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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]

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.

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.

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]

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 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 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]

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]

We present three-dimensional wave packet calculations for the photodissociation of FNO in the first excited singlet state S1 using a new ab initio potential surface. While the calculated absorption spectrum agrees satisfactorily with the measured spectrum, the energy dependence of the partial cross sections for particular NO product states is only in fair agreement with experiment. The same is true for the vibrational and rotational state distributions of NO for selected energies. Because of the interference between direct and indirect dissociation, details of the cross sections are highly sensitive to subtleties of the potential surface. Altogether, the new calculation reproduces the available experimental data more satisfactorily than a previous one. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present quantitative results on photodissociation of CH2 (X 3B1) and its isotopomers CHD and CD2 through the first excited triplet state (1 3A1). A two-dimensional wave packet method employing the light–heavy–light approximation was used to perform the dynamics. The potential energy surfaces and the transition dipole moment function used were all taken from ab initio calculations. The peak positions in the calculated CH2 and CD2 spectra nearly coincide with the positions of unassigned peaks in experimental CH2 and CD2 3+1 resonance enhanced multiphoton ionization spectra, provided that the experimental peaks are interpreted as two-photon transitions. Comparing the photodissociation of CH2 and its isotopomers to photodissociation of water in the first absorption band, we find these processes to be very similar in all aspects discussed in this work. These aspects include the origin of the diffuse structure and the overall shape of the total absorption spectra of vibrationless and vibrationally excited CH2 , trends seen in the fragment vibrational level distribution of the different isotopomers, and selectivity of photodissociation of both vibrationless and vibrationally excited CHD. In particular, we find that the CD/CH branching ratio exceeds two for all wavelengths in photodissociation of vibrationless CHD. [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]

We study experimentally and theoretically reflection-type structures in the rotational distributions of NO following the photodissociation of FNO via excitation of the S1 state. Exciting quasibound states with zero quanta of bending vibration in the FNO(S1) state yields Gaussian-type rotational distributions, while excitation of states with one bending quantum leads to bimodal distributions. In the latter case, the ratio of the two intensity maxima depends on the number of NO stretching quanta in the S1 state. The accompanying calculations employing a three-dimensional ab initio potential energy surface for the S1 state of FNO are performed in the time-dependent wave packet approach. They reproduce the main features of the experimental distributions, especially the bimodality. The analysis of two-dimensional calculations for a frozen NO bond distance shows that the final rotational state distributions can be explained as the result of a dynamical mapping of the stationary wave function on the transition line onto the fragment rotational quantum number axis. Here the transition line is defined as the line which separates the inner part of the FNO(S1) potential energy surface from the strongly repulsive F+NO product channel. [ABSTRACT FROM AUTHOR]

Reflection-type structures are investigated in fragment rotational state distributions following the decay of long-lived resonances with k*=0–2 quanta of bending vibration of the intermediate complex. The calculations are performed in relation to the photodissociation of ClNO(T1). In the absence of an ab initio potential energy surface we construct a model potential energy surface which reproduces some of the main features of this system in qualitative agreement with experimental data. In particular, excitation of the lowest bending state of the ClNO(T1) complex yields a Gaussian rotational distribution while excitation of the first two excited bending states gives rise to bi- and trinodal distributions, respectively. Employing a simple classical model demonstrates that the structures of the final distributions can be explained as a mapping of the corresponding wave function along the transition line which separates the complex region from the exit channel. Within this model the final fragmentation step will be treated as direct dissociation which starts at the transition state with the quantum mechanical coordinate distribution serving for the weighting function. In order to illustrate the sensitivity of the rotational NO distributions we carry out calculations for three potentials with different anisotropy in the exit channel. [ABSTRACT FROM AUTHOR]

