Re-analysis of the ultraviolet absorption spectrum of ozone

Signatures of a conical intersection in photofragment distributions and absorption spectra: photodissociation in the Hartley band of ozone

Photodissociation of ozone in the near UV is studied quantum mechanically in two excited electronic states coupled at a conical intersection located outside the Franck-Condon zone. The calculations, performed using recent ab initio PESs, provide an accurate description of the photodissociation dynamics across the Hartley/Huggins absorption bands. The observed photofragment distributions are reproduced in the two electronic dissociation channels. The room temperature absorption spectrum, constructed as a Boltzmann average of many absorption spectra of rotationally excited parent ozone, agrees with experiment in terms of widths and intensities of diffuse structures. The exit channel conical intersection contributes to the coherent broadening of the absorption spectrum and directly affects the product vibrational and translational distributions. The photon energy dependences of these distributions are strikingly different for fragments created along the adiabatic and the diabatic paths through the intersection. They can be used to reverse engineer the most probable geometry of the non-adiabatic transition. The angular distributions, quantified in terms of the anisotropy parameter β, are substantially different in the two channels due to a strong anticorrelation between β and the rotational angular momentum of the fragment O2.

Spectroscopic signatures of ozone at the air-water interface and photochemistry implications

First-principles simulations suggest that additional OH formation in the troposphere can result from ozone interactions with the surface of cloud droplets. Ozone exhibits an affinity for the air-water interface, which modifies its UV and visible light spectroscopic signatures and photolytic rate constant in the troposphere. Ozone cross sections on the red side of the Hartley band (290- to 350-nm region) and in the Chappuis band (450-700 nm) are increased due to electronic ozone-water interactions. This effect, combined with the potential contribution of the O3 + hν → O((3)P) + O2(X(3)Σg(-)) photolytic channel at the interface, leads to an enhancement of the OH radical formation rate by four orders of magnitude. This finding suggests that clouds can influence the overall oxidizing capacity of the troposphere on a global scale by stimulating the production of OH radicals through ozone photolysis by UV and visible light at the air-water interface.

Photodissociation of ozone from 321 to 329 nm: the relative yields of O(3P2) with O2(X 3Σg(-)), O2(a 1Δg) and O2(b 1Σg(+))

Product imaging of O((3)P2) following dissociation of ozone has been used to determine the relative yields of the product channels O((3)P2) + O2(X (3)Σg(-)) of ozone. All three channels are prominent at all wavelengths investigated. O2 vibrational distributions for each channel and each wavelength are also estimated assuming Boltzmann rotational distributions. Averaged over wavelength in the measured range, the yields of the O((3)P2) + O2(X (3)Σg(-)), O((3)P2) + O2(a (1)Δg), and O((3)P2) + O2(b (1)Σg(+)) channels are 0.36, 0.31,and 0.34, respectively. Photofragment distributions in the spin-allowed channel O((3)P) + O2(X (3)Σg(-)) are compared with the results of quantum mechanical calculations on the vibronically coupled PESs of the singlet states B (optically bright) and R (repulsive). The experiments suggest that considerably more vibrational excitation and less rotational excitation occur than predicted by the quantum calculations. The rotational distributions, adjusted to fit the experimental images, suggest that the dissociation takes place from a more linear configuration than the Franck-Condon bending angle of 117°. The dissociation at most wavelengths results in a positive value of the anisotropy parameter, β, both in the experiment and in the calculations. Calculations indicate that both nonadiabatic transitions and intersystem crossings substantially reduce β below the nominal value of 2.

Comparison of the Huggins band for six ozone isotopologues: vibrational levels and absorption cross section

