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The angular distribution of products from the ultraviolet photodissociation of nitrous oxide yielding O(1D) and N2(X Σg+1) was investigated using classical trajectory calculations. The calculations modeled absorption only to the 2 1A′ electronic state but used surface-hopping techniques to model nonadiabatic transitions to the ground electronic state late in the dissociation. Observed values of the anisotropy parameter β, which decrease as the product N2 rotational quantum number j increases, could be well reproduced. The relatively low observed β values arise principally from nonaxial recoil due to the very strong bending forces present in the excited state. In the main part of the product rotational distribution near 203 nm, an unusual dynamical effect produces the decrease in β with increasing j; nonaxial recoil effects remain approximately constant while higher j product molecules arise from parent molecules that had their transition dipole moments aligned more closely along the molecular axis. In both low and high j tails of the rotational distribution, the variations in β with j are caused by changes in the extent of nonaxial recoil. In the high-j tail, additional torque present on the ground state potential energy surface following nonadiabatic transitions causes both the additional rotational excitation and the lower β values. [ABSTRACT FROM AUTHOR]

The diffuse vibrational bands, observed in the ultraviolet photodissociation spectrum of nitrous oxide by exciting the molecule in the first 1A′ state, have recently been attributed to resonances localized mainly in the NN stretch and bend degrees of freedom. To further investigate the origin of this localization, fundamental families of periodic orbits emanating from several stationary points of the 1A′ potential energy surface and bifurcations of them are computed. We demonstrate that center-saddle bifurcations of periodic orbits are the main mechanism for creating stable regions in phase space that can support the partial trapping of the wave packet, and thus they explain the observed spectra. A non-linear mechanical methodology, which involves the calculation of equilibria, periodic orbits, and transition states in normal form coordinates, is applied for an in detail exploration of phase space. The fingerprints of the phase space structures in the quantum world are identified by solving the time dependent Schrödinger equation and calculating autocorrelation functions. This demonstrates that different reaction channels could be controlled if exact knowledge of the phase space structure is available to guide the initial excitation of the molecule. [ABSTRACT FROM AUTHOR]

Product state properties from the photodissociation of ozone in the ultraviolet Hartley band are investigated by trajectory surface-hopping calculations. The diabatic B and R state potential energy and coupling surfaces of Schinke and McBane [J. Chem. Phys. 132, 044305 (2010)] are employed. The properties computed include rotational and vibrational distributions in both the singlet and triplet channels, the total internal energy distribution in the triplet channel, and the photodissociation anisotropy parameter β in the singlet channel. A method for computing β from trajectories computed in internal Jacobi coordinates is described. In the singlet channel, the vibrational distribution is in good agreement with the experimental results. The observed increase in β with increasing photolysis wavelength is reproduced by the calculations and is attributed to the effects of the bending potential on the B state late in the fragmentation. The computed β values are too high with respect to experiment, and the peaks jmax of the singlet-channel rotational distributions are too low; these discrepancies are attributed to a too steep bending potential at long O-O distances. In the triplet channel, the main part of the internal energy distribution is described well by the calculations, although the detailed structures observed in the experiment are not reproduced. The experimental rotational distributions are well reproduced, although the maxima appear at slightly too high j. The triplet state product energy distributions are shown to depend largely on the distribution of hopping points onto the R state surface. A Landau-Zener model constructed as a function of the O2 bond distance provides a good physical description of the two-state dynamics. The high internal energy O2 products that cannot be attributed to the excitation of the Herzberg states remain unexplained. [ABSTRACT FROM AUTHOR]

Ab initio quantum calculations have been carried out on the helium ionic trimer. The potential energy surface is accurately fitted, especially in the vicinity of the three equivalent minima. The spectrum of bound states for the zero angular momentum is computed and analyzed in detail. Energies and wave functions reveal several interesting features related to the fact that He3+ represents one of the few homonuclear ionic trimers that are linear in their ground vibrational state. At low energies, the triply degenerate eigenfunctions are localized at the potential minimum. With growing excitation energy, however, the wave functions exhibit stronger spatial delocalization. [ABSTRACT FROM AUTHOR]

The recombination of ozone via the chaperon mechanism, i.e., ArO+O2→Ar+O3 and ArO2+O→Ar+O3, is studied by means of classical trajectories and a pairwise additive Ar-O3 potential energy surface. The recombination rate coefficient has a strong temperature dependence, which approximately can be described by T -n with n≈3. It is negligible for temperatures above 700 K or so, but it becomes important for low temperatures. The calculations unambiguously affirm the conclusions of Hippler et al. [J. Chem. Phys. 93, 6560 (1990)] and Luther et al. [Phys. Chem. Chem. Phys. 7, 2764 (2005)] that the chaperon mechanism makes a sizable contribution to the recombination of O3 at room temperature and below. The dependence of the chaperon recombination rate coefficient on the isotopomer, studied for two different isotope combinations, is only in rough qualitative agreement with the experimental data. The oxygen atom isotope exchange reaction involving ArO and ArO2 van der Waals complexes is also investigated; the weak binding of O or O2 to Ar has only a small effect. [ABSTRACT FROM AUTHOR]

The energy transfer between argon atoms and ozone complexes O3*, excited in the region of the dissociation threshold, is calculated for fixed temperatures (100 K≤T≤2500 K) using classical trajectories. The internal energy of ozone is resolved in terms of vibrational and rotational energies. For all temperatures, energy flows from O3* to Ar. The vibrational energy transfer, relative to kBT, is very small below 500 K, but gradually increases towards high temperatures. The relative rotational energy transfer, on the other hand, monotonously decreases with T; around 1100 K it falls below the relative vibrational energy transfer. Thermally averaged cross sections for vibrational and rotational energy transfers are also calculated. The implications for the stabilization of ozone complexes in the energy transfer model are discussed. [ABSTRACT FROM AUTHOR]

