonical ensemble fails to describe anisotropic materials such as oxides. We have developed a general and versatile model using activated complex kinetics with a microcanonical approach. This approach handles the kinetics issue in cluster growth when the transient nature of the processes hinders the use of the canonical ensemble. The model efficiently reproduces experimental size distributions of alumina clusters formed by laser ablation with different buffer gas densities, including magic numbers. We show that the thermodynamic equilibrium is not reached during the growth. The bounding energy measured is 10 times lower than the one deduced from DFT calculation, but also the one expected from the bulk cohesive energy.

Maxwell-Boltzmann versus non-ergodic events in the velocity distribution of water molecules evaporated from protonated water nanodroplets

Measurement of velocity distributions of evaporated water monomers from small mass- and energy-selected protonated water clusters allows probing the extent of thermalization after excitation of these ultimately small nanodroplets. Electronic excitation of a molecule in the cluster is here induced by a single collision with an argon atom in the keV energy range. The measured velocity distributions of the departing neutral molecules exhibit bimodal shapes with a lower-velocity part consistent with a complete redistribution of the deposited energy in the entire cluster and a higher-velocity contribution corresponding to evaporation before complete energy redistribution. Statistical molecular dynamics calculations reproduce the bimodal shape of the velocity distributions by assuming an initial spreading of the excitation energy among all modes, thereby reproducing the lower velocity contribution of the distribution. By contrast, assuming the deposited energy to be initially localized among the modes of a single molecule leads to calculated distributions with two components whose shape is in accordance with the experimental results. The characteristics and the relative abundance of these two contributions in the velocity distributions obtained are presented and discussed as a function of the number of molecules (n = 2-10) in the ionized nanodroplet H⁺(H₂O) _n .

Vibrational characterization of simple peptides using cryogenic infrared photodissociation of H2-tagged, mass-selected ions

We present infrared photodissociation spectra of two protonated peptides that are cooled in a ~10 K quadrupole ion trap and "tagged" with weakly bound H(2) molecules. Spectra are recorded over the range of 600-4300 cm(-1) using a table-top laser source, and are shown to result from one-photon absorption events. This arrangement is demonstrated to recover sharp (Δν ~6 cm(-1)) transitions throughout the fingerprint region, despite the very high density of vibrational states in this energy range. The fundamentals associated with all of the signature N-H and C=O stretching bands are completely resolved. To address the site-specificity of the C=O stretches near 1800 cm(-1), we incorporated one (13)C into the tripeptide. The labeling affects only one line in the complex spectrum, indicating that each C=O oscillator contributes a single distinct band, effectively "reporting" its local chemical environment. For both peptides, analysis of the resulting band patterns indicates that only one isomeric form is generated upon cooling the ions initially at room temperature into the H(2) tagging regime.

Phase space theory of evaporation in neon clusters: the role of quantum effects

Unimolecular evaporation of neon clusters containing between 14 and 148 atoms is theoretically investigated in the framework of phase space theory. Quantum effects are incorporated in the vibrational densities of states, which include both zero-point and anharmonic contributions, and in the possible tunneling through the centrifugal barrier. The evaporation rates, kinetic energy released, and product angular momentum are calculated as a function of excess energy or temperature in the parent cluster and compared to the classical results. Quantum fluctuations are found to generally increase both the kinetic energy released and the angular momentum of the product, but the effects on the rate constants depend nontrivially on the excess energy. These results are interpreted as due to the very few vibrational states available in the product cluster when described quantum mechanically. Because delocalization also leads to much narrower thermal energy distributions, the variations of evaporation observables as a function of canonical temperature appear much less marked than in the microcanonical ensemble. While quantum effects tend to smooth the caloric curve in the product cluster, the melting phase change clearly keeps a signature on these observables. The microcanonical temperature extracted from fitting the kinetic energy released distribution using an improved Arrhenius form further suggests a backbending in the quantum Ne(13) cluster that is absent in the classical system. Finally, in contrast to delocalization effects, quantum tunneling through the centrifugal barrier does not play any appreciable role on the evaporation kinetics of these rather heavy clusters.

