Rovibronic Coupling Research Articles

Correction to: Kinetic rate coefficients for electron-driven collisions with CH+: dissociative recombination and rovibronic excitation

Correction to: Kinetic rate coefficients for electron-driven collisions with CH+: dissociative recombination and rovibronic excitation

Kinetic rate coefficients for electron-driven collisions with CH+: dissociative recombination and rovibronic excitation

ABSTRACT Cross-sections and rate coefficients for rovibronic excitation of the CH+ ion by electron impact and dissociative recombination of CH+ with electrons are evaluated using a theoretical approach combining an R-matrix method and molecular quantum defect theory. The method has been developed and tested, comparing the theoretical results with the data from the recent Cryogenic Storage Ring experiment. The obtained cross-sections and rate coefficients evaluated for temperatures from 1 to 10 000 K could be used for plasma modelling in the interpretation of astrophysical observations and also in the technological applications where the molecular hydrocarbon plasma is present.

Intensities of KCs [formula omitted] band system up to dissociation threshold: an interplay between spin-orbit, hyperfine and rovibronic coupling effects

The relative intensity distribution in the rotationally resolved laser-induced fluorescence spectra belonging to the E(4)1Σ+→(a3Σ1+,X1Σ+) band systems of the KCs molecule was analyzed. The experimental intensities in doublet P,R progressions assigned to spin-allowed E→X and spin-forbidden E→a transitions up to their common ground dissociation limit were described in the framework of a coupled-channels (CC) deperturbation model applied for the interacting X1Σ+ and a3Σ+ states. The CC intensity simulation was based solely on fixed electronic structure parameters as functions of the internuclear distance R, namely: accurate empirical potential energy curves for all three states, ab initio estimates for matrix elements A(R) of the hyperfine structure (HFS), and transition dipole moments dEX(R) and dEa(R). A comparison between the measured intensities and their theoretical counterparts demonstrates a strong competition between different intramolecular interactions. A weak spin-orbit coupling of the upper E(4)1Σ+ state with the remote 3Π states is responsible for appearance of the E→a vibrational bands for the intermediate va-values. In turn, the HFS coupling between X1Σ+ and a3Σ+ states leads to peculiarities in E→(X,a) intensities, which are pronounced for high vX/a-values in the vicinity of K(42S)+Cs(62S) dissociation threshold. Both adiabatic ro-vibrational and non-adiabatic electronic-rotational interactions explain the abrupt deviation of some observed P/R intensity ratios from the expected Hönl-London factors.

Dissociative recombination and vibrational excitation of molecular cations with electrons: application to H2 +, BeH+ and their isotopomers

SynopsisCross sections and Maxwell rate coefficients for the low-energy reactive collisions of electrons with Hydrogen and Beryllium hydride cations will be presented, as well as a discussion on the major relevant mechanisms and features – direct/indirect process, ro-vibronic couplings, ro-vibrational dependence.

Hyperfine coupling of the iodine and β1g ion-pair states

Detailed studies of I2(β1g, vβ = 13, Jβ ∼ vD = 12, JD and D, 48, JD ∼ β, 47, Jβ) rovibronic state coupling have been carried out using two-step two-color, hν1 + hν2 and hν1 + 2hν2, optical–optical double resonance excitation schemes, respectively. The hyperfine interaction satisfying the = 0, 1 selection rules (magnetic-dipole interaction) has been observed. No electric-quadrupole hyperfine coupling ( = 2) has been found. The dependences of ratios of luminescence intensities from the rovibronic states populated due to the hyperfine coupling to those from optically populated ones on energy gaps between these states have been experimentally determined. The matrix elements as well as the hyperfine structure constant have been obtained using these dependences. It is shown that they increase slightly with the vibrational quantum number of the states.