By use of the 3(1)A' ab initio potential energy surface (PES) of ozone and the multi-configuration time-dependent Hartree program for wavepacket propagation, we have determined numerous eigenstates of this state for six ozone isotopologues. These bound vibrational levels are the upper levels of the Huggins band, which covers the range from 27,000 to ~33,000 cm(-1). This study extends our previous work on the Hartley band, which was limited to the range ~32,000-50,000 cm(-1). Four isotopologues, (16)O(3), (16)O(17)O(16)O, (16)O(18)O(16)O, and (18)O(3) (noted hereafter 666, 676, 686, and 888), are symmetric, and two are asymmetric, (17)O(16)O(2) and (18)O(16)O(2) (noted hereafter 667 and 668). The PES of the 3(1)A' state has two equivalent minima of C(s) symmetry located at ~27,000 cm(-1) above the X(1)A(1) ground state. The equilibrium geometry of these two minima is r(e(1)) = 2.28 a(0), r(e(2)) = 3.2 a(0), and θ(e) = 107°. The dissociation limit of this PES, which correlates to the O((1)D) + O(2) ((1)Δ) "singlet" channel, is about 4300 cm(-1) above the two minima. For the (16)O(3) isotopologue, the 120 lowest bound eigenstates have been calculated and partially assigned up to 800 cm(-1) below the dissociation limit. The 60 lower eigenstates are easily assignable in term of three normal modes, the "long" bond (ν(1)), the bending (ν(2)), and the "short" bond (ν(3)). A new family of wave functions, aligned along the dissociation channels, appears at 3782 cm(-1) above the 3(1)A' (0,0,0) level. The 3(1)A' vibrational levels and the corresponding intensity factors from the (000), (010), (100), and (001) levels of the X(1)A(1) ground state have been calculated for the six isotopologues. The Huggins absorption cross sections of the six isotopologues have been calculated from the 3(1)A' vibrational energy levels and the corresponding intensity factors. The rotational envelope of each vibronic band has been empirically described by an ad hoc function. The ratio of the Huggins cross section of each ozone isotopologue with one of (16)O(3) provides the fractionation factor of each ozone isotopologue as a function of the photon energy. These various fractionation factors will allow predicting enrichments due to photolysis by various light sources like the actinic flux.

Ab initio quantum mechanical study of the O((1)D) formation in the photolysis of ozone between 300 and 330 nm

Spin-allowed production of O((1)D) in the near-UV photolysis of ozone is studied using ab initio potential energy surfaces and quantum mechanics. The O((1)D) quantum yield, reconstructed from the absolute cross sections for eight initial vibrational states in the ground electronic state, is shown to agree with the measurements in a broad range of photolysis wavelengths and temperatures. Relative contributions of one- and two-quantum stretching and bending initial excitations are quantified, with the contribution of the antisymmetric stretch being dominant for lambda < 330 nm. Large scale structures in the low-resolution quantum yield are shown to reflect excitations in the high-frequency short bond stretch in the upper electronic state. Spin-forbidden contribution to the O((1)D) quantum yield at wavelengths lambda > 320 nm is estimated using ab initio energies of the triplet states and their spin-orbit couplings.

Probing the O2 (a 1Delta g) photofragment following ozone dissociation within the long wavelength tail of the Hartley band

The technique of resonance enhanced multiphoton ionization (REMPI) has been used in conjunction with time-of-flight mass spectrometry (TOFMS), to investigate the dynamics of ozone photolysis in the long wavelength region of the Hartley band (301-311 nm). Specifically, both the translational anisotropy and the rotational angular momentum orientation of the O(2) (a (1)Delta(g); nu=0, J=16-20) fragments have been measured as a function of photolysis wavelength. Within this region, the thermodynamic thresholds for the formation of these products in combination with O ((1)D(2)) are approached and passed, and consequently these studies have allowed an investigation into the effects on the dynamics of slowing fragment recoil velocities and the increasing importance of vibrationally mediated photolysis. The determined beta parameters for all the J states probed follow a similar trend, decreasing from a value typical for the initial (1)B(2)<--(1)A(1) excitation responsible for the Hartley band [for example, beta=1.40+/-0.12 for the O(2) (a (1)Delta(g); J=18) fragment], to a much lower value beyond the thermodynamic threshold for the fragment's production (for example, beta=0.63+/-0.19 for the J=18 fragment following photolysis at 311 nm). This trend, similar to that observed when probing the atomic fragment in a previous set of experiments, [Horrocks et al., J. Chem. Phys. 125, 133313 (2006); Denzer et al., Phys. Chem. Chem. Phys. 16, 1954 (2006)] is consistent with the photodissociation of vibrationally excited ozone molecules beyond the threshold wavelengths and we estimate approximately 1/3 of this to be from excitation in the nu(3) asymmetric stretching mode. These observations are substantiated by the values of the beta(0) (2)(2,1) orientation moment measured, which for photolysis at 301 nm are negative, indicating that a bond opening mechanism provides the key torque for the departing O(2) fragment. The orientation moment becomes positive again for photolysis beyond threshold, however, as the increasing impulsive dissociation again begins to dominate the nature of the rotation of the departing molecular fragment. In addition, a (2+2) REMPI scheme has been utilized to probe the O(2) (a (1)Delta(g)) "low" J fragments, where the majority of the population resides following photolysis within this region. The REMPI-TOFMS technique has been used to confirm the rotational character of a spectral feature through examination of the signal line shapes obtained using different experimental geometries. The dynamical information subsequently obtained, probing the "low" J O(2) (a (1)Delta(g)) fragments on these rotational transitions, has unified previous translational anisotropy results obtained by detecting the O ((1)D(2)) atomic fragment with data for the O(2) (a (1)Delta(g); J=16-20) fragments.

