82 research outputs found
The determination of the infrared radiative lifetimes of a vibrationally excited neutral molecule using stimulated-emission-pumping, molecular-beam time-of-flight.
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
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 nâł100 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
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
A New Sensitive Detection Scheme for Helium Nanodroplet Isolation Spectroscopy: Application to Benzene
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 â 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
Shaped Laser Pulses for Microsecond Time-Resolved Cryo-EM: Outrunning Crystallization During Flash Melting
Water vitrifies if cooled at rates above K/s. Surprisingly,
this process cannot simply be reversed by heating the resulting amorphous ice
at a similar rate. Instead, we have recently shown that the sample transiently
crystallizes even if the heating rate is more than one order of magnitude
higher. This may present an issue for microsecond time-resolved cryo-electron
microscopy experiments, in which vitreous ice samples are briefly flash melted
with a laser pulse, since transient crystallization could potentially alter the
dynamics of the embedded proteins. Here, we demonstrate how shaped microsecond
laser pulses can be used to increase the heating rate and outrun
crystallization during flash melting of amorphous solid water (ASW) samples. We
use time-resolved electron diffraction experiments to determine that the
critical heating rate is about K/s, more than two orders of magnitude
higher than the critical cooling rate. Our experiments add to the toolbox of
the emerging field of microsecond time-resolved cryo-electron microscopy by
demonstrating a straightforward approach for avoiding crystallization during
laser melting and for achieving significantly higher heating rates, which paves
the way for nanosecond time-resolved experiments
Visualizing Nanoscale Dynamics with Time-resolved Electron Microscopy
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 laboratoryâs 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
Electron Diffraction of Water in No Man's Land
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
In Situ Melting and Revitrification as an Approach to Microsecond Time-Resolved Cryo-Electron Microscopy
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
Flash Melting Amorphous Ice
Water can be vitrified if it is cooled at rates exceeding K/s. This
makes it possible to outrun crystallization in so-called no man's land, a range
of deeply supercooled temperatures where water crystallizes rapidly. One would
naively assume that the process can simply be reversed by heating the resulting
amorphous ice at a similar rate. We demonstrate that this is not the case. When
amorphous ice samples are flash melted with a microsecond laser pulse,
time-resolved electron diffraction reveals that the sample transiently
crystallizes despite a heating rate of more than K/s, demonstrating
that the critical heating rate for outrunning crystallization is significantly
higher than the critical cooling rate during vitrification. Moreover, we
observe different crystallization kinetics for amorphous solid water (ASW) and
hyperquenched glassy water (HGW), which suggests that the supercooled liquids
formed during laser heating transiently retain distinct non-equilibrium
structures that are associated with different nucleation rates. These
experiments open up new avenues for elucidating the crystallization mechanism
of water and studying its dynamics in no man's land. They also add important
mechanistic details to the laser melting and revitrification process that is
integral to the emerging field of microsecond time-resolved cryo-electron
microscopy.Comment: arXiv admin note: text overlap with arXiv:2211.0441
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