Rotational Excitation Energy Research Articles

Charging and Electric Field Effects on Hydrogen Molecules Physisorbed on Graphene

Molecular hydrogen adsorbed on graphene was investigated by analyzing rotational excitation spectra obtained with a gate-tunable scanning tunneling microscope (STM). Through the shift of the rotational excitation energy, the tunability of physisorbed H2 on graphene was evaluated in response to electric fields and surface charging. Since pristine graphene and molecular hydrogen are both inert in nature, direct observation of physisorbed H2 on graphene remains elusive as H2 binds too weakly to flat graphene, even at low temperatures. Here, molecular hydrogen was exposed to wrinkled graphene at 5 K. Localized charges in the wrinkled graphene stabilize the H2–graphene bonding and thus enable observation using gate-tunable STM. Gate biasing modifies the charge carrier density in graphene and the electric field applied to the system, allowing for control of the physisorption of H2 molecules on graphene. The interaction between the molecule and surface is altered significantly as the gate voltage changes from −40 to +40 V, which results in a shift of the J = 0 → 2 rotational excitation of H2 from 47 to 53 meV. Our theoretical results show that the rotational energy barrier follows a parabolic function of the electric field whose maximum shifts with surface charge. This trend of electric field and charge dependence was experimentally observed by STM. Modulating the electric field and the amount of surface charging offers unique opportunities to control physisorbed molecules, which is reversible and nondestructive.

Strong Anisotropy in Liquid Water upon Librational Excitation Using Terahertz Laser Fields.

Tracking the excitation of water molecules in the homogeneous liquid is challenging due to the ultrafast dissipation of rotational excitation energy through the hydrogen-bonded network. Here we demonstrate strong transient anisotropy of liquid water through librational excitation using single-color pump-probe experiments at 12.3 THz. We deduce a third-order response of χ3 exceeding previously reported values in the optical range by 3 orders of magnitude. Using a theory that replaces the nonlinear response with a material property amenable to molecular dynamics simulation, we show that the rotationally damped motion of water molecules in the librational band is resonantly driven at this frequency, which could explain the enhancement of the anisotropy in the liquid by the external terahertz field. By addition of salt (MgSO4), the hydration water is instead dominated by the local electric field of the ions, resulting in reduction of water molecules that can be dynamically perturbed by THz pulses.

Excitation of Molecular Hydrogen in the Orion Bar PhotodissociationRegion from a Deep Near-infrared IGRINS Spectrum

Abstract We present a deep near-infrared spectrum of the Orion Bar Photodissociation Region (PDR) taken with the Immersion Grating INfrared Spectrometer (IGRINS) on the 2.7 m telescope at the McDonald Observatory. IGRINS has high spectral resolution ( R ∼ 45,000 ) and instantaneous broad wavelength coverage (1.45–2.45 μm), enabling us to detect 87 emission lines from rovibrationally excited molecular hydrogen (H2) that arise from transitions out of 69 upper rovibration levels of the electronic ground state. These levels cover a large range of rotational and vibrational quantum numbers and excitation energies, making them excellent probes of the excitation mechanisms of H2 and physical conditions within the PDR. The Orion Bar PDR is thought to consist of cooler high density clumps or filaments ( T = 50 – 250 K, n H = 10 5 – 10 7 cm−3) embedded in a warmer lower density medium ( T = 250 – 1000 K, n H = 10 4 – 10 5 cm−3). We fit a grid of constant temperature and density Cloudy models, which recreate the observed H2 level populations well, to constrain the temperature to a range of 600–650 K and the density to n H = 2.5 × 10 3 – 10 4 cm−3. The best-fit model gives T = 625 K and n H = 5 × 10 3 cm−3. This well-constrained warm temperature is consistent with kinetic temperatures found by other studies for the Orion Bar’s lower density medium. However, the range of densities well fit by the model grid is marginally lower than those reported by other studies. We could be observing lower density gas than the surrounding medium, or perhaps a density-sensitive parameter in our models is not properly estimated.

