Agathe Untch

The vibrational predissociation of cis‐methyl nitrite in the S1 state: A comparison of exact quantum mechanical wave packet calculations with classical trajectory calculations and detailed experimental results

We present quantum mechanical wave packet calculations for the vibrational predissociation of cis‐CH3ONO in the S1 state including three degrees of freedom—the CH3O–NO dissociation bond, the N=O stretching coordinate, and the CH3O–N–O bending angle. We calculate the autocorrelation function, the absorption spectrum, the lifetimes of the excited complex as a function of the internal excitation, and the final vibrational‐rotational state distributions of the NO fragment. The lifetimes and the product state distributions are compared with experimental data as well as with previous results obtained from classical trajectory calculations. The calculated vibrational state distributions of the NO product satisfactorily reproduce the systematic variation with the initially prepared quasibound state of the CH3ONO(S1) complex found experimentally; however, they are considerably narrower than the experimental distributions. The theoretical rotational state distributions of NO, all being highly inverted and having th...

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...

Photodissociation of CH3ONO in the first absorption band: A three‐dimensional classical trajectory study

The photodissociation of c i s‐CH3 ONO following excitation into the first absorption band near 350 nm is investigated by means of classical trajectories and an a b i n i t i opotential 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.

Diffuse vibrational structures in photoabsorption spectra: A comparison of CH3ONO and CH3SNO using two‐dimensional ab initio potential energy surfaces

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...

Theoretical investigation of the photodissociation dynamics of HONO: Vibrational predissociation in the electronically excited state S1

Abstract We report the results of a two-dimensional, quantum mechanical study of the photodissociation of HONO within the first continuum (S 0 →S 1 ). The S 0 and S 1 potential energy surfaces are calculated by means of ab initio methods with variations of the ON and the NO bond lengths while keeping all other coordinates fixed at their ground state values. The dominant band of diffuse structure in the experimental absorption spectrum is unambiguously explained by scattering resonances resulting from a shallow potential well in the S 1 potential energy surface. The fragmentation of the excited complex proceeds via vibrational predissociation in the upper electronic state. Particular attention is paid to the sensitive dependence of the resonance widths as well as to the NO vibrational state distributions on the vibrational-translational coupling strength. In order to elucidate the fragmentation process we performed also classical trajectory calculations. The main trends of the vibrational state distribution in the NO fragment are well reproduced.

The photodissociation of ClNO through excitation in the T1 state: An ab initio study

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 Schrodinger 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 obta...

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...

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.

The vibrational reflection principle in direct photodissociation of triatomic molecules: Test of classical models

Abstract The applicability of ordinary classical mechanics to treating vibrational excitation in direct photodissociation of triatomic molecules is tested. Simple classical calculations for a collinear model system agree very well with exact quantum mechanical calculations for the same potential energy surface. We find that the necessity to include non-zero initial momenta depends very much on the interaction potential in the transition region. The final vibrational state distributions are explained by the vibrational reflection principle which is completely analogous to the rotational reflection principle and to the well-known one-dimensional reflection principle for the energy dependence of the total absorption cross section in direct photodissociation. The vibrational distributions “reflect” in a direct way the intermolecular forces in the exit channel.