Transition Dipole Moment Surfaces Research Articles

Conical intersections involving the dissociative1πσ* state in 9H-adenine: a quantum chemical ab initio study

The conical intersections of the dissociative 1pisigma* excited state with the lowest 1pipi* excited state and the electronic ground state of 9H-adenine have been investigated with multireference electronic structure calculations. Adiabatic and quasidiabatic potential energy surfaces and coupling elements were calculated as a function of the NH stretch coordinate of the azine group and the out-of-plane angle of the hydrogen atom, employing MultiReference Configuration-Interaction (MRCI) as well as Complete-Active-Space Self-Consistent-Field (CASSCF) methods. Characteristic properties of the 1pipi*-1pisigma* and 1pisigma*-S0 conical intersections, such as the diabatic-to-adiabatic mixing angle, the geometric phase of the adiabatic electronic wavefunctions, the derivative coupling, as well as adiabatic and diabatic transition dipole moment surfaces were investigated in detail. These data are a prerequisite for future quantum wavepacket simulations of the photodissociation and internal-conversion dynamics of adenine.

Potential energy surface, van der Waals motions, and vibronic transitions in phenol-argon complex

The structure and intermolecular vibrational energy levels of the phenol-Ar complex are calculated from its potential energy surface. This surface is constructed from a large set of the interaction energy values computed using second-order Moller-Plesset perturbation theory with the augmented correlation consistent polarized valence double-zeta basis set. The global minimum in the potential energy surface corresponds to a cluster structure with Ar located over the geometric center of the phenol ring at a distance of 3.510 A and shifted by 0.1355 A towards oxygen. The calculated dissociation energy of 371 cm(-1) is in accordance with the experiment. Additional local minima higher in energy are with Ar placed in the phenol plane. However, they are too shallow to form the bound states corresponding to planar isomers. The deformation of the potential energy surface shape, created by the interaction of Ar with the phenolic oxygen, is responsible for a pronounced intermode mixing. As a result, a set of hybrid stretching-bending states appears which cannot be described in terms of the standard models. The intermode coupling is reflected in the vibronic structure of the S1-S0 electronic transition. The intensities of the vibronic bands are calculated from the electronic transition dipole moment surfaces determined using the ab initio single-excitation configuration interaction method. They allow us to correct and complete the assignment of the spectra observed in phenol-Ar, as well as in the analogous complexes of phenol with Kr and Xe.

Potential-energy surface, van der Waals energy spectrum, and vibronic transitions in s-tetrazine-argon complex

The van der Waals vibrational states and the structure of the vibronic spectrum of s-tetrazine-argon complex have been studied by the ab initio methods. The potential-energy surface of the ground S(0) electronic state of the complex has been constructed by fitting the analytical many-body expansion to a large set of the interaction energy values computed using the second-order Møller-Plesset perturbation theory combined with the standard aug-cc-pVDZ basis set. The equilibrium structure of the complex found is that with argon located above the tetrazine ring at a distance of 3.394 A. The calculated dissociation energy of 354 cm(-1) is compatible with the experiment. The van der Waals energy spectrum calculated from the potential-energy surface is explained analyzing a correlation with a simpler energy spectrum of benzene-argon. A new assignment of the S(0)-S(1) vibronic spectrum is proposed on the basis of the rigorous selection rules, vibrational energy levels in S(0) and S(1) states and vibronic transition intensities calculated from the electronic transition dipole moment surfaces.

Theory of the photodissociation of ozone in the Hartley continuum: Potential energy surfaces, conical intersections, and photodissociation dynamics

