233 research outputs found

    Rotational energy compensates the energy defect in near-resonant vibration-vibration energy transfer: a state-to-state study of NO(v) + N20.

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    Our lack of understanding of the factors that compensate energy defects in near resonant V-V energy transfer constrains our ability to accurately predict resonance widths and, thus, the overall importance of such processes. We have carried out one of the first truly state-to-state measurements of near resonant V-V energy transfer under single collision conditions, employing the crossed molecular beams, stimulated emission pumping technique. We have varied the energy defect ΔE for the process: NO

    The determination of the infrared radiative lifetimes of a vibrationally excited neutral molecule using stimulated-emission-pumping, molecular-beam time-of-flight.

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    The authors present a new experimental method for measurement of collision-free infrared radiative lifetimes for single quantum states of a vibrationally excited sample. This method provides a more direct route to the infrared Einstein A coefficients than has been previously possible. Results for NO(X (2) Pi upsilon=21 and upsilon=7) are presented. Comparison to results of ab initio calculations shows excellent agreement. A controversy regarding the relative intensities of first overtone and fundamental emission intensities in NO is laid to rest. The most complete least squares analysis of existing data was carried out to derive the electric dipole moment function (EDMF) to an accuracy of +/-0.02 D between 0.9 and 1.7 Angstrom

    Unusual Rydberg System Consisting of a Positively Charged Helium Nanodroplet with an Orbiting Electron

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    Evidence is presented of an unusual Rydberg system consisting of a helium nanodroplet containing a positively charged sodium ion and an orbiting electron. Rydberg states of this system with principal quantum number n<20 are found to be unstable on a nanosecond timescale. In contrast, Rydberg states with n≳100 are found to have a lifetime of 1.1 s. In addition, it is found that the ionization threshold of sodium doped helium is broadened and red-shifted with respect to that of the free atom. These observations are successfully reproduced using a pseudo-diatomic description of the system in which the interactions of the sodium and its ion with the helium are calculated as the sum of pair potentials

    Barium Ions and Helium Nanodroplets: Solvation and Desolvation

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    The solvation of Ba+ ions created by the photoionization of barium atoms located on the surface of helium nanodroplets has been investigated. The excitation spectra corresponding to the 6p 2P1/2 2S1/2 and 6p 2P3/2 2S1/2 transitions of Ba+ are found to be identical to those recorded in bulk He II [Phys. Lett. A 115, 238 (1986)], indicating that the ions formed at the surface of the helium droplets become fully solvated by the helium. Time-of flight mass spectra suggest that following the excitation of the solvated Ba+ ions, these are being ejected from the helium droplets either as bare Ba+ ions or as small Ba+Hen (n<20) complexes

    Electron Diffraction of Water in No Man's Land

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    A generally accepted understanding of the anomalous properties of water will only emerge if it becomes possible to systematically characterize water in the deeply supercooled regime, from where the anomalies appear to emanate. This has largely remained elusive because water crystallizes rapidly between 160 K and 232 K. Here, we present an experimental approach to rapidly prepare deeply supercooled water at a well-defined temperature and probe it with electron diffraction before crystallization occurs. We show that as water is cooled from room temperature to cryogenic temperature, its structure evolves smoothly, approaching that of amorphous ice just below 200 K. Our experiments narrow down the range of possible explanations of the origin for the water anomalies and open up new avenues for studying supercooled water

    Visualizing Nanoscale Dynamics with Time-resolved Electron Microscopy

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    The large number of interactions in nanoscale systems leads to the emergence of complex behavior. Understanding such complexity requires atomic-resolution observations with a time resolution that is high enough to match the characteristic timescale of the system. Our laboratory’s method of choice is time-resolved electron microscopy. In particular, we are interested in the development of novel methods and instrumentation for high-speed observations with atomic resolution. Here, we present an overview of the activities in our laboratory

    A New Sensitive Detection Scheme for Helium Nanodroplet Isolation Spectroscopy: Application to Benzene

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    A new method is presented for recording excitation spectra of molecules embedded in helium nanodroplets. The method relies on the complete evaporation of the droplets following excitation of an dissolved molecule and the subsequent detection of the remaining unsolvated molecule by mass spectrometry. The technique has been successfully applied to record the S1 1B2u ← S0 1A1g transition in benzene. The transition frequencies determined by this new method, beam depletion spectroscopy and REMPI spectroscopy have been found to differ slightly from each other. It is argued that these differences in transition frequency are related to the different droplet sizes probed by the spectroscopic techniques

    EUV ionization of pure He nanodroplets: Mass-correlated photoelectron imaging, Penning ionization and electron energy-loss spectra

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    The ionization dynamics of pure He nanodroplets irradiated by EUV radiation is studied using Velocity-Map Imaging PhotoElectron-PhotoIon COincidence (VMI-PEPICO) spectroscopy. We present photoelectron energy spectra and angular distributions measured in coincidence with the most abundant ions He+, He2+, and He3+. Surprisingly, below the autoionization threshold of He droplets we find indications for multiple excitation and subsequent ionization of the droplets by a Penning-like process. At high photon energies we evidence inelastic collisions of photoelectrons with the surrounding He atoms in the droplets

    In Situ Melting and Revitrification as an Approach to Microsecond Time-Resolved Cryo-Electron Microscopy

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    Proteins typically undergo conformational dynamics on the microsecond to millisecond timescale as they perform their function, which is much faster than the time-resolution of cryo-electron microscopy and has thus prevented real-time observations. Here, we propose a novel approach for microsecond time-resolved cryo-electron microscopy that involves rapidly melting a cryo specimen in situ with a laser beam. The sample remains liquid for the duration of the laser pulse, offering a tunable time window in which the dynamics of embedded particles can be induced in their native liquid environment. After the laser pulse, the sample vitrifies in just a few microseconds, trapping particles in their transient configurations, so that they can subsequently be characterized with conventional cryo-electron microscopy. We demonstrate that our melting and revitrification approach is viable and affords microsecond time resolution. As a proof of principle, we study the disassembly of particles after they incur structural damage and trap them in partially unraveled configurations
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