Vibrational Rotational Excitation Research Articles

Modeling of the ion in cold helium plasma II: transport properties of in helium

A detailed modeling of N transport properties in helium gas has been performed by employing Monte Carlo calculations based on ab initio collision cross-sections reported by our group in a preceding paper (S Paláček et al 2022 Plasma Sources Sci. Technol. 31 105004). A broad range of the reduced electric field () is considered to provide data directly usable in macroscopic modeling of processes in cold helium plasmas. The N mobility in helium gas at room temperature (T = 300 K), the characteristic energies of its longitudinal and transversal diffusion, and the rate constant of the N dissociation induced by collisions with helium atoms have been calculated. The effect of the N initial rotational-vibrational excitation is investigated as well as the effect of the rotational alignment of the N molecule. A direct comparison with N/He mobility experimental data is performed as well as indirect tests of theoretical estimates of the characteristic diffusion energies by comparing the latter with pseudo-experimental data obtained from mobility experiments via an inverse-method approach.

Quantum Optimal Control of Rovibrational Excitations of a Diatomic Alkali Halide: One-Photon vs. Two-Photon Processes

We investigated the roles of one-photon and two-photon processes in the laser-controlled rovibrational transitions of the diatomic alkali halide, 7Li37Cl. Optimal control theory calculations were carried out using the Hamiltonian, including both the one-photon and two-photon field-molecule interaction terms. Time-dependent wave packet propagation was performed with both the radial and angular motions being treated quantum mechanically. The targeted processes were pure rotational and vibrational–rotational excitations: (v = 0, J = 0) → (v = 0, J = 2); (v = 0, J = 0) → (v = 1, J = 2). Total time of the control pulse was set to 2,000,000 atomic units (48.4 ps). In each control excitation process, weak and strong optimal fields were obtained by means of giving weak and strong field amplitudes, respectively, to the initial guess for the optimal field. It was found that when the field is weak, the control mechanism is dominated exclusively by a one-photon process, as expected, in both the targeted processes. When the field is strong, we obtained two kinds of optimal fields, one causing two-photon absorption and the other causing a Raman process. It was revealed, however, that the mechanisms for strong fields are not simply characterized by one process but rather by multiple one- and two-photon processes. It was also found that in the rotational excitation, (v = 0, J = 0) → (v = 0, J = 2), the roles of one- and two-photon processes are relatively distinct but in the vibrational–rotational excitation, (v = 0, J = 0) → (v = 1, J = 2), these roles are ambiguous and the cooperative effect associated with these two processes is quite large.

Ab initio potential energy surface and quantum scattering studies of $$\hbox {Li}^{+}$$ Li + with $$\hbox {N}_{2}$$ N 2 : comparison with experiments at $$E_{\mathrm{c.m.}}= 2.47\,\hbox {eV}$$ E c . m . = 2.47 eV and 3.64 eV

A new ground electronic state potential energy surface of $$\hbox {Li}^{+}+\hbox {N}_{2}$$ system is presented in the Jacobi scattering coordinates at MRCI level of accuracy employing the augmented correlation-consistent polarized valence quadrupole zeta (aug-cc-pVQZ) basis set. An analytic fit of the computed ab initio surface has also been obtained. The surface has a global minimum for the collinear geometry at the internuclear distance of $$\hbox {N}_{2}, r = 2.078a_{0}$$ , and the distance between $$\hbox {Li}^{+}$$ and $$\hbox {N}_{{2}}$$ , $$R = 4.96a_{0}$$ . Quantum dynamics studies have been performed within the vibrational close coupling-rotational infinite-order sudden approximation at $$E_{\mathrm{c.m.}} = 3.64$$ eV, and the collision attributes have been analyzed. The computed total differential cross-sections are found in quantitative agreement with those available from the experiments at $$E_{\mathrm{c.m.}}= 3.64\,\hbox {eV}$$ . The other dynamic attributes such as angle dependent opacities and integral cross-sections are also reported. Preliminary rigid-rotor and vibrational–rotational coupled-state calculations at $$E_{\mathrm{c.m.}} = 2.47\,\hbox {eV}$$ also support the experimental observation that the system exhibits a large number of rotational excitations in the vibrational manifold $$v = 0$$ . SYNOPSIS Quantum scattering dynamics studies for vibrational-rotational excitations of N2 upon collisions of Li+ have been carried out on a newly computed ground electronic state potential energy surface and the computed collision attributes are compared with those available from the experiments at collision energies, $$E_{\mathrm{c.m.}} = 2.47$$ eV and 3.64 eV

