The effect of parent internal motion on photofragment rotational distributions: Vector correlation of angular momenta and C2v symmetry breaking in dissociation of AB2 molecules

Photodissociation of the N2-NO Complex between 225.8 and 224.0 nm

Our primary goal was to measure the NO (A) photoproduct appearance energy and ground-state dissociation energy of the N₂-NO complex. We recorded velocity map ion images of NO photofragments resulting from the dissociation of the N₂-NO complex excited between ∼225.8 and 224.0 nm, which ranged from the photodissociation threshold to about 342 cm^-1 above the threshold. In the experiment, one photon dissociated the complex through the N₂ (X ¹Σ_g⁺)-NO (A ²Σ⁺) ← N₂ (X ¹Σ_g⁺)-NO (X ²Π) transition, and a second photon nonresonantly ionized the NO (A) photoproduct. The lowest-energy photons near 225.8 nm did not have sufficient energy to photodissociate the lowest excited state of the complex; however, dissociation was observed with increasing photon energy. On the basis of the experiments, we determined the appearance energy for the NO (A) photoproduct to be 44 284.7 ± 2.8 cm^-1. From the appearance energy and the NO A ← X origin band transition, we determined a ground-state dissociation energy of 85.8 ± 2.8 cm^-1. As we increased the photon energy, the excess energy was partitioned into rotational modes of the diatomic products as well as product translational energy. We found good agreement between the average fraction of rotational energy and the predictions of a simple pseudo three atom impulsive model. Finally, at all photon energies, we observed some contribution from internally excited complexes in the resulting P(E_T). The maximum internal energy of these complexes was consistent with the ground-state dissociation energy.

Photodissociation Dynamics of CO-Forming Channels on the Ground-State Surface of Methyl Formate at 248 nm: Direct Dynamics Study and Assessment of Generalized Multicenter Impulsive Models

The photodissociation dynamics of methyl formate in the electronic ground state S₀, initiated by a 248 nm-wavelength laser, is studied by direct dynamics simulations. We analyze five channels, where four of them have as products CH₃OH + CO, one leading to the formation of three fragments, H₂CO + H₂ + CO, and a channel characterized by a roaming transition state. The analysis of energy distribution among the degrees of freedom of the product and the comparison with experimental results previously published by other groups provide the ingredients to distinguish the examined dissociation pathways. The interpretation of the results proves that the characterization of dissociation mechanisms must rely on a dynamics approach involving multiple electronic states, including considerations on the features of the S₁/S₀ conical intersection. Here, we also assess the generalized multicenter impulsive model, GMCIM, that has been designed for dissociation processes with exit barriers, and the energy distribution in the products is predicted on the basis of information from the saddle points and the intrinsic reaction coordinates. Main features, advantages, limits, and future perspectives of the method are reported and discussed.

Evidence for lambda doublet propensity in the UV photodissociation of ozone

The photodissociation of O₃ at 266 nm has been studied using velocity mapped ion imaging. We report temperature-dependent vector correlations for the O₂(a¹Δ_g, v = 0, j = 18-20) fragments at molecular beam temperatures of 70 K, 115 K, and 170 K. Both the fragment spatial anisotropy and the v-j correlations are found to be increasingly depolarized with increasing beam temperature. At all temperatures, the v-j correlations for the j = 19 state were shown to be reduced compared to those of j = 18 and 20, while no such odd/even rotational state difference was observed for the spatial anisotropy, consistent with previous measurements. We find that temperature-dependent differences in the populations and v-j correlations between the odd and even rotational states can be explained by a Λ-doublet propensity model. Although symmetry conservation should lead to formation of only the A' Λ-doublet component, and only even rotational states, out-of-plane rotation of the parent molecule breaks the planar symmetry and permits the formation of the A″ Λ-doublet component and odd rotational states. A simple classical model to treat the effect of parent rotation on the v-j correlation and the odd/even rotational population alternation reproduces both the current measurements and previously reported rotational distributions, suggesting that the "odd" behavior originates from a Λ-doublet propensity, and not from a mass independent curve crossing effect, as previously proposed.