The effect of zero-point energy differences (ΔZPE) between the possible fragmentation channels of highly excited O3 complexes on the isotope dependence of the formation of ozone is investigated by means of classical trajectory calculations and a strong-collision model. ΔZPE is incorporated in the calculations in a phenomenological way by adjusting the potential energy surface in the product channels so that the correct exothermicities and endothermicities are matched. The model contains two parameters, the frequency of stabilizing collisions ω and an energy dependent parameter Δdamp, which favors the lower energies in the Maxwell–Boltzmann distribution. The stabilization frequency is used to adjust the pressure dependence of the absolute formation rate while Δdamp is utilized to control its isotope dependence. The calculations for several isotope combinations of oxygen atoms show a clear dependence of relative formation rates on ΔZPE. The results are similar to those of Gao and Marcus [J. Chem. Phys. 116, 137 (2002)] obtained within a statistical model. In particular, like in the statistical approach an ad hoc parameter η≈1.14, which effectively reduces the formation rates of the symmetric ABA ozone molecules, has to be introduced in order to obtain good agreement with the measured relative rates of Janssen et al. [Phys. Chem. Chem. Phys. 3, 4718 (2001)]. The temperature dependence of the recombination rate is also addressed. [ABSTRACT FROM AUTHOR]

The Huggins band of ozone is investigated by means of dynamics calculations using a new (diabatic) potential energy surface for the 3 1A′(1B2) state. The good overall agreement of the calculated spectrum of vibrational energies and intensities with the experimental spectrum, especially at low to intermediate excitation energies, is considered as evidence that the Huggins band is due to the two Cs potential wells of the 1B2 state rather than the single C2v well of the 2 1A′(1A1) state. The vibrational assignment of the “cold bands,” based on the nodal structure of wave functions, on the whole supports the most recent experimental assignment [J. Chem. Phys. 115, 9311 (2001)]. The quantum mechanical spectrum is analyzed in terms of classical periodic orbits and the structure of the classical phase space.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

New electronic structure calculations for the transition-state region of the lowest ozone potential energy surface are reported. A two-dimensional potential energy surface in the asymptotic channel is calculated with the O2 bond distance being fixed. The calculations are performed at the multireference average quadratic coupled cluster level of theory using full-valence complete active space self-consistent field wave functions and the augmented correlation consistent polarized V6Z atomic basis set. The general shape of the potential energy surface as predicted in earlier studies, that is, a narrow transition state below the O+O2 asymptote, is confirmed by the present calculations. The transition state is 181 cm-1 below the asymptote and 72 cm-1 above the van der Waals–like minimum. The changes in the O+O2→O3* capture cross section and rate constant when the new potential energy surface is employed are investigated by means of classical trajectory calculations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The rovibrational spectrum assigned to the low-lying 3B2 electronic state of ozone is measured with intracavity laser absorption spectroscopy. The experimental results are interpreted by means of quantum dynamical calculations on a global ab initio potential energy surface. The observed spectrum is shown to originate from the vibrational ground state in the local minimum of the 3B2 potential. The spectrum of short-lived resonance states in this local minimum is analyzed. Additionally, the global minimum of the surface is shown to lie in the dissociation channel in the van der Waals region. This region supports a short sequence of weakly bound vibrational states. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The energy transfer of highly excited ozone molecules is investigated by means of classical trajectories. Both intramolecular energy redistribution and the intermolecular energy transfer in collisions with argon atoms are considered. The sign and magnitude of the intramolecular energy flow between the vibrational and the rotational degrees of freedom crucially depend on the projection Ka of the total angular momentum of ozone on the body-fixed a axis. The intermolecular energy transfer in single collisions between O3 and Ar is dominated by transfer of the rotational energy. In accordance with previous theoretical predictions, the direct vibrational de-excitation is exceedingly small. Vibration–rotation relaxation in multiple Ar+O3 collisions is also studied. It is found that the relaxation proceeds in two clearly distinguishable steps: (1) During the time between collisions, the vibrational degrees of freedom are “cooled” by transfer of energy to rotation; even at low pressure equilibration of the internal energy is slow compared to the time between collisions. (2) In collisions, mainly the rotational modes are “cool” by energy transfer to argon. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present the analysis and the semiclassical quantization of the van der Waals states of ozone in the ground electronic state X 1A1. Progressions of these states dominate the spectrum of O3 at threshold. Periodic orbits are used to perform assignment and quantization of the vibrational states. Semiclassical quantization is numerically accurate despite the fact that the classical phase space is chaotic while the nodal patterns of the quantum mechanical wave functions are regular. The lifetimes of recombination of the van der Waals states into the “normal” ozone are also discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present the analysis and the semiclassical quantization of the van der Waals states of ozone in the ground electronic state X 1A1. Progressions of these states dominate the spectrum of O3 at threshold. Periodic orbits are used to perform assignment and quantization of the vibrational states. Semiclassical quantization is numerically accurate despite the fact that the classical phase space is chaotic while the nodal patterns of the quantum mechanical wave functions are regular. The lifetimes of recombination of the van der Waals states into the “normal” ozone are also discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Huggins band of ozone is investigated by means of exact dynamics calculations using a new (diabatic) potential energy surface for the 1B2 state. The remarkable agreement with the measured spectrum strongly suggests that the Huggins band is due to the two Cs potential wells of the 1B2 state. The vibrational assignment, based on the nodal structure of wave functions, supports the most recent experimental assignment. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effect of spin–orbit coupling on O([sup 3]P)+O[sub 2]([sup 3]Σ[sub g][sup -]) collisions is investigated for J=0 using time-dependent wave packets. The probability of forming O[sub 3] complexes, which is important for understanding the atom exchange reaction mechanism, is calculated in two different ways. The first approach follows the standard treatment in that only the reactive ground electronic state is included. In the second approach all 27 states correlating with O([sup 3]P)+O[sub 2]([sup 3]Σ[sub g][sup -]) and the nonadiabatic transitions induced by spin–orbit coupling are taken into account; all the excited electronic states are repulsive and thus do not lead to complex formation if nonadiabatic transitions are neglected. The required nine diabatic potential energy surfaces (not including spin–orbit coupling) for the electronic states 1 [sup s]A[sup ′], 2 [sup s]A[sup ′], and [sup s]A[sup ″] (s=1, 3, and 5) are constructed by high-level electronic structure calculations in the asymptotic O+O[sub 2] channel with the O[sub 2] bond length being fixed. The two sets of calculations show that spin–orbit coupling is not an important effect. The probability for forming ozone complexes when the oxygen atom is initially in the excited fine structure state O([sup 3]P[sub j=1]) state is only 10% of that for the lowest state O([sup 3]P[sub j=2]), and it is below 1% for O([sup 3]P[sub j=0]). The single-surface calculation, with the excited states phenomenologically taken into account by a statistical factor, gives a rather accurate value for the thermally averaged complex formation rate coefficient. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Threshold spectra of several isotopomers of ozone are studied using accurate quantum mechanical calculations and an ab initio potential energy surface. Shallow van der Waals minima in the dissociation channels, separated from the deep main wells by an 80 cm[sup -1] high barrier, are shown to accommodate long progressions of assignable states. As a result, dense vibrational spectrum of ozone near dissociation is dominated by van der Waals-type states for all studied isotope compositions. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The isotope dependence of the O+O[sub 2] exchange reaction is investigated by means of kinetic experiments and classical trajectory calculations on an accurate potential energy surface. The measurements confirm the previously reported negative temperature dependence and yield the rate coefficients for both the exothermic [sup 18]O+[sup 16]O[sub 2]→[sup 18]O[sup 16]O+[sup 16]O and the endothermic [sup 16]O+[sup 18]O[sub 2]→[sup 16]O[sup 18]O+[sup 18]O reaction between 233 and 353 K: k[sub 8]=(3.4±0.6)×10[sup -12] (300 K/T)[sup 1.1±0.5] cm[sup 3] s[sup -1] and k[sub 6]=(2.7±0.4)×10[sup -12] (300 K/T)[sup 0.9±0.5] cm[sup 3] s[sup -1]. In addition, the ratio of these two rates, R, has been measured with comparatively higher precision. It is 1.27±0.04 at 300 K and also shows a distinct negative temperature dependence. Four types of classical trajectory calculations are performed in order to interpret the experimental result. They differ by the way in which the quantum mechanical zero-point energy of the reactants and the differences of zero-point energies between reactants and products, ΔE[sub ZPE]≈±22 cm[sup -1], are phenomenologically incorporated. Only calculations which account for ΔE[sub ZPE] are successful in reproducing the temperature dependence of R. This emphasizes that ΔE[sub ZPE] is an important factor in low-temperature ozone kinetics. At energies slightly above threshold a dynamical effect is found which additionally favors the exothermic over the endothermic reaction. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We discuss the vibrational spectrum of rotationless cyclic ozone, i.e., ozone in the so-called ring minimum of the ground electronic state. The bound state calculations utilize the ab initio potential energy surface constructed by Siebert et al. on the basis of high-level electronic structure calculations [J. Chem. Phys. 116, 9749 (2002)]. The spectrum is qualitatively similar to that of H[SUP+,SUB3], except that the energy spacing between the states belonging to the same polyad [ν[SUB1], ν[SUB2]] is much smaller; ν[SUB1] is the breathing mode and ν[SUB2] is the doubly degenerate bending mode. [ABSTRACT FROM AUTHOR]

