K. Weide

An experimental and theoretical study of the bond selected photodissociation of HOD

Experimental and theoretical studies of the photodissociation of single vibrational states in HOD provide a qualitative and quantitative understanding of the dissociation dynamics and bond selectivity of this process. Vibrationally mediated photodissociation, in which one photon prepares a vibrational state that a second photon dissociates, can selectively cleave the O–H bond in HOD molecules containing four quanta of O–H stretching excitation. Dissociation of HOD(4νOH) with 266 or 239.5‐nm photons produces OD fragments in at least a 15 fold excess over OH, but photolysis of the same state with 218.5‐nm photons produces comparable amounts of OH and OD. Wave packet propagation calculations on an ab initio potential energy surface reproduce these observations quantitatively. They show that the origin of the selectivity and its energy dependence is the communication of the initial vibrational state with different portions of the outgoing continuum wave function for different photolysis energies.Experimental and theoretical studies of the photodissociation of single vibrational states in HOD provide a qualitative and quantitative understanding of the dissociation dynamics and bond selectivity of this process. Vibrationally mediated photodissociation, in which one photon prepares a vibrational state that a second photon dissociates, can selectively cleave the O–H bond in HOD molecules containing four quanta of O–H stretching excitation. Dissociation of HOD(4νOH) with 266 or 239.5‐nm photons produces OD fragments in at least a 15 fold excess over OH, but photolysis of the same state with 218.5‐nm photons produces comparable amounts of OH and OD. Wave packet propagation calculations on an ab initio potential energy surface reproduce these observations quantitatively. They show that the origin of the selectivity and its energy dependence is the communication of the initial vibrational state with different portions of the outgoing continuum wave function for different photolysis energies.

Nonadiabatic effects in the photodissociation of H2S in the first absorption band: An ab initio study

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 ...

Photodissociation dynamics of water in the second absorption band. I. Rotational state distributions of OH(2Σ) and OH(2Π)

The photodissociation of H2O and D2O in the second band (λ≳120 nm) is investigated using two‐dimensional (translation and rotation) classical trajectories. The calculations include all electronic states which are involved in the dissociation dynamics, i.e., B 1A1, X  1A1, and A 1B1. The nonadiabatic transitions B→X and B→A near linearity are modeled in a very simple way, which does not yield the OH(2Σ)/OH(2Π) branching ratio. The rotational distributions for OH(2Σ) and OD(2Σ) agree qualitatively very well with the measurements. They are highly inverted and peak close to the highest accessible state. Comparing the OH(2Π) rotational distributions with recent experimental results we conclude that B→X is probably the main dissociation pathway, although contributions from a B→A transition cannot be excluded. The OH(2Π) distribution is also highly inverted with a peak near j∼43 in excellent agreement with experiment. The majority of trajectories on all three potential energy surfaces is direct. The s...

Photodissociation dynamics of water in the second absorption band. II: Ab initio calculation of the absorption spectra for H2O and D2O and dynamical interpretation of diffuse vibrational structures

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 ene...

The direct photodissociation of ClNO(S1): An exact three‐dimensional wave packet analysis

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 Schrodinger 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 section...

Unstable periodic orbits, recurrences, and diffuse vibrational structures in the photodissociation of water near 128 nm

The photodissociation of H2O in the second absorption band (X→B) is investigated in a completely time‐dependent approach. The Schrodinger 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...

Nonadiabatic effects in the photodissociation of H2S

The photodissociation of H2 S in the first absorption band is studied by time‐dependent wave packets evolving in two electronic states; the lower state is dissociative and the upper one is bound. The adiabatic potential energy surfaces and transition dipole functions are constructed from ab initio calculations while the nonadiabatic coupling is adjusted. The diffuse structure superimposed on the broad absorption spectrum is due to symmetric stretch motion in the upper (bound) electronic state which is strongly quenched by nonadiabatic coupling. This is different from the photodissociation of water in the first band.

Photodissociation of vibrationally excited water in the first absorption band

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.

3D wavepacket study of the photodissociation of CH3ONO(S1)

Abstract We report the first results of a three-dimensional wavepacket study for the photodissociation of methyl nitrite via the S 1 state. The calculation includes the ON and the NO bond distances as well as the ONO bending angle and uses a previously published ab initio potential energy surface. We present the calculated absorption spectrum and the final vibrational- and rotational-state distributions of the NO fragment for one particular photon wavelength. They agree well with the corresponding experimental data.

Photodissociation dynamics of water in the second absorption band: vibrational excitation of OH(A2Σ)

A three-dimensional potential energy surface for the second excited state of water, B1A1, is calculated by the MRD-CI method. It is employed in classical trajectory calculations to study the photodissociation in the second absorption band. We find, in accordance with experimental data, vibrational excitation of OH(2Σ) to be generally weak. This result can be readily explained in terms of the calculated potential surface.