Physics

Water clusters are known to form through hydrogen bonding. However, this study shows that the formation of very small water clusters significantly deviates from this mechanism and instead involves both hydrogen bonding and electron delocalization. Our density functional theory calculations show that small water rings (H2O)n of n=3 or 4 show strong electron delocalization originating from both the hydrogen and oxygen atoms and extending to the ring center. This is very different from larger rings. Further energy decomposition of rings with n=3-6 demonstrates an upward trend in the polarization component but an decrease in the electrostatic and exchange repulsion components, presenting a minimum and accounting for 33% of interaction energy at n=3. This significantly promotes stability of the small water rings. Our findings provide a comprehensive analysis and improve our understanding of the stability characteristics of water clusters.

Net ionization and net capture cross-section calculations are presented for proton collisions from methane molecules and the DNA/RNA nucleobases adenine, cytosine, guanine, thymine, and uracil. We use the recently introduced independent-atom-model pixel counting method to calculate these cross sections in the 10 keV to 10 MeV impact energy range and compare them with results obtained from the simpler additivity rule, a previously used complete-neglect-of-differential-overlap method, and with experimental data and previous calculations where available. It is found that all theoretical results agree reasonably well at high energies, but deviate significantly in the low-to-intermediate energy range. In particular, the pixel counting method which takes the geometrical overlap of atomic cross sections into account is the only calculation that is able to describe the measurements for capture in proton-methane collisions down to 10 keV impact energy. For the nucleobases it also yields a significantly smaller cross section in this region than the other models. New measurements are urgently required to test this prediction.

Hydrogen-release by photoexcitation, excited-state- hydrogen-transfer (ESHT), is one of the important photo- chemical processes that occur in aromatic acids and is responsible for photoprotection of biomolecules. The mecha- nism is described by conversion of the initial state to a charge- separated state along the O(N)-H bond elongation, leading to dissociation. Thus ESHT is not a simple H-atom transfer in which a proton and a 1s electron move together. Here we show that the electron-transfer and the proton-motion are decoupled in gas-phase ESHT. We monitor electron and proton transfer independently by picosecond time-resolved near-infrared and infrared spectroscopy for isolated phenol--(ammonia)5, a benchmark molecular cluster. Electron transfer from phenol to ammonia occurred in less than 3 picoseconds, while the overall H-atom transfer took 15 picoseconds. The observed electron-proton decoupling will allow for a deeper understanding and control of of photochemistry in biomolecules

Electron-induced chemistry in imidazole (IMI) clusters embedded in helium nanodroplets (with an average size of 2 × 10 5 He atoms) has been investigated with high-resoluton time-of-flight mass spectrometry. The formation of both, negative and positive, ions was monitored as a function of the cluster size n. In both ion spectra a clear series of peaks with IMI cluster sizes up to at least 25 are observed. While the anions are formed by collisions of IMI n with He ⋆ − , the cations are formed through ionization of IMI n by He + as the measured onset for the cation formation is observed at 24.6 eV (ionization energy of He). The most abundant series of anions are dehydrogenated anions IMI n−1 (IMI-H) − , while other anion series are IMI clusters involving CN and C 2 H 4 moieties. The formation of cations is dominated by the protonated cluster ions IMI n H + , while the intensity of parent cluster cations IMI n + is also observed preferentially for the small cluster size n. The observation of series of cluster cations [IMI n CH 3 ] + suggests either CH 3 + cation to be solvated by n neutral IMI molecules, or the electron-induced chemistry has led to the formation of protonated methyl-imidazole solvated by (n-1) neutral IMI molecules.

Excited state hydrogen transfer (ESHT) is responsible to various photochemical processes of aromatics including photoprotection of nuclear basis. Its mechanism is explained by the internal conversion from aromatic π π * to π σ * states via conical intersection. It means that the electron is transferred to a diffuse Rydberg like σ * orbital apart from the proton migration. This picture means the electron and the proton are not move together and its dynamics are different in principle. Here, we have applied the picosecond time-resolved near infrared (NIR) and infrared (IR) spectroscopies to the phenol--(NH 3) 5 cluster, the bench mark system of ESHT, and monitored the electron transfer and proton motion independently. The electron transfer monitored by the NIR transition rises within 3 ps while the overall H transfer detected by the IR absorption of NH vibration appears with the lifetime of ≈ 20 ps. It clearly proves that the electron motion and proton migration are decoupled. Such the difference of the time-evolutions between the NIR absorption and the IR transition has not been detected in the cluster with three ammonia molecules. We will report full of our observation together with theoretical calculations of potential energy surfaces of π π * and π σ * states, and will discuss the ESHT mechanism and its cluster size-dependence between n = 3 and 5. It is suggested that the presence and absence of a barrier in the proton transfer coordinate cause the different dynamics. 2

