Rovibrational State Distributions Research Articles

Exact three-dimensional quantum mechanical calculation of ozone photodissociation in the Hartley band

Exact three-dimensional time-dependent wave packet calculation for ozone photodissociation in the Hartley band has been carried out for total angular momentum J=0. Fully converged absorption spectrum, autocorrelation function, and O2(a 1Δg) photofragment rovibrational state distributions over wide range of wavelengths are obtained using the Sheppard–Walker potential energy surface (PES). Excellent agreements as well as some deviations have been found in comparison to the various experimental results. From these successes and failings, the ways for further improving the PES have been suggested, and the dynamics has been discussed in some detail in conjunction with time-dependent pictures of the wave packet.

Predicting inelastic rovibrational state distributions from an energy constrained angular momentum mechanism

We present a quantitative version of the velocity–angular momentum plots of Besley et al. that we have used extensively to represent the key processes at work in collisional transfer mechanisms. Rotational state distributions are obtained by incorporating probability distributions of the relevant variables, and the Monte Carlo (MC) trajectory technique is used to sample these distributions. The method is illustrated with the case of weakly quasi-resonant vibration rotation transfer in A( 1Σ u +) Li 2+ Ne collisions. The results show excellent agreement with published experimental data, indicating the apparent dominance of the factors governing angular momentum (AM) conversion in shaping rovibrational distributions.

Quantum state distributions of HCl from the ultraviolet photodissociation of HCl dimer

The rovibrational state distributions for the HCl product of the 193 nm photodissociation of (HCl)2, (HCl)2+hν→H+Cl+HCl, have been measured. The HCl dimer is prepared in a supersonic expansion of HCl in Ar, and its photoproduct detected by resonant multiphoton ionization under collisionless conditions. The state distributions are extremely “cold,” with very little of the available energy deposited in either rotation or vibration of the surviving HCl molecule. Only v′=0 product is observed, and linear rotational surprisal analysis yields an extremely large surprisal parameter, θr′ of 95(8). The results are in excellent agreement with theoretical predictions.

Dynamics of the O(3P) + CS2(XΣg+) → SO(X3Σ-) + CS(X1Σ+) Reaction

The rovibrational state distributions of both the CS and the SO products following the reaction of O(3P) + CS2 have been investigated. The O(3P) atoms are generated by photolysis of NO2 using a frequency-tripled Nd:YAG laser at 355 nm. The SO(X3Σ-, v‘ ‘ = 0−6) and CS(X1Σ+, v‘ ‘ = 0−3) are observed directly using laser induced fluorescence (LIF) spectroscopy on the (B3Σ- − X3Σ-) and (A1Π − X1Σ+) transitions, respectively. The SO(X3Σ-) product is found to be highly excited with the vibrational state distribution inverted at v‘ ‘ = 1 and detectable population up to v‘ ‘ = 6, while the CS(X1Σ+) product vibrational state distribution is characterized as Boltzmann with a vibrational temperature of 1230 ± 155 K. The total vibrational excitation of both products accounts for 21% of the available energy to the products. The near nascent SO(X3Σ-, v‘ ‘ = 0−4) rotational state distributions are characterized by rotational temperatures in the range of 882−1312 K, and the near nascent CS(X3Σ+, v‘ ‘ = 0) is characterized by a temperature of 2986 ± 607 K. The total rotational energy of the products accounts for 34.8% of the available energy. Correlated ab initio calculations of the reaction pathway have been performed, resulting in accurate energies for the reactants, products, intermediates, and transition states. Optimized geometries for the intermediates and transition states have been obtained. The inverted vibrational state distribution of the SO(X3Σ-) product and the excited rotational state distribution of the CS(X1Σ+) product suggest a short-lived, nonlinear intermediate structure as the primary pathway for the reaction. The results from the ab initio calculations corroborate this model.