High-resolution THz gain measurements in optically pumped ammonia.

This study is aimed at the evaluation of THz gain properties in an optically pumped NH3 gas. NH3 molecules undergo rotational-vibrational excitation by mid-infrared (MIR) optical pumping provided by a MIR quantum cascade laser (QCL) which enables precise tuning to the NH3 infrared transition around 10.3 μm. Pure inversion transitions, (J = 3, K = 3) at 1.073 THz and (J = 4, K = 4) at 1.083 THz were selected. The THz measurements were performed using a THz frequency multiplier chain. The results show line profiles with and without optical pumping at different NH3 pressures, and with different MIR tuning. The highest gain at room temperature under the best conditions obtained during single pass on the (3,3) line was 10.1 dB×m-1 at 26 μbar with a pumping power of 40 mW. The (4,4) line showed lower gain of 6.4 dB×m-1 at 34 μbar with a pumping power of 62 mW. To our knowledge these THz gains are the highest measured in a continuous-wave MIR pumped gas.

Quantum optimal control of the isotope-selective rovibrational excitation of diatomic molecules

We carry out optimal control theory calculations for isotope-selective pure rotational and vibrational-rotational excitations of diatomic molecules. The fifty-fifty mixture of diatomic isotopologues, 7Li37Cl and 7Li35Cl, is considered and the molecules are irradiated with a control pulse. In the wave packet propagation we employ the method quantum mechanically rigorous for the two-dimensional system including both the radial and angular motions. We investigate quantum controls of the isotope-selective pure rotational excitation for two total times 1280000 and 2560000a.u. (31.0 and 61.9ps) and the vibrational-rotational excitation for three total times, 640000, 1280000, and 2560000a.u. (15.5, 31.0, and 61.9ps) The initial state is set to the situation that both the isotopologues are in the ground vibrational and rotational levels, v=0 and J = 0. The target state for pure rotational excitation is set to 7Li37Cl (v=0, J = 1) and 7Li35Cl (v=0, J = 0); that for vibrational-rotational excitation is set to 7Li37Cl (v=1, J = 1) and 7Li35Cl (v=0, J = 0). The obtained final yields are quite high and those for the longest total time are calculated to be nearly 1.0. When total time is 1280000a.u., the final yields for the pure rotational excitation are slightly smaller than those for the vibrational-rotational excitation. This is because the isotope shift (difference in transition energy between the two isotopologues) for the pure rotational transition between low-lying levels is much smaller than that for the vibrational-rotational transition. We thus theoretically succeed in controlling the isotope-selective excitations of diatomic molecules using the method including both radial and angular motions quantum mechanically.

Stereo-selective partitioning of translation-to-internal energy conversion in gas ensembles.

