We review recent theoretical studies of the photodissociation of ozone in the wavelength region from 200 nm to 1100 nm comprising four major absorption bands: Hartley and Huggins (near ultraviolet), Chappuis (visible), and Wulf (near infrared). The quantum mechanical dynamics calculations use global potential energy surfaces obtained from new high-level electronic structure calculations. Altogether nine electronic states are taken into account in the theoretical descriptions: four 1A′, two 1A″, one 3A′ and two 3A″ states. Of particular interest is the analysis of diffuse vibrational structures, which are prominent in all absorption bands, and their dynamical origin and assignment. Another focus is the effect of non-adiabatic coupling on lifetimes in the excited states and on the population of the specific electronic product channels.

New theoretical investigations of the photodissociation of ozone in the Hartley, Huggins, Chappuis, and Wulf bands

Visualising a collision between an atom or a molecule or a photodissociation (half-collision) of a molecule on a single particle and single quantum level is like watching the collision of billiard balls on a pool table: Molecular beams or monoenergetic photodissociation products provide the colliding reactants at controlled velocity before the reaction products velocity is imaged directly with an elaborate camera system, where one should keep in mind that velocity is, in general, a three-dimensional (3D) vectorial property which combines scattering angles and speed. If the processes under study have no cylindrical symmetry, then only this 3D product velocity vector contains the full information of the elementary process under study.

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Imaging chemical reactions – 3D velocity mapping

In the late 1980s, Dixon, Hall, Houston, Simons, and others recognized that experiments that probe the angular distribution of photofragments with spectroscopic techniques could be used to obtain the angular distribution of angular momentum polarization. These were generalized as “vector correlation” experiments, wherein one examines the correlations among the angular momentum polarization vector, the product recoil direction, the transition moment in the parent molecule, the laser polarization, the fluorescence polarization, or in scattering experiments, the relative velocity vector. Virtually all of the early studies were based on Doppler spectroscopy and emphasized rotational angular momentum polarization. Semiclassical models were used to interpret the experiments, largely, based on the bipolar harmonics approach introduced by Dixon. Around the same time, Chandler and Houston introduced the ion-imaging technique, in which the full recoil velocity distribution of stateresolved photofragments could be detected in a highly multiplexed approach. Soon thereafter, large orbital alignment effects in atoms were seen in imaging experiments, and it was immediately recognized that the semiclassical picture, developed to describe high-J rotational polarization, would not be suitable for interpretation of angular momentum polarization in atoms, and a new, fully quantum theory was required. The foundation for this theory was established in a paper in 1994 by Siebbeles et al. (see also the earlier paper by Vasyutinskii). The development of the theory is described fully in section 2.

Imaging atomic orbital polarization in photodissociation.

We present an overview of developments using the nonresonant dynamic Stark effect within the fields of time-resolved molecular dynamics and quantum control, drawing on examples from our own recent ...

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A Stark future for quantum control.

The lowest five A1′ states of ozone, involved in the photodissociation with UV light, are analyzed on the basis of multireference configuration interaction electronic structure calculations with emphasis on the various avoided crossings in different regions of coordinate space. Global diabatic potential energy surfaces are constructed for the lowest four states termed X, A, B, and R. In addition, the off-diagonal potentials that couple the initially excited state B with states R and A are constructed to reflect results from additional electronic structure calculations, including the calculation of nonadiabatic coupling matrix elements. The A/X and A/R couplings are also considered, although in a less ambitious manner. The photodissociation dynamics are studied by means of trajectory surface hopping (TSH) calculations with the branching ratio between the singlet, O(D1)+O2(Δ1g), and triplet, O(P3)+O2(Σ3g−), channels being the main focus. The semiclassical branching ratio agrees well with quantum mechanical ...

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Photodissociation of ozone in the Hartley band: Potential energy surfaces, nonadiabatic couplings, and singlet/triplet branching ratio.

The angular momentum polarization of atomic photofragments provides a detailed insight into the dynamics of the photodissociation process. In this article, the origins of electronic angular momentum polarization are introduced and experimental and theoretical methods for the measurement or calculation of atomic orientation and alignment parameters described. Many diatomic photodissociation systems are surveyed, in order to provide an overview both of the historical development of the field and of the most state-of-the-art contemporary studies.

