Initial Rotational State Research Articles

Rotational energy transfer in collisions between CO(X 1Sigma+, v= 2 , J = 0, 1, 4, and 6) and He at temperatures from 294 to 15 K.

Infrared-vacuum ultraviolet double resonance experiments have been implemented in the ultracold environment provided by a Cinetique de Reaction en Ecoulement Supersonique Uniforme apparatus. With this technique rate coefficients of two kinds have been measured for rotational energy transfer in collisions between CO and He: (a) those for total removal from the selected rotational states J = 0, 1, 4, and 6 in the vibronic state X 1Sigma+, v = 2, and (b) those for transfer between selected initial and specific final states. Using different Laval nozzles, results have been obtained at several different temperatures: 294, 149, 63, 27, and 15 K. The thermally averaged cross sections for total removal by collisions with He show only slight variations both with initial rotational state and with temperature. The variation of state-to-state rate coefficients with DeltaJ show several general features: (i) a decrease with increasing DeltaJ; (ii) a propensity to favor odd DeltaJ over even DeltaJ; and (iii) at lower temperatures, the distribution of rate coefficients against DeltaJ becomes narrower, and decreases in J are increasingly favored over increases in J, a preference which is most strongly seen for higher initial values of J. The results are shown to be in remarkably good agreement with those obtained in ab initio scattering calculations by Dalgarno and co-workers [Astrophys. J. 571, 1015 (2002)].

Collisional Energy Transfer between Hot Pyrazine and Cold CO: A Classical Trajectory Study

Vibrational energy transfer from hot pyrazine (E‘ = 40 322 cm-1) to cold CO is modeled using classical trajectories. Collisional energy transfer properties are studied as a function of the initial rotational state J‘ of the CO, the length of the CO, the energy E‘ in pyrazine, the relative kinetic energy, the temperature, isotopic substitution on pyrazine, and the intermolecular potential. The energy transfer probability function P(E,E‘) exhibits distinct deviation from single exponential behavior. Collisions that transfer particularly large energy are associated with large amplitude out-of-plane motion of a C−H bond, imparting a kick to the departing CO. Slower collisions are particularly effective in relaxing pyrazine; faster collisions can add energy as often as they remove it. Energy transfer properties also depend on the initial rotational state of the CO. Temperature effects on 〈E − E‘〉 are weak in the 200−500 K range; this results from an increase in the magnitudes of both 〈ΔE〉down and 〈ΔE〉up. The f...

A reduced dimensionality quasiclassical and quantum study of the proton transfer reaction H3O(+)+H2O-->H2O+H3O(+).

We report quantum and quasiclassical calculations of proton transfer in the reaction H(3)O(+)+H(2)O in three degrees of freedom, the two OH(+) bond lengths and the OH(+)O angle. The reduced dimensional potential energy surface is obtained from the full dimensional OSS3(p) energy function of H(5)O(2) (+) [L. Ojamae, I. Shavitt, and S. J. Singer, J. Chem. Phys. 109, 5547 (1998)], with an additional long-range correction to reproduce the correct ion-molecule interaction. This surface is used to perform both quasiclassical trajectory and quantum reactive scattering calculations of the zero total angular momentum cumulative reaction probability and cross sections for initial rotational states 0, 1, and 2. Comparison of these quantities are made to assess the importance of quantum effects in this reduced dimensional reaction. Additional quasiclassical cross sections are calculated to obtain the thermal rate constant for the reaction.

Role of Initial Vibrational and Rotational Reactant Excitation for the Reaction Dynamics of H2(ν0,J0) with Si+(2P)

Molecular dynamics simulations with semiempirical quantum chemistry have been used to investigate the reaction dynamics of ground-state silicon ions with molecular hydrogen for translational impact energies between 0.5 and 10 eV. The validity of the employed PM3 method is demonstrated in comparison to high-level ab initio CCSD(T) calculations. Reaction cross sections for both SiH+(ν,J) formation and complete dissociation are determined for different initial vibrational and rotational excitation states of the H2(ν0,J0) molecules. Heating the commonly used room temperature hydrogen plasma to a maximum possible experimental value of 1000 K only results in a very modest increase of the reactive cross section for SiH+ production by about 25%. The use of selective laser excitation of the reactants, however, permits us to increase the reactivity drastically. In this latter case, initial rotational laser excitation enhances the reactivity of our system as much as vibrational excitation, illustrating that only the amount of the initial excitation and not its precise nature influences the reaction dynamics. Furthermore, we show how the initial vibrational, rotational, and translational energies of the H2 reactants control the final energy distributions of the SiH+ products. The mechanisms leading to the observed reaction dynamics are discussed.