A recent computational study of translation-to-internal energy transfer to H2 (v = 0,j = 0), hereinafter denoted H2 (0;0), in a bath of H atoms [A. J. McCaffery and R. J. Marsh, J. Chem. Phys. 139, 234310 (2013)] revealed an unexpected energy partitioning in which the H2 vibrational temperature greatly exceeds that of rotation. This occurs despite rotation and vibration distributions being close to Boltzmann from early in ensemble evolution. In this work, the study is extended to include H2 (0;0), O2 (0;0), and HF (0;0) in a wide range of atomic bath gases comprising some 22 ensembles in all. Translation-to-internal energy conversion in the systems studied was found to be relatively inefficient, falling approximately with (√μ')(-1) as bath gas mass increases, where μ' is the reduced mass of the diatomic-bath gas pair. In all 22 systems studied, T(v) exceeds T(r)--by a factor > 4 for some pairs. Analysis of the constraints that influence (0;0) → (1;j) excitation for each diatomic-atom pair in momentum-angular momentum space demonstrates that a vibrational preference results from energy constraints that limit permitted collision trajectories to those of low effective impact parameter, i.e., to those that are axial or near axial on impact with the Newton surface. This implies that a steric constraint is an inherent feature of vibration-rotation excitation and arises because momentum and energy barriers must be overcome before rotational states may be populated in the higher vibrational level.

Genetic algorithm optimization of laser pulses for molecular quantum state excitation

Conventionally optimal control theory has been used in the theoretical design of laser pulses through the direct variation in the electric field of the laser pulse as a function of time. This often leads to designed laser pulses which contain a broad and seemingly arbitrary frequency structure that varies in time in a manner which may be difficult to realize experimentally. In contrast, the experimental design of laser pulses has used a genetic algorithm (GA) approach, varying only those laser parameters actually available to the experimentalist. We investigate in this paper the possibility of using GA optimization methods in the theoretical design of laser pulses to bring about quantum state transitions in molecules. This allows us to select only a small limited number of parameters to vary and to choose these parameters so that they correspond to those available to the experimentalist. In the paper we apply our methods to the vibrational-rotational excitation of the HF molecule. We choose a small limited number of frequencies and vary only the associated electric field amplitudes and pulse envelopes. We show that laser pulses designed in this way can lead to very high transition probabilities.

The molecular and ionic sublimation of holmium tribromide

The molecular and ionic composition of vapor over holmium tribromide was studied by high-temperature mass spectrometry under the conditions of sublimation from a Knudsen effusion cell and from the open single crystal surface. The partial pressures of HoBr3 and Ho2Br6 in saturated vapor and the ratio between their sublimation coefficients under free vaporization conditions were determined. The intensities of lines in the electron impact mass spectra and their temperature dependences were different under Knudsen and Langmuir conditions, which was evidence of superthermal vibrational-rotational excitation of molecules sublimed from the open surface. The enthalpies and activation energies of sublimation of HoBr3 crystals as monomers and dimers were calculated. The emission of HoBr4− and Ho2Br7− negative ions was observed under both sublimation conditions. Ion-molecular equilibria in the LaBr3-HoBr3 system were studied. The enthalpies of formation of the HoBr3 and Ho2Br6 molecules and HoBr4− and Ho2Br7− negative ions in the gas phase were calculated.

Vibration–rotation excitation of CO by hot hydrogen atoms: Comparison of two potential energy surfaces

Collision cross sections for rotational and vibrational excitation of CO by fast H atoms are calculated for two potential energy surfaces, the older Bowman–Bitman–Harding potential and the recently constructed surface of Werner, Keller, and Schinke. Both quantum mechanical and classical calculations are performed. The results obtained with the new potential energy surface are very similar to those obtained with the older potential; in particular, they do not rectify the discrepancies between the experimental and theoretical cross sections for vibrationally elastic transitions into small rotational states of CO.

Rotational-vibrational rainbows in impulsive electron ? diatomic molecule collisions: I. state-to-state transitions

Collisional induced combined rotational-vibrational excitation of diatomic molecules is discussed in a simple quantum mechanical spectator model with applications to electron-molecule collisions at intermediate collision energies (≈ 102 eV) within the rigid rotor/harmonic oscillator approximation. Quantum mechanical transition probabilities of the rotational-vibrational excitation, which show typical vibrational and rotational rainbow patterns, are calculated and compared with the structure of classical rainbow singularities.