Stepwise hydration and evaporation of adenosine monophosphate nucleotide anions: a multiscale theoretical study

The structure and finite-temperature properties of hydrated nucleotide anion adenosine 5'-monophosphate (AMP) have been theoretically investigated with a variety of methods. Using a polarizable version of the Amber force field and replica-exchange molecular dynamics simulations, putative lowest-energy structures have been located for the AMP(-)(H(2)O)(n) cluster anions with n = 0-20. The hydration energies obtained with the molecular mechanics potential slightly overestimate experimental measurements. However, closer values are found after reoptimizing the structures locally at more sophisticated levels, namely semi-empirical (PM6) and density-functional theory (B3LYP/6-31+G*). Upon heating the complexes, various indicators such as the heat capacity, number of hydrogen bonds or surface area provide evidence that the water cluster melts below 200 K but remains bonded to the AMP anion. The sequential loss of water molecules after sudden heating has been studied using a statistical approach in which unimolecular evaporation is described using the orbiting transition state version of phase space theory, together with anharmonic densities of vibrational states. The evaporation rates are calibrated based on the results of molecular dynamics trajectories at high internal energy. Our results indicate that between 4 and 10 water molecules are lost from AMP(-)(H(2)O)(20) after one second depending on the initial heating in the 250-350 K range, with a concomitant cooling of the remaining cluster by 75-150 K.

Photoinduced evaporation of mass-selected aniline+(water)n (n=4-20) clusters

Photofragmentation of mass-selected aniline(+)(water)n (An(+)Wn, n=4-20) clusters is investigated over photon energies ranging from 1.65 to 4.66 eV by linear tandem time-of-flight mass spectrometry. The aniline ring turns out to survive irradiation of photons, and most of the absorbed photon energy flows to the hydrogen-bonding networks to be used up for liberation of water molecules. The average number of ejected water molecules measured as a function of photon energy reveals that the loss of water molecules is a photoevaporation process. The distributions of internal energies for parent ions and binding energies of water molecules are estimated from the plots of photofragment branching ratio versus photon energy, which give nice Gaussian fits. Also, density functional theory calculations are performed to obtain optimized structures of isomers for An(+)Wn clusters and binding energies. The authors find that the An(+)W6 cluster has a highly symmetric structure and its binding energy in An(+)W6-->An(+)W5+W stands out. This is in line with the experimental results showing that n=6 is a magic number in the mass distribution and An(+)W6 is relatively stable in metastable decay.

Formation and properties of metal clusters isolated in helium droplets

The unique conditions forming atomic and molecular complexes and clusters using superfluid helium nanodroplets have opened up an innovative route for studying the physical and chemical properties of matter on the nanoscale. This review summarizes the specific characteristics of the formation of atomic clusters partly generated far from equilibrium in the helium environment. Special emphasis is on the optical response, electronic properties as well as dynamical processes which are mostly affected by the surrounding quantum matrix. Experiments include the optical induced response of isolated cluster systems in helium under quite different excitation conditions ranging from the linear regime up to the violent interaction with a strong laser field leading to Coulomb explosion and the generation of highly charged atomic fragments. The variety of results on the outstanding properties in the quantum size regime highlights the peculiar capabilities of helium nanodroplet isolation spectroscopy.

