Physics

We extend studies of micro-solvation of carbon monoxide by a combination of high-resolution IR spectroscopy and ab initio calculations. Spectra of the (H2O)4-CO and (D2O)4-CO pentamers are observed in the C-O stretch fundamental region (~2150 cm-1). The H2O containing spectrum is broadened by predissociation, but that of D2O is sharp, enabling detailed analysis which gives a precise band origin and rotational parameters. Ab initio calculations are employed to confirm the assignment to (water)4-CO and to determine the structure, in which the geometry of the (water)4 fragment is a cyclic ring very similar to the isolated water tetramer. The CO fragment is located "above" the ring plane, with a partial hydrogen bond between the C atom and one of the "free" protons (deuterons) of the water tetramer. Together with previous results on D2O-CO, (D2O)2-CO, and (D2O)3-CO, this represents a probe of the four initial steps in the solvation of carbon monoxide at high resolution.

We have studied the explosion dynamics of methane clusters irradiated by intense, femtosecond, 38 nm (32.6 eV) XUV laser pulses. The ion time-of-flight spectrum measured with a Wiley-McLaren-type time-of-flight spectrometer reveals undissociated molecular CH + 4 ions, fragments which are missing hydrogen atoms due to the breakage of one or more C-H bonds ( CH + 3 , CH + 2 and CH + ) and the recombination product CH + 5 . Also visible on the time-of-flight traces are atomic and molecular hydrogen ions ( H + and H + 2 ) , carbon ions, and larger hydrocarbons such as C 2 H + 2 and C 2 H + 3 . No doubly-charged parent ions ( CH 2+ 4 ) were detected. The time-of-flight results show that total and relative ion yields depend strongly on cluster size. The absolute yields of CH + 5 and H + scale linearly with the yields of the other generated fragments up to a cluster size of ⟨N⟩=70,000 molecules , then begin to decrease, whereas the yields of the CH + n (n=1−4) fragments plateau at this cluster size. The behavior of H + may be understood through the electron recombination rate, which depends on the electron temperature and the cluster average charge. Moreover, the CH + 5 behavior is explained by the depletion of both CH + 4 and H + via electron-ion recombination in the expanding nanoplasma.

We use laser-induced rotation of single molecules embedded in superfluid helium nanodroplets to reveal angular momentum dynamics and transfer in a controlled setting, under far-from-equilibrium conditions. As an unexpected result, we observe pronounced oscillations of time-dependent molecular alignment that have no counterpart in gas-phase molecules. Angulon theory reveals that these oscillations originate from the unique rotational structure of molecules in He droplets and quantum-state-specific transfer of rotational angular momentum to the many-body He environment on picosecond timescales. Our results pave the way to understanding collective effects of macroscopic angular momentum exchange in solid state systems in a bottom-up fashion.

Single-shot X-ray imaging of short-lived nanostructures such as clusters and nanoparticles near a phase transition or non-crystalizing objects such as large proteins and viruses is currently the most elegant method for characterizing their structure. Using hard X-ray radiation provides scattering images that encode two-dimensional projections, which can be combined to identify the full three-dimensional object structure from multiple identical samples. Wide-angle scattering using XUV or soft X-rays, despite yielding lower resolution, provides three-dimensional structural information in a single shot and has opened routes towards the characterization of non-reproducible objects in the gas phase. The retrieval of the structural information contained in wide-angle scattering images is highly non-trivial, and currently no efficient rigorous algorithm is known. Here we show that deep learning networks, trained with simulated scattering data, allow for fast and accurate reconstruction of shape and orientation of nanoparticles from experimental images. The gain in speed compared to conventional retrieval techniques opens the route for automated structure reconstruction algorithms capable of real-time discrimination and pre-identification of nanostructures in scattering experiments with high repetition rate -- thus representing the enabling technology for fast femtosecond nanocrystallography.

Fullerene complexes may play a key role in the design of future molecular electronics and nanostructured devices with potential applications in light harvesting using organic solar cells. Charge and energy flow in these systems is mediated by many-body effects. We studied the structure and dynamics of laser-induced multi-electron excitations in isolated C 60 by two-photon photoionization as a function of excitation wavelength using a tunable fs UV laser and developed a corresponding theoretical framework on the basis of \emph{ab initio} calculations. The measured resonance line width gives direct information on the excited state lifetime. From the spectral deconvolution we derive a lower limit for purely electronic relaxation on the order of τ el = 10 +5 −3 fs. Energy dissipation towards nuclear degrees of freedom is studied in time-resolved techniques. The evaluation of the non-linear autocorrelation trace gives a characteristic time constant of τ vib =400±100 fs for the exponential decay. In line with the experiment, the observed transient dynamics is explained theoretically by nonadiabatic (vibronic) couplings involving the correlated electronic, the nuclear degrees of freedom (accounting for the Herzberg-Teller coupling), and their interplay.

