Engineering topics

Wester, Roland

Publications and source records attributed to Wester, Roland.

8 records

DOE OSTI2021

Temperature-dependent rotationally inelastic collisions of OH – and He

We have studied the fundamental rotational relaxation and excitation collision of OH – J = 0 ↔ 1 with helium at different collision energies. Using state-selected photodetachment in a cryogenic ion trap, the collisional excitation of the first excited rotational state of OH – has been investigated and absolute inelastic collision rate coefficients have been extracted for collision temperatures between 20 and 35 K. Furthermore, the rates are compared with accurate quantum scattering calculations for three different potential-energy surfaces. Good agreement is found within the experimental accuracy, but the experimental trend of increasing collision rates with temperature is only in part reflected in the calculations.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2019

Controlling internal degrees: general discussion

Temperature is a very important concept in ion spectroscopy and its control has seen tremendous advances in recent years. Cryogenic ion traps are now found in many experiments and serve as thermostats, reaction vessels, clustering sources and spectroscopic sample chambers. The control over internal degrees of freedom provides routes to forming ions that are relevant at cryogenic temperatures such as in the interstellar medium and to control and lock-in conformations. It also provides a condensing environment and allows weakly bound clusters to be formed, which in turn provides new routes to action spectroscopy. This session will discuss the most recent developments in cryogenic spectroscopy and the research it enables and how new theoretical methods allow the prediction of experimental spectra as a function of temperature. The main discussion topics will cover: the best cooling methods; how temperature can be used to form exotic clusters and weakly interacting systems; and the possibilities for probing conformational locking and metastable structures.

Ahmed, Musahid↗

DOE OSTI2019

Pushing resolution in frequency and time: general discussion

One of the most important aspects of spectroscopy in general is resolution: the higher it is, the more information can be attained. At one end, spectral resolution provides geometric and electronic structural information and insights into weak interactions such as hydrogen bonds. At the other end, temporal resolution provides insight into geometric and electronic structural dynamics. The aim of this session is to provide a forum to discuss the state of the art in terms of resolution in general and how recent advances in light sources and ion mass spectroscopy can be used to study unexplored phenomena and interactions. Theoretical developments also have a key role in this area, particularly in relation to time-resolved measurements, since theoretical predictions are of key importance for providing molecular-insight into the observed experimental dynamics. The key discussion points will revolve around how experiment and theory can best combine to achieve the deepest insight, both in terms of structure and dynamics; and how new light sources may open new horizons.

Simons, Jack↗

DOE OSTI2019

Going large(r): General Discussion

With the introduction of new ion sources and advanced mass-spectrometric techniques coupled to spectroscopy, ever larger complexes are being probed. This increase in size is coupled with an increase in complexity requiring new methodologies (both experimental and theoretical) but also providing new opportunities. This session will primarily focus on the application of ion spectroscopy of non-covalent interactions in clusters and biological systems. These include solvent clusters, secondary and tertiary structure of proteins, and weakly-bound molecular clusters. Topics covered will include: structural probes for weak interactions; advances in action spectroscopy; and conformationally selective spectroscopy. A key discussion point will concern the marriage of mass-spectrometery with optical spectroscopy. The interpretation of experimental data obtained for these larger systems demands concomitant developments in theoretical methods, and the session will provide an opportunity for comparative discussions of new theoretical approaches in this field.

Ahmed, Musahid↗

DOE OSTI2017

C{sub 60}{sup +} and the Diffuse Interstellar Bands: An Independent Laboratory Check

In 2015, Campbell et al. presented spectroscopic laboratory gas phase data for the fullerene cation, C{sub 60}{sup +}, that coincide with the reported astronomical spectra of two diffuse interstellar band (DIB) features at 9633 and 9578 Å. In the following year, additional laboratory spectra were linked to three other and weaker DIBs at 9428, 9366, and 9349 Å. The laboratory data were obtained using wavelength-dependent photodissociation spectroscopy of small (up to three) He-tagged C{sub 60}{sup +}--He{sub n} ion complexes, yielding rest wavelengths for the bare C{sub 60}{sup +} cation by correcting for the He-induced wavelength shifts. Here we present an alternative approach to derive the rest wavelengths of the four most prominent C{sub 60}{sup +} absorption features, using high-resolution laser dissociation spectroscopy of C{sub 60}{sup +} embedded in ultracold He droplets. Accurate wavelengths of the bare fullerene cation are derived from linear wavelength shifts recorded for He{sub n} C{sub 60}{sup +} species with n up to 32. A careful analysis of all of the available data results in precise rest wavelengths (in air) for the four most prominent C{sub 60}{sup +} bands: 9631.9(1) Å, 9576.7(1) Å, 9427.5(1) Å, and 9364.9(1) Å. The corresponding bandwidths have been derived and the relative band intensity ratios are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

Chemical dynamics simulations of the monohydrated OH{sup −}(H{sub 2}O) + CH{sub 3}I reaction. Atomic-level mechanisms and comparison with experiment