Quantitative spectroscopic and theoretical study of the optical absorption spectra of H2O, HOD, and D2O in the 125-145 nm region.

The room temperature absorption spectra of water and its isotopomers D2O and HOD have been determined in absolute cross section units in the 125 to 145 nm wavelength region using synchrotron radiation. The experimental results for these B band spectra are compared with results from quantum mechanical calculations using accurate diabatic ab initio potentials. A Monte Carlo sampling over the initial rotational states of the molecules is applied in order to calculate the cross sections at a temperature of 300 K. The overall rotation of the water molecule is treated exactly. Both for the experimental and for the theoretical spectrum an analysis is made in terms of a component attributed to rapid direct dissociation processes and a component attributed to longer-lived resonances. The agreement between the results from experiment and theory is excellent for H2O and D2O. In the case of HOD in the results of theory two more resonances are found at low energy. It is demonstrated that the width of the resonances of 0.04 eV is the result of overlapping and somewhat narrower resonances in the spectra of molecules differing in rotational ground state.

Theoretical Study of the X+YCl (X, Y=H, D) Reactions

AbstractTime‐dependent wave packet calculations for the reaction H+HCl and its isotopic reactions are carried out on the potential energy surface (PES) of Bian and Werner (BW2) [Bain, W.; Werner, H.‐J. J. Chem. Phys. 2000, 112, 220]. Reaction probabilities for the exchanged and abstraction channels are calculated from various initial rotational states of the reagent. Those have then been used to estimate reaction cross sections and rate constants which also are calculated and explained by the zero‐point energy and the tunneling effect. The results of this work were compared with that of previous quasiclassical trajectory calculations and reaction dynamics experiments on the abstraction channel. In addition, the calculated rate constants are in reasonably good agreement with experimental measurements for both channels.

The Magnetorotational Instability in Core‐Collapse Supernova Explosions

We investigate the action of the magnetorotational instability (MRI) in the context of iron-core collapse. Exponential growth of the field on the timescale Ω-1 by the MRI will dominate the linear growth process of field-line "wrapping" with the same characteristic time. We examine a variety of initial rotation states, with solid-body rotation or a gradient in rotational velocity, that correspond to models in the literature. A relatively modest value of the initial rotation, a period of ~10 s, will give a very rapidly rotating proto-neutron star and hence strong differential rotation with respect to the infalling matter. We assume conservation of angular momentum on spherical shells. Rotational distortion and the dynamic feedback of the magnetic field are neglected in the subsequent calculation of rotational velocities. In our rotating and collapsing conditions, a seed field is expected to be amplified by the MRI and to grow exponentially to a saturation field. Results are discussed for two examples of saturation fields, a fiducial field that corresponds to vA = rΩ and a field that corresponds to the maximum growing mode of the MRI. We find, as expected, that the shear is strong at the boundary of the newly formed proto-neutron star and, unexpectedly, that the region within the stalled shock can be subject to strong MHD activity. Modest initial rotation velocities of the iron core result in sub-Keplerian rotation and a sub-equipartition magnetic field that nevertheless produce substantial MHD luminosity and hoop stresses: saturation fields of order 1015-1016 G can develop ~300 ms after bounce with an associated MHD luminosity of ~1052 ergs s-1. Bipolar flows driven by this MHD power can affect or even cause the explosions associated with core-collapse supernovae.

