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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1977

Rotational excitation of linear molecules by collisions with atoms - Comparison of classical and quantum methods

Exact quantum results for the rotational excitation of rigid linear molecules by collisions with atoms are compared with classical trajectory results. The systems studied are CO-He, CS-H2, OCS-H2, HCl-He, and HCl-Ar at collision energies up to 500/cm. Total cross sections and state to state rate constants are compared. The classical results are found to be in good agreement with the quantum results on the average. Differences arising from the existence of purely quantum effects are clearly evident, but consistent and predictable. Two methods of extracting state selective information from moments of the classical distribution are examined and found to be less reliable than the usual histogram method. In conjunction with previous comparisons of classical and quantum results these calculations provide a useful measure of the limitations and reliability of classical trajectories.

Chapman, S.↗
NASA NTRS2007

Electron-Molecule Col1isions: Quantitative Approaches, and the Legacy of Aaron Temkin

This article, on electron-molecule collisions, is dedicated to the legacy of my good friend and sometime collaborator, Aaron Temkin on his retirement from the NASA-Goddard Space Flight Center after many years of work at the highest intellectual level in the theoretical treatment of electron-atom and electron-molecule scattering. Aaron's contributions to the manner in which we think about electron-molecule collisions is clear to all of us who have worked in this field. I doubt that the great progress that has occurred in the computational treatment of such complex collision problems could have happened without these contributions. For a brief historical account, see the discussion of Temkin's contribution to electron-molecule scattering in the first article of this volume by Dr. A. K. Bhatia. In this article, I will concentrate on the application of the so called, non-adiabatic R-matrix theory, to vibrational excitation and dissociative attachment, although I will also present some results applying the Linear Algebraic and Kohn-Variational methods to vibrational excitation. As a starting point for almost all computationally effective approaches to electron-molecule collisions, is the fixed nuclei approximation. That is, one recognizes, just as one does with molecular bound states, that there is a separation of electronic(fast) and nuclear(s1ow) degrees of freedom. This separation makes it possible to "freeze" the nuclei in space, calculate the collision parameters for the frozen molecule and then, somehow to add back the vibrations and rotations. The manner in which this is done, depends on the details of the collision problem. It is the work of Aaron and a number of other researchers that has provided the guidance necessary to resolve these issues.

Schneider, B.I.↗
DOE OSTI2024

State-to-State Molecular Reactions in the Ultracold Regime (Final Scientific Report)

We have achieved the proposed goal to experimentally probe the AB + CD and AB + C types of reactions with state-to-state resolution, which we also compared to advanced theoretical calculations to help elucidate the role of quantum mechanics in the processes of bond breakage and formation. Our approach uses reactants that are prepared at ultracold temperatures (< 1µK) such that the quantum effects of translational motion are an important factor. Specific example reactions, including the potassium-rubidium metathesis reaction KRb + KRb → K 2 + Rb 2 as well as the atom exchange reaction Rb + KRb → Rb2 + K, are chosen because the technology of quantum internal and motional state control of these types of molecules is particularly advanced. The results for the entire funding period are fruitful. For the majority of this grant, we have constructed a one-of-the-kind quantum degenerate gas apparatus that integrates ion detection and velocity map imaging capabilities, allowing us to explore the KRb + KRb → K 2 + Rb 2 bimolecular reaction in detail. Specifically, we first verified such a reaction indeed proceed at ultracold temperatures by direct detection of reaction products. We then mapped out the complete product state distribution, which was compared to a state-counting model based on statistical theory. Our results show an overall agreement with the statistical state counting model, but also reveal several deviating state-pairs. An exact quantum calculation for molecule-molecule collisions, that is needed to understand these deviations, is however beyond the current state-of-the-art. Beside scrutinizing the reaction products, we also directly observe the reaction intermediate complex, which was quite a surprise to us. The intermediate complexes are long-lived and can interact with the inferred light that we use to trap the ultracold gas. After molecule-molecule collisions, we then explored the more theoretically tractable Rb + KRb reaction, which is endothermic. Surprisingly, we observed an exceedingly long-lived KRb$^*_2$ collisional complexes, with our experimentally measured complex lifetime deviating from conventional theoretical calculations by five orders of magnitude. This discrepancy has motivated many explorations of possible underlying causes, though no model yet captures this phenomenon completely. In the final year and the work that continues today, we extend upon these atom-molecule collision experiments to explore the origin of the long-lived KRb$^*_2$ complex lifetime and develop means to control the outcome of the reaction complex. The 5-year funded work advanced our understanding of chemical reactions at the lowest possible temperatures and at the same time opened up many new questions that are beyond our initial imaginations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1982

Test of Sigmund scaling for low collision energies

A scaling law that describes the most probable vibrorotational excitation in ion-molecule collisions has recently been extended by Sigmund (1981) to cover polyatomic targets. The predictions of this scaling law are examined for collisions of Cl(-) and K(+) with the targets O2, N2, CO, CO2, and CH4 in the energy range 50-200 eV. The experimental results do not all scale according to the theory. The study does demonstrate the usefulness of the scaling variables when examining the most probable vibrorotational excitation in ion-molecule collisions. A collection of isoelectronic scattering results is presented.