A clear explanation for an anomalous isotope effect in ozone formation is given in terms of the energy transfer mechanism, where the metastable states of ozone are formed first, and then stabilized by collisions with other atoms. Unusual nonstatistical properties of metastable states spectra discovered earlier [J. Chem. Phys. 118, 6298 (2003)] are incorporated into the kinetics model, where different metastable states are treated as different species, and the stabilization step is treated approximately. The population of the ozone metastable states builds up and decays through three possible O[SUB2] + O channels. When different isotopes of oxygen are involved the three channels become open at different energies because of the differences in the quantum zero-point-energies (ΔZPE) of the different O[SUB2] molecules. The spectrum of metastable states is anomalously dense below the DZPE threshold and these states are accessible only from the lower entrance channel. Also, these low-lying metastable states are stabilized very efficiently (by collisions with third body) because they are energetically close to the bound O[SUB3] states. Such processes significantly enhance the formation rates of ozone isotopologues through the lower channels over the formation rates through the upper channels. Numerical results obtained for J = 0 give isotope effects in the right direction and of the right order of magnitude. Consideration of J>0 should improve the comparison with experiment. [ABSTRACT FROM AUTHOR]

Global ab initio potential energy surfaces for the ground (1 [SUP2]A[SUB1]) and the first excited (1 [SUP2]B[SUB2]) electronic states of NO[SUB2] are presented. The calculations are performed at a multireference configuration interaction level with the correlation consistent quadruple zeta atomic basis set of Dunning (cc-pVQZ). A total of about 5000 ab initio points were calculated. The dissociation energy corresponding to NO[SUB2])→ NO + O and other features of the surfaces are found to be in close agreement with experimental values. The adiabatic ground and the first excited electronic states form a conical intersection and have been diabatized to form two coupled diabatic surfaces. Quantum dynamics calculations are performed to obtain the fundamental frequencies and the energies of low lying bound states. Comparison of bound state energies with experimental values show good agreement. [ABSTRACT FROM AUTHOR]