Photodissociation of warm water: ab initio calculations of the room-temperature absorption spectrum

This Letter presents cross sections of water in the second absorption band obtained from quantum mechanical calculations. A Monte-Carlo sampling over the initial rotational state is applied in order to calculate the cross section for water at a temperature of 300 K (warm water). This makes a fair comparison with the experimental spectrum, only available for water at room temperature, possible. The overall rotation of the water molecule is treated exactly. The inclusion of initial rotational motion, which reduces the resonance structure in the spectrum, significantly improves the agreement between theory and experiment.

Four-dimensional quantum scattering calculations on the D+CD4→CD3+D2 reaction

The dynamics for the D+CD4→CD3+D2reaction have been studied using reduced dimensionality quantum-mechanical theory. By the theory, the reactive polyatomic molecule CD4was treated as a diatomic molecule D—CD 3 , so the system can be treated as a linear atom_diatom reaction, reducing the sy stem to a four_dimensional scattering problem. In calculations, the Hamiltonian of the reaction system has been carried out using the time-dependent wave packet method, and the propagation of wave packets by the split-operator method. The semiempirical potential energy surface which has been developed by Jordan and Gi lbert is employed. The energy dependence of the calculated reaction probability shows oscillatory structures, similar to those observed in abstraction reactions H+H2, H+CH4,etc. The excitation of the stretching vib ration of reac tive molecule D—CD3gives a significant enhancement of reaction pro babilit y, the reaction threshold decreases with the enhancement of the vibrating excita tion. Detailed study of the influence of initial rotational states on reaction p robability shows a strong steric effect. The integral cross sections of translat ional energy for CD4at both v=0 and v=1 at ground rotational state show th at the vibrational excitation significantly enhances the reaction cross section. And the reaction threshold decreases by about 0.2eV, consistent with that of re action probability. The significant enhancement of the reaction probability of r eaction H+CH4at ground state when compared with D+CD4 can be explain ed reasonably in terms of quantum mechanical zero-point energies and the tunneli ng effect.

Selective dissociation of HCl in Kr from vibrational overtones

Vibrational levels v=1, 2, and 3 of HCl in Kr matrices are populated with tunable IR radiation and the excited molecules are dissociated by UV excitation to the repulsive A 1∏ state. Cl fragments are recorded by laser induced fluorescence of Kr2Cl and dissociation rates are determined from the increase in LIF with UV dose. The enlarged UV Franck–Condon range for overtones allows the study of cage exit of H fragments with small kinetic energy Ekin. A threshold at Ekin=1.4 eV and a steep rise indicate a predominant sudden exit. Monomers, different initial rotational states and transients in the relaxation cascade are preselected with overtone excitation and the feasibility of a discrimination between isotopes, aggregates, and local structures is illustrated.

Trajectory-Surface-Hopping Study of the Renner−Teller Effect in the N(2D) + H2 Reaction

We present a semiclassical study of the rovibronic Renner-Teller (RT) effect on the reaction N( 2 D) + H 2 (X 1 Σ + g ) → NH 2 (X 2 + A 2 A') → NH(X 3 Σ - + a'A) + H( 2 S). A new approach for describing RT coupling within the framework of trajectory surface hopping is developed for this application. The calculations include the two lowest-lying electronic states of NH 2 as derived from ab initio calculations together with a semiclassical description of the RT coupling. We investigated the role of the RT effect on the integral and differential reactive cross sections for formation of both NH electronic states and on the product distribution by varying the collision energy and the initial rotational state of H 2 . Our results show that the RT coupling does not affect the reactivity of trajectories starting on the X 2 potential but considerably affects those starting on A 2 A', leading to an almost exclusive formation of NH(X 3 Σ - ) at collision energies below 5 kcal/mol and a maximum contribution of ∼10% to the overall formation of NH(X 3 Σ - ) for collision energies between 5 and 10 kcal/mol. The dependence of the RT effect on the collision energy and on the rovibrational state of H 2 has also been analyzed, and a simple classical interpretation of most of these features based on the time spent by trajectories in the region where the RT coupling is active is proposed. Our results predict that the RT effect is not important to the thermal rate constant but has measurable consequences for certain state-resolved experiments.