IMPROVED LINE DATA FOR THE SWAN SYSTEM 12 C 13 C ISOTOPOLOGUE

We present new, accurate predictions for rotational line positions, excitation energies, and transition probabilities of the 12C13C isotopologue Swan d3Π-a3Π system 0-0, 0–1, 0–2, 1–0, 1–1, 1–2, 2–0, 2–1, and 2–2 vibrational bands. The line positions and energy levels were predicted through new analyses of published laboratory data for the 12C13C lines. Transition probabilities were derived from recent computations of transition dipole moments and related quantities. The 12C13C line data were combined with similar data for 12C2, reported in a companion paper, and applied to produce synthetic spectra of carbon-rich metal-poor stars that have strong C2 Swan bands. The matches between synthesized and observed spectra were used to estimate band head positions for a few of the 12C13C vibrational bands and to verify that the new computed line data match observed spectra. The much weaker C2 lines of the bright red giant Arcturus were also synthesized in the band head regions.

Theoretical and Experimental Studies of Circular Dichroism of Mono- and Diazonia[6]helicenes

Combined experimental and theoretical studies revealed the characteristic circular dichroism (CD) spectral profiles of mono- and diazonia[6]helicenes, which were distinctly different from those reported for parent [6]helicene and neutral (di)aza-analogues. Aza[6]helicenes and [6]helicene showed bisignate Cotton effects (CEs) at the (1)Ba and (1)Bb bands, along with a weak CE at the (1)Lb band, where the signs of the former bands are responsible for the helical chirality of the helicenes while the sign of the latter is susceptive to the various factors such as electronic and steric effects. Protonation to monoaza[6]helicenes produces azonia[6]helicenes, showing dramatic changes in the CE pattern from the two bisignate to a three positive, two negative CE extremum series of comparable magnitudes, while dual protonation to diaza[6]helicenes forming diazonia[6]helicenes led to only nominal changes (slightly different rotational strength and excitation energy) in the CE pattern. Such rather complicated and contrasting CE behaviors of mono- versus diazoniahelicenes are derived mostly from the electronic effects of (unsymmetrical) protonation because the structures of neutral, mono-, and dicationic species are essentially identical to each other. Compared with those of neutral (di)aza[6]helicenes, the experimental CD spectra of (di)azonia[6]helicenes were less satisfactorily reproduced by the theoretical calculations at the state-of-the-art RI-CC2/TZVPP//DFT-D2-B97-D/TZVP level, most probably due to the inadequate incorporation of the effects of solvation. Nevertheless, the bytheoretical predictions were reasonably accurate and highly valuable in assigning the observed CE and elucidating the origin of the elaborate CD spectral behaviors upon protonation through inspection of the molecular orbital configuration of each transition, encouraging the extended use of the present protocol for analyzing the CD spectral behavior of aza- and other heteroatom-incorporated helicenes upon protonation. The CD spectral behavior upon metal ligation will also be explained through further theoretical and experimental studies.

Influence of rotational excitation and collision energy on the stereo dynamics of the reaction: N(4S)+H2 (v = 0, j = 0, 2, 5, 10)→NH(X3Σ−) + H

The N+H2 reaction has attracted a great deal of attention from both the experimental and the theoretical community, and most of the attention has been paid to the first excited state N(2D) atoms in collisions with hydrogen molecules and the scalar properties of the reaction. In this paper, we study the stereo dynamical properties and calculate the reaction cross sections of the N(4S) + H2 (v = 0, j = 0, 2, 5, 10) → NH(X3Σ−) + H using the quasi-classical trajectory (QCT) method on an accurate NH2 potential energy surface (PES) reported by Poveda and Varandas [Poveda L A and Varandas A J C 2005 Phys. Chem. Chem. Phys. 7 2867], in a collision energy range of 25 kcal·mol−1−140 kcal·mol−1. Results indicate that the reactant rotational excitation and initial collision energy both have a considerable influence on the distributions of the k—j′ correlation, the k—k′—j′ correlation and k—k′ correlation. The differential cross section is found to be sensitive to collision energy.