Time-, angle- and kinetic-energy-resolved photoelectron spectroscopy of highly excited states of NO

We investigate non-adiabatic dynamics of NO molecules that are photo-excited in the vacuum ultraviolet photon energy range using time-resolved velocity map imaging. Highly excited valence and Rydberg states are populated with a tunable (147–151 nm) femtosecond laser pulse and then ionized by a time-delayed near-IR laser pulse. Three main contributions are observed in the photoelectron kinetic spectra with corresponding electron yields that show pronounced oscillations. Two oscillations are assigned to ro-vibronic coupling in the valence-Rydberg mixture of the B and 4dδ N2Δ(v = 0) states and the B2Π(v = 25) and 4pπ K2Π(v = 1) states, respectively. We assign a third oscillation to originate from a coupling between two Rydberg states.

Renner-Teller Quantum Dynamics of NH(a1Δ) + H Reactions on the NH2 Ã2A1 and X̃2B1 Coupled Surfaces

Four reactions NH(a1Delta) + H′(2S) are investigated by the quantum mechanical real wavepacket method, taking into account nonadiabatic Renner-Teller (RT) and rovibronic Coriolis couplings between the involved states. We consider depletion (d) to N(2D) + H2(X1Sigmag+), exchange (e) to NH′(a1Delta) + H(2S), quenching (q) to NH(X3Sigma-) + H′(2S), and exchange-quenching (eq) to NH′(X3Sigma-) + H(2S). We extend our RT theory to a general AB + C collision using a geometry-dependent but very simple and empirical RT matrix element. Reaction probabilities, cross sections, and rate constants are presented, and RT results are compared with Born-Oppenheimer (BO), experimental, and semiclassical data. The nonadiabatic couplings open two new channels, (q) and (eq), and increase the (d) and (e) reactivity with respect to the BO one, when NH(a1Delta) is rotationally excited. In this case, the quantum cross sections are larger than the semiclassical ones at low collision energies. The calculated rate constants at 300 K are k(d) = 3.06, k(e) = 3.32, k(q) = 1.44, and k(eq) = 1.70 in 10(-11) cm3 s(-1) compared with the measured values k(d) = (3.2 =/- 1.7), k(q + eq) = (1.7 +/- 0.3), and k(total) = (4.8 +/- 1.7). The theoretical depletion rate is thus in good agreement with the experimental value, but the quenching and total rates are overestimated, because the present RT couplings are too large. This discrepancy is probably due to our simple and empirical RT matrix element.

ANALYSIS OF ELECTRON-IMPACT EXCITATION AND EMISSION OF THE np σ 1 Σ + u AND np π 1 Π u RYDBERG SERIES OF H 2

Calculated and recently measured photoabsorption transition probabilities of the H2 npσ1Σ+u and npπ1Πu − X1Σ+g band systems have been examined with high-resolution (Δλ = 95–115 mÅ) electron-impact induced emission spectra obtained previously by Jonin et al. and Liu et al. When localized rovibronic coupling is insignificant, transition probabilities calculated with the adiabatic approximation are found to be generally consistent with experiment. However, in the presence of significant coupling, the transition probabilities obtained from a nonadiabatic calculation of B'1Σ+u, D1Π+u, , D'1Π+u, and 5pσ1Σ+u state coupling give better agreement with the experimental spectra. Emission yields obtained by comparison of the calculated and experimental spectra are also consistent with the measured predissociation and autoionization yields. In addition, more accurate excitation and emission cross sections and nonradiative yields have been obtained for a number of the npσ1Σ+u and npπ1Πu states. The results obtained in the present investigation lead to a significantly more accurate calibration of the Cassini UVIS instrument and laboratory spectrometers in the range 790–920 Å. They are also an important step toward an accurate model of extreme ultraviolet H2 auroral and dayglow emissions in the outer planet atmospheres.

