Radiative lifetimes for the B1sigma(u)+ and C1pi states of the H2 molecule

With RKR electronic potentials for the B1sigma(u)+, C1pi(u) and X1sigma(u)+ states in conjunction with Huffaker's correction and appropriate asymptotic functions, the unperturbed radiative lifetimes of rovibrational levels of the B1sigma(u)+ and C1pi(u) states of H2 are calculated. Comparison with previous calculations is presented. Better lifetimes for B1sigma(u)+ are obtained in present work.     

The rovibrational distribution of H2 and HD formed on a graphite surface at 15-50 K

The rotational distributions of H2 and HD formed on a highly oriented pyrolitic graphite surface at temperatures of 15-50 K have been measured using laser spectroscopy. The population of the rovibrational levels nu=1, J=0-4 and nu=2, J=0-4 has been observed and the average rotational temperatures of the nascent H2 and HD molecules have been determined. We find that the average rotational temperature of the newly formed molecules is much higher than the surface temperature on which they have formed. We compare our results with other recent experimental data and theoretical calculations.     

Fluorescence reabsorption in anthracene single crystals: Lifetime variations with emission wavelength and temperature

Fluorescence spectra and lifetimes of anthracene melt-grown single crystals and sublimation flakes have been examined at 298 and 77°K, using a mono-photon counting technique for the lifetime measurements. The observed emission decay times were nearly independent of the excitation wavelength, though a small dependence of the fluorescence spectrum on the excitation wavelength was noted. By contrast, large variations of fluorescence lifetimes in thick crystals were found as a function of emission wavelength. For thick melt-grown single crystals at 298°K the lifetime was found to increase from 9.8 nsec at 405 nm to 20.4 nsec at 445 nm. For sublimation flakes at 77°K and at 298°K and for thick melt-grown crystals at 77°K, the lifetimes were less than 10 nsec and were nearly independent of emission wavelength. Despite these relatively large variations in lifetimes, the decay rates at each separate wavelength remained exponential, within experimental error. Theoretical calculations were made of emission lifetimes based on a model with one reabsorbing state. The calculations are in substantial agreement with the experimental results.     

Insights Gained by Modeling the Kinetics of Archetypal Hydrogen Atom Transfer Reactions: NH3 + ·H ⇆ H2N· + H2 and CH4 + ·H ⇆ H3C· + H2

Experimental measurements of the kinetics of the title reactions extend to temperature ranges of 1360 K for the ammonia‐hydrogen reaction and of 1602 K for the methane‐hydrogen reaction. Curved plots of ln(k) versus 1/T are obtained. Many theoretical calculations modeling these reactions routinely use tunneling corrections to match experiment. The steepness and curvatures of the plots are modeled successfully in this work and are shown to be caused solely by changes in the bond dissociation energies of the bonds involved in the reactions without invoking tunneling or any other adjustable parameters. The conclusion that tunneling does not contribute significantly to the rates in the temperature range of the measurements is in stark contrast with those theoretical calculations invoking large tunneling factors in the experimental temperature range. Support for the conclusion is provided by theoretical calculations of harmonic quantum transition state theory implementing instanton theory. There is direct experimental evidence that significant tunneling occurs in some H atom transfers, as with isotopomers of H₂ + ·H and other H transfers at very low temperatures. However, there is no direct experimental evidence of significant tunneling contributions to the rates of the title reactions in the temperature range of the measurements. Insights are gained into what specific forces must be overcome by the enthalpy of activation for reaction to occur.     

Statistical modelling of NH+/ND+ + H2/HD/D2 branching ratios

In this work we study hydrogen isotope fractionation along the proposed gas-phase ammonia formation pathway at temperatures relevant to interstellar modelling. Specifically we study the isotopically substituted variants of the NH(+) + H(2) reaction, for which the primary product is NH(2)(+) + H. Adiabatic capture theory calculations have been performed for the association reaction. A new potential-energy surface has been determined for the NH(2)(+) product. An extensive set of rovibrational energy levels has been calculated for the NH(2)(+) isotopologues described by this potential-energy surface. These rovibrational energy levels have been used to determine energy- and angular-momentum-resolved numbers of accessible product states from the NH(3)(+) isotopologue intermediates, which in turn have been averaged to give statistical branching fractions for all isotopically-substituted NH(+) + H(2) reactions. It is determined that in all cases where both hydrogen and deuterium are present, the NHD(+) product is preferred.     

