Publications by authors named "Alexander Millard"

Recently, a combined study of high-resolution molecular crossed beam experiment and accurate full-dimensional time-dependent theory, including full spin-orbit characteristics on the effect of electronic spin and orbital angular momenta in the F + HD reaction, was reported by some of us, focusing on the partial wave resonance phenomenon ( 936-940). It revealed that the time-dependent theory could explain all of the details observed in the high-resolution experiment. Here, we develop two time-independent close-coupling methods using hyperspherical coordinates, including the two-state model, where only a part of the spin-orbit characteristics is considered, and the six-state model, where the full spin-orbit characteristics is considered.

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A combined experimental and theoretical study of quantum state-resolved rotational energy transfer kinetics of optically centrifuged CO molecules is presented. In the experiments, inverted rotational distributions of CO in rotational states up to = 80 were prepared using two different optical centrifuge traps, one with the full spectral bandwidth of the optical centrifuge pulses, and one with reduced bandwidth. The relaxation kinetics of the high- tail of the inverted distribution from each optical trap was determined based on high-resolution transient IR absorption measurements.

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Objectives: Defects in human cognition commonly result in clinical reasoning failures that can lead to diagnostic errors.

Case Presentation: A 43-year-old female was brought to the emergency department with 4-5 days of confusion, disequilibrium resulting in several falls, and hallucinations. Further investigation revealed tachycardia, diaphoresis, mydriatic pupils, incomprehensible speech and she was seen picking at the air.

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Chemical reactions are important in the evolution of low-temperature interstellar clouds, where the quantum tunnelling effect becomes significant. The F + para-H → HF + H reaction, which has a significant barrier of 1.8 kcal mol, is an important source of HF in interstellar clouds; however, the dynamics of this quantum-tunnelling-induced reactivity at low temperature is unknown.

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Article Synopsis
  • A new potential energy surface (PES) for the excited ã B state of the sulfur monoxide (SO) molecule was created using advanced computational methods to ensure accurate energy calculations.
  • This PES was used to find the lowest ro-vibrational energy levels of different sulfur isotopes, showing improved accuracy compared to previous models.
  • The newly developed PES closely aligns with experimental results for the three lowest vibrational transitions, differing by only 1-3 cm, indicating its effectiveness.
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The kinetics and dynamics of the collisional electronic quenching of O(D) atoms by Kr have been investigated in a joint experimental and theoretical study. The kinetics of quenching were measured over the temperature range 50-296 K using the Laval nozzle method. O(D) atoms were prepared by 266 nm photolysis of ozone, and the decay of the O(D) concentration was monitored through vacuum ultraviolet fluorescence at 115.

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The optical and electronic properties of atomically thin materials such as single-walled carbon nanotubes and graphene are sensitively influenced by substrates, the degree of aggregation, and the chemical environment. However, it has been experimentally challenging to determine the origin and quantify these effects. Here we use time-dependent density-functional-theory calculations to simulate these properties for well-defined molecular systems.

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  • The study explores how sulfur monoxide (SO) breaks apart on a specific energy surface created from detailed computational data.
  • The energy surface reveals unique molecular structures for SO that are hindered from breaking apart by a barrier linked to a more energetic state.
  • The investigation finds that the internal energy of the resulting SO molecules varies significantly depending on the states before dissociation, with higher energy photons leading to more excitation.
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  • The low-energy absorption spectra of SO in the ultraviolet range are calculated for different isotopes using advanced potential energy surfaces (PESs).
  • Contributions from various ro-vibrational states lead to broadened thermally averaged spectra while preserving important features.
  • The calculated absorption peaks show a linear increase in isotope shifts with energy, aligning well with experimental results.
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We determine from first principles two sets of four-dimensional diabatic potential energy surfaces (PES's) for the interaction of NO(XΠ) with H, under the assumption of fixed NO and H bond distances. The first set of PES's was computed with the explicitly correlated multi-reference configuration interaction method [MRCISD-F12 + Q(Davidson)], and the second set with an explicitly correlated, coupled-cluster method [RCCSD(T)-F12a] with the geometry scan limited to geometries possessing a plane of symmetry. The calculated PES's are then fit to an analytical form suitable for bound state and scattering calculations.

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The high resolution spectroscopy of the SO molecule is of great topical interest, in a wide variety of contexts ranging from origins of higher life, to astrophysics of the interstellar medium, to environmental chemistry. In particular, the C̃B ← X̃A UV photoabsorption spectrum has received considerable attention. This spectrum exhibits a highly regular progression of ∼20 or so strong peaks, spaced roughly 350 cm apart, which is comparable to the C̃B bending vibrational frequency.

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Transport properties for collisions of oxygen atoms with hydrogen atoms and hydrogen molecules have been computed by means of time-independent quantum scattering calculations. For the O(P)-H(S) interaction, potential energy curves for the four OH electronic states emanating from this asymptote were computed by the internally-contracted multi-reference configuration interaction method, and the R-dependent spin-orbit matrix elements were taken from Parlant and Yarkony [J. Chem.