Double photoionization and dication fragmentation of CF3I: Experiment and theory

The double photoionization of CF(3)I and the electronic structure and the dissociation dynamics of the CF(3)I(++) dication have been investigated using large ab initio calculations and coincidence techniques. The double photoionization spectrum of CF(3)I consists of a continuous background with a number of narrow bands superimposed. The spectrum is attributed here to the population of groups of close lying electronic states interacting mutually by spin-orbit, spin-spin, and rovibronic couplings. At energies near the vertical double ionization threshold, CF(3) (+)+I(+) ionic fragments are produced. At higher energies, a very specific dissociation with double charge retained on one fragment, CF(3)I(++)-->CF(2)I(++)+F becomes dominant and is attributed to a specific group of dication electronic states.

Investigation of four carbon monoxide isotopomers in natural abundance by laser-induced fluorescence in a supersonic jet

The four carbon monoxide (CO) isotopomers 12C 16O, 13C 16O, 12C 18O and 12C 17O have been detected simultaneously in a CO gas sample of natural isotopic abundance by measuring rovibronic excitation spectra of six vibronic bands in the Fourth Positive System. The CO sample was flow cooled by adiabatic expansion in a pulsed supersonic jet. The rovibronic excitation spectra were obtained using a novel pulsed laser source (pulse duration ∼25 ns, spectral bandwidth ∼5 GHz) continuously tunable in the 139–155 nm vacuum ultraviolet wavelength region for excitation and recording the total fluorescence. In the present paper we report on the spectroscopic results obtained, including transition wavelengths of three forbidden rovibronic bands (e 3Σ − − X 1Σ +(1, 0), d 3Δ − X 1Σ +(5, 0), a′ 3Σ + − X 1Σ +(14, 0)) of 12C 16O and band origins of six rovibronic bands (A 1Π( v′ = 0–5) − X 1Σ +( v″ = 0)) of the rare isotopomer 12C 17O, and on the experimental conditions facilitating the high sensitivity of the measurements. The exceptional sensitivity demonstrated by the results has been achieved by fine tuning experimental conditions including the conditions in the supersonic expansion, the jet pulse duration and the laser pulse timing.

Elimination of Translational and Rotational Motions in Nuclear Orbital Plus Molecular Orbital Theory: Contribution of the First-Order Rovibration Coupling.

The translation- and rotation-free nuclear orbital plus molecular orbital (TRF-NOMO) theory was developed to determine nonadiabatic nuclear and electronic wave functions. This study implemented a computational program for the TRF-NOMO method including first-order rotational terms, which corresponds to rovibronic coupling. Numerical assessments of first-order TRF-NOMO Hartree-Fock as well as second-order Møller-Plesset perturbation methods were carried out for several small molecules. The first-order contributions give small corrections in energy. Thus, we confirm that the approximate zeroth-order treatment is sufficient for eliminating the rotational contamination.

A Simple Model for N2Line Oscillator Strengths of the \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $b^{\prime }\, {}^{1}\Sigma ^{+}_{u}( 1) $ \end{document} , \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty}

A simple model, based on fitting experimental energy term values, has been developed to calculate the line oscillator strengths (f) of N 2 b' 1Σ(1), c 1Σ(0), b 1Π(4), b 1Π(5), and c3 1Π(0)-X 1Σ(0) bands. The strong rovibronic coupling among these states leads to quantum interference in the transition dipole matrix elements and results in anomalous P/R branch oscillator strength ratios. The calculated line oscillator strengths are in very good agreement with experimental values. The calculated values can be used for modeling atmospheric emission. Calculated line oscillator strengths for J' up to 30 are provided.