We present the results of a three-dimensional wave packet study on the photodissociation of ClNO through excitation of the first singlet state S1. The calculations employ an ab initio potential energy surface depending on the Cl–N and N–O bond coordinates and the ClNO bending angle. By expanding the wave packet in terms of the eigenfunctions of the NO rotor, the time-dependent Schrödinger equation is transformed into a coupled set of 60 two-dimensional partial differential equations which are solved by discretization on a grid. The wave packet yields the absorption spectrum and all partial dissociation cross sections for producing the NO fragment in a particular vibrational–rotational state (nj). The photodissociation of ClNO via the S1 state is a relatively fast process and the necessary propagation time is on the order of 50 fs. The calculated data agree well with recent experimental results. For the first time, we can directly compare the wavelength dependence of partial photodissociation cross sections for a single vibrational–rotational fragment state state with experiment. Our results suggest that the photodissociation of ClNO(S1) proceeds mainly adiabatically for the vibrational degree of freedom. [ABSTRACT FROM AUTHOR]

We investigate the photodissociation of highly excited vibrational states of water in the first absorption band. The calculation includes an ab initio potential energy surface for the A-state and an ab initio X→A transition dipole function. The bending angle is fixed at the equilibrium value within the ground electronic state. Most interesting is the high sensitivity of the final vibrational distribution of OH on the initially prepared vibrational state of H2 O. At wavelengths near the onset of the absorption spectrum the vibrational state distribution can be qualitatively understood as a Franck–Condon mapping of the initial H2 O wave function. At smaller wavelengths final state interaction in the excited state becomes stronger and the distributions become successively broader. Our calculations are in satisfactory accord with recent measurements of Vander Wal and Crim. [ABSTRACT FROM AUTHOR]

The photodissociation of H2O in the second absorption band (X→B) is investigated in a completely time-dependent approach. The Schrödinger equation is solved by a time-dependent close-coupling method expanding the two-dimensional wave packet in terms of free rotor states. The vibrational degree of freedom of the OH fragment is fixed and only motion on the B-state potential-energy surface is considered. The calculated absorption spectrum exhibits a long progression of diffuse structures, ΔE∼0.1 eV, in very good agreement with the experimental spectrum. The structure is readily explained in terms of a recurrence of the autocorrelation function after about 40 fs. The recurrence, in turn, is attributed to special indirect trajectories which on the average perform one oscillation within the deep potential well before they dissociate into products H+OH. These trajections are ‘‘guided’’ by so-called unstable periodic orbits which persist to energies high above the H+OH(2 Σ) threshold. The existence of unstable periodic orbits leading to a recurrence of the autocorrelation function gives, for the first time, a consistent explanation of the diffuse structure in the absorption spectrum of H2 O in the second band. [ABSTRACT FROM AUTHOR]

The photodissociation of cis-CH3 ONO following excitation into the first absorption band near 350 nm is investigated by means of classical trajectories and an ab initio potential energy surface. The calculations include the O–N coordinate, the N=O coordinate, and the ONO bending angle as variables whilst the internal degrees of freedom of the CH3 O moiety are kept fixed. The calculated lifetimes range from 120 to 410 femtoseconds for excitation of the n*=4 to n*=0 vibrational states of the terminal NO group in the intermediate complex. They agree well with the lifetimes estimated from the anisotropy parameter β. The ONO bending degree of freedom has only a small effect on the lifetime of the complex. The final vibrational state (n) distribution of the NO fragment exhibits a systematic energy dependence which manifests itself in a propensity for the excitation of level n=n*-1 that is in excellent agreement with the measurement. Two-dimensional calculations for a fixed ONO bending angle cannot satisfactorily reproduce these experimental findings. The rotational state distributions are highly inverted with maxima around j∼30–35 depending slightly on the initial state (n*) and the final state (n) of NO. The overall agreement with the measured distributions is satisfactory. The results of this study emphasize the importance of the bending degree of freedom in the dissociation of CH3 ONO and by revealing the interplay of the three active vibrational modes they provide a detailed picture of the predissociation mechanism in a polyatomic molecule. [ABSTRACT FROM AUTHOR]