Atomic polarization in the photodissociation of diatomic molecules

The dissociation of OCS has been investigated subsequent to excitation at 248nm. Speed distributions, speed dependent translational anisotropy parameters, angular momentum alignment, and orientation are reported for the channel leading to S(D21). In agreement with previous experiments, two product speed regimes have been identified, correlating with differing degrees of rotational excitation in the CO coproducts. The velocity dependence of the translational anisotropy is also shown to be in agreement with previous work. However, contrary to previous interpretations, the speed dependence is shown to primarily reflect the effects of nonaxial recoil and to be consistent with predominant excitation to the 2A′1 electronic state. It is proposed that the associated electronic transition moment is polarized in the molecular plane, at an angle greater than ∼60° to the initial linear OCS axis. The atomic angular momentum polarization data are interpreted in terms of a simple long-range interaction model to help ide...

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Photodissociation dynamics of OCS at 248nm: The S(D21) atomic angular momentum polarization

The dissociation of OCS has been investigated subsequent to excitation at 248nm using velocity map ion imaging. Speed distributions, speed dependent translational anisotropy parameters, and the atomic angular momentum orientation and alignment are reported for the channel leading to S(PJ3). The speed distributions and β parameters are in broad agreement with previous work and show behavior that is highly sensitive to the S-atom spin-orbit state. The data are shown to be consistent with the operation of at least two triplet production mechanisms. Interpretation of the angular momentum polarization data in terms of an adiabatic picture has been used to help identify a likely dissociation pathway for the majority of the S(PJ3) products, which strongly favors production of J=2 fragment atoms, correlated, it is proposed, with rotationally hot and vibrationally cold CO cofragments. For these fragments, optical excitation to the 2A′1 surface is thought to constitute the first step, as for the singlet dissociatio...

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Photodissociation dynamics of OCS at 248 nm: the S((1)D(2)) atomic angular momentum polarization.

The depolarization of the rotational angular momentum of electronically excited OH(2Σ) radicals through collisions with water molecules has been measured using Zeeman quantum beat spectroscopy. The new data have permitted the evaluation of OH(A) state-specific quenching and angular momentum depolarization cross-sections for superthermal OH(A) radicals with mean relative velocities centred around 3500 m s-1. The quenching cross-sections are compared both with values available in the literature, and with predictions based on a simple harpoon model, and are found to be in good qualitative accord with previous findings. For the lowest rotational levels studied, the depolarization cross-sections (which include contributions from both elastic and inelastic processes in OH(A)) are found to approach ∼100 A2, only slightly below the high-temperature cross-sections for rotational energy transfer determined elsewhere. The data suggest that under the present conditions rotational energy transfer is accompanied by sig...

Collisional depolarization of OH(A) studied by Zeeman quantum beat spectroscopy

Polarized laser photolysis, coupled with resonantly enhanced multiphoton ionization detection of O(D21) and velocity-map ion imaging, has been used to investigate the photodissociation dynamics of ozone at 193nm. The use of multiple pump and probe laser polarization geometries and probe transitions has enabled a comprehensive characterization of the angular momentum polarization of the O(D21) photofragments, in addition to providing high-resolution information about their speed and angular distributions. Images obtained at the probe laser wavelength of around 205nm indicate dissociation primarily via the Hartley band, involving absorption to, and diabatic dissociation on, the BB21(3A11) potential energy surface. Rather different O(D21) speed and electronic angular momentum spatial distributions are observed at 193nm, suggesting that the dominant excitation at these photon energies is to a state of different symmetry from that giving rise to the Hartley band and also indicating the participation of at lea...

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F. Quadrini

Papers

Atomic polarization in the photodissociation of diatomic molecules

Photodissociation dynamics of OCS at 248nm: The S(D21) atomic angular momentum polarization

Photodissociation dynamics of OCS at 248 nm: the S((1)D(2)) atomic angular momentum polarization.

Collisional depolarization of OH(A) studied by Zeeman quantum beat spectroscopy

The photodissociation dynamics of ozone at 193 nm: an O(1D2) angular momentum polarization study.