Nascent O2 (a 1Δg, v = 0, 1) rotational distributions from the photodissociation of jet-cooled O3 in the Hartley band

We report rotational distributions for the O₂ (a ¹Δ_g) fragment from the photodissociation of jet-cooled O₃ at 248, 266, and 282 nm. The rotational distributions show a population alternation that favors the even states, as previously reported for a 300 K sample by Valentini et al. [J. Chem. Phys. 86, 6745 (1987)]. The alternation from the jet-cooled precursor is much stronger than that observed by Valentini et al. and in contrast to their observations does not depend strongly on the O₂ (a ¹Δ_g) vibrational state or photolysis wavelength. The odd/even alternation diminishes substantially when the ozone beam temperature is increased from 60 to 200 K, confirming its dependence on parent internal energy. The magnitude of the even/odd alternation in product rotational states from the cold ozone sample, its temperature dependence, and other experimental and theoretical evidence reported since 1987 suggest that the alternation originates from a Λ-doublet propensity and not from a mass independent curve crossing effect, as previously proposed.

A generalized unimolecular impulsive model for curved reaction path

This work aims to introduce a generalized impulsive model for unimolecular dissociation processes. This model allows us to take into account the curvature of the reaction path explicitly. It is a generalization of the previously developed multi-center impulsive model [P.-Y. Tsai and K.-C. Lin, J. Phys. Chem. A 119, 29 (2015)]. Several limitations of conventional impulsive models are eliminated by this study: (1) Unlike conventional impulsive models, in which a single molecular geometry is responsible for the impulse determination, the gradients on the whole dissociation path are taken into account. The model can treat dissociation pathways with large curvatures and loose saddle points. (2) The method can describe the vibrational excitation of polyatomic fragments due to the bond formation by multi-center impulse. (3) The available energy in conventional impulsive models is separated into uncoupled statistical and impulsive energy reservoirs, while the interplay between these reservoirs is allowed in the new model. (4) The quantum state correlation between fragments can be preserved in analysis. Dissociations of several molecular systems including the roaming pathways of formaldehyde, nitrate radical, acetaldehyde, and glyoxal are chosen as benchmarks. The predicted photofragment energy and vector distributions are consistent with the experimental results reported previously. In these examples, the capability of the new model to treat the curved dissociation path, loose saddle points, polyatomic fragments, and multiple-body dissociation is verified. As a cheaper computational tool with respect to ab initio on-the-fly direct dynamic simulations, this model can provide detailed information on the energy disposal, quantum state correlation, and stereodynamics in unimolecular dissociation processes.

The stability of allyl radicals following the photodissociation of allyl iodide at 193 nm

The photodissociation of allyl iodide (C3H5I) at 193 nm was investigated by using a combination of vacuum-ultraviolet photoionization of the allyl radical, resonant multiphoton ionization of the iodine atoms, and velocity map imaging. The data provide insight into the primary C-I bond fission process and into the dissociative ionization of the allyl radical to produce C3H3+. The experimental results are consistent with the earlier results of Szpunar et al. [J. Chem. Phys. 119, 5078 (2003)], in that some allyl radicals with internal energies higher than the secondary dissociation barrier are found to be stable. This stability results from the partitioning of available energy between the rotational and vibrational degrees of freedom of the radical, the effects of a centrifugal barrier along the reaction coordinate, and the effects of the kinetic shift in the secondary dissociation of the allyl radical. The present results suggest that the primary dissociation of allyl iodide to allyl radicals plus I*(2P(1/2)) is more important than previously suspected.

Internal state distribution of the CF fragment from the 193nm photodissociation of CFCl and CFBr

The dynamics of the 193 nm photodissociation of the CFCl and CFBr molecules have been investigated in a molecular beam experiment. The CFCl and CFBr parent molecules were generated by pyrolysis of CHFCl2 and CFBr3, respectively, and the CFCl and the CF photofragment were detected by laser fluorescence excitation. The 193 nm attenuation cross section of CFCl was determined from the reduction of the CF photofragment signal as a function of the photolysis laser fluence. The internal state distribution was derived from the analysis of laser fluorescence excitation spectra in the A 2Sigma+-X 2Pi band system. A very low degree of rotational excitation, with essentially equal A' and A" Lambda-doublet populations, and no vibrational excitation were found in the CF photofragment. The energy available to the photofragments is hence predominantly released as translational energy. The CF internal state distribution is consistent with the dissociation of a linear intermediate state. Considerations of CFCl electronic states suggest that a bent Rydberg state is initially excited.