Tunneling chemical reactions in solid parahydrogen: direct measurement of the rate constants of R + H2 --> RH + H (R = CD3,CD2H,CDH2,CH3) at 5 K

Tunneling chemical reactions between deuterated methyl radicals and the hydrogen molecule in a parahydrogen crystal have been studied by Fourier transform infrared spectroscopy. The tunneling rates of the reactions R + H2 --> RH + H (R = CD3,CD2H,CDH2) in the vibrational ground state were determined directly from the temporal change in the intensity of the rovibrational absorption bands of the reactants and products in each reaction in solid parahydrogen observed at 5 K. The tunneling rate of each reaction was found to differ definitely depending upon the degree of deuteration in the methyl radicals. The tunneling rates were determined to be 3.3 x 10(-6) s(-1), 2.0 x 10(-6) s(-1), and 1.0 x 10(-6) s(-1) for the systems of CD3, CD2H, and CDH2, respectively. Conversely, the tunneling reaction between a CH3 radical and the hydrogen molecule did not proceed within a week's time. The upper limit of the tunneling rate of the reaction of the CH3 radical was estimated to be 8 x 10(-8) s(-1).     

A new ab initio interaction energy surface and high-resolution spectra of the H2-CO van der Waals complex

A new four-dimensional intermolecular potential-energy surface for the H(2)-CO complex is presented. The ab initio points have been computed on a five-dimensional grid including the dependence on the H-H separation (the C-O separation was fixed). The surface has then been obtained by averaging over the intramolecular vibration of H(2). The coupled-cluster supermolecular method with single, double, and noniterative triple excitations has been used to calculate the interaction energy. The correlation part of the interaction energy has been obtained from extrapolations based on calculations in a series of basis sets. An analytical fit of the ab initio potential-energy surface has the global minimum of -93.049 cm(-1) at the intermolecular separation of 7.92 bohr for the linear geometry with the C atom pointing toward the H(2) molecule. For the other linear geometry, with the O atom pointing toward H(2), the local minimum of -72.741 cm(-1) has been found for the intermolecular separation of 7.17 bohr. The potential has been used to calculate the rovibrational energy levels of the para-H(2)-CO complex. The results agree very well with those observed by McKellar [A. R. W. McKellar J. Chem. Phys. 108, 1811 (1998)]: the discrepancies are smaller than 0.1 cm(-1). The calculated dissociation energy is equal to 19.527 cm(-1) and significantly smaller than the value of 22 cm(-1) estimated from the experiment. Predictions of rovibrational energy levels for ortho-H(2)-CO have also been done and can serve as a guidance to assign recorded experimental spectra. The interaction second virial coefficient has been calculated and compared with the experimental data.     

Three-dimensional quantum dynamics study of vibrational predissociation of HeI_2 van der Waals molecule for low vibrational excitation using the time-dependent wave packet method

Three-dimensional quantum mechanical calculations for vibrational predissociation of HeI2(B) van der Waals molecules are presented using the time-dependent wave packet technique within the golden rule approxima tion.The total and partial decay widths,lifetimes,rates and their dependence on initial vibrational states were obtained for HeI2 at low initial vibrational excited levels.Our calculations show that the calculated tota decay widths,lifetimes and rates agree well with those extrapolated from experimental data available The predicted total decay widths as a function of initial vibrational states exhibit highly nonlinear behavior.The very short propagation time (less.than 1 ps) required in the golden rule wave packet calculation is determined by the duration time of the final state inter-action between the fragments on the vibrationally deexcited adiabatic potential surface.The final state interaction between the fragments is shown to play an important role in determining the final rotational distri     

Experimental and theoretical study of the electronic spectrum of the methylene amidogen radical (H2CN): verification of the 2A1 <-- 2B2 assignment

A collaborative experimental and theoretical study of the electronic spectrum and excited-state photochemistry of H(2)CN has been carried out. The absorption spectrum, in the range of 287-278 nm, was measured through cavity ring-down spectroscopy. The radical was prepared by 193 nm photolysis of monomeric formaldoxime vapor. Two diffuse features were observed in the 34800-35800 cm(-1) spectral range, along with the A-X (1,0) band of the OH cofragment. The broad features were assigned through high-level ab initio calculations as vibronic transitions to the ground and 2b(1) (umbrella mode) levels of the second excited B (2)A(1) state from the ground X (2)B(2) state of H(2)CN. Rotational constants for the lower and upper levels of these transitions were computed from the expectation values of the moments of inertia tensor, using the appropriate vibrational wave functions. Experimental and simulated rotational profiles of these bands agree extremely well with each other for an assumed type-B electric dipole-allowed (2)A(1) <-- (2)B(2) transition appropriate to this transition. The former assignment to the dipole-forbidden (2)B(1) <-- (2)B(2) transition can be ruled out by these results. A theoretical investigation of the dissociation pathways for electronically excited H(2)CN is also presented. The upper states of the observed bands cannot dissociate directly but rather decay through internal conversion and subsequent dissociation to H + HCN fragments; higher b(1) levels are above the excited-state dissociation limit.     