We report the results of a two-dimensional, quantal study of the photodissociation of CH3O–NO within the first continuum (S0→S1, 300–400 nm) taking into account only the O–N and the N=O separations. The S1 potential energy surface is taken from recent ab initio calculations. The calculated absorption spectrum consists of two band progressions of narrow resonance lines with widths of ∼0.3 and ∼5 meV, respectively. These resonances can be associated with excitation of the O–N bond (m=0,1) and excitation of the N=O chromophore (n*=0,1,2,...). The intensities of the m=1 band are negligibly small compared to those of the m=0 band. The decay mechanism in the two cases is different: The m=0 resonances decay primarily via vibrational predissociation, i.e., a nonadiabatic transition from n* to n*-1, and yield NO products with a preferential population of the (n*-1) level. The m=1 resonances decay mainly via tunneling through a potential barrier yielding preferentially NO products in state n*. Several of the theoretical results agree qualitatively (ratio of peak intensities) or even quantitatively (energy spacing between peaks) with the measurements. Most important, however, is the good agreement found for the vibrational NO distributions at several excitation wavelengths of the parent, which reveals that vibrational predissociation within the S1 state is the main decay mechanism. [ABSTRACT FROM AUTHOR]

Vibrational state distributions following the direct photodissociation of a collinear, triatomic molecule is investigated with particular emphasis on the so-called final state interaction, i.e., the translational–vibrational coupling due to the excited state interaction potential. In order to separate the various effects which determine the state distribution we performed calculations on three levels of accuracy: The energy sudden (ES) approximation, the modified sudden (MS) approximation, and the exact close-coupling (CC) formulation. The pure ES distributions peak at high states and are very broad. They are explained within the semiclassical limit as a mapping of an amplitude onto the quantum number axis. We call this effect vibrational reflection principle in analogy to the equivalent effect in rotational excitation processes. It is a direct and sensitive probe of the parameters of the system, most importantly the potential energy surface. Energy conservation strongly modifies the ES distributions. The MS and CC distributions are much narrower and peak at considerably lower states. A detailed analysis is given within the MS approximation. Based on these general conclusions we suggest a particular excited state potential for the dissociation of CF3I which qualitatively reproduces the recently reported experimental CF3 distribution. The essential feature of this potential is a distance dependent local frequency which we find necessary to obtain distributions as broad as in the experiment. Because of the inherent difficulties with the time-of-flight technique if several energy transfer channels are involved we certainly do not know how realistic our final potential energy surface is. However, the general trends found in this study should be valid for a larger class of systems. [ABSTRACT FROM AUTHOR]

Studies the photodissociation of ozone in the Chappuis band. Time-dependent wave-packet calculations; Interpretation of diffuse vibrational structures; Symmetric stretch excitation.

Studies electronic structure calculations of the photodissociation of ozone in the Chappuis band. Potential energy surfaces and corresponding transition-dipole-moment surfaces; Function of the two bond distances and the bond angle; Electronic-structure models.

We present a first principles study of the carbon dioxide (CO2) photodissociation process in the 150- to 210-nm wavelength range, with emphasis on photolysis below the carbon monoxide + O(1D) singlet channel threshold at ~167 nm. The calculations reproduce experimental absorption cross-sections at a resolution of ~0.5 nm without scaling the intensity. The observed structure in the 150- to 210-nm range is caused by excitation of bending motion supported by the deep wells at bent geometries in the 21A' and 11A“ potential energy surfaces. Predissociation below the singlet channel threshold occurs via spin-orbit coupling to nearby repulsive triplet states. Carbon monoxide vibrational and rotational state distributions in the singlet channel as well as the triplet channel for excitation at 157 nm satisfactorily reproduce experimental data. The cross-sections of individual CO2 isotopologues (12C16O2, 12C17O16O, 12C18O16O, 13C16O2, and 13C18O16O) are calculated, demonstrating that strong isotopic fractionation will occur as a function of wavelength. The calculations provide accurate, detailed insight into CO2 photoabsorption and dissociation dynamics, and greatly extend knowledge of the temperature dependence of the cross-section to cover the range from 0 to 400 K that is useful for calculations of propagation of stellar light in planetary atmospheres. The model is also relevant for the interpretation of laboratory experiments on mass-independent isotopic fractionation. Finally, the model shows that the mass-independent fractionation observed in a series of Hg lamp experiments is not a result of hyperfine interactions making predissociation of 17O containing CO2 more efficient. [ABSTRACT FROM AUTHOR]