The H+n-C5H12/n-C6H14→H2(v′,j′)+C5H11/C6H13 reactions: State-to-state dynamics and models of energy disposal

The rovibrational state distributions for the H2 product of the H+n-C5H12/n-C6H14→H2+C5H11/C6H13 reactions at 1.6 eV collision energy are reported. The results are compared to measurements made on the kinematically and energetically similar H+RH→H2+R (RH=CH4, C2H6, and C3H8) reactions as well as the atom–diatom reactions H+HX→H2+X(HX=HCl, HBr). For the title reactions, as for all the comparison reactions, the product appears in few of the energetically accessible states. This is interpreted as the result of a kinematic constraint on the product translational energy. Characteristic of the H+RH reactions we have previously studied, the title reactions show increasing rotational excitation of the H2 product with increasing vibrational excitation of it, a correlation that gets stronger as the size of the alkane increases. Trends and variations in the product energy disposal are analyzed and explained by a localized reaction model. This model predicates a truncation of the opacity function due to competing reactive sites in the polyatomic alkane reactant, and a relaxation of the otherwise tight coupling of energy and angular momentum conservation, because the polyatomic alkyl radical product is a sink for angular momentum.

Associative detachment in low-energy collisionsbetween hydrogen atoms and atomic halogen anions

Using an improved nonlocal resonance theory, total crosssections, rovibrational final state distributions and electronspectra for low-energy (<2 eV) associative detachment (AD)collisions between hydrogen atoms and negative halogen ionsX- (X = F, Cl, Br) as well as deuterium atoms and F- ions have been calculated. The model involves realisticlong-range interactions for the H + X- systems andtakes into account the dependence of the dipole-modifiedthreshold exponent on internuclear distance. Good agreement isfound between experimental data for the total AD cross sectionsand the theoretical results. In the calculated electron energyspectra nonlocal effects are manifest through step structurewhich is (almost) independent of collision energy; thisstructure should therefore be observable in experiments withrather modest collision energy resolution. For illustration,electron spectra are presented which correspond to realisticexperimental conditions with regard to the energy widths of theelectron spectrometer and the collision energy distribution. Therovibrational final-state distributions in the HX(v, J)molecules formed are reported for a thermal kinetic energy distributionat T ≈ 300 K; for HF, DF and HCl the rotational-summedvibrational populations pv are compared with previoustheoretical and experimental results.

Vibrationally excited state spectroscopy of radicals in a supersonic plasma

A plasma source based on a multilayer discharge geometry in combination with a time-of-flight REMPI experiment is used to study rotationally cold spectra of highly excited vibrational states of mass selected radicals. The rovibrational state distributions upon discharge excitation are characterised for the example of NO for ground state vibrational levels up to v″=18. Whereas rotational temperatures are lower than 50 K, a vibrational temperature of T vib=6700±700 K is found.

Time evolution of reactants, intermediates, and products in the vibrational predissociation of Br2⋯Ne: A theoretical study

A hybrid quantum/classical simulation of the vibrational predissociation of the Br2⋯Ne cluster in the B state is carried out. The resulting lifetimes and final rovibrational state distributions compare very well with the experimental measurements, as well as with accurate quantum mechanical results. The time-evolution of the reactants, products, and intermediates is analyzed by a kinetic mechanism, comporting three elementary steps: direct vibrational predissociation (VP), intramolecular vibrational redistribution (IVR), and evaporative cooling (EC). The importance of intramolecular vibrational redistribution followed by evaporative cooling relative to direct vibrational predissociation is shown to evolve from 100% of VP for the lowest initial vibrational level v=10 to 53% for the highest one v=27. In the cases where IVR is important, the complexes are shown to explore the whole configuration space, in contrast with the cases where dynamics are governed by direct vibrational predissociation for which the complexes mainly evolve in the region around the T-shaped equilibrium configuration. A time-dependent picosecond experiment is proposed to detect the IVR intermediates, based on their different structure. It consists of exciting the complex with a first laser and probing the intermediates with a second laser to an electronic state with a minimum in the collinear configuration where the initially excited state wave function has no weight. The ground state of the positive ion is proposed as the final state, so that ions are detected. An appreciable population of intermediates is predicted for initial excited levels with v⩾20.