Coulomb reorientation in near-barrier fusion of deformed+spherical systems in classical dynamical approach

A classical rigid-body dynamics model which takes into account all the translational and the rotational degrees of freedom is developed to study Coulomb reorientation of deformed nuclei in heavy-ion collisions. Various aspects of the collision dynamics in the case of near-barrier fusion of 24Mg + 208Pb system due to the Coulomb reorientation are studied; the dependence of the extent of reorientation of the symmetry axis of the deformed nucleus, isotropy of the initial orientations, barrier parameters, and rotational excitation energy are discussed in detail. It is found that the barrier parameters not only depend on the initial orientations of the deformed nucleus but also on the collision energy; with maximum reorientation effect at near- and below-barrier energies. Even small amount of the rotational excitation energy gained by the deformed nucleus at large separation distances is crucial in determining the conditions at the barrier. Study of 154Sm + 16O and 238U + 16O systems involving heavier deformed nuclei shows that the extent of reorientation also depends on the moment of inertia of the deformed nucleus.

The ν1 band system of HCN

The emission spectrum of HCN has been recorded at 1463 K using hot gas molecular emission (HOTGAME) spectroscopy in the wavenumber region of 2900–3500 cm −1 with a resolution of 0.01 cm −1. The dense emission spectrum was analyzed with the spectrum analysis software SyMath™ implemented in the Mathematica™ computer algebra system. This work reports the analysis of the 1 v 2 l 0 → 0 v 2 l 0 band series up to v 2 = 8 and of the 1 v 2 l 1 → 0 v 2 l 1 band series up to v 2 = 6.36 rovibronic ( v 1, v 2, l, e/ f, v 3) substates of HCN including all l = 0, 2, 4, 6, 8 sublevels of the v 1 v 2 l v 3 = 18 l 0 highly excited bending combination mode have been characterized for the first time and for the 22 known vibrational sublevels it was possible to improve the existing spectroscopic constants substantially. 18 ( v 1, v 2, l, v 3) vibrational sublevels are located for the first time relative to the 00 00 state. The analysis reported here includes rovibrational states up to very large rotational excitations of J = 60–80. For the 1 v 2 l 0 combination states the rotational states have been determined up to J = 86 which corresponds to 7000 cm −1 rotational excitation energy, this state is only 2000 cm −1 below the isomerization barrier. It was possible to determine for the first time the L v high order rotational constant for many states reported in this work.

Dynamics of the Gas−Liquid Interfacial Reaction of O(1D) with a Liquid Hydrocarbon

The dynamics of the gas-liquid interfacial reaction of the first electronically excited state of the oxygen atom, O((1)D), with the surface of a liquid hydrocarbon, squalane (C(30)H(62); 2,6,10,15,19,23-hexamethyltetracosane) has been studied experimentally. Translationally hot O((1)D) atoms were generated by 193 nm photolysis of a low pressure (nominally 1 mTorr) of N(2)O a short distance (mean = 6 mm) above a continually refreshed liquid squalane surface. Nascent OH (X(2)Π, v' = 0) reaction products were detected by laser-induced fluorescence (LIF) on the OH A(2)Σ(+)-X(2)Π (1,0) band at the same distance above the surface. The speed distribution of the recoiling OH was characterized by measuring the appearance profiles as a function of photolysis-probe delay for selected rotational levels, N'. The rotational (and, partially, fine-structure) state distributions were also measured by recording LIF excitation spectra at selected photolysis-probe delays. The OH v' = 0 rotational distribution is bimodal and can be empirically decomposed into near thermal (~300 K) and much hotter (~6000 K) Boltzmann-temperature components. There is a strong positive correlation between rotational excitation and translation energy. However, the colder rotational component still represents a significant fraction (~30%) of the fastest products, which have substantially superthermal speeds. We estimate an approximate upper limit of 3% for the quantum yield of OH per O((1)D) atom that collides with the surface. By comparison with established mechanisms for the corresponding reactions in the gas phase, we conclude that the rotationally and translationally hot products are formed via a nonstatistical insertion mechanism. The rotationally cold but translationally hot component is most likely produced by direct abstraction. Secondary collisions at the liquid surface of products of either of the previous two mechanisms are most likely responsible for the rotationally and translationally cold products. We do not think it likely, a priori, that they could be produced in the observed significant yield via a statistical insertion mechanism for a molecule the size of squalane embedded in a surrounding liquid surface.