Stereodynamics and rovibrational effect for H+CH4(v,j,K,n)→H2+CH3 reaction

In this work, we employ the semirigid vibrating rotor target (SVRT) model to study the influence of rotational and vibrational excitation of the reagent on reactivity for the benchmark reaction H+CH4(v,j,K,n). The excitation of the pseudo H–CH3 stretching vibration of the SVRT model gives significant enhancement of reaction probability, consistent with the later position of the reaction barrier on the potential energy surface. The vibrationally thermal-averaged rate constant is much larger than the rate constant of the ground vibrational state. Detailed study of the influence of initial rotational states on reaction probability shows strong steric effect. The reaction probability is directly correlated with the angular distribution of the initial wave function determined by different angular momentum relationships among three vectors j, R, and r. The steric effect of polyatomic reactions, treated by the SVRT model, is more complex and richer than theoretical calculations involving linear molecular models.

Direct observation of OH photofragment from triplet hydroxyacetone

In contrast to the photoexcitation of hydroxyacetone (HA) at 193 nm resulting in an instantaneous dissociation of the Rydberg 1(n,3s) state, on photoexcitation at 248 nm the singlet 1(n,π*) excited HA molecule first undergoes intersystem crossing (ISC) to the triplet state, followed by a minor dissociation channel to CH3COCH2 and OH radicals. The real time formation of OH, which is probed by laser-induced fluorescence (LIF), shows a rate constant to be ⩾108s−1. The initial rotational state distribution of OH(X2Π) is found to be Boltzmann-like, characterized by a single rotational temperature Trot of 450±40 K. The average relative translational energy of the photofragments is determined by Doppler spectroscopy to be 8.7±2.0kcalmol−1. The observation of OH with a modest rotational energy, no vibrational energy, and a large amount of translational energy suggests significant exit energy barrier with the dissociating surface.

Quenching of vibrationally excited CO( ν=2) molecules by ultra-cold collisions with 4He atoms

The quenching of vibrational excited CO( ν=2) molecules by He atoms at ultra-low temperature is examined to determine the relative efficiency of the processes that involve the exchange of a double or a single quantum of vibrational energy. We find that at low temperature the double quantum exchange cross-section is orders of magnitude smaller than for the single quantum process. This result is valid for different initial rotational states of the CO molecule.

H2 dissociation dynamics on an alloy surface – controlling the dynamics via orientation

We demonstrate how we could influence the H2 dissociation dynamics through the H–H bond orientation (with respect to the surface) dependence of the potential energy (hyper-) surface (PES), by considering the interaction of H2 with an alloy surface (e.g., an ordered Cu3Pt(111)). Our preliminary results show that by changing the dependence of the PES on the H–H orientation (e.g., such that preference for H–H orientation perpendicular to the surface over intermediate H–H orientations between perpendicular and parallel occurs), not only did we observe a general increase in dissociation probability of H2, we also observe an increase in the dissociation probability of cartwheel-like rotating H2, relative to helicopter-like rotating H2. Depending on the initial state of the impinging H2 (initial rotational state and incidence energy), we also observed that the dissociation probability of cartwheel-like rotating H2 becomes even greater than the dissociation probabilities for rotating H2 with orientations intermediate between cartwheeling and helicoptering H2. Based on these results, we suggest how further information regarding the orientation dependence of the PES could be obtained experimentally.

A reduced dimensionality, six-degree-of-freedom, quantum calculation of the H+CH4→H2+CH3 reaction

A reduced dimensionality, time-dependent wave-packet calculation is reported for the H+CH4→H2+CH3 reaction in six degrees of freedom and for zero total angular momentum, employing the Jordan–Gilbert potential energy surface. Reaction probabilities for seven initial vibrational states of nonrotating “CH4,” and for the three lowest energy vibrational states and numerous initial rotational states are presented. Excitation of the C–H stretch, and the bending of H–CH3, enhances the reaction probability more than excitation of the umbrella mode. The six-degree-of-freedom cumulative reaction probability (CRP) for zero total angular momentum is obtained by direct summation over initial state-resolved reaction probabilities. An approximate full-dimensional CRP for zero total angular momentum is obtained using the energy-shift approximation to account for the contribution of degrees of freedom missing in the reduced dimensionality calculations. Then J–K shifting is applied to this CRP to obtain the thermal rate constant which is compared with previous calculations.

