V. Engel

Photodissociation dynamics of H2O and D2O in the first absorption band: A complete abinitio treatment

We report a detailed theortical study of the photodissociation of H2O and D2O in the first absorption band (λ∼165 nm). The calculations are three dimensional and purely quantum mechanical. They include an ab initio potential energy surface for the A state and a calculated SCF dipole moment function for the X→A transition. The dynamical calculations are performed within the infinite‐order‐sudden approximation for the rotational degree of freedom of OH and the LHL approximation for the masses. The resulting vibrational–translational motion is then treated exactly in two dimensions using hyperspherical coordinates. This study does not include any adjustable parameters. The thermally averaged total absorption spectra for H2O and D2O agree perfectly with the experimental spectra. Even finer details such as the progression of ‘‘vibrational’’ structures are well reproduced. They are not induced by any selective absorption but can be explained on the basis of the A state potential energy surface and details o...

Isotope effects in the fragmentation of water: The photodissociation of HOD in the first absorption band

We investigate the photofragmentation of HOD in the first absorption band. Full three‐dimensional quantum mechanical calculations on an ab initio excited state potential are presented. They are based on the IOS approximation for the rotational (bending) degree of freedom. The remaining two‐dimensional problem is treated exactly using hyperspherical (polar) coordinates. HOD absorption spectra are compared with recently published results for H2O and D2O. Dissociation of the vibrational ground state favors the production of OD fragments, in agreement with physical intuition. The final vibrational distributions for OH and OD are relatively broad indicating strong final state interaction. They are broader for OD but inverted for OH products. Cross sections for the dissociation of vibrationally excited stretching states are also compared with those for H2O and a strong isotope effect is found. The branching ratio depends significantly on the initial vibrational state of the parent molecule. The quantum mechanic...

Photodissociation dynamics of methylnitrite (CH3O–NO) in the 300–400 nm range: An abinitio quantum mechanical study

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 theore...

Rotational state distributions in the photolysis of water: Influence of the potential anisotropy

We report a detailed investigation of rotationally inelastic effects in the photodissociation of water in the first absorption band using a recently calculated ab initio potential energy surface of the A 1B1 state. Although the excited state potential anisotropy is large it has only very weak influence on the rotational state distributions which thus simply reflect the angular behavior of the ground state wave function of the parent molecule. The reason is that both potential energy surfaces have roughly the same equilibrium angle. Strong inelastic effects are observed, however, for dissociation out of excited bent states because the corresponding ground state wave function extends over a considerably wider angular range and thus the more anisotropic regions of the excited state potential are probed. Calculations are performed on three levels of accuracy for the continuum wave function: close‐coupling, coupled‐states (CS), and infinite‐order‐sudden (IOS) approximation. The ground state wave function is t...

Vibrational state distributions following the photodissociation of (collinear) triatomic molecules: The vibrational reflection principle in model calculations for CF3I

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...

The c- a emission in water - theory and experiment

The C→A emission spectra for H2O and D2O are measured and calculated. The theoretical model is based on an exact treatment of the dissociation dynamics of the A state using a calculated potential energy surface. Agreement with the measurements is excellent. The spectra extend from λ∼380 nm up to λ∼600 nm with maxima around 425 (H2O) and 440 nm (D2O).

An ab initio calculation of the absorption cross section of water in the first absorption continuum

Abstract We present a complete ab initio study of the photodissociation of water in the first absorption band including a calculated excited-state potential and a calculated transition dipole function. The dynamical treatment is based on the IOS approximation for the rotational degree of freedom. The vibrationally coupled equations are solved exactly using polar coordinates. Agreement with the experimental absorption spectrum is excellent and even the “vibrational” structures are reproduced. A dynamical interpretation of these structures in terms of features of the A 1B1 surface is given.

Semiclassical analysis of rotational state distributions in the photolysis of triatomic molecules: Mapping of ground state wave function and potential anisotropy

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 b...

Test of the Wigner method for the photodissociation of symmetric triatomic molecules

The validity of the semiclassical Wigner method for photodissociation of symmetric triatomic molecules is studied. The photodissociation of H2O in the first absorption band using an ab initio potential energy surface and a collinear model for CO2 are considered. In both cases comparison is made with exact quantum mechanical calculations. The overall agreement with the exact results is reasonable, and the general features of the partial and total cross sections are reproduced. However, finer details such as ‘‘vibrational’’ structures superimposed on the broad absorption cross section are poorly resolved. The agreement is generally better in the case of CO2 for which the potential energy surface is more harmonic.

Reactive resonances in the photodissociation of symmetric triatomic molecules: an interpretation of collinear CO2 in terms of polar coordinates

Abstract We discuss the origin of reactive scattering resonances in absorption cross sections following the collinear photodissociation of symmetric triatomic molecules. The analysis of spectra for CO2 dissociation, previously calculated by Kulander and Light, becomes very simple in terms of polar coordinates. Of particular interest is a symmetry effect which is not observable in ordinary scattering calculations.