Venus' atomic hot oxygen environment

We present a Monte Carlo study of the atomic hot oxygen corona of Venus. In this model we consider elastic, inelastic, and quenching collisions between the traced hot particle and the ambient neutral atmosphere as well as differential cross sections to determine the scattering angle in the collisions. We also include rotational and vibrational excitation energies for the calculation of the initial energy of the produced hot oxygen atoms. Our results indicate that the differential cross sections and the fraction between elastic, inelastic, and quenching collisions are the most sensitive parameters which effect the corona density. We found that the hot O densities inferred from PVO observations can only be reproduced during high solar activity based on a forward scattering model but without inelastic and quenching collisions. The corona densities for low solar activity (VEX solar conditions) are about a factor of 2–3 smaller than for high solar activity.

Rotationally resolved electronic spectroscopy of 5-methoxyindole

Rotationally resolved electronic spectra of the vibrationless origin and of eight vibronic bands of 5-methoxyindole (5MOI) have been measured and analyzed using an evolutionary strategy approach. The experimental results are compared to the results of ab initio calculations. All vibronic bands can be explained by absorption of a single conformer, which unambiguously has been shown to be the anti-conformer from its rotational constants and excitation energy. For both anti- and syn-conformers, a (1)L(a)/(1)L(b) gap larger than 4000 cm(-1) is calculated, making the vibronic coupling between both states very small, thereby explaining why the spectrum of 5MOI is very different from that of the parent molecule, indole.

Rotational dependence of the proton-transfer reaction HBr++CO2→HOCO++Br. I. Energy versus angular momentum effects

Cross sections for the endothermic proton-transfer reactions of rotationally state-selected HBr(+) and DBr(+) ions with CO(2) were measured in a guided ion beam apparatus in order to determine the influence of rotational excitation and collision energy in the center of mass (c.m.) system on the cross section. Ab initio calculations were performed to obtain energetic information about reactants, intermediates, and products. In the experiment HBr(+) and DBr(+) ions were prepared with the same mean rotational quantum number but different mean rotational energies as the rotational constants differ by about a factor of two. The mean rotational energy was varied from 1.4 to 66.3 meV for HBr(+) and from 0.7 to 43.0 meV for DBr(+). Collision energies (E(c.m.)) ranged from 0.32 to 1.00 eV. Under all conditions considered, an increase in the rotational excitation leads to a decrease in the cross section for both reactions. However, the effect is more pronounced for the higher collision energies. For E(c.m.)=1.00 and 0.85 eV; a comparison between the results for HBr(+) and DBr(+) indicates that the cross section is dominated by effects of rotational energy rather than angular momentum. For lower collision energies the cross sections for the deuteron transfer and the proton transfer are in best agreement if not compared for the same c.m. collision energy but for the same value of the difference between the collision energy and the reaction enthalpy.

Rotational effects in complex-forming bimolecular substitution reactions: A quantum-mechanical approach