Theoretical studies of energy transfer and reaction in H+H2O and H+D2O collisions

We present the results of a quasiclassical trajectory study of vibration–rotation excitation and reaction in H+H2O(000) and H+D2O(000) collisions, including detailed comparisons with experiment. All calculations have used a semiempirical potential surface due to Schatz and Elgersma, and the H2O initial and final states were numerically determined by solving for the good action variables associated with vibrational motions. Our studies of collisional excitation emphasize comparisons with recent experiments by Lovejoy, Goldfarb, and Leone [J. Chem. Phys. 96, 7180 (1992)] in which fast hydrogen atoms produce vibrationally and rotationally excited water. As in the experiments, we find a propensity for the production of rotational states in which the rotational angular momentum vector is predominantly aligned perpendicular to the water molecule plane (c-axis excitation). This propensity is found for all excited vibrational states of H2O, but it is significantly stronger in the experiments [where only the (001) state was studied] than in the calculations. An analysis of trajectory motions indicates that the primary excitation mechanism for states which show the c-axis propensity involves a nearly planar collision in which the incoming H impulsively strikes one of the water hydrogens. Failed reactive collisions associated with either abstraction or exchange as well as reactive exchange collisions give the same propensity but they are not the dominant mechanism for producing aligned water. In studies of the reaction H+D2O→OD+HD, we analyze product vibrational and rotational state distributions in detail, making comparison with recent studies of Adelman, Filseth, and Zare [preceding paper, J. Chem. Phys. 98, 4636 (1993)] as well as earlier work. The product HD energy partitioning is found to be in excellent average agreement with experiment, with the HD receiving much more of the available energy than does OD. There are, however, differences in some of the HD rotational distributions, with the experiment showing a much stronger inverse correlation between HD rotational and vibrational excitation than is found in the calculations.

Algebraic eikonal approach to electron-molecule scattering. II. Rotational-vibrational excitations.

The algebraic eikonal approach to electron-molecule scattering is applied to electron scattering off HF and HCl, for which rotational-vibrational excitation cross sections were recently measured. Calculations are done with realistic interactions that include improved dipole, quadrupole, and polarization interactions. Good agreement with the data is obtained in the vibrational elastic channels.

Low energy ion–molecule reaction dynamics: Complex and direct collisions of O− with NH3

Reactive and nonreactive collisions of O− with NH3 are studied at relative collision energies of 0.65 and 1.24 eV. We observed a significant contribution to the collision dynamics from nonreactive encounters between the reagents. In addition to elastic scattering, we observed a direct contribution to this nonreactive scattering with a very strong dependence of energy transfer on scattering angle. A third contribution to nonreactive scattering arose from O−⋅NH3 collision complexes that regenerate the reactants. In these collisions, ∼80% of the incident translational energy is transformed into vibrational–rotational excitation of the NH3 reagent. The kinetic energy distribution is in reasonable agreement with statistical phase space theory calculations. We also observed reactive collisions. The hydrogen atom transfer process to yield OH− is exothermic by 0.11 eV and exhibits direct dynamics at all collision energies. Proton transfer to form NH−2, endothermic by 0.9 eV, was studied as its deuterium analog and was observed only at the higher collision energy, and took place with very small cross section. The product kinetic energy distributions for the hydrogen atom transfer reaction approach a Gaussian form at the higher collision energy, and we ascribe that behavior to the impulsive nature of reactive collisions in which the ground state vibrational wave function of the N–H bond to be broken is reflected onto product translational energy states through the ‘‘corner’’ of the potential energy surface. Such a Franck–Condon picture of the reaction is a consequence of the highly skewed potential energy surface associated with the heavy–light–heavy mass combination.

Multiphoton vibration-rotation excitation of SH, SH- and SH+ ions

The Floquet method (non-perturbative) is used to investigate the multiphoton vibration-rotation excitation of SH, SH- and SH+ ions in the presence of an intense infrared laser beam. A number of interesting features such as line broadening and the dynamic Stark shift are studied as a function of laser intensity and frequency.

