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

Protonated mixed pyrene-water clusters, (Py)(HO)H, where = [1-3] and = [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 ≥ 3, whereas they are absent for = 1 and 2.

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.

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 = 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.

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 , , 2019, , 014303], collision simulations between low-energy protonated uracil water clusters (HO)UH 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.

Dependence upon charge of the vibrational spectra of small Polycyclic Aromatic Hydrocarbon clusters: the example of pyrene.

Infrared spectra are computed for neutral and cationic clusters of Polycyclic Aromatic Hydrocarbon clusters, namely , using the Density Functional based Tight Binding scheme combined with a Configuration Interaction scheme (DFTB-CI) in the double harmonic approximation. Cross-comparison is carried out with DFT and simple DFTB. Similarly to the monomer cation, the IR spectra of cluster cations are characterized by a depletion of the intensity of the CH stretch modes around 3000 cm, with a weak revival for = 3 and 4.

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.

Thermal evaporation of pyrene clusters.

This work presents a study of the thermal evaporation and stability of pyrene (CH) 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.

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

A collision-induced dissociation study of hydrated protonated uracil (HO)UH 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.

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 (HO)H (n = 20-23).

A gas aggregation source for the production of heterogeneous molecular clusters.

We present the design of a versatile gas aggregation source that allows producing molecular beams of charged clusters containing a controlled amount of chosen impurities. Several examples of clusters production using this source characterized by time of flight mass spectrometry are presented here. We demonstrate the source ability to produce homogeneous clusters, such as pure protonated water and alcohol clusters, as well as inhomogeneous ones such as water clusters containing a few units of uracil, glycine, sulfuric acid, or pyrene.

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.

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.

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.

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.

Scaling laws for photoelectron holography in the midinfrared wavelength regime.

Midinfrared strong-field laser ionization offers the promise of measuring holograms of atoms and molecules, which contain both spatial and temporal information of the ion and the photoelectron with subfemtosecond temporal and angstrom spatial resolution. We report on the scaling of photoelectron holographic interference patterns with the laser pulse duration, wavelength, and intensity. High-resolution holograms for the ionization of metastable xenon atoms by 7-16  μm light from the FELICE free electron laser are presented and compared to semiclassical calculations that provide analytical insight.

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.

Time-resolved holography with photoelectrons.

Ionization is the dominant response of atoms and molecules to intense laser fields and is at the basis of several important techniques, such as the generation of attosecond pulses that allow the measurement of electron motion in real time. We present experiments in which metastable xenon atoms were ionized with intense 7-micrometer laser pulses from a free-electron laser. Holographic structures were observed that record underlying electron dynamics on a sublaser-cycle time scale, enabling photoelectron spectroscopy with a time resolution of almost two orders of magnitude higher than the duration of the ionizing pulse.

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.

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.

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.

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.

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.

Single-shot measurement of revival structures in femtosecond laser-induced alignment of molecules.

We present a single-shot detection technique for field-free molecular alignment. The method is based on probing the time-varying birefringence of an aligned sample by use of a chirped probe pulse, thus encoding the dependence of the alignment on time onto the spectral domain. The technique is applied to alignment of O2.

Response of polyatomic molecules to ultrastrong laser- and ion-induced fields.

The exposure of molecules to short, ultrastrong electric fields leads to multiple ionization and a subsequent Coulomb explosion. We present a comparative study where uracil molecules are exposed to fields generated by high-power laser pulses (tau approximately 75 fs, I > 10(16) W/cm2) or swift highly charged ions (0.5 MeV Xe25+) representing a half-cycle pulse of less than 10 fs duration.

Observation of large quadrupolar effects in a slow photoelectron imaging experiment.

We have studied nondipolar effects in resonance-enhanced multiphoton ionization of Xe and have observed an azimuthal dependence of the photoelectron angular distribution on a quadrupole resonance, as well as a very large asymmetry with respect to the direction of the laser propagation close to the resonance, which is understood in terms of interference between dipole- and quadrupole-allowed ionization channels. The observed asymmetry in the photoelectron angular distribution provides insight into the ejection of slow photoelectrons near an ionization threshold.