Ab initio potential energy and transition dipole moment surfaces are presented for the five lowest singlet even symmetry electronic states of ozone. The surfaces are calculated using the complete active space self consistent field method followed by contracted multireference configuration interaction (MRCI) calculations. A slightly reduced augmented correlation consistent valence triple-zeta orbital basis set is used. The ground and excited state energies of the molecule have been computed at 9282 separate nuclear geometries. Cuts through the potential energy surfaces, which pass through the geometry of the minimum of the ground electronic state, show several closely avoided crossings. Close examination, and higher level calculations, very strongly suggests that some of these seemingly avoided crossings are in fact associated with non-symmetry related conical intersections. Diabatic potential energy and transition dipole moment surfaces are created from the computed ab initio adiabatic MRCI energies and transition dipole moments. The transition dipole moment connecting the ground electronic state to the diabatic B state surface is by far the strongest. Vibrational-rotational wavefunctions and energies are computed using the ground electronic state. The energy level separations compare well with experimentally determined values. The ground vibrational state wavefunction is then used, together with the diabatic B<--X transition dipole moment surface, to form an initial wavepacket. The analysis of the time-dependent quantum dynamics of this wavepacket provides the total and partial photodissociation cross sections for the system. Both the total absorption cross section and the predicted product quantum state distributions compare well with experimental observations. A discussion is also given as to how the observed alternation in product diatom rotational state populations might be explained.

Ab initio potential energy surfaces, total absorption cross sections, and product quantum state distributions for the low-lying electronic states of N2O

Adiabatic potential energy surfaces for the six lowest singlet electronic states of N(2)O (X (1)A('), 2 (1)A('), 3 (1)A('), 1 (1)A("), 2 (1)A(") and 3 (1)A(")) have been computed using an ab initio multireference configuration interaction (MRCI) method and a large orbital basis set (aug-cc-pVQZ). The potential energy surfaces display several symmetry related and some nonsymmetry related conical intersections. Total photodissociation cross sections and product rotational state distributions have been calculated for the first ultraviolet absorption band of the system using the adiabatic ab initio potential energy and transition dipole moment surfaces corresponding to the lowest three excited electronic states. In the Franck-Condon region the potential energy curves corresponding to these three states lie very close in energy and they all contribute to the absorption cross section in the first ultraviolet band. The total angular momentum is treated correctly in both the initial and final states. The total photodissociation spectra and product rotational distributions are determined for N(2)O initially in its ground vibrational state (0,0,0) and in the vibrationally excited (0,1,0) (bending) state. The resulting total absorption spectra are in good quantitative agreement with the experimental results over the region of the first ultraviolet absorption band, from 150 to 220 nm. All of the lowest three electronically excited states [(1)Sigma(-)(1 (1)A(")), (1)Delta(2 (1)A(')), and (1)Delta(2 (1)A("))] have zero transition dipole moments from the ground state [(1)Sigma(+)(1 (1)A('))] in its equilibrium linear configuration. The absorption becomes possible only through the bending motion of the molecule. The (1)Delta(2 (1)A('))<--X (1)Sigma(+)((1)A(')) absorption dominates the absorption cross section with absorption to the other two electronic states contributing to the shape and diffuse structure of the band. It is suggested that absorption to the bound (1)Delta(2 (1)A(")) state makes an important contribution to the experimentally observed diffuse structure in the first ultraviolet absorption band. The predicted product rotational quantum state distribution at 203 nm agrees well with experimental observations.

Theory of the photodissociation of ozone in the Hartley continuum; effect of vibrational excitation and O(1D) atom velocity distribution

The effect of vibrational excitation on the photodissociation cross section of ozone in the Hartley continuum is examined. The calculations make use of newly computed potential energy and transition dipole moment surfaces. The initial vibrational states of the ozone are computed using grid based techniques and the first few ab initio computed vibrational energy level spacings agree to within 10 cm(-1) with experimental values. The computed total absorption cross sections arising from different initial vibrational states of ozone are discussed in the light of the nature of the transition dipole moment surface. The computed cross section for excitation from the ground vibrational-rotational state is in good agreement with the experimentally measured cross section. Excitation of the asymmetric stretching vibration of ozone has a marked effect on both the form and magnitude of the photodissociation cross section. The velocity distributions of highly reactive O(1D) atoms arising from the photodissociation process in different wavelength ranges is also presented. The results show that the O(1D) atoms travel with a most probable translational velocity of 2.030 km s(-1) corresponding to a translational energy of 0.342 eV or 33.0 kJ mol(-1).