We present new calculations of the branching ratios betweenthevarious electronic and isotopic photodissociation channels of ozone.Special emphasis is placed on the isotopic/isotopologue differencesbecause the contribution of the ozone photodissociation to the oxygenisotope and ozone isotopologue enrichments or fractionations is importantfor atmospheric applications. These branching ratios, which dependon photon energy, have been calculated with a full quantum mechanicalwavepacket propagation approach: the multiconfiguration time-dependentHartree (MCTDH) method. Five ozone isotopologues are considered: threesymmetric, 16O3(noted 666), 16O17O16O (676), and 16O18O16O (686); two asymmetric, 16O217O (noted 667) and 16O218O (668).The 668 and 667 asymmetric isotopologues can dissociate into either66 + 8 or 68 + 6 for 668 and into 66 + 7 or 67 + 6 for 667. In theranges of the Chappuis and Hartley bands, the dissociation is veryfast and electronic and isotopic branching ratios are obtained fromthe wavepacket fluxes through complex absorbing potentials (CAPs)located perpendicular to the dissociation channels of the potentialenergy surfaces (PESs) of the A 1B1(Chappuis)and B 31A′ (Hartley/Huggins) electronic states.In the range of the Huggins band the dissociation is much slower andthe isotopic branching ratios of 667 and 668 asymmetric isotopologues,(e.g; 668 → 66 + 8 or 86 + 6) are obtained from the ratiosof two partial absorption cross sections corresponding to the selectiveexcitation of one or the other of the two isomers of Cssymmetry, which dissociate respectively into 66+ 8 and 86 + 6. We find that the photodissociation of the 668 asymmetricisotopologue favors the 68 + 6 channel with a propensity varying between52% (Hartley) and 54% (Huggins) as a function of the photon energy.The electronic branching ratios to the singlet channel (O3+ hυ → O(1D) + O2(1Δ)) are all close to 90% above ≈32 000cm–1. Below this energy, the singlet channel isenergetically closed and only the triplet channel (O3+ hυ → O(3P) + O2(3Σ)) is open. These branching ratios are required tocalculate the photolysis rates of each ozone isotopologue, which inturn contribute to the atomic oxygen and the ozone isotopic enrichmentsin the atmosphere. [ABSTRACT FROM AUTHOR]

We investigate the contributions of the lowest two 1A″ states in the UV photodissociation ofN2O employing three-dimensional potential energy surfacesand transition dipole moment functions. Because the transition dipolemoments are much smaller than for the 2 1A′ state,we conclude that excitation of the 1A″ states hasa marginal effect. The dense vibrational spectrum of the quasi-bound21A″ state possibly explains some of the tiny, noise-likestructures of the measured absorption spectrum. [ABSTRACT FROM AUTHOR]

We present new calculations on the Renner-Teller induced decay of the vibrational states of HCO(A˜ [sup 2]A[sup ″]) using accurate ab initio potential energy surfaces. The dynamics calculations are performed by employing filter diagonalization and an absorbing optical potential in the exit channel. The objective of this investigation is twofold: the completion of earlier time-dependent wave packet calculations by determining resonance widths for all vibrational states for projection quantum number K=1--up to 2.75 eV above the H+CO(r[sub e]) dissociation threshold--and the determination of the widths for the long-lived K=0 states. In the latter case, a clear-cut J[sup 2](J+1)[sup 2] dependence, where J is the total angular momentum, is observed indicating that the rate determining step is K-resonance interaction between K=0 and 2 states. The experimentally observed J-independent contribution (0.22-0.5 cm-1), which dominates the linewidth for small values of J, is not accounted for by our calculations. Arguments are put forward, that it is caused by spin-orbit interaction, which is not included in our treatment. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have calculated one-dimensional potential cuts of twenty-five singlet/triplet A′/A″ states of ozone. The calculations are performed at the multi-reference configuration interaction level of electronic structure theory with the aug-cc-pVTZ set of atomic basis functions. It is found that many triplet potentials cross the 31A′(1B2) potential, on which the fragmentation after excitation in the Huggins–Hartley band system primarily proceeds. These crossings allow the population of the three spin-forbidden product channels O(3P)+O2(a1Δg), O(3P)+O2(b1Σg+) and O(1D)+O2(X3Σg−). Possible consequences of singlet–triplet crossings for the Wulf band are also briefly discussed. [Copyright &y& Elsevier]