Theoretical and experimental analysis of ammonia ionic clusters produced by 252Cf fragment impact on an NH3 ice target

Positive and negatively charged ammonia clusters produced by the impact of (252)Cf fission fragments (FF) on an NH(3) ice target have been examined theoretical and experimentally. The ammonia clusters generated by (252)Cf FF show an exponential dependence of the cluster population on its mass, and the desorption yields for the positive (NH(3))(n)NH(4)(+) clusters are 1 order of magnitude higher than those for the negative (NH(3))(n)NH(2)(-) clusters. The experimental population analysis of (NH(3))(n)NH(4)(+) (n = 0-18) and (NH(3))(n)NH(2)(-) (n = 0-8) cluster series show a special stability at n = 4 and 16 and n = 2, 4, and 6, respectively. DFT/B3LYP calculations of the (NH(3))(0)(-)(8)NH(4)(+) clusters show that the structures of the more stable conformers follow a clear pattern: each additional NH(3) group makes a new hydrogen bond with one of the hydrogen atoms of an NH(3) unit already bound to the NH(4)(+) core. For the (NH(3))(0)(-)(8)NH(2)(-) clusters, the DFT/B3LYP calculations show that, within the calculation error, the more stable conformers follow a clear pattern for n = 1-6: each additional NH(3) group makes a new hydrogen bond to the NH(2)(-) core. For n = 7 and 8, the additional NH(3) groups bind to other NH(3) groups, probably because of the saturation of the NH(2)(-) core. Similar results were obtained at the MP2 level of calculation. A stability analysis was performed using the commonly defined stability function E(n)(-)(1) + E(n)(+1) - 2E(n), where E is the total energy of the cluster, including the zero point correction energy (E = E(t) + ZPE). The trend on the relative stability of the clusters presents an excellent agreement with the distribution of experimental cluster abundances. Moreover, the stability analysis predicts that the (NH(3))(4)NH(4)(+) and the even negative clusters [(NH(3))(n)NH(2)(-), n = 2, 4, and 6] should be the most stable ones, in perfect agreement with the experimental results.

On the statistical nature of collision and surface-induced dissociation: a theoretical investigation of aluminum clusters

The unimolecular dissociation dynamics of aluminum clusters following collision with either a rare gas atom or a surface is investigated by classical trajectory simulations with model potentials. Two conformers of Al(6) with very distinct shapes, i.e., the spherical O(h) and planar C(2)(h) clusters, are considered in this work. The initial vibrational energy and angular momentum distributions resulting from collision, as well as the energy and angular momentum resolved lifetime distributions, of excited clusters were determined for both collision-induced dissociation (CID) and surface-induced dissociation (SID) processes. The partitioning of excitation energy acquired upon collision was found to depend on the excitation mechanism (CID or SID), as well as on the cluster molecular shape, especially in the case of CID. For both types of processes, the energy and angular momentum resolved excited cluster lifetime distributions were found to decay exponentially, in agreement with statistical theories of chemical reactions, suggesting intrinsic Rice-Ramsperger-Kassel-Marcus (RRKM) behavior. Moreover, the simulated microcanonical rate constants determined from the cluster lifetime distributions are in good agreement with the predictions of the orbiting transition state model of phase space theory (OTS/PST), which further supports the statistical character of cluster CID and SID. Thus, in the CID and SID of highly fluxional systems such as aluminum clusters, the rate of intramolecular vibrational energy redistribution (IVR) is much faster than the dissociation rate, which validates one of the key assumptions, i.e., post-collision statistical behavior, underlying the models that are routinely used to determine cluster binding energies from experimental CID/SID cross sections.

Statistical evaporation of rotating clusters. III. Molecular clusters

Unimolecular evaporation of weakly bound clusters made of rigid molecules is considered from the points of view of statistical theories and molecular dynamics simulations. We explicitly work out expressions for the kinetic energy released and product angular momentum distributions within the sphere+sphere and sphere+linear rigid body assumptions of phase space theory (PST). Various approximations are investigated, including the shape of the interaction potential between the two fragments and the anharmonicity of the vibrational density of states. The comparison between phase space theory and simulation for nitrogen and methane clusters shows a quantitative agreement, thereby suggesting that PST is accurate in predicting statistical observables in a wide range of systems under various physical conditions.