We describe a setup to study ultrafast dynamics in gas-phase molecules using time-resolved photoelectron and photoion spectroscopy. The vacuum ultraviolet (VUV) probe pulses are generated via strong field high-order harmonic generation from infrared femtosecond laser pulses. The band pass characteristic in transmission of thin indium (In) metal foil is exploited to isolate the 9 th harmonic of the 800 nm fundamental (H9, 14 eV, 89 nm) from all other high harmonics. The 9 th harmonic is obtained with high conversion efficiencies and has sufficient photon energy to access the complete set of valence electron levels in most molecules. The setup also allows for direct comparison of VUV single-photon probe with 800 nm multi-photon probe without influencing the delay of excitation and probe pulse or the beam geometry. We use a magnetic bottle spectrometer with high collection efficiency for electrons, serving at the same time as a time of flight spectrometer for ions. Characterization measurements on Xe reveal the spectral width of H9 to be 190±60 meV and a photon flux of ∼1⋅ 10 7 photons/pulse after spectral filtering. As a first application, we investigate the S 1 excitation of perylene using time-resolved ion spectra obtained with multi-photon probing and time-resolved electron spectra from VUV single-photon probing. The time resolution extracted from cross-correlation measurements is 65±10 fs for both probing schemes and the pulse duration of H9 is found to be 35±8 fs.

Lowest bound S-state energy of Coulomb three-body systems ( N Z+ μ ??e ??) having a positively charged nucleus of charge number Z ( N Z+ ), a negatively charged muon ( μ ??) and an electron ( e ??), is investigated in the framework of hyperspherical harmonics expansion method. A Yukawa type Coulomb potential with an adjustable screening parameter ( λ ) is chosen for the 2-body subsystems. In the resulting Schrödinger equation (SE), the three-body relative wave function is expanded in the complete set of hyperspherical harmonics (HH). Thereafter use of orthonormality of HH in the SE, led to a set of coupled differential equations which are solved numerically to get the energy (E) of the systems investigated.

I present a photon statistics method for quasi-one dimensional sub-diffraction limited nanofluidic motions of single molecules using Feynman-Enderlein path integral approach. The theory is validated in Monte Carlo simulation platform to provide fundamental understandings of Knudsen type flow and diffusion of single molecule fluorescence in liquid. Distribution of single molecule burst size can be precise enough to detect molecular interaction. Realisation of this theoretical study considers several fundamental aspects of single-molecule nanofluidics, such as electrodynamics, photophysics, and multi-molecular events/molecular shot noise. I study two different sizes of molecules, one with 2 nm and another with 20 nm hydrodynamic radii driven by a wide range of flow velocities. The study reports distinctly different velocity dependent nanofluidic regimes, which have not been theoretically as well as experimentally reported earlier. Experimental single-molecule fluorescence bursts inside all-silica nanofluidic channels are used to validate the robustness of the method. It is not restricted to single molecule environment of uniform electrodynamic interactions and can be used to investigate complex refractive index mismatch related non-uniform single-molecule electrodynamic interactions as well. This fundamental investigation of single-molecule nanofluidics has a potential to accelerate the progress of dynamic and complex single-molecule experiments, such as dynamic heterogeneity, biomolecular interactions of misfolded proteins, and nanometric cavity electrodynamics.

Utilising Car-Parrinello molecular dynamics simulations, the finite temperature behaviour of water trimers, (H2O)3, in both neutral and anionic frameworks, has been investigated. A significant structural change in the anionic structure has been observed at temperatures above 100 K where a chain geometry has formed and stabilised entropically. On the other hand, neutral trimers have remained in their ring structure, as predicted theoretically at low temperatures, for long time periods.

In this work we undertake a comprehensive numerical study of the ground state structures and optical absorption spectra of isomers of B 12 cluster. Geometry optimization was performed at the coupled-cluster-singles-doubles (CCSD) level of theory, employing cc-pVDZ extended basis sets. Once the geometry of a given isomer was optimized, its ground state energy was calculated more accurately at the coupled-cluster-singles-doubles along with perturbative treatment of triples (CCSD(T)) level of theory, employing larger cc-pVTZ basis sets. Thus, our computed values of binding energies of various isomers are expected to be quite accurate. Our geometry optimization reveals eleven distinct isomers, along with their point group, and electronic ground state symmetries. We also performed vibrational frequency analysis on the three lowest energy isomers, and found them to be stable. Therefore, we computed the linear optical absorption spectra of these isomers of B 12 , employing large-scale multi-reference singles-doubles configuration-interaction (MRSDCI) approach, and found a strong structure-property relationship. This implies that the spectral fingerprints of the geometries can be utilized for optical detection, and characterization, of various isomers of B 12 . We also explored the stability of the isomer with with the structure of a perfect icosahedron, with I h symmetry. In bulk boron icosahedron is the basic structural unit, but, our vibrational frequency analysis reveals that it is unstable in the isolated form. We speculate that this instability could be due to Jahn-Teller distortion because five-fold degenerate HOMO orbitals in I h structure are unfilled.