Based on an accurate potential energy surface [J. Chem. Phys. 113, 4598 (2000)] we calculated ca. 700 bound state energies and wave functions of nonrotating HOBr using the filter-diagonalization method. Similar to HOCl, a 1:2 anharmonic resonance between the HOBr bending and the OBr stretching mode determines the general structure of the level spectrum. One of the results of this resonance is a saddle-node bifurcation at which a new class of states (''dissociation states'') comes into existence, which advance along the HO-Br dissociation path. Because the resonance condition at low energies is better fulfilled for HOBr, the bifurcation occurs at considerably lower energies than for HOCl. The results of the quantum mechanical calculations are interpreted in terms of classical periodic orbits (continuation/bifurcation diagram) and a semiclassical analysis based on a spectroscopic Hamiltonian, which is fitted to the exact energy levels and also taking into account the wave functions. [ABSTRACT FROM AUTHOR]

A new potential energy surface for ozone is developed. It is based on high level ab initio data and includes an accurate description of the barrier region. Full quantum reactive scattering calculations using a coupled channel approach and hyperspherical coordinates are performed on this surface for various isotopic compositions of ozone. Collision lifetimes are obtained over a wide energy range, which gives the spectrum of rovibrational metastable states (scattering resonances). This spectrum is discovered to be very nonstatistical. The spectrum of resonances is dense below the isotopic zero-point-energy threshold and sparse above it. This feature is explained by the opening of additional dissociation channels at higher energies. This behavior is a general quantum mechanical effect that should occur in other triatomic molecules. [ABSTRACT FROM AUTHOR]

The exchange reaction [sup 16]O+[sup 18]O[sub 2]→[sup 16]O[sup 18]O+[sup 18]O and, in particular, its dependence on the transition state region is investigated by classical trajectories on three potential energy surfaces, all based on high-level electronic structure calculations. The first one is the original potential recently constructed by Siebert, Schinke, and Bittererová [Phys. Chem. Chem. Phys. 3, 1795 (2001)]; it has a very small barrier above the O + O[sup 2] asymptote. The second potential is a modification of the first one in that the transition state region is adjusted according to new electronic structure calculations on higher levels of theory; it has a small barrier below the asymptote. The third potential is obtained by artificially removing this barrier. The variation of the exchange reaction cross section with collision energy and the magnitude of the thermal rate constant at and below room temperature depend drastically on the shape of the potential at intermediate distances. The second potential, which is believed to represent the transition state structure of the true ground-state potential of ozone best, yields a reaction rate for room temperature that is about a factor of three smaller than the experimental rate. The neglect of nonadiabatic transitions between the several electronic states in the entrance channel may explain this discrepancy. The very slight negative temperature dependence found in the calculations is caused by the strong decrease of the reaction cross section with the initial rotational excitation of the oxygen molecule. Statistical calculations give poor agreement with the classical energy and initial-state dependent cross sections. Nevertheless, the statistical thermal rates are in fair agreement with the classical ones, because the overestimation of the cross sections for low j's and the underestimation for high j's partly compensate. [ABSTRACT FROM AUTHOR]

An effective two-dimensional potential energy surface has been constructed for the S[sub N]2 reaction Cl[sup -]+CH[sub 3]Br→ClCH[sub 3]+Br[sup -] from coupled-cluster calculations with a large basis set. In the quantum dynamics calculations Radau coordinates were employed to describe the Cl-C and C-Br stretching modes. Making use of the filter diagonalization method and an optical potential, bound states as well as resonance states up to energies far above the dissociation threshold have been calculated. The resonance widths fluctuate over several orders of magnitude. In addition to a majority of Feshbach-type resonances there are also exceedingly long-lived shape resonances, which can only decay by tunneling. Owing to a smaller width of the potential barrier and a larger density of states, tunneling through the barrier is more important for Cl[sup -] + CH[sub 3]Br than for Cl[sup -] + CH[sub 3]Cl despite the larger total mass of this system. Excitation of the C-Br stretching vibration enhances the tunneling probability of the entrance channel complex. [ABSTRACT FROM AUTHOR]

The potential energy functions for all states of ozone correlating with the lowest O(³P) +O[sub 2](X³Σ[sup -][sub g]) asymptote have been calculated in the asymptotic region employing correlated electronic wave functions. For linear ozone, the [sup s]Σ states (s = 1, 3, and 5) lie above the corresponding [sup s]Π states. For bent geometries the H states split into Renner-Teller components with A' and A' symmetry, respectively. While the ¹Π and ³Π states lead to bent-bent Renner-Teller pairs, the [sup 5]Π state gives rise to a linear/linear pair of states. The different spin multiplets emerging from the H states cross for valence angles around 160° and the ¹A' component becomes the lowest one. The matrix elements of the spin-orbit operator have also been calculated. They are dominated by the atomic ³p contributions and their dependence on the mutual orientation of the O[sub 2] molecule and the O atom is small. In the regions where the states correlating to the linear [sup 1,3,5]Π states cross, i.e., for valence angles between 150° and 180° and close to 90°, the mixing among the singlet, triplet, and quintet states is strong and the electron spin quantum number is no longer a good quantum number. [ABSTRACT FROM AUTHOR]

We present an ab initio potential energy surface for the ground electronic state of ozone. It is global, i.e., it covers the three identical C[sub 2v] (open) minima, the D[sub 3h] (ring) minimum, as well as the O([sup 3]P) + O[sub 2]([sup 3]σ[sup -, sub g]) dissociation threshold. The electronic structure calculations are performed at the multireference configuration interaction level with complete active space self-consistent-field reference functions and correlation consistent polarized quadruple zeta atomic basis functions. Two of the O-O bond distances, R[sub 1] and R[sub 2], and the O-O-O bending angle are varied on a regular grid (ca. 5000 points with R[sub 1]≥R[sub 2]). An analytical representation is obtained by a three-dimensional cubic spline. The calculated potential energy surface has a tiny dissociation barrier and a shallow van der Waals minimum in the exit channel. The ring minimum is separated from the three open minima by a high potential barrier and therefore presumably does not influence the low-temperature kinetics. The dissociation energy is reproduced up to 90% of the experimental value. All bound states of nonrotating ozone up to more than 99% of the dissociation energy are calculated using the filter diagonalization technique and employing Jacobi coordinates. The three lowest transition energies for [sup 16]O[sub 3] are 1101.9 cm[sup -1] (1103.14 cm[sup -1]), 698.5 cm[sup -1] (700.93 cm[sup -1]), and 1043.9 cm[sup -1] (1042.14 cm[sup -1]) for the symmetric stretch, the bending, and the antisymmetric stretch modes, respectively; the numbers in parentheses are the experimental values. The root-mean-square error for all measured transition energies for [sup 16]O[sub 3] is only 5 cm[sup -1]. The comparison is equally favorable for all other isotopomers, for which experimental frequencies are available. The assignment is made in terms of normal modes, despite the observation that with increasing energy an increasing number of states... [ABSTRACT FROM AUTHOR]

