Ozone visible photodissociation dynamics

Ultrafast dissociation of ammonia: Auger Doppler effect and redistribution of the internal energy

We study vibrationally-resolved resonant Auger (RAS) spectra of ammonia recorded in coincidence with the NH₂⁺ fragment, which is produced in the course of dissociation either in the core-excited 1s^-14a11 intermediate state or the first spectator 3a^-24a11 final state. Correlation of the NH₂⁺ ion flight times with electron kinetic energies allows directly observing the Auger-Doppler dispersion for each vibrational state of the fragment. The median distribution of the kinetic energy release E_KER, derived from the coincidence data, shows three distinct branches as a function of Auger electron kinetic energy E_e: E_e + 1.75E_KER = const for the molecular band; E_KER = const for the fragment band; and E_e + E_KER = const for the region preceding the fragment band. The deviation of the molecular band dispersion from E_e + E_KER = const is attributed to the redistribution of the available energy to the dissociation energy and excitation of the internal degrees of freedom in the molecular fragment. We found that for each vibrational line the dispersive behavior of E_KERvs. E_e is very sensitive to the instrumental uncertainty in the determination of E_KER causing the competition between the Raman (E_KER + E_e = const) and Auger (E_e = const) dispersions: increase in the broadening of the finite kinetic energy release resolution leads to a change of the dispersion from the Raman to the Auger one.

Photoacoustic studies of energy transfer from ozone photoproducts to bath gases following Chappuis band photoexcitation

Photoacoustic spectroscopy (PAS) is a sensitive technique for the detection of trace gases and aerosols and measurements of their absorption coefficients. The accuracy of such measurements is often governed by the fidelity of the PAS instrument calibration. Gas samples laden with O3 of a known or independently measured absorption coefficient are a convenient and commonplace route to calibration of PAS instruments operating at visible wavelengths (λ), yet the accuracy of such calibrations remains unclear. Importantly, the photoacoustic detection of O3 in the Chappuis band (λ ∼ 400-700 nm) depends strongly on the timescales for energy transfer from the nascent photoproducts O(3P) and O2(X, v > 0) to translational motion of bath gas species. Significant uncertainties remain concerning the dependence of these timescales on both the sample pressure and the bath gas composition. Here, we demonstrate accurate characterisation of microphone response function dependencies on pressure using a speaker transducer to excite resonant acoustic modes of our photoacoustic cells. These corrections enable measurements of photoacoustic response amplitudes (also referred to as PAS sensitivities) and phase shifts with variation in static pressure and bath gas composition, at discrete visible wavelengths spanning the Chappuis band. We develop and fit a photochemical relaxation model to these measurements to retrieve the associated variations in the aforementioned relaxation timescales for O(3P) and O2(X, v > 0). These timescales enable a full assessment of the accuracy of PAS calibrations using O3-laden gas samples, dependent on the sample pressure, bath gas composition and PAS laser modulation frequency.

Predicting the effect of angular momentum on the dissociation dynamics of highly rotationally excited radical intermediates

We present a model which accurately predicts the net speed distributions of products resulting from the unimolecular decomposition of rotationally excited radicals. The radicals are produced photolytically from a halogenated precursor under collision-free conditions so they are not in a thermal distribution of rotational states. The accuracy relies on the radical dissociating with negligible energetic barrier beyond the endoergicity. We test the model predictions using previous velocity map imaging and crossed laser-molecular beam scattering experiments that photolytically generated rotationally excited CD2CD2OH and C3H6OH radicals from brominated precursors; some of those radicals then undergo further dissociation to CD2CD2 + OH and C3H6 + OH, respectively. We model the rotational trajectories of these radicals, with high vibrational and rotational energy, first near their equilibrium geometry, and then by projecting each point during the rotation to the transition state (continuing the rotational dynamics at that geometry). This allows us to accurately predict the recoil velocity imparted in the subsequent dissociation of the radical by calculating the tangential velocities of the CD2CD2/C3H6 and OH fragments at the transition state. The model also gives a prediction for the distribution of angles between the dissociation fragments' velocity vectors and the initial radical's velocity vector. These results are used to generate fits to the previously measured time-of-flight distributions of the dissociation fragments; the fits are excellent. The results demonstrate the importance of considering the precession of the angular velocity vector for a rotating radical. We also show that if the initial angular momentum of the rotating radical lies nearly parallel to a principal axis, the very narrow range of tangential velocities predicted by this model must be convoluted with a J = 0 recoil velocity distribution to achieve a good result. The model relies on measuring the kinetic energy release when the halogenated precursor is photodissociated via a repulsive excited state but does not include any adjustable parameters. Even when different conformers of the photolytic precursor are populated, weighting the prediction by a thermal conformer population gives an accurate prediction for the relative velocity vectors of the fragments from the highly rotationally excited radical intermediates.

Absorption spectrum and assignment of the Chappuis band of ozone

New global diabatic potential energy surfaces of the electronic states 1B1 and 1A2 of ozone and the non-adiabatic coupling surface between them are constructed from electronic structure calculations. These surfaces are used to study the visible photodissociation in the Chappuis band by means of quantum mechanical calculations. The calculated absorption spectrum and its absolute intensity are in good agreement with the experimental results. A vibrational assignment of the diffuse structures in the Chappuis band system is proposed on the basis of the nodal structures of the underlying resonance states.

A Quantum Dynamical Treatment of Symmetry-Induced Kinetic Isotope Effects in the Formation of He2+

Kinetic isotope effects for He(2)(+) formation are calculated quantum dynamically using high-quality Born-Oppenheimer (BO) potentials for two electronic states of He(2)(+) and an accurate treatment of all nonadiabatic BO corrections. The two potentials are coupled only when the helium isotopes are different, and the calculations reveal that this coupling is sufficient to allow the two sets of distinguishable reactants, (4)He(+) + (3)He or (3)He(+) + (4)He, to yield He(2)(+) with comparable efficiency over a wide temperature range. Consequently, the potential coupling provides a significant formation rate enhancement for the low isotopic symmetry reactants, as compared to the symmetrical cases (e.g., (4)He(+) + (4)He or (3)He(+) + (3)He). The computed symmetry-induced kinetic isotope effects (SIKIEs) are in substantial agreement with the available experimental results and represent the first theoretical demonstration of this unusual kinetic phenomenon. Possible application of SIKIE to ozone formation and other chemical systems is discussed.

A New Laboratory Source of Ozone and Its Potential Atmospheric Implications

Although 248-nanometer radiation falls 0.12 electron volt short of the energy needed to dissociate O(2) large densities of ozone (O(3)) can be produced from unfocused 248-nanometer KrF excimer laser irradiation of pure O(2). The process is initiated in some undefined manner, possibly through weak two-photon O(2) dissociation, which results in a small amount of O(3) being generated. As soon as any O(3) is present, it strongly absorbs the 248-nanometer radiation and dissociates to vibrationally excited ground state O(2) (among other products), with a quantum yield of 0.1 to 0.15. During the laser pulse, a portion of these molecules absorb a photon and dissociate, which results in the production of three oxygen atoms for one O(3) molecule destroyed. Recombination then converts these atoms to O(3), and thus O(3) production in the system is autocatalytic. A deficiency exists in current models of O(3) photochemistry in the upper stratosphere and mesosphere, in that more O(3) iS found than can be explained. A detailed analysis of the system as it applies to the upper atmosphere is not yet possible, but with reasonable assumptions about O(2) vibrational distributions resulting from O(3) photodissociation and about relaxation rates of vibrationally excited O(2) a case can be made for the importance of incuding this mechanism in the models.