Algebraic-eikonal approach to the electron-molecule-collision process: Vibrational excitation and quadrupole interaction.

The vibrational excitation mechanism has been included in the algebraic-eikonal approach to the electron--polar-molecule collision process. A dipole operator that can induce vibrational transitions has been used, and a technique for the evaluation of the matrix elements required for the calculation of the scattering amplitude in the Glauber approximation has been developed. In addition to the long-range dipole interaction, the quadrupole interaction has been included in the same framework in order to improve the potential description of the collision process. Numerical calculations are shown for electron-induced rotational-vibrational excitation through the dipole and dipole-plus-quadrupole interactions.

Fluorescent vibration-rotation excitation of cometary C2

The statistical equilibrium equations that determine the population densities of the energy levels in cometary C2 molecules due to fluorescent excitation are examined in detail. The adopted model and molecular parameters are discussed, and a theoretical estimate is made of the two intercombination transition moments. From the theoretical population densities in the various rotational levels, flux ratios and synthetic emission profiles are calculated as functions of the a 3Pi(u) - X 1Sigma(g)+ and the c 3Sigma(u)+ - X 3Sigma(g)+ intercombination transition moments. The influence of each of these two transitions separately on the vibrational and rotational excitation temperatures is investigated. The observed emission spectra of the (0,0) Swan band in Comet Halley are presented and compared to the synthetic profiles.

Laser induced vibration–rotation excitation of hydroxyl ions: OH− and OH+

The quantum dynamics of vibration–rotation excitation of hydroxyl ions (e.g., OH− and OH+ in the gaseous phase) in the presence of a laser beam is investigated. The nonperturbative Floquet method is used to solve the equation of motion with an explicitly time-dependent Hamiltonian. Convergent results are obtained for the long-time averaged transition probabilities for various transitions by including a sufficiently large number of rovibrational and photon states. In order to understand the roles played by power broadening, dynamic stark shifts, and non-Lorentzian line shapes, higher laser intensities (in the GW/cm2 range) are considered in the molecular excitation process.

State-to-state chemistry with fast hydrogen atoms. Reaction and collisional excitation in H + CO2

We present a detailed theoretical study of vibrational–rotational excitation and reaction in collisions of CO2 with 1.9–2.6 eV hydrogen atoms. Minima and saddle points on the potential surface have been characterized using ab initio configuration interaction calculations, and a global surface has been developed by a combination of many-body expansion and surface-fitting methods. The collision dynamics have been studied using quasiclassical trajectories, with the CO2 vibrational states characterized by a Fourier-transform action calculation. For non-reactive scattering there is reasonable correlation between the vibrational modes that are excited and the regions of the potential surface sampled during the collisions. Most of the lower states of CO2 are excited by direct collisions that do not sample potential wells. Collisions which do sample wells lead to short-lived HOCO and HCO2 complexes, in which either the H dissociates to produce highly excited overtone and combination states of CO2, or a CO bond breaks to give OH + CO having close to statistical vibrational–rotational distributions. Comparison of cross-sections and final-state distributions with experiment is excellent for the reactive collisions, and is good on a relative but not absolute basis for the non-reactive collisions.

Multiphoton vibration-rotation excitation of the OH molecule in the presence of an infrared laser beam

The Floquet method (non-perturbative) is used to investigate the multiphoton vibration—rotation excitation of the OH molecule in the presence of an infrared laser beam. A number of interesting features, such as line broadening and the dynamic Stark shift, are studied as a function of laser intensity and frequency.

Energy transfer in ammonia-dimer-helium collisions

In a crossed molecular beam experiment differential time-of-flight spectra have been measured for (NH 3) 2 + He collisions at E = 108 meV. The energy transfer in the backward direction is large, Δ E E ≈ 0.60. Only about half of it can be explained by simple rotational-vibrational excitation of the dimer without incorporating the internal degrees of freedom of each individual monomer.