Analysis of the Ultraviolet Absorption Cross Sections of Six Isotopically Substituted Nitrous Oxide Species Using 3D Wave Packet Propagation

The ultraviolet absorption cross sections of six isotopically substituted nitrous oxide species ( 1 4 N 1 4 N 1 6 O, 1 4 N 1 4 N 1 7 O, 1 4 N 1 4 N 1 8 O, 1 4 N 1 5 N 1 6 O, 1 5 N 1 4 N 1 6 O, and 1 5 N 1 5 N 1 6 O) were computed using the wave packet propagation technique to explore the influence of excited-state dynamics, transition dipole surface, and initial vibrational state. Three-dimensional potential energy surfaces for the electronic states of N 2 O related to the experimentally observed photoabsorption between 170 and 220 nm were calculated using the ab initio molecular orbital configuration interaction method. The transition dipole moment surfaces between these states were also calculated. Numerous wave packet simulations were carried out and used to calculate the temperature-dependent photodissociation cross sections of the six isotopically substituted species. The photolytic isotopic fractionation constants determined using the calculated cross sections are in good agreement with recent experiments. The results show that, in addition to the effect of the changed shape of the ground-state vibrational wave function with isotopic substitution, photodissociation dynamics play a central role in determining isotopic fractionation constants.

Photodissociation of DOBr—Time-dependent wavepacket calculations

DOBr photoabsorption cross-sections and product rotational state distributions of the OD fragment resulting from excitation to the two lowest-lying excited singlet electronic states, 11A″ and 21A′, are computed using time-dependent wavepacket dynamics. The dynamical calculations are based on two-dimensional ab initio potential energy and transition dipole moment surfaces, in which the OD bond length is held fixed. The computed absorption band for DOBr is very similar to that of HOBr for excitation from the ground vibrational state. For vibrationally mediated photofragmentation spectra in which the initial state is a vibrationally excited state, the absorption line shape for DOBr differs markedly from the corresponding HOBr absorption. For all initial vibrational states, the resulting OD fragments are produced more rotationally “hot” than their OH counterparts. The resulting OD and OH rotational distributions agree qualitatively with experimental measurements at 266 nm, where the excitation is dominated by the parallel 2 1A′←1A′ transition. Predictions are also made for the rotational distributions at 355 nm, where the perpendicular transition 11A″←1A′ is dominant and no experimental product state distributions are as yet available.

Absorption and resonance emission spectra of [formula omitted] calculated from ab initio potential energy and transition dipole moment surfaces

We report here an analytical fit of the transition dipole moments between X ̃ 1 A 1 and C ̃ 1 B 2 electronic states of SO 2 calculated using a high level ab initio method. The absorption spectrum as well as the resonance emission spectra from several low-lying C ̃ 1 B 2 vibrational levels are calculated using a newly developed ab initio potential energy surface (PES) and the transition dipole functions. The calculated spectra are in semi-quantitative agreement with available experimental data. A strong non-Condon effect is found for the emission spectra.

Photodissociation of N2O. I. Ab Initio Potential Energy Surfaces for the Low-Lying Electronic States X̃ 1A‘, 2 1A‘, and 1 1A‘ ‘

Adiabatic potential energy surfaces of the three lowest lying singlet states, X̃ 1A‘ 2 1A‘, and 1 1A‘ ‘, of N2O have been computed as a function of the RN2-O bond distance and the Jacobi angle. The calculations are performed using the complete-active-space self-consistent field (CASSCF) and the multireference configuration interaction (MRCI) electronic structure methods. It is shown that there is a wide avoided crossing between the ground, X̃ 1A‘, and lowest excited, 2 1A‘, electronic state. This avoided crossing is thought to give rise to a seam of conical intersection at other N−N separations. Both excited state surfaces display important conical intersections at linear geometries. The transition dipole moment surfaces for the two excitation processes (2 1A‘ ← X̃ 1A‘ and 1 1A‘ ‘ ← X̃ 1A‘) are also presented. These calculations provide the basic data needed to compute the dynamics of the N2O + hν → N2 + O(1D) photodissociation process for photon frequencies in the range 5.2 eV (240 nm) to 7.3 eV (170 nm).