It was recently shown that the exact potential driving the electron's dynamics in enhanced ionization of H + 2 can have large contributions arising from dynamical electron-nuclear correlation, going beyond what any electrostatics-based model can provide[1]. This potential is defined via the exact factorization of the molecular wavefunction that allows the construction of a Schrödinger equation for the electronic system, in which the potential contains exactly the effect of coupling to the nuclear system and any external fields. Here we study enhanced ionization in isotopologues of H + 2 in order to investigate nuclear-mass-dependence of these terms for this process. We decompose the exact potential into components that naturally arise from the conditional wavefunction, and also into components arising from the marginal electronic wavefunction, and compare the performance of propagation on these different components as well as approximate potentials based on the quasi-static or Hartree approximation with the exact propagation. A quasiclassical analysis is presented to help analyse the structure of different non-electrostatic components to the potential driving the ionizing electron.

The electronic structure calculation for the nanoclusters of (AsSiTeB/SiAsBTe) quaternary semiconductor alloy belonging to the (III-V Group elements) is performed. The two clusters one in the linear form and the other in the bent form have been studied under the framework of Density Functional Theory (DFT) using the B3LYP functional and LANL2DZ basis set with the software packaged GAUSSIAN 16 . We have discussed the Optimised Energy, Frontier Orbital Energy Gap in terms of HOMO-LUMO, Dipole Moment, Ionisation Potential, Electron Affinity, Binding Energy and Embedding Energy value in the research work and we have also calculated the Density of States (DoS) spectrum for the above quaternary system for two nanoclusters. The application of these compounds or alloys are mainly in the Light emitting diodes. Motivation for this research work is to look for electronic and geometric data of nanocluster (AsSiTeB/SiAsBTe).

Hydrogen bond (H-bond) covalency has recently been observed in ice and liquid water, while the penetrating molecular orbitals (MOs) in the H-bond region of most typical water dimer system, (H2O)2, have also been discovered. However, obtaining the quantitative contribution of these MOs to the H-bond interaction is still problematic. In this work, we introduced the orbital-resolved electron density projected integral (EDPI) along the H-bond to approach this problem. The calculations show that, surprisingly, the electronic occupied orbital (HOMO-4) of (H2O)2 accounts for about 40% of the electron density at the bond critical point. Moreover, the charge transfer analysis visualizes the electron accumulating effect of the orbital interaction within the H-bond between water molecules, supporting its covalent-like character. Our work expands the classical understanding of H-bond with specific contributions from certain MOs, and will also advance further research into such covalency and offer quantitative electronic structure insights into intermolecular systems.

Electronic transitions of the title molecules were measured between 250 and 710 nm using a mass-resolved 1+1' resonant two-photon ionization technique at a resolution of 0.1 nm. Calculations at the B3LYP/aug-cc-pVQZ level of theory support the analyses. Because of their spectral properties, SiC 2 , linear Si 2 C 2 , Si 3 C, and SiC 6 H 4 are interesting target species for astronomical searches in the visible spectral region. Of special relevance is the Si--C 2 --Si chain, which features a prominent band at 516.4 nm of a strong transition ( f=0.25 ). This band and one from SiC 6 H 4 at 445.3 nm were also investigated at higher resolution (0.002 nm).

Various ways to analyze the dynamical response of clusters and molecules to electromagnetic perturbations exist. Particularly rich information can be obtained from measuring the properties of electrons emitted in the course of the excitation dynamics. Such an analysis of electron signals covers total ionization, Photo-Electron Spectra, Photoelectron Angular Distributions, and ideally combined PES/PAD, with a long history in molecular physics, also increasingly used in cluster physics. Recent progress in the design of new light sources (high intensity and/or frequency, ultra short pulses) opens new possibilities for measurements and thus has renewed the interest on the analysis of dynamical scenarios through these observables, well beyond a simple access to a density of states. This, in turn, has motivated many theoretical investigations of the dynamics of electronic emission for molecules and clusters. A theoretical tool of choice is here Time-Dependent Density Functional Theory (TDDFT) propagated in real time and on a spatial grid, and augmented by a Self-Interaction Correction. This provides a pertinent, robust, and efficient description of electronic emission including the detailed pattern of PES and PAD. A direct comparison between experiments and well founded elaborate microscopic theories is thus readily possible, at variance with more demanding observables such as for example fragmentation or dissociation cross sections. The aim of this paper is to review the available experimental results motivating such studies, describe the theoretical tools developed on the basis of real-time and real-space TDDFT to address in a realistic manner the analysis of electronic emission following irradiation of clusters and molecules by various laser pulses, discuss representative results, and finally give some future directions of investigations.