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We report new and more accurate adiabatic potential energy surfaces (PESs) for the ground X̃(1)A1 and electronically excited C̃(1)B2(2(1)A(')) states of the SO2 molecule. Ab initio points are calculated using the explicitly correlated internally contracted multi-reference configuration interaction (icMRCI-F12) method. A second less accurate PES for the ground X̃ state is also calculated using an explicitly correlated single-reference coupled-cluster method with single, double, and non-iterative triple excitations [CCSD(T)-F12].

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An optical centrifuge pulse drives carbon dioxide molecules into ultrahigh rotational states with rotational frequencies of ω ≈ 32 THz based on the centrifuge frequency at the full width at half-maximum of the spectral chirp. High-resolution transient IR absorption spectroscopy is used to measure the time-evolution of translational and rotational energy for a number of states in the range of J = 0-100 at a sample pressure of 5-10 Torr. Transient Doppler profiles show that the products of super rotor collisions contain substantial amounts of translational energy, with J-dependent energies correlating to a range of ΔJ propensities.

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We use molecular beams and ion imaging to determine quantum state resolved angular distributions of NO radicals after inelastic collision with Kr. We also determine both the sense and the plane of rotation (the rotational orientation and alignment, respectively) of the scattered NO. By full selection and then detection of the quantum parity of the NO molecule, our experiment is uniquely sensitive to quantum interference.

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The dynamics of the O((1)D) + Xe electronic quenching reaction was investigated in a crossed beam experiment at four collision energies. Marked large-scale oscillations in the differential cross sections were observed for the inelastic scattering products, O((3)P) and Xe. The shape and relative phases of the oscillatory structure depend strongly on collision energy.

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We present the quantum close-coupling treatment of spin-orbit induced transitions between the (1)D and (3)P states of an atom in collisions with a closed-shell spherical partner. In the particular case of O colliding with Xe, we used electronic structure calculations to compute the relevant potential energy curves and spin-orbit coupling matrix elements. We then carried out quantum scattering calculations of integral and differential quenching cross sections as functions of the collision energy.

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Article Synopsis
  • The study examines the scattering resonances in the collisions of NH3 and ND3 with H2 molecules using a quantum close-coupling method.
  • The researchers calculated integral and differential cross sections for collision energies ranging from 5 to 70 cm(-1) based on a prior potential energy surface.
  • They identified the resonances as shape or Feshbach resonances and indicated that their characteristics could make them detectable in a specific molecular beam experiment.
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Photodetachment spectroscopy of the FH2(-) and FD2(-) anions allows for the direct observation of reactive resonances in the benchmark reaction F + H2 → HF + H. Using cooled anion precursors and a high-resolution electron spectrometer, we observe several narrow peaks not seen in previous experiments. Theoretical calculations, based on a highly accurate F + H2 potential energy surface, convincingly assign these peaks to resonances associated with quasibound states in the HF + H and DF + D product arrangements and with a quasibound state in the transition state region of the F + H2 reaction.

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We present an experimental and theoretical investigation of rotationally inelastic transitions of OH, prepared in the X(2)Π, v = 0, j = 3/2 F1f level, in collisions with molecular hydrogen (H2 and D2). In a crossed beam experiment, the OH radicals were state selected and velocity tuned over the collision energy range 75-155 cm(-1) using a Stark decelerator. Relative parity-resolved state-to-state integral cross sections were determined for collisions with normal and para converted H2.

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This is the second in a series of papers detailing a MATLAB based implementation of the finite element method applied to collinear triatomic reactions. Here, we extend our previous work to reactions on coupled potential energy surfaces. The divergence of the probability current density field associated with the two electronically adiabatic states allows us to visualize in a novel way where and how nonadiabaticity occurs.

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  • The study investigates how the bending angle affects the interaction of the methylene radical (CH2) in its excited state with helium.
  • Using a quantum close-coupled method, the researchers calculated the cross sections for ro-vibrational relaxation.
  • The findings show that the cross sections for losing vibrational quanta are significantly smaller compared to those for rotational relaxation, with no clear dependence on the energy gap.
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For the interaction of OH(X(2)Π) with H2, under the assumption of fixed OH and H2 bond distances, we have determined two new sets of four-dimensional ab initio potential energy surfaces (PES's). The first set of PES's was computed with the multi-reference configuration interaction method [MRCISD+Q(Davidson)], and the second set with an explicitly correlated coupled cluster method [RCCSD(T)-F12a] sampling the subset of geometries possessing a plane of symmetry. Both sets of PES's are fit to an analytical form suitable for bound state and scattering calculations.

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Collisional energy transfer between the ground (X³B₁) and first excited (ã¹A₁) states of CH2 is facilitated by strong mixing of the rare pairs of accidentally degenerate rotational levels in the ground vibrational manifold of the [Formula: see text] state and the (020) and (030) excited bending vibrational manifolds of the X state. The simplest model for this process involves coherent mixing of the scattering T-matrix elements associated with collisional transitions within the unmixed ã and X states. From previous calculations in our group, we have determined cross sections and room-temperature rate constants for intersystem crossing of CH2 by collision with He.

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