Close-coupling calculations of low-energy inelastic and elastic processes in He4 collisions with H2: A comparative study of two potential energy surfaces

The two most recently published potential energy surfaces (PESs) for the HeH2 complex, the so-called MR (Muchnick and Russek) and BMP (Boothroyd, Martin, and Peterson) surfaces, are quantitatively evaluated and compared through the investigation of atom-diatom collision processes. The BMP surface is expected to be an improvement, approaching chemical accuracy, over all conformations of the PES compared to that of the MR surface. We found significant differences in inelastic rovibrational cross sections computed on the two surfaces for processes dominated by large changes in target rotational angular momentum. In particular, the H2(nu=1,j=0) total quenching cross section computed using the BMP potential was found to be a factor of 1000 larger than that obtained with the MR surface. A lesser discrepancy persists over a large range of energies from the ultracold to thermal and occurs for other low-lying initial rovibrational levels. The MR surface was used in previous calculations of the H2(nu=1,j=0) quenching rate coefficient and gave results in close agreement with the experimental data of Audibert et al. which were obtained for temperatures between 50 and 300 K. Examination of the rovibronic coupling matrix elements, which are obtained following a Legendre expansion of the PES, suggests that the magnitude of the anisotropy of the BMP potential is too large in the interaction region. However, cross sections for elastic and pure rotational processes obtained from the two PESs differ typically by less than a factor of 2. The small differences may be ascribed to the long-range and anharmonic components of the PESs. Exceptions occur for (nu=10,j=0) and (nu=11,j=1) where significant enhancements have been found for the low-energy quenching and elastic cross sections due to zero-energy resonances in the BMP PES which are not present in the MR potential.

Extremely narrow peaks in predissociation of sodium dimer due to rovibronic coupling.

In sodium dimer the 2 (3)Pi(g), 3 (3)Pi(g), and 4 (3)Sigma(g) (+) electronic states are coupled; the coupling of the two (3)Pi(g) states is due to vibrational motion while the nonadiabatic interaction between the (3)Sigma(g) (+) and the (3)Pi(g) states-in particular, the 3 (3)Pi(g) state-is mediated by rotational interaction. The resulting vibronic problem is studied in some detail. The bound vibrational states of the 3 (3)Pi(g) and 4 (3)Pi(g) (+) states lie in the dissociation continuum of the 2 (3)Pi(g) state and become resonances due to the prevailing nonadiabatic coupling. The resonances are calculated using the complex scaling method and the available ab initio adiabatic potential energy curves. It is demonstrated that the resonances associated with rotational nonadiabatic coupling are narrower by several orders of magnitude than those that emerge from the vibrational nonadiabatic coupling. The predissociation cross section is computed and compared with experiment.

He 79 Br 2 B,v=8←X,v″=0 excitation spectrum: Ab initio prediction and spectroscopic manifestation of a linear isomer

The B←X rovibronic excitation spectrum of the HeBr2 van der Waals complex is calculated using an ab initio potential energy surface for the ground electronic state. The coupled-cluster single double triple calculations predict double-minimum topology (linear and T-shaped wells) for the X-state potential with a low isomerization barrier. The two lowest vibrational levels, assigned to T-shaped and linear isomers using the localization patterns of the corresponding wave functions, are almost degenerated and lie slightly above the isomerization barrier. This indicates that T-shaped and linear isomers can coexist even at low temperatures and give rise to two separated bands in the excitation spectrum. The main band of the B←X excitation spectrum is assigned to transitions from the T-shaped isomer, whereas the very good agreement between the observed and calculated spectrum, using the ab initio X-state potential, demonstrates that the unassigned secondary band corresponds to excitation of the linear isomer of the HeBr2(X) complex. The complete assignment of the spectrum in terms of individual rovibronic transitions is presented.

Kramers-Type Splitting in the X2Π and a4Σ− States of CH and CD Calculated in a Hund's Case (a) Basis

We present relativistic ab initio calculations of spectroscopic parameters and rotational term energies of the CH and CD molecules. Potential energy curves have been obtained for a scalar relativistic no-pair Hamiltonian using a multireference configuration interaction (MRCI) approach. Rovibronic couplings have been computed for the ground and first excited states X2Π and a4Σ−. These have been determined through perturbation theory based upon vibronic CI wavefunctions. All important angular momentum couplings including spin–orbit interaction are accounted for, which leads to a correct description of intermediate states between Hund's cases (a) and (b). Experimental values are reproduced very well, and we give some predictions for excited states.