Photodissociation of HOBr. I. Ab initio potential energy surfaces for the three lowest electronic states and calculation of rotational–vibrational energy levels and wave functions

Potential energy surfaces are presented for the three lowest lying singlet electronic states of HOBr. The surfaces are computed using the recently developed multireference averaged quadratic coupled clusters method and a TZ2P orbital basis set. They provide the basic data needed to compute the dynamics of the HOBr+hν→OH+Br photodissociation process, which plays a key role in the bromine chemistry of the stratosphere. A pseudopotential is used for the core electrons of the Br atom; this is shown not to introduce any errors in the shape of the surfaces through direct comparison with sample all-electron calculations. Transition dipole moment surfaces for the two excitation processes (1 1A″←X̃ 1A′ and 2 1A′←X̃ 1A′) are also presented. These are computed using a multireference singles and doubles configuration interaction method. A grid based method is developed to compute the vibrational–rotational states of the molecule and spectroscopic constants extracted from the computed molecular energy level spacings are compared with experimentally determined quantities.

Absorption cross section of NO2 by the reflection method from ab initio calculations involving the three low lying electronic states

The potential energy surfaces of the three low lying electronic states of NO2, namely the X̃ 2A1, Ã 2B2, and B̃ 2B1 states, and the transition dipole moment surfaces between the ground state and both excited states have been calculated at two levels of ab initio theory; complete active space self-consistent field (CASSCF) and internally contracted multireference configuration interaction (CMRCI). Only 9 points of these surfaces, located around the equilibrium geometry of X̃ 2A1 and corresponding to C2v geometries, have been found sufficient for calculating the cross section, in the 10000–45000 cm−1 energy range, by means of a 2D version of the reflection method. The agreement between the experimental low resolution data and the ab initio absorption cross section is satisfactory, mainly at the CMRCI level, at which the energy at maximum amplitude, the width, the maximum amplitude and the effective transition dipole moment describing both involved electronic transitions are predicted within 4%, 6%, 20%, and 11%, respectively. The sources of errors coming from the reflection approximation and from the level of ab initio approximation are analyzed on the basis of test calculations. The relative importance of the different contributions to the total cross section (both electronic transitions, cold and hot bands) is also discussed. In addition, quantum calculations based on Franck–Condon factors have been performed in order to improve the description of the low energy part of the cross section and to interpret the observed radiative lifetimes.

Ultrafast excitation process in NaNH 3: a combined theoretical and experimental analysis

The conditions for spectroscopic studies of fast decaying excited states with femtosecond laser pulses are examined both experimentally and by quantum dynamical calculations. The sodium-ammonia complex (NaNH 3) acts as a model system which can be treated in quantum chemical calculations and is of chemical relevance for metal-molecular interaction in polar liquids. The state selective dynamics is studied in a two colour pump-probe experiment. The fundamental wavelength of a Ti:sapphire generated femtosecond laser pulse excites the NaNH 3 complex and the second harmonic is used for ionization. The calculations performed on ab initio potential energy and transition dipole moment surfaces show that for laser intensities with a Rabi cycle less than π the pump-probe process is a pure two-photon ionization process. Under these conditions the observed ion signal reflects directly the population of the excited state and the observed wavelength dependence resembles the vibrational spectrum of the NaNH 3 complex.

Theoretical study of the photodissociation cross sections and the photodissociation dynamics of hypochlorous acid

Potential energy surfaces for electronic ground and excited states of HOCl, which are related to the experimentally observed photoabsorption between 400 and 200 nm, were calculated with the ab initio molecular orbital (MO) configuration interaction (CI) method. The transition dipole moment surfaces between these states were also calculated. Two types of model calculations were carried out for evaluation of the theoretical photodissociation cross section. Theoretical spectra exhibit two peaks at about 335 and 245 nm. They are in better agreement with the experimental results than those in the previous study. The present calculation supports the assumption that the HOCl molecule in the stratosphere may contribute to ozone depletion. The rotational state distributions of the product OH({sup 2}II) were explored with classical trajectory calculations. The rotational state distributions are dependent on the excited state. 36 refs., 14 figs., 3 tabs.