Preparation and probing of Ar–Cl radical complexes from UV photodissociation of the Ar–HCl cluster

Photodissociation of Ar–HCl is simulated by exact wavepacket calculations. The asymptotic wavepacket is projected out onto the product states corresponding to several excitation energies of Ar–HCl. A high relative efficiency of Ar–Cl formation is found at low and high cluster excitation energies. The ro-vibrational state distribution of the Ar–Cl radical formed is reflected in the kinetic-energy distribution of the H fragment. Photolysis of Ar–HCl then appears as an efficient method for preparing Ar–Cl radicals, simultaneously probing their final state distribution.

Photodissociation of hydrogen sulfide at 157.6 nm: Observation of SH bimodal rotational distribution

Photodissociation of the H2S molecule at 157.6 nm was studied experimentally using the Rydberg tagging technique. Translational energy distributions of the H-atom product from the H2S photodissociation were measured, and the SH(X 2Π)+H(2S) channel was found to be the dominant dissociation process. Spin-orbit and rovibrational state distributions were also obtained for the SH product, which was found to be both vibrationally and rotationally excited. An intriguing bimodal rotational distribution in the lowest two vibrational states, v=0 and 1, has been clearly observed for the SH product, indicating that there are two distinctive dissociation mechanisms involved in the photodissociation of H2S at 157 nm excitation.

Quantum state-resolved reactive scattering of F+H2 in supersonic jets: Nascent HF(v,J) rovibrational distributions via IR laser direct absorption methods

Supersonically cooled discharge radical atom sources are combined with high-sensitivity IR absorption methods to investigate state-to-state reactive scattering of F+n-H2→HF(v,J)+H in low-density crossed supersonic jets at center-of-mass collision energies of 2.4(6) kcal/mole. The product HF(v,J) is probed with full vibrational and rotational quantum state selectivity via direct absorption of a single mode (Δν≈0.0001 cm−1), tunable F-center laser in the Δv=1 fundamental manifold with near shot noise limited detection levels of 108 molecules/cm3/quantum state per pulse. The high absorption sensitivity, long mean free path lengths, and low-density conditions in the intersection region permit collision-free HF(v,J) rovibrational product state distributions to be extracted for the first time. Summed over all rotational levels, the HF vibrational branching ratios are 27.0(5)%, 54.2(23)%, 18.8(32)%, and &amp;lt;2(2)%, respectively, into vHF=3:2:1:0. The nascent vibrational distributions are in good agreement with rotationally unresolved crossed-beam studies of Neumark et al. [J. Chem. Phys. 82, 3045 (1985)], as well as with full quantum close-coupled calculations of Castillo and Manolopoulos [J. Chem. Phys. 104, 6531 (1996)] on the lowest adiabatic F+H2 potential surface of Stark and Werner [J. Chem. Phys. 104, 6515 (1996)]. At a finer level of quantum state resolution, the nascent rotational distributions match reasonably well with full quantum theoretical predictions, improving on the level of agreement between theory and experiment from early arrested relaxation studies. Nevertheless, significant discrepancies still exist between the fully quantum state-resolved experiment and theory, especially for the highest energetically allowed rotational levels.

Multi-photon dissociation of CHBr 3 at 248 and 193 nm: observation of the electronically excited CH( [formula omitted]) product

The CH(A 2 Δ ) multi-photon dissociation channel of CHBr 3 following the excitation at 193 and 248 nm has been studied. Two experimental techniques are applied: time-resolved Fourier transform emission spectroscopy (TR-FTES) and two-laser experiments. The rovibrational state distribution of the nascent CH(A 2 Δ ) fragments generated by multi-photon dissociation of CHBr 3 in a supersonic jet is determined through an analysis of the CH(A 2 Δ –X 2 Π ) emission spectrum obtained by TR-FTES with resolutions between 0.5 and 4.0 cm −1. From the power dependence of the fluorescence the order of the multi-photon dissociation is determined to be two for 193 nm and three for 248 nm radiation. At both wavelengths the rovibrational distributions are power dependent but the effect is most pronounced for 193 nm excitation. At the lowest power investigated, the rotational temperatures are T=1450(70) K for 193 nm and T=2090(90) K for 248 nm multi-photon dissociation. While only v′=0 is observed in the two-photon dissociation at 193 nm, the vibrational temperature at low laser energy is 2400(200) K for three-photon dissociation at 248 nm. In experiments with two spatially separated volumes of the supersonic jet excited by two delayed laser pulses, the dissociation mechanism is shown to be a sequential fragmentation for 193 nm and to involve at least one intermediate species for 248 nm multi-photon dissociation. The possible dissociation channels for the production of CH(A 2 Δ ) by multi-step dissociation of CHBr 3 are discussed with respect to the observed rovibrational distributions.