The unimolecular dissociation of HCO in its ground electronic state is considered. By performing calculations (filter diagonalization, absorbing potential) for nonzero angular momentum quantum numbers, it is demonstrated how mixings with near-by ”background“ states affect the linewidths of the narrower resonance states. This explains qualitatively the theoretically as well as experimentally observed behavior of the linewidth as a function of the vibrational quantum numbers. [ABSTRACT FROM AUTHOR]

The predissociation dynamics in the A˜ [sup 1]A[sup ″] electronic state of HNO is investigated. The quantum mechanical dynamics calculations take into account the Renner–Teller (or electronic Coriolis) coupling with the electronic ground state X˜ [sup 1]A[sup ′], which appears to be the dominant decay mechanism for the quasi-bound vibrational states in the A˜ [sup 1]A[sup ″] upper state. All three internal vibrational degrees of freedom are incorporated and two ab initio potential energy surfaces are used. The linewidths, Γ, are directly calculated by the filter diagonalization method and an absorbing optical potential in the exit channel. They are generally small (∼1 cm[sup -1]) and increase with excitation of the bending mode (bent-to-bent transition). On average, Γ increases with K, the a-axis rotational quantum number. However, for some vibrational states the linewidth shows a non-monotonic behavior with K, which is the result of mixing with highly excited vibrational levels in the continuum of the ground electronic state. This effect is even more striking, when the total angular momentum quantum number J is varied: In some cases, the linewidth exhibits a pronounced resonancelike behavior as function of J. The agreement with recent experimental spectroscopic data is satisfactory. The calculated linewidths are of the same order (within a factor of 2 or so) as the experimental ones. However, because the two ab initio potential energy surfaces do not reproduce sufficiently well the X˜–A˜ excitation energies, the resonancelike effects are not quantitatively described. Potential energy surfaces with considerably higher accuracy are required. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the photodissociation of isocyanic acid, HNCO, in the S[sub 1] electronic state. Potential energy surface; Vibrational-rotational state distributions of the diatomic fragments; Effects of the pre-excitation of the HN stretching mode by several quanta.

Presents a variational method for calculating complex resonance states in unimolecular dissociation reactions. Basis of the approach on the log-derivative version of the Kohn variational principle; Formulation of the basic matrix equations in terms of scattering-wave boundary conditions yielding the resonance states as homogenous solutions for singular energies.

A comprehensive study of the photodissociation of HF in ArnHF van der Waals clusters, with n=1-14,54, for an ultrashort δ(t)-pulse excitation, is presented. The emphasis is on the dependence of the photodissociation dynamics of the HF solute molecule on the size and geometry of the Arn solvent cluster. This cluster size range encompasses formation and closing of the first solvation shell, which occurs for n=12, the addition of the complete second solvent layer (n=54), as well as the change of the HF location in the cluster, from a surface site for n≤8 to the interior of a cage for n≥9 clusters. Evolution of the fragmentation dynamics is revealed by following how the H-atom kinetic energy and angular distributions, the survival probability, and cluster fragmentation patterns change as a function of the cluster size and structure. Classical trajectories are used to simulate the photodissociation dynamics. The probability distributions of the initial coordinates and momenta of the H and F atom are defined by accurate quantum five-dimensional eigenstates of the coupled, very anharmonic large amplitude intermolecular vibrations of HF in the cluster. All aspects of the dissociation process studied here are found to exhibit a strong dependence on the size and geometry of the ArnHF clusters. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The emission spectroscopy of H2S excited in the first absorption band around 195 nm is investigated theoretically using ab initio potential energy surfaces (PES) and transition dipole moment functions. As shown in our previous studies, the photodissociation involves two excited electronic states, one which is binding and another one which is dissociative. The nonadiabatic coupling between these two states is very strong and described in a diabatic representation in which only the binding state is optically excited while the dissociative state is dark. As in the case of H2O excited in the 165 nm band, the emission spectrum shows a long progression of stretching states up to seven HS vibrational quanta. In contrast to water, however, some weak activity in the bending mode is also observed. Most remarkable is a prominent wavelength dependence which is attributed to the strong nonadiabatic coupling between the two excited electronic states. The agreement with experimental data is only fair; the essential features of the measurements are qualitatively reproduced, finer details such as the wavelength dependence are, however, not well described. It is concluded that more accurate ab initio input data are required in order to reproduce all details of the measured emission spectra. [ABSTRACT FROM AUTHOR]