Radiative relaxation and fragmentation dynamics of S 2p-excited hydrogen sulfide

Radiative relaxation of S 2p-excited hydrogen sulfide (H(2)S) is investigated by dispersed ultraviolet and visible fluorescence spectroscopies. We observe distinct changes in the fluorescence spectra as a function of excitation energy. Excitation to Rydberg states below the S 2p ionization threshold yields intense fluorescence from neutral and ionic atomic fragments (H, S(+), and S(2+)). In addition to the atomic emission, fluorescence of the molecular fragment ion HS(+) is preferably found after excitation of the S 2p electron into the unoccupied 6a(1) and 3b(2) orbitals with sigma(*) character. This is interpreted as evidence for ultrafast dissociation of the core-excited molecule prior to electronic relaxation. The rotationally resolved fluorescence spectra of the A (3)Pi-->X (3)Sigma(-) transition are analyzed in terms of the fragmentation dynamics leading to the formation of the excited molecular fragment ion, where changes in bond angle are discussed in terms of the rotational population.

Dynamics of the 193nm photodissociation of dichlorocarbene

The dynamics of the 193 nm photodissociation of the CCl2 molecule have been investigated in a molecular beam experiment. The CCl2 parent molecule was generated in a molecular beam by pyrolysis of CHCl3, and both CCl2 and the CCl photofragment were detected by laser fluorescence excitation. The 193 nm attenuation cross section was estimated from the reduction of the CCl2 signal as a function of the photolysis laser fluence. The internal state distribution of the CCl photofragment was derived from analysis of laser fluorescence excitation spectra in the A 2Delta-X 2Pi band system. Most of the energy available to the CCl(X 2Pi)+Cl fragments appears as translational energy. The CCl fragment rotational energy is much less than predicted in an impulsive model. The excited electronic state appears to dissociate indirectly, through coupling with a repulsive state arising from the ground-state CCl(X 2Pi)+Cl asymptote. The identity of the initially excited electronic state is discussed on the basis of what is known about the CCl2 electronic states.

Photodissociation dynamics of dichlorocarbene at 248 nm

The dynamics of the 248 nm photodissociation of the CCl(2) molecule have been investigated in a molecular beam experiment. The CCl(2) parent molecule was generated in a molecular beam by pyrolysis of CHCl(3), and both CCl(2) and the CCl photofragment were detected by laser fluorescence excitation. The 248 nm attenuation cross sections was estimated from the reduction of the CCl(2) signal as a function of the photolysis laser fluence. The internal state distribution of the CCl photofragment was derived from analysis of laser fluorescence excitation spectra in the A (2)Delta- X (2)Pi band system. The CCl(X (2)Pi, nu = 0) rotational state distribution was found to be bimodal, with maximum populations at N approximately 10 and 85, and was dependent upon the source backing pressure, and hence upon the internal state distribution of the CCl(2) precursor. The 248 nm photodissociation dynamics appears to involve two separate channels, namely nearly impulsive rotational energy release and predissociation with little rotational energy imparted to the CCl fragment.

State-resolved dynamics of photofragmentation

We discuss experiments on the dynamics of photodissociation that employ methods to select the energy, sometimes quantum states, of the reactant and to determine the quantum states and energy, sometimes also the orientation and alignment, of products. A summary of new advances of experimental methods is followed by applications to photodissociation of various types. Representative examples of simple bond fission, molecular elimination, and three-body dissociation with determined electronic states-sometimes the orientation of their angular momentum-of product atoms or distributions of electronic and internal states of product molecules illustrate the detailed information and insight that one can derive from such experiments. Photodissociation of van der Waals complexes, ions, species adsorbed on surfaces, and species in solution is excluded from this review.