Direct dynamics simulations, with B97-1/ECP/d theory, were performed to study the role of microsolvation for the OH{sup −}(H{sub 2}O) + CH{sub 3}I reaction. The S{sub N}2 reaction dominates at all reactant collision energies, but at higher collision energies proton transfer to form CH{sub 2}I{sup −}, and to a lesser extent CH{sub 2}I{sup −} (H{sub 2}O), becomes important. The S{sub N}2 reaction occurs by direct rebound and stripping mechanisms, and 28 different indirect atomistic mechanisms, with the latter dominating. Important components of the indirect mechanisms are the roundabout and formation of S{sub N}2 and proton transfer pre-reaction complexes and intermediates, including [CH{sub 3}--I--OH]{sup −}. In contrast, for the unsolvated OH{sup −} + CH{sub 3}I S{sub N}2 reaction, there are only seven indirect atomistic mechanisms and the direct mechanisms dominate. Overall, the simulation results for the OH{sup −}(H{sub 2}O) + CH{sub 3}I S{sub N}2 reaction are in good agreement with experiment with respect to reaction rate constant, product branching ratio, etc. Differences between simulation and experiment are present for the S{sub N}2 velocity scattering angle at high collision energies and the proton transfer probability at low collision energies. Equilibrium solvation by the H{sub 2}O molecule is unimportant. The S{sub N}2 reaction is dominated by events in which H{sub 2}O leaves the reactive system as CH{sub 3}OH is formed or before CH{sub 3}OH formation. Formation of solvated products is unimportant and participation of the (H{sub 2}O)CH{sub 3}OH---I{sup −} post-reaction complex for the S{sub N}2 reaction is negligible.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2013

Simulation studies of the Cl{sup -}+ CH{sub 3}I S{sub N}2 nucleophilic substitution reaction: Comparison with ion imaging experiments

In the previous work of Mikosch et al.[Science 319, 183 (2008)], ion imaging experiments were used to study the Cl{sup -}+ CH{sub 3}I {yields} ClCH{sub 3}+ I{sup -} reaction at collision energies E{sub rel} of 0.39, 0.76, 1.07, and 1.9 eV. For the work reported here MP2(fc)/ECP/d direct dynamics simulations were performed to obtain an atomistic understanding of the experiments. There is good agreement with the experimental product energy and scattering angle distributions for the highest three E{sub rel}, and at these energies 80% or more of the reaction is direct, primarily occurring by a rebound mechanism with backward scattering. At 0.76 eV there is a small indirect component, with isotropic scattering, involving formation of the pre- and post-reaction complexes. All of the reaction is direct at 1.07 eV. Increasing E{sub rel} to 1.9 eV opens up a new indirect pathway, the roundabout mechanism. The product energy is primarily partitioned into relative translation for the direct reactions, but to CH{sub 3}Cl internal energy for the indirect reactions. The roundabout mechanism transfers substantial energy to CH{sub 3}Cl rotation. At E{sub rel}= 0.39 eV both the experimental product energy partitioning and scattering are statistical, suggesting the reaction is primarily indirect with formation of the pre- and post-reaction complexes. However, neither MP2 nor BhandH/ECP/d simulations agree with experiment and, instead, give reaction dominated by direct processes as found for the higher collision energies. Decreasing the simulation E{sub rel} to 0.20 eV results in product energy partitioning and scattering which agree with the 0.39 eV experiment. The sharp transition from a dominant direct to indirect reaction as E{sub rel} is lowered from 0.39 to 0.20 eV is striking. The lack of agreement between the simulations and experiment for E{sub rel}= 0.39 eV may result from a distribution of collision energies in the experiment and/or a shortcoming in both the MP2 and BhandH simulations. Increasing the reactant rotational temperature from 75 to 300 K for the 1.9 eV collisions, results in more rotational energy in the CH{sub 3}Cl product and a larger fraction of roundabout trajectories. Even though a ClCH{sub 3}-I{sup -} post-reaction complex is not formed and the mechanistic dynamics are not statistical, the roundabout mechanism gives product energy partitioning in approximate agreement with phase space theory.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2013

Indirect dynamics in a highly exoergic substitution reaction

The highly exoergic nucleophilic substitution reaction F - + CH 3 I shows strikingly different reaction dynamics than substitution reactions of larger halogen ions. Over a wide range of collision energies, a large fraction of indirect scattering via a long-lived hydrogen-bonded complex is found both in crossed-beam imaging experiments and in direct chemical dynamics simulations. Our measured differential scattering cross sections show dominant large-angle scattering and low product velocities for all collision energies, resulting from efficient transfer of the collision energy to internal energy of the CH 3 F reaction product. Both findings are in strong contrast to the previously studied substitution reaction of Cl - + CH 3 I [Science 2008, 319, 183] at all but the lowest collision energies, a discrepancy that was not captured in a subsequent study at only a low collision energy [J. Phys. Chem. Lett. 2010, 1, 2747]. Our direct chemical dynamics simulations at the DFT/B97-1 level of theory show that the reaction is dominated by three atomic-level mechanisms, an indirect reaction proceeding via an F - –HCH2I hydrogen-bonded complex, a direct rebound and a direct stripping reaction. The indirect mechanism is found to contribute ~ 60% of the overall substitution reaction at both low and high collision energies. This large fraction of indirect scattering at high collision energy is particularly surprising, since the barrier for the F - –HCH 2 I complex to form products is only 0.10 eV. Overall, experiment and simulation agree very favorably in both the scattering angle and the product internal energy distributions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