Potential surfaces and dynamics of the O(3P)+H2O(X1A1)-->OH(X2pi)+OH(X2pi) reaction

We present global potential energy surfaces for the three lowest triplet states in O(3P)+H2O(X1A1) collisions and present results of classical dynamics calculations on the O(3P)+H2O(X1A1)-->OH(X2pi)+OH(X2pi) reaction using these surfaces. The surfaces are spline-based fits of approximately 20,000 fixed geometry ab initio calculations at the complete-active-space self-consistent field+second-order perturbation theory (CASSCF+MP2) level with a O(4s3p2d1f)/H(3s2p) one electron basis set. Computed rate constants compare well to measurements in the 1000-2500 K range using these surfaces. We also compute the total, rovibrationally resolved, and differential angular cross sections at fixed collision velocities from near threshold at approximately 4 km s(-1) (16.9 kcal mol(-1) collision energy) to 11 km s(-1) (122.5 kcal mol(-1) collision energy), and we compare these computed cross sections to available space-based and laboratory data. A major finding of the present work is that above approximately 40 kcal mol(-1) collision energy rovibrationally excited OH(X2pi) products are a significant and perhaps dominant contributor to the observed 1-5 micro spectral emission from O(3P)+H2O(X1A1) collisions. Another important result is that OH(X2pi) products are formed in two distinct rovibrational distributions. The "active" OH products are formed with the reagent O atom, and their rovibrational distributions are extremely hot. The remaining "spectator" OH is relatively rovibrationally cold. For the active OH, rotational energy is dominant at all collision velocities, but the opposite holds for the spectator OH. Summed over both OH products, below approximately 50 kcal mol(-1) collision energy, vibration dominates the OH internal energy, and above approximately 50 kcal mol(-1) rotation is greater than vibrational energy. As the collision energy increases, energy is diverted from vibration to mostly translational energy. We note that the present fitted surfaces can also be used to investigate direct collisional excitation of H2O(X1A1) by O(3P) and also OH(X2pi)+OH(X2pi) collisions.     

Five-dimensional ab initio potential energy surface and predicted infrared spectra of H2-CO2 van der Waals complexes

The authors present a new five-dimensional potential energy surface for H2-CO2 including the Q3 normal mode for the nu3 antisymmetric stretching vibration of the CO2 molecule. The potential energies were calculated using the supermolecular approach with the full counterpoise correction at the CCSD(T) level with an aug-cc-pVTZ basis set supplemented with bond functions. The global minimum is at two equivalent T-shaped coplanar configurations with a well depth of 219.68 cm-1. The rovibrational energy levels for four species of H2-CO2 (paraH2-, orthoH2-, paraD2-, and orthoD2-CO2) were calculated employing the discrete variable representation (DVR) for radial variables and finite basis representation (FBR) for angular variables and the Lanczos algorithm. Our calculations showed that the off-diagonal intra- and intermolecular vibrational coupling could be neglected, and separation of the intramolecular vibration by averaging the total Hamiltonian with the wave function of a specific vibrational state of CO2 should be a good approximation with high accuracy. The calculated band origin shift in the infrared spectra in the nu3 region of CO2 is -0.113 cm-1 for paraH2-CO2 and -0.099 cm-1 for orthoH2-CO2, which agrees well with the observed values of -0.198 and -0.096 cm-1. The calculated rovibrational spectra for H2-CO2 are consistent with the available experimental spectra. For D2-CO2, it is predicted that only a-type transitions occur for paraD2-CO2, while both a-type and b-type transitions are significant for orthoD2-CO2.     

Electronic states of MgO: Spectroscopy, predissociation, and cold atomic Mg and O production

We used multiconfigurational methods and a large basis set to compute the potential energy curves of the valence and valence-Rydberg electronic states of MgO molecule. New bound electronic states are found. Using these highly correlated wave functions, we evaluated their mutual spin-orbit couplings and transition moment integrals. For the bound electronic states of MgO, we deduced an accurate set of spectroscopic constants that agree remarkably well with experimental results. Moreover, our potentials, transition moments, and spin-orbit coupling evolutions are incorporated into Fermi golden rule calculations to deduce the radiative lifetimes of MgO(B?(1)Σ(+)) rovibrational levels and the natural lifetimes of MgO(A?(1)Π) vibrational levels, where a good agreement is found with experimental values. Finally, we suggest new routes for the production of cold Mg and O atoms and cold MgO molecules.     

