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 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

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 ← 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

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 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

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

Communication: Nucleation of quantized vortex rings in 4He nanodroplets

Whereas most of the phenomena associated with superfluidity have been observed in finite-size helium systems, the nucleation of quantized vortices has proven elusive. Here we show using time-dependent density functional simulations that the solvation of a Ba+ ion created by photoionization of neutral Ba at the surface of a 4He nanodroplet leads to the nucleation of a quantized ring vortex. The vortex is nucleated on a 10 ps timescale at the equator of a solid-like solvation structure that forms around the Ba+ ion. The process is expected to be quite general and very efficient under standard experimental conditions

Critical Landau Velocity in Helium Nanodroplets

The best-known property of superfluid helium is the vanishing viscosity that objects experience while moving through the liquid with speeds below the so-called critical Landau velocity. This critical velocity is generally considered a macroscopic property as it is related to the collective excitations of the helium atoms in the liquid. In the present work we determine to what extent this concept can still be applied to nanometer-scale, finite size helium systems. To this end, atoms and molecules embedded in helium nanodroplets of various sizes are accelerated out of the droplets by means of optical excitation, and the speed distributions of the ejected particles are determined. The measurements reveal the existence of a critical velocity in these systems, even for nanodroplets consisting of only a thousand helium atoms. Accompanying theoretical simulations based on a time-dependent density functional description of the helium confirm and further elucidate this experimental finding