A Tensorial Formalism Adapted to the Rovibronic Couplings in the Colored Hexafluorides: Application to the ν5(F2g) and ν3(F1u) Modes

A tensorial formalism adapted to the case of transition-metal hexafluorides in a degenerate electronic state has been developed on the basis of preceding works about spherical-top molecules in a nondegenerate electronic state. We have introduced electronic operators constructed using group theory features and some physical considerations. Vibronic couplings (Jahn–Teller effect, etc.) have been reviewed for the triply degenerate vibrational modes ν5(F2g) and ν3(F1u) leading to the identification of the main vibronic parameters. For the first time, an effective rovibronic Hamiltonian as well as the effective transition moment operators (dipole moment and polarizability) for rovibronic transitions have been studied qualitatively thanks to the tensorial algebra properties.

Molecular π pulses: Population inversion with positively chirped short pulses

Detailed theoretical analysis and numerical simulation indicate that nearly complete electronic population inversion of molecular systems can be achieved with intense positively chirped broadband laser pulses. To provide a simple physical picture, a two-level model is used to examine the condition for the so-called π pulses and a four-level model is designed to demonstrate for molecular systems the correlation between the sign of the chirp and the excited state population. The proposed molecular π pulse is the combined result of vibrational coherence in the femtosecond regime and adiabatic inversion in the picosecond regime. Numerical results for a displaced oscillator, for LiH and for I2, show that the proposed molecular π pulse scheme is robust with respect to changes in field parameters such as the linear positive chirp rate, field intensity, bandwidth, and carrier frequency, and is stable with respect to thermal and condensed phase conditions including molecular rotation, rovibronic coupling, and electronic dephasing.

Photoinitiated unimolecular decomposition of NO2: Rotational dependence of the dissociation rate

Photoinitiated unimolecular decomposition rate constants of rotationally excited NO2 molecules have been measured near dissociation threshold (D0) by employing a double resonance technique. Rotational selectivity has been achieved by using narrow-linewidth (0.015 cm−1) infrared excitation to prepare specific rotational levels (N′=1,3,…,15, Ka′=0) of the (1,0,1) vibrational level. The picosecond-resolution pump–probe technique has then been used to photodissociate the molecules thus tagged and to monitor the appearance of the NO product. Data have been obtained for two progressions of average excess energies, 〈E〉−D0: (i) 10 cm−1+E101rot and (ii) 75 cm−1+E101rot, where 〈E〉 denotes an average over the pump laser linewidth and E101rot is the rotational energy of the (1,0,1) X̃ 2A1 intermediate vibrational level. The measured rate constants do not display any noticeable dependence on N′, which is a reflection of significant rovibronic interaction. Spin–rotation interaction, which has been implicated as the main source of rovibronic coupling for small values of N′, is not likely to yield such a result. A model is proposed to describe the influence of rotation on the dissociation rate. The experimental data are consistent with a Coriolis coupling mechanism causing transitions to occur between Ka levels.

Rovibronic coupling in the Na3 B system

The rovibronic spectrum of the Na3 B system is computed taking into account full rovibronic coupling between different vibronic states. The rovibronic coupling matrix elements are calculated using the vibronic states following from a pseudo Jahn–Teller model for the vibronic dynamics in the B system. Comparison with results of optical–optical double resonance measurements [W. E. Ernst and S. Rakowsky, Can. J. Phys. 72, 1307 (1994)] shows good agreement. In particular, certain properties of the Coriolis splittings, so far explained by an ad hoc spin–rotation interaction, are now well understood as the consequence of rovibronic coupling between two vibronic states. A general discussion of rovibronic dynamics on electronic potential energy surfaces with three equivalent minima is presented.