Three-dimensional quantum dynamics study of vibrational predissociation of HeI2 van der Waals molecule for low vibrational excitation using the time-dependent wave packet method

Three-dimensional quantum mechanical calculations for vibrational predissociation of He1₂(B) van der Waals molecules are presented using the time-dependent wave packet technique within the golden rule approximation. The total and partial decay widths, lifetimes, rates and their dependence on initial vibrational states were obtained for HeI₂ at low initial vibrational excited levels. Our calculations show that the calculated total decay widths, lifetimes and rates agree well with those extrapolated from experimental data available. The predicted total decay widths as a function of initial vibrational states exhibit highly nonlinear behavior. The very short propagation time (less than 1 ps) required in the golden rule wave packet calculation is determined by the duration time of the final state interaction between the fragments on the vibrationally deexcited adiabatic potential surface. The final state interaction between the fragments is shown to play an important role in determining the final rotational distribution. This interpretation clearly explains the dynamical effect that the final rotational distribution shifts to the lower rotational energy levels as the initial vibrational quantum numberu increases.     

Accurate quantum calculation of the bound and resonant rovibrational states of Li-(H2)

In a recent paper [B. Poirier, Chem. Phys. 308, 305 (2005)] a full-dimensional quantum method for computing the rovibrational dynamics of triatomic systems was presented, incorporating three key features: (1) exact analytical treatment of Coriolis coupling, (2) three-body "effective potential," and (3) a single bend angle basis for all rotational states. In this paper, these ideas are applied to the Li-(H2) electrostatic complex, to compute all of the rovibrational bound state energies, and a number of resonance energies and widths, to very high accuracy (thousandths of a wave number). This application is very challenging, owing to the long-range nature of the interaction and to narrow level spacings near dissociation. Nevertheless, by combining the present method with a G4 symmetry-adapted phase-space-optimized representation, only modest basis sizes are required for which the matrices are amenable to direct diagonalization. Several new bound levels are reported, as compared with a previous calculation [D. T. Chang, G. Surratt, G. Ristroff, and G. I. Gellene, J. Chem. Phys. 116, 9188 (2002)]. The resonances exhibit a clear-cut separation into shape and Feshbach varieties, with the latter characterized by extremely long lifetimes (microseconds or longer).     

Experimental detection and theoretical characterization of the H2-NHX van der Waals complex

The H2-NH(X) van der Waals complex has been examined using ab initio theory and detected via fluorescence excitation spectroscopy of the A(3)Pi-X(3)Sigma(-) transition. Electronic structure calculations show that the minimum energy geometry corresponds to collinear H2-NH(X), with a well depth of D(e)=116 cm(-1). The potential-energy surface supports a secondary minimum for a T-shaped geometry, where the H atom of NH points towards the middle of the H2 bond (C(2v) point group). For this geometry the well depth is 73 cm(-1). The laser excitation spectra for the complex show transitions to the H2+NH(A) dissociative continuum. The onset of the continuum establishes a binding energy of D(0)=32+/-2 cm(-1) for H2-NH(X). The fluorescence from bound levels of H2-NH(A) was not detected, most probably due to the rapid reactive decay [H2-NH(A)-->H+NH2]. The complex appears to be a promising candidate for studies of the photoinitiated H2+NH abstraction reaction under conditions were the reactants are prealigned by the van der Waals forces.     

A six-dimensional H(2)-H(2) potential energy surface for bound state spectroscopy

We present a six-dimensional potential energy surface for the (H(2))(2) dimer based on coupled-cluster electronic structure calculations employing large atom-centered Gaussian basis sets and a small set of midbond functions at the dimer's center of mass. The surface is intended to describe accurately the bound and quasibound states of the dimers (H(2))(2), (D(2))(2), and H(2)-D(2) that correlate with H(2) or D(2) monomers in the rovibrational levels (v,j)=(0,0), (0,2), (1,0), and (1,2). We employ a close-coupled approach to compute the energies of these bound and quasibound dimer states using our potential energy surface, and compare the computed energies for infrared and Raman transitions involving these states with experimentally measured transition energies. We use four of the experimentally measured dimer transition energies to make two empirical adjustments to the ab initio potential energy surface; the adjusted surface gives computed transition energies for 56 experimentally observed transitions that agree with experiment to within 0.036 cm(-1). For 26 of the 56 transitions, the agreement between the computed and measured transition energies is within the quoted experimental uncertainty. Finally, we use our potential energy surface to predict the energies of another 34 not-yet-observed infrared and Raman transitions for the three dimers.     