Nitrogen recombination dynamics at Cu(111): Rotational energy release and product angular distributions

Nitrogen atoms adsorbed on Cu(111) desorb thermally from an ordered Cu(100)−c(2×2)N phase in a sharp, zero order desorption feature near 700 K with an activation barrier of 143 kJ mol−1. Detailed N2 product rovibrational state distributions have been measured following recombinative desorption from a 700 K Cu(111) surface exposed to a N atom beam, with an equilibrium N coverage θN⩽10−2 ML. Although desorbing N2 is translationally and vibrationally hot, with a vibrational temperature of 5100 K and 4.2 eV of translational excitation perpendicular to the surface, rotation is excited with a temperature of just 910(±50) K for the vibrational ground state and 840(±250) K for (v=1). The energy released during recombinative desorption channels effectively into translational and vibrational motion, but not into rotational excitation. The angular distribution of recombinatively desorbed N2 is sharply peaked along the surface normal, P(θ)=cos(28±1) θ, indicating a mean energy release of 0.28 eV into translation parallel to the surface. This is inconsistent with 1D models of the translational energy release based on thermal motion parallel to the surface and a repulsive energy release directed along the surface normal. The dynamics can be described by a direct, repulsive model with a transition state at extended N2 separation, similar to the models developed for H2 dissociation on the same surface. We discuss the application of detailed balance to determine N2 sticking functions S(E,v,J) and, using a simple model for these functions, estimate a rotational efficacy of ∼0.23 for sticking of N2(v=0, J⩽24) and a vibrational efficacy of 0.7 for N2(v=1). The dynamics are compared to the models developed for H2 dissociation and the role of molecular chemisorption states and the local desorption site discussed.

Reactant-product decoupling approach to half-scattering problems: Photodissociation of H2O in three dimensions

In this paper, we present the RPD (reactant-product decoupling) approach to the calculation of final-state distribution in photodissociation of H2O in three-dimensional space. Although the RPD approach was recently developed for bimolecular state-to-state reactive scattering calculations, its application to photodissociation dynamics is very attractive. Specifically in photodissociation, the interaction (reactant) component wavefunction ψr (which in the present case of photodissociation is replaced by the interaction component ψint) is nonzero only in the strong interaction region, which greatly simplifies the numerical calculation for ψint in comparison to that for ψr in a full bimolecular reactive scattering calculation. In the following report, the time-dependent implementation of the RPD approach to the photodissociation of H2O in three dimensions is given and the calculated rovibrational state distributions of the OH fragment are presented.

Rovibrational state distribution of deuterium molecules resulting from D-atom interaction with Ni(110)

Using the REMPI technique we have studied the internal state distribution of deuterium molecules produced by the interaction of atomic deuterium with chemisorbed deuterium on Ni(110) at 180 K. We observed molecules in vibrational states up to v = 3 with a mean vibrational energy of 220 meV. The mean rotational energies of the molecules in the vibrational states v = 0 to 3 are 185 meV, 133 meV, 75 meV and 37 meV, respectively. The overall mean rotational energy amounts to 150 meV, again far in excess of the mean rotational energy of molecules accommodated to the surface. The data are consistent with direct interactions of the impinging particles with the adsorbed particles (Eley-Rideal reaction and collision induced desorption), for which it is assumed that a considerable amount of the potential and kinetic energy of the impinging atoms is channeled into translational and internal energy of the reaction products.

