Reinhard Schinke

The vibrational energies of ozone up to the dissociation threshold: Dynamics calculations on an accurate potential energy surface

We present an ab initio potential energy surface for the ground electronic state of ozone. It is global, i.e., it covers the three identical C2v (open) minima, the D3h (ring) minimum, as well as the O(3P)+O2(3Σg−) dissociation threshold. The electronic structure calculations are performed at the multireference configuration interaction level with complete active space self-consistent-field reference functions and correlation consistent polarized quadruple zeta atomic basis functions. Two of the O–O bond distances, R1 and R2, and the O–O–O bending angle are varied on a regular grid (ca. 5000 points with R1⩾R2). An analytical representation is obtained by a three-dimensional cubic spline. The calculated potential energy surface has a tiny dissociation barrier and a shallow van der Waals minimum in the exit channel. The ring minimum is separated from the three open minima by a high potential barrier and therefore presumably does not influence the low-temperature kinetics. The dissociation energy is reproduce...

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...

State to state photodissociation of H2O in the first absorption band

The photodissociation of H2O in the first absorption band is studied from single rotational states of vibrationally excited water. A tunable IR laser is used to prepare single rotational states in the asymmetric stretch mode. The subsequent photodissociation at 193 nm favors product formation from these single prepared states. The formation of the OH product in different rotational, Λ‐doublet, and spin states is analyzed for a series of initial rotational states of H2O. This is the first direct photodissociation studied on a state to state level. The product state distributions depend sensitively upon the prepared state in the parent molecule H2O and exhibit pronounced quantum structure. The experimental results are understood almost quantitatively in terms of theory. The photodissociation of water turns out to be a limiting case of a dissociation which is governed by transfer of parent motion to products. The experiment leads to a highly improved understanding for the selective population of Λ‐doublet st...

The unimolecular dissociation of HCO: I. Oscillations of pure CO stretching resonance widths

The unimolecular dissociation of the formyl radical HCO in the electronic ground state is investigated using a completely new ab initio potential energy surface. The dynamics calculations are performed in the time‐independent picture by employing a variant of the log‐derivative Kohn variational principle. The full resonance spectrum up to energies more than 2 eV above the vibrational ground state is explored. The three fundamental frequencies (in cm−1) for the H–CO and CO stretches, and the bending mode are 2446 (2435), 1844 (1868), and 1081 (1087), where the numbers in parentheses are the measured values of Sappey and Crosley obtained from dispersed fluorescence excitation spectra [J. Chem. Phys. 93, 7601 (1990)]. In the present work we primarily emphasize the dissociation of the pure CO stretching resonances (0v20) and their decay mechanisms. The excitation energies, dissociation rates, and final vibrational–rotational state distributions of CO agree well with recent experimental data obtained from stim...

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.

Resolution of interference effects in the rotational excitation of NO (N = O) by Ar

Pulsed nozzle sources are used to analyze the rotational excitation of NO by Ar. Integral cross sections are determined and compared with the results of a coupled‐state calculation. (AIP)

THE UNIMOLECULAR DISSOCIATION OF HCO. II. COMPARISON OF CALCULATED RESONANCE ENERGIES AND WIDTHS WITH HIGH-RESOLUTION SPECTROSCOPIC DATA

We present a theoretical study of the unimolecular dissociation resonances of HCO in the electronic ground state, X1A′, using a new ab initio potential energy surface and a modification of the log‐derivative version of the Kohn variational principle for the dynamics calculations. Altogether we have analyzed about 120 resonances up to an energy of ≊2 eV above the H+CO threshold, corresponding to the eleventh overtone in the CO stretching mode (v2=11). The agreement of the resonance energies and widths with recent stimulated emission pumping measurements of Tobiason et al. [J. Chem. Phys. 103, 1448 (1995)] is pleasing. The root‐mean‐square deviation from the experimental energies is only 17 cm−1 over a range of about 20 000 cm−1 and all trends of the resonance widths observed in the experiment are satisfactorily reproduced by the calculations. The assignment of the states is discussed in terms of the resonance wave functions. In addition, we compare the quantum mechanical state‐resolved dissociation rates ...

Electronic fine structure transitions and rotational excitation in NO rare gas collisions

The excitation of rotationally cold NO in the ( j=1/2,2Π1/2) state by collisions with different rare gases is studied for multiplet conserving and multiplet changing transitions. In the crossed beam experiment we use jet cooling to prepare the cold NO and LIF to measure the state distribution of the collisionally excited NO in the scattering center. The measured integral state to state cross sections are compared to theory. The calculations are performed in the CS approximation and based on recent advances in treating open shell molecules. For multiplet conserving transitions the agreement between experiment and theory is good. Although for the multiplet changing collisions the general structure is in qualitative agreement, the strength of the Ω=1/2→3/2 transitions is underestimated in the theory.

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...

Metastable states of ozone calculated on an accurate potential energy surface

A new potential energy surface for ozone is developed. It is based on high level ab initio data and includes an accurate description of the barrier region. Full quantum reactive scattering calculations using a coupled channel approach and hyperspherical coordinates are performed on this surface for various isotopic compositions of ozone. Collision lifetimes are obtained over a wide energy range, which gives the spectrum of rovibrational metastable states (scattering resonances). This spectrum is discovered to be very nonstatistical. The spectrum of resonances is dense below the isotopic zero-point-energy threshold and sparse above it. This feature is explained by the opening of additional dissociation channels at higher energies. This behavior is a general quantum mechanical effect that should occur in other triatomic molecules.