Quantum dynamics of rovibrational transitions in H2-H2 collisions: internal energy and rotational angular momentum conservation effects

We present a full dimensional quantum mechanical treatment of collisions between two H(2) molecules over a wide range of energies. Elastic and state-to-state inelastic cross sections for ortho-H(2)?+ para-H(2) and ortho-H(2)?+ ortho-H(2) collisions have been computed for different initial rovibrational levels of the molecules. For rovibrationally excited molecules, it has been found that state-to-state transitions are highly specific. Inelastic collisions that conserve the total rotational angular momentum of the diatoms and that involve small changes in the internal energy are found to be highly efficient. The effectiveness of these quasiresonant processes increases with decreasing collision energy and they become highly state-selective at ultracold temperatures. They are found to be more dominant for rotational energy exchange than for vibrational transitions. For non-reactive collisions between ortho- and para-H(2) molecules for which rotational energy exchange is forbidden, the quasiresonant mechanism involves a purely vibrational energy transfer albeit with less efficiency. When inelastic collisions are dominated by a quasiresonant transition calculations using a reduced basis set involving only the quasiresonant channels yield nearly identical results as the full basis set calculation leading to dramatic savings in computational cost.     

Quantum mechanical investigation of rovibrational relaxation of H2 and D2 by collisions with Ar atoms

We report quantum mechanical calculations of cross sections and rate coefficients for rovibrational relaxation of H2 and D2 by collisions with Ar atoms over a wide range of temperatures including the ultracold limit. Limiting values of the rate coefficients for vibrational and rotational quenching at zero temperature were computed and sensitivity of the results to the choice of the interaction potential is investigated. We also demonstrate dramatic change in the behavior of the rate coefficients at low temperatures when the van der Waals potential supports a quasibound level very close to the dissociation threshold.     

Communication: New insight into the barrier governing CO2 formation from OH + CO

Despite its relative simplicity, the role of tunneling in the reaction OH + CO → H + CO(2) has eluded the quantitative predictive powers of theoretical reaction dynamics. In this study a one-dimensional effective barrier to the formation of H + CO(2) from the HOCO intermediate is directly extracted from dissociative photodetachment experiments on HOCO and DOCO. Comparison of this barrier to a computed minimum-energy barrier shows that tunneling deviates significantly from the calculated minimum-energy pathway, predicting product internal energy distributions that match those found in the experiment and tunneling lifetimes short enough to contribute significantly to the overall reaction. This barrier can be of direct use in kinetic and statistical models and aid in the further refinement of the potential energy surface and reaction dynamics calculations for this system.     

On the stability of IrCl6(3-) and other triply charged anions: solvent stabilization versus ionic fragmentation and electron detachment for the IrCl6(3-).(H2O)n n = 0-10 microsolvated clusters

The intrinsic gas-phase stability of the IrCl(6)(3-) trianion and its microsolvated clusters, IrCl(6)(3-).(H(2)O)(n) n = 1-10, have been investigated using density functional theory (DFT) calculations. Although IrCl(6)(3-) is known to exist as a stable complex ion in bulk solutions, our calculations indicate that the bare trianion is metastable with respect to decay via both electron detachment and ionic fragmentation. To estimate the lifetime of IrCl(6)(3-), we have computed the electron tunneling probability using an adaption of the Wentzel-Kramer-Brillouin theory and predict that the trianion will decay spontaneously via electron tunneling on a time scale of 2.4 x 10(-13) s. The global minimum structure for IrCl(6)(3-).H(2)O was found to contain a bifurcated hydrogen bond, whereas for IrCl(6)(3-).(H(2)O)(2), two low energy minima were identified; one involving two bifurcated water-ion hydrogen bonds and a second combining a bifurcated hydrogen bond with a water-water hydrogen bond. Clusters based on each of these structural motifs were obtained for all of the n = 3-10 systems, and the effect of solvation on the possible decay pathways was explored. The calculations reveal that solvation stabilizes IrCl(6)(3-) with respect to both electron detachment decay and ionic fragmentation, with the magnitude of the repulsive Coulomb barrier for ionic fragmentation increasing smoothly with sequential solvation. This study is the first to compare the propensity for electron detachment versus ionic fragmentation decay for a sequentially solvated triply charged anion.