The quantum dynamics of the complex-forming S(N)2 reaction Cl(-)+CH(3)Br-->ClCH(3)+Br(-) is studied with emphasis on rotational effects. The pseudotriatomic system Cl-Me-Br is treated with a corresponding three-dimensional (3D) potential energy surface as a function of the two scattering coordinates and the enclosed angle where the geometry of the methyl group Me is optimized at each point. The 3D space is divided into three different parts, the interaction region, an intermediate region, and the asymptotic region. In line with simple classical-mechanical arguments and previous classical trajectory calculations, initial rotational motion of CH(3)Br seemingly decreases the reaction probability. However, the dynamical inclusion of the rotational degree of freedom and the presence of the many rovibrational product states overall lead to a large increase in reactivity compared to our previous collinear study on this reaction. If the reactant is rotationally excited, the higher vibrational product states are depleted in favor of lower-lying levels. Starting the reaction with rotationless reactants may end up in significant rotational excitation in the product molecules (translation-to-rotation energy transfer). On the other hand, initial rotational energy in rotationally highly excited reactants is to a large amount converted into translational and vibrational energy. The average amount of rotational energy in the products shows a twofold vibrational excitation-independent saturation (i.e., memorylessness), with respect to both initial rotational excitation and translational energy. Since only about one-half of all reactant states end in rotationless products, the reaction probability should be increased by a factor of 2; the actually larger reactivity points to other dynamical effects that play an important role in the reaction.

Rotational spectrum of cyanoacetylene solvated with helium atoms

The high resolution microwave spectra of He(N)-HCCCN clusters were studied in the size ranges of 1-18 and 25-31. In the absence of an accompanying infrared study, rotational excitation energies were computed by the reptation quantum Monte Carlo method and used to facilitate the search and assignment of R(0) transitions from N > 6, as well as R(1) transitions with N > 1. The assignments in the range of 25-31 are accurate to +/-2 cluster size units, with an essentially certain relative ordering. The rotational transition frequencies decrease with N = 1-6 and then show oscillatory behavior for larger cluster sizes, which is now recognized to be a manifestation of the onset and microscopic evolution of superfluidity. For cluster sizes beyond completion of the first solvation shell the rotational frequencies increase significantly above the large-droplet limit. This behavior, common to other linear molecules whose interaction with He features a strong nearly equatorial minimum, is analyzed using path integral Monte Carlo simulations. The He density in the incipient second solvation shell is shown to open a new channel for long permutation cycles, thus increasing the decoupling of the quantum solvent from the rotation of the dopant molecule.

Hyperspherical elliptic coordinates for the theory of light atom transfer reactions in atom-diatom collisions

We formulate and demonstrate a new method for quantum 3D calculations of light atom transfer reactions in atom-diatom collisions. The method follows a general scheme of the hyperspherical method, in common with other hyperspherical formulations in the field. The main novelty consists in the hyperspherical elliptic coordinates (ξ,η) used to parametrize the hypersphere. These coordinates have been introduced recently for studying three-body Coulomb systems, and here we apply them to study a system of three atoms. The coordinates are defined and their relation with the Smith-Whitten and Delves coordinates is explored. On account of a big difference between vibrational and rotational excitation energies in molecules, the hyperspherical adiabatic Hamiltonian allows adiabatic separation between ξ and η. This not only greatly facilitates solution of the hyperspherical adiabatic eigenvalue problem, but also provides an approximate classification of the states by a pair of indices (nξ,nη) representing vibrational and rotational quantum numbers simultaneously for a reagent and a product. Another novel technology exploited here is the Slow/Smooth Variable Discretization (SVD) method. The SVD is used for treating nonadiabatic couplings between the ξ and η motions, as well as between the motions with respect to the hyperradius and the hyperangular variables. The whole scheme is illustrated by calculations for the reaction O(3P)+HCl→OH+Cl for zero total angular momentum. It is shown to be very efficient, accurate, and providing a framework of choice for elucidating light atom transfer reaction mechanisms.

Rotational excitations of methane molecules in porous media

Rotational tunneling of CH 4 molecules in microporous and mesoporous silicate structures has been studied by inelastic neutron scattering. The rotational excitation energies of the spherical quantum rotor CH 4 are a sensitive probe of the strength and symmetry of local potentials. Measurements of methane molecules isolated in micropores of dodecasil 3C and of solid methane condensates in mesoporous silicagel and in the mesoporous channels of MCM-41 show broad distributions of rotational energies. A simple model which is based on a distribution of local potentials was applied to the data. In dodecasil 3C time-resolved measurements did not show spin conversion behaviour in a measuring time of 15 h. Even in pores with diameters as large as 100 Å no tendency towards the ordering of the CH 4 phase II was found.