Formula omitted] product energy disposal in the desulfurization reaction of thiirane with O( 3P) atoms

The SO( X 3 Σ −) product energy disposal in the desulfurization reaction of thiirane (C 2H 4S) with O( 3P) atom has been studied by probing its rovibrational state distributions via laser-induced fluorescence spectroscopy on the B 3 Σ − − X 3 Σ − transition. The O( 3P) atom was produced by 351 nm photolysis of NO 2. The nascent SO( X 3 Σ −) product vibrational state distribution was found to be inverted with a maximum at v″ = 1 and measurable population was detected up to v″ = 8. The rotational state distributions of SO( X 3 Σ −, v″ = 1/2–5) produced within two gas kinetic collisions can be characterized by an average rotational temperature, T R = 1163 ± 311 K. The observed SO( X 3 Σ −) internal energy accounts for 16% of the total energy available to the products. No evidence for the production of other species, e.g. OH, was observed. The non-statistical energy distribution in SO( X 3 Σ −) is consistent with an abstraction mechanism for the title reaction.

State-specific unimolecular dissociation dynamics of HFCO. II. CO rotational distribution and Doppler widths

Rovibrational state distributions and Doppler widths of CO fragments formed from unimolecular dissociation of HFCO in its ground electronic state are measured by vacuum ultraviolet laser-induced fluorescence, following state-selective preparation of the molecule in a single quantum state in the energy region of 2000 to 3000 cm−1 above the dissociation threshold by stimulated emission pumping. CO fragments are rotationally hot and distributed over J≤15 to J=63 with distributions peaking at J=45 to 50 depending upon the initial HFCO dissociative state. Although CO rotational distributions are significantly different for different initial states, about 20% of the total available energy is released on average as rotational energy of CO for all three initial states studied. The yield of CO(v=1) fragments is determined to be about 10% and CO(v≥2) fragments are not observed. The average Doppler width of CO fragments is 0.85 cm−1, which indicates that ∼50% of the total available energy is released as translation. The CO product state distributions and Doppler widths may be rationalized using a modified impulsive model with the ab initio transition state geometry. The dependence of product state distributions on the initial HFCO quantum state may reflect incomplete intramolecular vibrational energy redistribution.

Flat surface study of the Eley–Rideal dynamics of recombinative desorption of hydrogen on a metal surface

The dynamics of a direct reaction between a gas phase H atom and an adsorbed H atom, often referred to as an Eley–Rideal mechanism, is explored using a fully three-dimensional flat surface model for Cu(111). The model is based on a flat-surface approximation for a single electronically adiabatic potential energy surface (PES). This reduces the inherently six-dimensional reactive scattering problem (for a rigid surface) to a three-dimensional problem by introducing three constants of motion. The resulting scattering problem is treated quantum mechanically by solving the time-dependent Schrödinger equation, and also by quasiclassical trajectory calculations. We have considered four different model PESs which are all more or less compatible with available knowledge about the interactions between hydrogen atoms and molecules and Cu(111). We have studied the dependence of the reactive cross section and product translational energy and rovibrational state distributions on the kinetic energy and angle of the incident H atom, and the vibrational state of the adsorbed H atom.

Rovibrational State Distribution of SiO Desorbed from a Silicon Surface during O2 Molecular Beam Scattering.

Rovibrational state distributions of SiO desorbed from a Si(111) surface during O2 molecular beam scattering at 1250±50K are studied using resonance enhanced multiphoton ionization mass spectroscopy. Results show that the vibrational temperature is comparable with the surface temperature but the rotational temperature is lower than that.

Rovibrational state distributions for H 2 photoeliminated from 1,3-butadiene. Evidence for a symmetric transition state

The rovibrational distributions of H 2 photoeliminated from 1,3-butadiene upon excitation at 212.8 nm have been measured via 1 + 1 resonance-enhanced two-photon ionization. The measured rotational distributions are in reasonable agreement with a prior distribution, while the experimental vibrational distribution is substantially hotter than the prior. These results indicate that the photoelimination occurs via a concerted mechanism through a symmetric transition state in which the HH distance is greater than the equilibrium H 2 value.