We present a novel approach to molecular spectroscopy with light pulses of arbitrary strength and duration. The key quantity is the frequency-resolved net energy transfer dE/dω which reveals at which frequencies energy is transferred from the field to the molecule (absorption) or from the molecule to the field (stimulated emission). It is shown that dE/dω can be expressed as the Fourier transform of the cross-correlation function of the molecular polarization P(t) and the time derivative of the applied field, dE/dt. In this sense, it is formally equivalent to the absorption cross section under weak-field conditions which, as commonly known, can be represented as the Fourier transform of the autocorrelation function S(t). The time-dependent polarization P(t) is determined by exact integration of the time-dependent Schrödinger equation including the light-matter interaction to all orders. It is shown that under weak-field conditions the expression for dE/dω reduces to the well-known cross section formula in the time-dependent picture of spectroscopy, multiplied by the spectral intensity of the light pulse. Therefore, we consider the expression for the frequency-resolved energy transfer, which is valid for arbitrary electric fields, as the natural extension of the absorption cross section in the weak-field limit. Furthermore, dE/dω is shown to be formally equivalent to the change of the spectral intensity, ΔI(ω), of an optical pulse after transmission through a sample, the latter being derived by solving Maxwell’s equations under well-known approximations. The theory is applied to a simple one-dimensional model with two electronic states and the frequency-resolved energy transfer is investigated as a function of the field strength. For sufficiently strong fields, dE/dω exhibits transitions between essentially all vibrational levels in the ground and all states in the excited electronic manifold. The new expression... [ABSTRACT FROM AUTHOR]

We investigate the cage effect in the ultraviolet (UV) photodissociation of the Ar...HCl van der Waals complex, especially the possibility of resonance structures caused by trapping of the hydrogen atom between its heavy partners as recently highlighted by Garcia-Vela and Gerber [J. Chem. Phys. 98, 427 (1993)]. The dynamics is described by solving the time-dependent Schrödinger equation employing the standard Jacobi coordinates which are routinely used for triatomic systems. Due to the large size of the required grid, exact three-dimensional (3D) wave packet calculations are extremely time consuming and could be followed up to 20 fs only. This time is sufficient for calculating the absorption spectrum, but too short for determining the final kinetic energy distributions of the fragment atoms. Therefore, the photodissociation dynamics is mainly treated in a vibrationally sudden approximation, in which the dynamical calculations are performed for a range of fixed ArCl bond distances, and the results averaged over this bond length. 3D classical trajectory calculations show that the energy transfer out of the dissociative HCl mode is very weak (∼5% of the total energy), supporting the application of the sudden approximation. In this approximation, both the absorption spectrum and the kinetic energy distribution associated with the dissociating HCl motion exhibit very weak diffuse structures (resonances) which, following the work of Garcia-Vela and Gerber, can be assigned to the transient vibrational motion of hydrogen between Ar and Cl. However, in our calculations these structures are much less pronounced than in the work of Garcia-Vela and Gerber. The very small amplitudes of the resonance features indicate that trapping in the dissociation of HCl in Ar...HCl is marginal, and much less important than suggested by the previous studies of Garcia-Vela et al. Furthermore, in contrast to the work reported by Garcia-Vela et al., we do not find any evidence... [ABSTRACT FROM AUTHOR]

We present quantum mechanical wave packet calculations for the vibrational predissociation of cis-CH3ONO in the S1 state including three degrees of freedom—the CH3O–NO dissociation bond, the N=O stretching coordinate, and the CH3O–N–O bending angle. We calculate the autocorrelation function, the absorption spectrum, the lifetimes of the excited complex as a function of the internal excitation, and the final vibrational-rotational state distributions of the NO fragment. The lifetimes and the product state distributions are compared with experimental data as well as with previous results obtained from classical trajectory calculations. The calculated vibrational state distributions of the NO product satisfactorily reproduce the systematic variation with the initially prepared quasibound state of the CH3ONO(S1) complex found experimentally; however, they are considerably narrower than the experimental distributions. The theoretical rotational state distributions of NO, all being highly inverted and having the overall shape of a Gaussian, agree well with the experimental data; this is the case for several quasibound vibrational states of CH3ONO(S1) as well as several final vibrational states of the NO product. In general, the classical trajectory calculations parallel the quantum mechanical results. The existing differences have to be attributed to the inability of the purely classical treatment in reproducing subtle quantum effects if the dissociation proceeds through a relatively long-lived complex. While the calculations yield satisfactory agreement with the experimental NO state distributions including the envelope of the absorption spectrum, they disagree with the experiment in that the resonance widths are about one order of magnitude narrower than in the measured spectrum. Additional calculations for which the torsional angle of NO with respect to the intermolecular dissociation vector R is approximately taken into account as a... [ABSTRACT FROM AUTHOR]

We reinvestigate the collinear photodissociation of a symmetric triatomic molecule studied a decade ago by Kulander and Light [J. Chem. Phys. 73, 4437 (1980)], within the time-dependent formalism. The diffuse structures in the absorption spectrum are uniquely related to three, well separated recurrences which, in turn, are explained by three generic periodic orbits of the corresponding classical Hamiltonian. One of these orbits describes pure symmetric stretch motion while the other two orbits combine symmetric stretch and hyperspherical motion. The applicability of simple one-dimensional, zeroth-order pictures is reviewed in the light of these new results. [ABSTRACT FROM AUTHOR]

We investigated the photodissociation of ClNO via the S1 electronic state using a three-dimensional (3D) ab initio potential-energy surface (PES). The dissociation is found to be fast and direct. In the Franck–Condon (FC) region the slope of the potential along the dissociation path is relatively small giving rise to narrow partial absorption peaks. The total absorption spectrum therefore exhibits a broad vibrational structure which is in perfect agreement with recent measurements. The vibrational excitation of the NO fragment is small and can be qualitatively described within the adiabatic approximation. It is found to be very sensitive to the vibrational FC factor in the transition region. The rotational state distribution of NO is highly inverted with a peak around j=30. It is readily explained by the rotational reflection principle. The experimental results are satisfactorily reproduced by our calculations which underlines the overall quality of the calculated PES. Minor adjustments are necessary, however, to quantitatively reproduce the vibrational branching ratio. [ABSTRACT FROM AUTHOR]

We report exact quantum mechanical close-coupling calculations for the photodissociation of Cl–CN using the ab initio potential of Waite and Dunlap. We find that the simple classical calculations of Barts and Halpern based on the rotational reflection principle agree very nicely with the exact rotational state distributions. Furthermore, it is shown that the impulsive model fails severely to account for the high degree of rotational excitation in the photodissociation of Cl–CN. [ABSTRACT FROM AUTHOR]