Vedder, M.↗
NASA NTRS1995

The Schwinger Variational Method

Variational methods have proven invaluable in theoretical physics and chemistry, both for bound state problems and for the study of collision phenomena. For collisional problems they can be grouped into two types: those based on the Schroedinger equation and those based on the Lippmann-Schwinger equation. The application of the Schwinger variational (SV) method to e-molecule collisions and photoionization has been reviewed previously. The present chapter discusses the implementation of the SV method as applied to e-molecule collisions.

Huo, Winifred M.↗
NASA NTRS1980

Electron-atom /molecule/ collision processes

Electron-atom (molecule) collision processes at low and intermediate energies, from near threshold to a few hundred electron volts, are discussed. Attention is given to experimental techniques and procedures, electron impact cross sections, impact excitation and electron-atom scattering in laser fields. Specific examples are presented that illustrate various experimental techniques and interpretations of observations.

Trajmar, S.↗
DOE OSTI2023

Signatures of Non-universal Quantum Dynamics of Ultracold Chemical Reactions of Polar Alkali Dimer Molecules with Alkali Metal Atoms: Li( 2 S) + NaLi( a 3 Σ + ) → Na( 2 S) + Li 2 ( a 3 Σ u + )

Ultracold chemical reactions of weakly bound triplet-state alkali metal dimer molecules have recently attracted much experimental interest. Here we perform rigorous quantum scattering calculations with a new ab initio potential energy surface to explore the chemical reaction of spin-polarized NaLi(a 3 Σ + ) and Li( 2 S) to form Li 2 (a 3 Σ u + ) and Na( 2 S). The reaction is exothermic and proceeds readily at ultralow temperatures. Significantly, we observe strong sensitivity of the total reaction rate to small variations of the three-body part of the Li 2 Na interaction at short range, which we attribute to a relatively small number of open Li 2 (a 3 Σ u + ) product channels populated in the reaction. This provides the first signature of highly non-universal dynamics seen in rigorous quantum reactive scattering calculations of an ultracold exothermic insertion reaction involving a polar alkali dimer molecule, opening up the possibility of probing microscopic interactions in atom+molecule collision complexes via ultracold reactive scattering experiments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1995

Rotational And Rovibrational Energy Transfer In Electron Collisions With Molecules

Air flows around a hypervelocity reentry vehicle undergo dissociation, rovibrational excitation and ionization. More specifically the air, initially 80% N2 and 20% O2, in the shock layer consists of species such as N, O, N2, O2, NO, N+, O+, N+, O+, NO+ and 2 free electrons. It was pointed out in multi temperature models'' that the temperature of the rotational energy modes and the gas-kinetic translational temperature are quickly equilibrated by a few collisions and rise rapidly to high temperatures as 50000K before falling off to equilibrium value of 10000K. Contrary, the electronic and vibrational temperatures state energy distributions remain low (less than 15000K) because of the slow equilibration. Electron vibrational energy transfer is thought to play a crucial role in such a ionizing flow regime since chemical reaction rates and dissociation depend strongly on the vibrational temperatures. Modeling of these flowfields in principle require the rovibrational excitation and de-excitation cross section data for average electron energies from threshold up to several eV (leV=11605.4 K). In this lecture we focus on theoretical description of rotational effects i.e. energy transfer of electrons to molecules such that the molecular rotational (vojo goes to voj) or vibrational and rotational (v(sub 0)j(sub 0) goes to vj) states are changed. Excitation and de-excitation of electronic states was discussed in a previous talk at this conference.

Thuemmel, Helmar T.↗
NASA NTRS1976

Rotational excitation of symmetric top molecules by collisions with atoms: Close coupling, coupled states, and effective potential calculations for NH3-He

The formalism for describing rotational excitation in collisions between symmetric top rigid rotors and spherical atoms is presented both within the accurate quantum close coupling framework and also the coupled states approximation of McGuire and Kouri and the effective potential approximation of Rabitz. Calculations are reported for thermal energy NH3-He collisions, treating NH3 as a rigid rotor and employing a uniform electron gas (Gordon-Kim) approximation for the intermolecular potential. Coupled states are found to be in nearly quantitative agreement with close coupling results while the effective potential method is found to be at least qualitatively correct. Modifications necessary to treat the inversion motion in NH3 are discussed.

Green, S.↗
NASA NTRS1979

Rotational excitation of symmetric top molecules by collisions with atoms. II - Infinite order sudden approximation

The infinite order sudden (IOS) approximation is extended to rotational excitation of symmetric tops by collisions with atoms. After development of a formalism for 'primitive' or 'one-ended' tops, proper parity-adapted linear combinations describing real rotors are considered and modifications needed for asymmetric rigid rotors are noted. The generalized spectroscopic relaxation cross sections are discussed. IOS calculations for NH3-He and H2CO-He are performed and compared with more accurate calculations, and the IOS approximation is found to provide a reasonably accurate description.