Quantum-state-resolved investigation of the UV photodesorption of NH 3

The photodesorption dynamics of ammonia from Cu(111) and GaAs(100)-(4 × 6) has been studied using (2 + 1) resonance-enhanced multiphoton ionization (REMPI) as a quantum-state-specific probe. Hyperthermal excitation of the v 2 mode is observed for both substrates. Photodesorption from Cu is characterized by a marked underpopulation of antisymmetric v 2 levels, whereas for NH 3 GaAs the population of the v 2 mode follows Boltzmann statistics. The t systems also differ in the amount of rotational excitation and translational energy. The experimental results are discussed referring to a model which was developed recently to account for the large isotope effect in ammonia photodesorption. The model describes correctly the different quantum state distributions within the v 2 manifold for the two systems.

Energy and momentum distributions and projections in the scattering of CO from Ag(111)

We have studied the cross correlations between the rotational state distributions, angular momentum alignment, and velocity distributions of monoenergetic, rotationally cold CO and N2 scattered at 0.75 eV from Ag(111). Measurements were made for both normal incidence and glancing incidence beams and for specular and off-specular detection. The comparison between N2 and CO is most dramatic. (1) For N2 the rotational state selected velocity distributions are very narrow while for CO the rotational state selected velocity distributions are wide. (2) For glancing incidence beams, N2 exhibits a higher degree of parallel momentum conservation than CO. (3) The velocity resolved rotational rainbows for N2 are more prominent for glancing incidence while the velocity resolved rotational rainbows for CO are more prominent for normal incidence. (4) There is 100% cartwheeling type alignment for N2 in medium and high exit rotational states while for CO the alignment is weak except at the very highest rotational states where it is still <100% cartwheeling. Our data can be interpreted as showing that the N2 molecules at these relatively high energies collide with a slightly corrugated surface and have nearly linear trajectories. Conversely, the CO scattering data are consistent with scattering from a more corrugated surface. The CO molecules have a permanent dipole moment, therefore the gradient for the CO–Ag(111) gas surface potential with respect to molecular orientation is larger. In addition, CO has a deeper physisorption well on Ag(111). Thus, the CO molecules probe deeper into the corrugated repulsive portion of the potential and have a more inelastic collision that results in greater rotational and phonon excitation but lower exit translational energy. The lower alignment for CO scattering into high J states is consistent with the CO molecules having curved exit trajectories and/or multiple collisions with the surface. For both N2 and CO, rotational excitation into high J states scales with the normal component of incident translational energy, but the phonons can be excited by both the parallel and normal components of the incident translational energy.

Improved Dunham coefficients for CO from infrared solar lines of high rotational excitation

About 4500 unblended CO lines have been selected and their wavenumbers accurately measured on high resolution solar spectra obtained from space with the ATMOS Fourier transform spectrometer. Half of these lines are of high rotational excitation energy and have never been observed before in the laboratory. Line positions of the fundamental bands of 12C 16O have been measured up to J = 133, those of 13C 16O and of 12C 18O up to J = 103 and 91, respectively. The first overtone bands of 12C 16O have been measured up to J = 110. These new solar CO wavenumbers, with an additional selected set of about 14 000 accurate laboratory measurements, have been simultaneously fitted to the Dunham expression utilizing 10 recently published relations between isotopically invariant parameters U ij . The present set of coefficients reproduces all accurate laboratory positions and our solar measurements of high rotational excitation with a standard deviation of about 10 −5 cm −1 (300kHz). This new set is particularly recommended for all high resolution studies of infrared laboratory and stellar spectra showing CO lines of high J-values.

Rotational and vibrational excitations of weak skyrmions

We perform a semiclassical quantization of weak skyrmions, classically stable solutions present in effective models of the weak interactions. In particular, we discuss the collective quantization of the zero modes associated with spatial and weak “vector” isospin rotations. We determine the collective rotational and vibrational excitation energies of weak skyrmions.