We use time-dependent quantum theory to interpret the absorption spectrum of CH3ONO in terms of the nuclear motion on the upper potential surface. The model uses one excited potential energy surface and two nuclear coordinates: the NO stretch and the CH3O–NO bond. The latter bond breaks upon excitation leading to dissociation. The spectrum consists of a broad band and two progressions corresponding to predissociation resonances. The band width is inverse proportional to the time scale on which the NO bond length increases to adjust to the longer equilibrium bond length of the upper potential energy surface. The progression of intense narrow resonances corresponds to the NO stretching motion. The other progression is due to oscillations along the reaction coordinate of the wave function temporarily trapped in the predissociation well. Our calculations show that important dynamic information can be obtained by ‘‘smearing off’’ the high resolution spectrum to generate a series of low resolution versions which reveal the time scales on which various spectral features develop in the spectrum. We also show that time-dependent theory can be used efficiently to calculate the lifetime of relatively long lived resonances. [ABSTRACT FROM AUTHOR]

We investigated the photodissociation of methyl nitrite (CH3 ONO) and methyl thionitrite (CH3 SNO) within the first absorption band (S1 ←S0 ). The calculations were based on a two-dimensional model including the O–NO/S–NO and N=O bond distances as active coordinates. The S1 -potential energy surfaces were calculated with quantum chemical methods and the dynamical calculations were performed exactly within the time-independent approach. The main emphasis is on the origin of diffuse vibrational structure in the photoabsorption spectrum of both molecules. A low potential barrier of 0.086 eV along the O–NO dissociation coordinate in CH3 ONO prevents immediate dissociation and leads to an initial state dependent lifetime for the excited complex of 100–250 fs corresponding to 3–8 NO vibrational periods. CH3 ONO decays nonadiabatically via vibrational predissociation. The absorption spectrum of CH3 ONO is dominated by narrow Feshbach-like scattering resonances which can be characterized by two quantum numbers, m and n*: m=0 and 1 specifies the quanta of excitation in the O–NO bond and n*=0,1,2,... specifies the excited vibrational level of the N=O bond. The potential barrier is absent in CH3 SNO and the dissociation is direct on the time scale of about 10 fs corresponding to only one third of a NO vibrational period. Nevertheless, the absorption spectrum exhibits diffuse vibrational structures. The shape of the individual absorption peaks is determined by the classical Franck–Condon reflection principle. The dissociation of CH3 SNO is primarily adiabatic which leads to a pronounced energy dependence of the final NO vibrational state distribution. The diffuse structures originate in both cases from excitation of the NO stretching vibration. In order to make contact with time-dependent theory we calculated the autocorrelation function of the time-dependent wave function by inverse Fourier transformation of the... [ABSTRACT FROM AUTHOR]

We calculated the absorption spectra of H2O and D2O in the second absorption band around 128 nm using a two-dimensional ab initio potential energy surface for the B(1A1) electronic state. Nonadiabatic coupling to the lower states A and X and the vibrational degree of freedom of the OH fragment are completely neglected. Despite these limitations the agreement with the measured spectra is very satisfactory. The overall shape, the width, and the energetical position of the maximum are well described. Most important, however, is the reproduction of the diffuse vibrational structures superimposed on the broad background. It is demonstrated that this structure is not caused by pure bending-excitation in the B state with associated bending quantum numbers ν’2=1,2,3,... as originally assumed. Because the equilibrium HOH bending angle and the equilibrium H–OH distance are very different in the ground and in the excited state, the main part of the spectrum and especially the diffuse structures occur at high energies within the continuum of the B state potential energy surface. Within the time-dependent approach, based on the autocorrelation function and simple classical trajectories, it is shown that the diffuse structures originate from the temporary excitation of a large amplitude bending and stretching oscillation embedded in the continuum (short lived quasiperiodic orbits). The vibrational period of this mode is approximately 40 fs and the lifetime of the trapped trajectories is on the average one vibrational period. [ABSTRACT FROM AUTHOR]

It was recently suggested that vibrational excitation of van der Waals molecules could lead to prereaction instead of predissociation. A mechanism for vibrational prereaction based on tunneling of a light atom is proposed and tested. The effect of van der Waals wells on the reactivity of collinear MuD2, ClHBr, and ClHCl systems is studied. We find that vibrational prereaction is sensitive to the ratio of tunneling and vibrationally nonadiabatic interaction. If tunneling dominates, prereaction will take place. This is the case for the ClHBr and ClHCl systems. The interplay between reaction probability and photodissociation cross sections is studied. We conclude that vibrational prereaction will lead to an increase in reaction probability and an increase in the formation of products in photodissociation. This study suggests that vibrational prereaction could be observed at least in principle in light atom transfer systems. [ABSTRACT FROM AUTHOR]

We investigate rotational state distributions following the direct photodissociation of triatomic model systems. In view of several recent measurements cases of very high rotational excitation are studied. The dissociation cross sections are calculated exactly by the quantal close-coupling method. All distributions are smooth and highly inverted depending sensitively on the anisotropy of the dissociative potential energy surface. They are explained as a mapping of the bound state wave function onto the quantum number axis. This mapping is mediated by the so-called classical excitation function which is determined by running classical trajectories on the potential energy surface within the dissociative state. We call this effect rotational reflection principle. It establishes a rather direct relation between the bound state wave function of the parent molecule, the anisotropy of the dissociative potential, and the final rotational state distribution. The classical calculations agree qualitatively and in most cases even quantitatively very well with the exact quantal results. Differences are most pronounced in the region of a rotational rainbow where the classical cross section is singular. Because of the special choice of initial conditions only very few trajectories are needed to get converged results. [ABSTRACT FROM AUTHOR]

Examines the photodissociation of water in the first absorption band. Impact of the process on the excitation of the first triplet state; Application of quantum mechanical methods in the calculation of potential-energy surface; Need for a comparison between measurement and absorption spectra levels and water photodissociation.