ArF laser photodissociation dynamics of furfuryl alcohol: LIF observation of OH state distribution

The dynamics of furfuryl alcohol (FURFUROL) photodissociation at 193 nm is reported, using laser-induced fluorescence (LIF) of the nascent OH radical and RRKM theory. The nascent OH fragments are probed by LIF under collisionless conditions, to determine the initial product state distributions. There is no significant population (<2%) in excited vibrational levels of OH (X 2 Π). However, the initial rotational state distribution is Boltzmann-like, characterized by a single rotational temperature T rot of 780±40 K . The average relative translational energy of the photofragments is determined to be 26±4 kJ mol −1 . The measured rate constant for the FURFUROL dissociation vis-a-vis statistical RRKM theory, suggests a threshold dissociation energy of 357±20 kJ mol −1 .

Orientational effects on the dynamics of H2 dissociation at the atop-Cu, atop-Pt, and Cu–Pt bridge sites of an ordered Cu3Pt(1 1 1)

To demonstrate how we could influence the H2 dissociation dynamics through the H–H bond orientation (with respect to the surface) dependence of H2 dissociation, we considered the dissociation of H2 on an ordered Cu3Pt(111). Our preliminary results show that by changing the dependence of the H2 dissociation on the H–H orientation, such that preference for H–H orientation perpendicular to the surface over intermediate H–H orientations between perpendicular and parallel occurs, not only did we observe a general increase in dissociation probability of H2, with initial rotational state (j=1), we also observe a corresponding increase in the dissociation probability of cartwheel-like rotating H2, relative to helicopter-like rotating H2. Based on these results, we suggest how further information regarding the orientation dependence of the H2 dissociation could be determined experimentally.

Photodissociation of water in the à band revisited with new potential energy surfaces

Theoretical calculations on the photodissociation of water in the first absorption band have been used to test the accuracy of three available potential energy surfaces for the first excited state of water: the well-known coupled electron pair approximation potential of Staemmler and Palma [Chem. Phys. 93, 63 (1985)], and two new multireference double excitation configuration interaction surfaces: the Dobbyn–Knowles surface (unpublished), and the Leiden surface [R. van Harrevelt and M. C. van Hemert, J. Chem. Phys. 112, 5777 (2000)]. Exact quantum mechanical calculations, using the wave packet approach, have been performed for J″&amp;gt;0, where J″ is the initial rotational state of the water molecule. The cross section was found not to depend strongly on the rotational state, so that it is reasonable to compare calculated cross sections for J″=0 with experimental room temperature cross sections. Small and simple corrections were applied to the potential energy surface to improve the agreement between theory and experiment for the cross section of H2O. Spectra for D2O and vibrationally excited water molecules calculated with all three corrected potential energy surfaces were in good agreement with experiments. A comparison between calculated OH(X) or OD(X) vibrational distributions, and recent kinetic energy release measurements of the H or D atoms produced in the 157.6 nm photodissociation of water and its isotopomers [Yang et al., J. Chem. Phys. 113, 10597 (2000)], however, suggests that the Leiden surface is more accurate than the two other surfaces.

Excitation functions of elementary chemical reactions: a direct link from crossed-beam dynamics to thermal kinetics ?

The excitation function refers to the translational energy dependence of the integral cross-section of a biomolecular collision process. Exemplified by a number of elementary chemical reactions, the information content of excitation functions is critically surveyed. Particular emphasis is placed on the close comparison with the available thermal kinetics data. The reactivity for an activated reaction was found to depend sensitively on the rotational state of the reagent, indicative of stereodynamical effects. The intramolecular isotope branching ratio, for the reaction A + HD, exhibits a strong dependence on the collision energy. Its isotopic propensity reverses between a non-rotating and a rotating reagent. By way of contrast, the reactive behaviour of a barrierless reaction shows little dependence on the initial rotational state, and the intramolecular isotope branching ratio instead becomes nearly independent of the collision energy. In this case, the excitation function obtained from crossed-beam experiments then provides a direct and reliable route to compare with available thermal rate constants or to extrapolate kinetics to a wider temperature range.