Green, S.↗
DOE OSTI2021

A molecular dynamics study of collisional heat transfer to nanoclusters in the gas phase

Nanoclusters in the gas phase grow by condensation and coagulation, which heat up nanoclusters, creating thermal non-equilibrium with the surrounding gas. The extent of non-equilibrium for nanoclusters is dependent upon the rate of heating relative to the rate of cooling brought about by nanocluster-gas molecule collisions, hence the latter process plays an important role in nanocluster energy evolution during growth. We apply molecular dynamics simulations to investigate heat transfer between metal nanoclusters and gas molecules relevant to high temperature aerosol systems. In analysis, we first define a thermal reemission coefficient (TRC), which is a correction factor for the portion of energy possessed by reemitted gas molecules after collision. Here, the TRC differs from the thermal accommodation coefficient (TAC), which is a correction factor for the maximum possible energy transfer rate from nanoclusters to gas molecules based on the diffusive collision model. We find the TRC is insensitive to nanocluster size but is affected by the nanocluster to gas temperature ratio and the nanocluster to gas atomic mass ratio. Comparison to the traditionally-defined TAC shows that the TAC is insensitive to temperature, a phenomenon that has been widely indicated in literature but not quantitatively explained. We show how the temperature insensitivity of the TAC arises because of the manner in which TRCs vary with both nanocluster and gas temperature, i.e. the manner in which the reemitted gas molecule energy from a surface varies with the temperature of that surface relative to that of the gas. In addition, while calculations ultimately reveal temperature insensitive TACs and justify their continued use in modeling gas-phase heat transfer, we argue that the TRC is more appropriate to calculate a priori in analyzing simulations; it is physically more realistic to first introduce a correction factor solely for the energy of reemitted gas molecules after a collision (as TRCs) instead of a correction factor applying to both the reemission energy and the initial energy (as TACs), as the latter is exactly calculable and independent of nanocluster properties. In line with prior work, for monoatomic gases we find the TAC decreases as the nanocluster to gas atomic mass increases, approaching 0.08 for gold nanoclusters in helium.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1975

Rotational excitation in H2-H2 collisions - Close-coupling calculations

Rotational excitation in molecule-molecule collisions has been treated for the first time by accurate quantum close-coupling scattering calculations, employing an expansion basis set of two to three rotational levels for each molecule and correctly accounting for exchange of identical particles. Elastic and inelastic cross sections have been computed for collisions of para-para, ortho-ortho, and para-ortho hydrogen molecules assuming an intermolecular potential suggested previously. The accuracy of recent 'effective potential' calculations is demonstrated by comparison with the exact quantum results.

Green, S.↗
NASA NTRS1979

Calculation of total cross sections for charge exchange in molecular collisions

Areas of investigation summarized include nitrogen ion-nitrogen molecule collisions; molecular collisions with surfaces; molecular identification from analysis of cracking patterns of selected gases; computer modelling of a quadrupole mass spectrometer; study of space charge in a quadrupole; transmission of the 127 deg cylindrical electrostatic analyzer; and mass spectrometer data deconvolution.

Ioup, J.↗
NASA NTRS1995

Vibrational and Electronic Energy Transfer and Dissociation of Diatomic Molecules by Electron Collisions

At high altitudes and velocities equal to or greater than the geosynchronous return velocity (10 kilometers per second), the shock layer of a hypersonic flight will be in thermochemical nonequilibrium and partially ionized. The amount of ionization is determined by the velocity. For a trans atmospheric flight of 10 kilometers per second and at an altitude of 80 kilometers, a maximum of 1% ionization is expected. At a velocity of 12 - 17 kilometer per second, such as a Mars return mission, up to 30% of the atoms and molecules in the flow field will be ionized. Under those circumstances, electrons play an important role in determining the internal states of atoms and molecules in the flow field and hence the amount of radiative heat load and the distance it takes for the flow field to re-establish equilibrium. Electron collisions provide an effective means of transferring energy even when the electron number density is as low as 1%. Because the mass of an electron is 12,760 times smaller than the reduced mass of N2, its average speed, and hence its average collision frequency, is more than 100 times larger. Even in the slightly ionized regime with only 1% electrons, the frequency of electron-molecule collisions is equal to or larger than that of molecule-molecule collisions, an important consideration in the low density part of the atmosphere. Three electron-molecule collision processes relevant to hypersonic flows will be considered: (1) vibrational excitation/de-excitation of a diatomic molecule by electron impact, (2) electronic excitation/de-excitation, and (3) dissociative recombination in electron-diatomic ion collisions. A review of available data, both theory and experiment, will be given. Particular attention will be paid to tailoring the molecular physics to the condition of hypersonic flows. For example, the high rotational temperatures in a hypersonic flow field means that most experimental data carried out under room temperatures are not applicable. Also, the average electron temperature is expected to be between 10,000 and 20,000 K. Thus only data for low energy electrons are relevant to the model.

Huo, Winifred M.↗