Hans Ulrich Suter

Photodissociation of ClNO in the S1 state: A quantum‐mechanical ab initio study

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, how...

Mapping of parent transition-state wave functions into product rotations : an experimental and theoretical investigation of the photodissociation of FNO

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 l...

A quantum mechanical, time‐dependent wave packet interpretation of the diffuse structures in the S0→S1 absorption spectrum of FNO: Coexistence of direct and indirect dissociation

We have investigated the photodissociation of FNO in the first absorption band (S0→S1) by a two‐dimensional wave packet study based on an ab initio potential energy surface. The quantum chemical calculations were performed in the multiconfiguration self‐consistent field (MCSCF) approach including the N–O and the F–NO bond distances with the FNO bond angle being fixed. The most striking feature of the time‐dependent dynamical analysis is a bifurcation of the wave packet near the Franck–Condon point: while one part of the wave packet leaves the inner region of the potential energy surface very rapidly, a second part remains trapped for several periods in an extremely shallow well at short F–NO distances. The direct part leads to a broad background in the absorption spectrum while the trapped portion of the wave packet gives rise to relatively narrow resonances, i.e., well resolved diffuse vibrational structures. The bandwidth decreases with the degree of internal excitation. The calculated spectrum agrees w...

Mapping of transition‐state wave functions: I. Rotational state distributions following the decay of long‐lived resonances in the photodissociation of HONO(S1)

We investigate final rotational state distributions following the decay of long‐lived resonance states with k*=0, 1, and 2 quanta of internal bending excitation. The calculations are related to the photodissociation of HONO on the S1 electronic state surface, truncated to two degrees of freedom namely the HO–NO dissociation bond and the ONO bending angle. The decay of the k*=0 resonance yields a smooth Gaussian‐type distribution, in very good agreement with recent measurements. The distributions following the decay of the excited bending states show a bimodal behavior with the main maxima at high rotational states. The final angular momentum distributions reflect the coordinate‐dependence of the dissociation wave function in the region of the transition state, mediated by the dynamics in the exit channel when the wave packet slides down the steep potential slope. A qualitative interpretation of the rotational state distributions is provided by a simple classical model which applies the transition‐state wa...

Photodissociation of CH3ONO by a direct and indirect mechanism

Abstract The S 2 potential-energy surface of CH 3 ONO has been calculated for the coordinates R NO and R ON using the MCSCF ab initio method. To complete the experimental data set of the S 2 photodissociation, we have measured the vibrational- and rotational-state distributions of the NO(X 2 Π) fragments, and discussed the distinctly different product energy distributions from the direct S 2 and the indirect S 1 dissociation on the basis of the potential-energy surfaces.

S1 potential energy surface of HONO: Absorption spectrum and photodissociation

Abstract The molecular geometry of cis- and trans-HONO in the S 0 and S 1 states as well as the S 1 (nπ*)←S 0 transition energy were calculated by ab initio methods (CI, MCSCF). Together with S 1 -potential energy surface calculations in three dimensions ( R ON , R NO , R OH ) the absorption spectrum and the photodissociation of trans-HONO into OH and NO are discussed.

Predissociation via conformational change: photodissociation of N,N-dimethylnitrosamine in the S(1) state.

We investigated the photodissociation mechanism of N,N-dimethylnitrosamine (CH(3))(2)NNO (DMN) by ab intio quantum chemical methods. Inspired by an earlier study we calculated two-dimensional potential energy surfaces of the S(1) state of DMN in its planar and pyramidal conformations. While the planar molecular geometry appears to possess no direct dissociation channel, the pyramidal configuration is dissociative yielding the products NO + (CH(3))(2)N. Using wave packet dynamics on the planar S(1) potential energy surface the experimental absorption spectrum was well reproduced which gives indirect but strong support for the nondissociative nature of this surface. The transition from the planar to the pyramidal conformation of DMN was then investigated by an ab initio molecular dynamics method which revealed the time evolution of the geometrical parameters of the molecule up to the dissociation of the N-N bond. This occurs about 90 fs after photon excitation. The calculated minimum energy path along the N-N coordinate and the structural changes of the molecule along this coordinate provided a detailed picture of this indirect dissociation or, more specific, predissociation process via conformational change.