Focuses on the vibrational and rotational state distributions of ground-state oxygen following the photodissociation of ozone in the Chappuis band. Observation of the lowest excited states causing photodissociation of ozone in the Chappuis band; Schematic way to view the excitation and dissociation processes; Description in performing dynamics calculations.

The rotational state distributions of H2 and CO following the photo-fragmentation of formaldehyde at wavelengths of about ∼29 500 cm-1 are analyzed using a simple model. The ingredients of the model are an ab initio potential energy surface in the excit channel and the known geometry of the H2CO→H2+CO transition state. Both the ortho H2 and the CO distributions agree very well with recent measurements. While the CO rotational distribution is directly determined by the excit channel dynamics (i.e., the potential energy surface) the H2 distribution is exclusively determined by the Franck–Condon principle (i.e., the transition state geometry). CO and H2 rotations are almost completely uncorrelated. All results are easily understandable in terms of the calculated potential energy surface. [ABSTRACT FROM AUTHOR]

We investigate the influence of final state interaction on rotational state distributions in the photolysis of (model) triatomic molecules. All calculations are performed within the infinite-order-sudden approximation (IOSA). We find distinct modal structures, which depend systematically on the parameters of the system such as anisotropy, energy, mass, etc. Most of the observations can be directly explained in the semiclassical limit of the IOSA. The key quantities are the classical excitation function and an amplitude, which represents the angular variation of the ground state wave function and the anisotropy of the excited state potential. Assuming hard core potentials analytical expressions for the excitation function can be derived which explain all the trends found in the numerical calculations. The observed structures are believed to exist whenever the dissociation process is impulsive and many rotational states are involved. They are closely related to rotational rainbows in normal scattering. We believe that recently reported experimental distributions for ICN at long laser wavelengths can be qualitatively explained by the structures investigated in this article. [ABSTRACT FROM AUTHOR]

We present a theoretical and experimental investigation of the emission spectrum of dissociating water after excitation in the second absorption band (X 1A1→B 1A1). The calculations are performed in the time-dependent wave packet formalism employing an ab initio potential energy surface. All three degrees of freedom (the two OH stretching modes and the HOH bending mode) are taken into account. The B 1A1 potential energy surface depends strongly on the HOH bending angle which leads to very fast opening of this angle after the water molecule is promoted to the excited electronic state. As a consequence, we observe, both experimentally and theoretically, the excitation of high bending states in the X ground state. According to the wave packet study the emission spectrum is determined in the first ten femtoseconds of the motion in the excited state. The agreement with the measured spectrum for an excitation wavelength of 141.2 nm is good. [ABSTRACT FROM AUTHOR]

The photodissociation of H2S through excitation in the first absorption band (λ≊195 nm) is investigated by means of extensive ab initio calculations. Employing the MRD-CI method we calculate the potential energy surfaces for the lowest two electronic states of 1A‘ symmetry varying both HS bond distances as well as the HSH bending angle. (In the C2v point group these states have electronic symmetry 1B1 and 1A2, respectively.) The lower adiabatic potential energy surface is dissociative when one H atom is pulled away whereas the upper one is binding. For the equilibrium angle of 92° in the electronic ground state they have two conical intersections, one occurring near the Franck–Condon point. Because of the very small energy separation between these two states nonadiabatic coupling induced by the kinetic energy operator in the nuclear degrees of freedom are substantial and must be incorporated in order to describe the absorption and subsequent dissociation process in a realistic way. In the present work we treat the coupling between the two electronic states in a diabatic representation extracting the coordinate-dependent mixing angle from the CI coefficients of the electronic wave functions. The nuclear motion is treated in three dimensions in an exact quantum mechanical approach by propagation of a two-component time-dependent wave packet. The calculated absorption spectra for H2S and D2S satisfactorily agree with the measured spectra. In particular, the calculations reproduce the diffuse structures with energy spacing of about 1200 and 850 cm-1 for H2S and D2S, respectively. Furthermore, the calculated rotational- and vibrational-state distributions of the HS and DS fragments reproduce recent measurements in a convincing way. The photodissociation of H2S is a prototype for very fast electronic predissociation. The photon preferentially excites the binding (diabatic) state. This state, however, is quickly depleted by strong... [ABSTRACT FROM AUTHOR]

An ab initio study of the photodissociation of ClNO following excitation of the T1 state is presented. The corresponding three-dimensional potential-energy surface has been calculated using extensive complete-active-space self-consistent-field (CASSCF) and multireference-averaged coupled pair functional (MR-ACPF) wave functions and large basis sets. The nuclear dynamics has been treated by solving the time-dependent Schrödinger equation in three dimensions. The steepness of the potential with respect to the dissociation coordinate in the Franck–Condon region is found to be very crucial for a realistic description of the dissociation dynamics. It controls directly the lifetime of the ClNO(T1) complex and therefore the widths of the vibrational structures in the absorption spectrum. Due to very large dynamical electron correlation effects, the CASSCF potential is found to be much too steep. Even with extended MR-ACPF wave functions an empirical scaling of the correlation energy is necessary in order to obtain a potential which is sufficiently flat in the transition region to allow for the splitting of each vibrational band into three bending peaks. Only the absorption spectrum calculated with the scaled MR-ACPF potential is in very good agreement with the measured spectrum. The dissociation proceeds adiabatically as far as the vibrational degree of freedom of NO is concerned with the result that excitation of ClNO(T1) within vibrational band n* yields almost exclusively products NO(n*). The degree of rotational excitation of the NO fragment is relatively low, i.e., the distributions peak at low rotational quantum numbers. The calculations reproduce the intriguing relation between the bending state in the ClNO(T1) complex (k*) and the multimodal structures in the final rotational-state distributions, first observed experimentally by Reisler and co-workers [J. Chem. Phys. 89, 6547 (1988)]. [ABSTRACT FROM AUTHOR]