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

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.

The photodissociation of ozone in the Hartley band: A theoretical analysis

Three-dimensional diabatic potential energy surfaces for the lowest four electronic states of ozone with 1A' symmetry-termed X, A, B, and R-are constructed from electronic structure calculations. The diabatization is performed by reassigning corresponding energy points. Although approximate, these diabatic potential energy surfaces allow one to study the uv photodissociation of ozone on a level of theory not possible before. In the present work photoexcitation in the Hartley band and subsequent dissociation into the singlet channel, O3X+hnu-->O(1D)+O2(a 1Deltag), are investigated by means of quantum mechanical and classical trajectory calculations using the diabatic potential energy surface of the B state. The calculated low-resolution absorption spectrum as well as the vibrational and rotational state distributions of O2(a 1Deltag) are in good agreement with available experimental results.

Theory of the photodissociation of ozone in the Hartley continuum: Potential energy surfaces, conical intersections, and photodissociation dynamics

Ab initio potential energy and transition dipole moment surfaces are presented for the five lowest singlet even symmetry electronic states of ozone. The surfaces are calculated using the complete active space self consistent field method followed by contracted multireference configuration interaction (MRCI) calculations. A slightly reduced augmented correlation consistent valence triple-zeta orbital basis set is used. The ground and excited state energies of the molecule have been computed at 9282 separate nuclear geometries. Cuts through the potential energy surfaces, which pass through the geometry of the minimum of the ground electronic state, show several closely avoided crossings. Close examination, and higher level calculations, very strongly suggests that some of these seemingly avoided crossings are in fact associated with non-symmetry related conical intersections. Diabatic potential energy and transition dipole moment surfaces are created from the computed ab initio adiabatic MRCI energies and transition dipole moments. The transition dipole moment connecting the ground electronic state to the diabatic B state surface is by far the strongest. Vibrational-rotational wavefunctions and energies are computed using the ground electronic state. The energy level separations compare well with experimentally determined values. The ground vibrational state wavefunction is then used, together with the diabatic B<--X transition dipole moment surface, to form an initial wavepacket. The analysis of the time-dependent quantum dynamics of this wavepacket provides the total and partial photodissociation cross sections for the system. Both the total absorption cross section and the predicted product quantum state distributions compare well with experimental observations. A discussion is also given as to how the observed alternation in product diatom rotational state populations might be explained.

The Huggins band of ozone: Assignment of hot bands

The "hot bands" of the Huggins band of ozone are assigned, in both the 218 K and the 295 K spectrum. The assignment is based on intensities calculated with three-dimensional vibrational wave functions for the electronic ground state (X) and the excited state (B). The hot-band structures in the 218 K spectrum all can be assigned to transitions starting from vibrational states with one quantum of stretching excitation in the ground electronic state. The 295 K spectrum shows new structures, which are due to transitions originating from vibrational states in the X state with two quanta of excitation of the stretching modes--despite very small Boltzmann factors. All structures in the low-energy range of the 295 K spectrum, even the very weak ones, thus can be uniquely interpreted. The significance of hot bands results from the strong increase of Franck-Condon factors with excitation of the stretching modes in both the lower and/or the upper electronic states, whose equilibrium bond lengths differ significantly.

Theory of the photodissociation of ozone in the Hartley continuum; effect of vibrational excitation and O(1D) atom velocity distribution

The effect of vibrational excitation on the photodissociation cross section of ozone in the Hartley continuum is examined. The calculations make use of newly computed potential energy and transition dipole moment surfaces. The initial vibrational states of the ozone are computed using grid based techniques and the first few ab initio computed vibrational energy level spacings agree to within 10 cm(-1) with experimental values. The computed total absorption cross sections arising from different initial vibrational states of ozone are discussed in the light of the nature of the transition dipole moment surface. The computed cross section for excitation from the ground vibrational-rotational state is in good agreement with the experimentally measured cross section. Excitation of the asymmetric stretching vibration of ozone has a marked effect on both the form and magnitude of the photodissociation cross section. The velocity distributions of highly reactive O(1D) atoms arising from the photodissociation process in different wavelength ranges is also presented. The results show that the O(1D) atoms travel with a most probable translational velocity of 2.030 km s(-1) corresponding to a translational energy of 0.342 eV or 33.0 kJ mol(-1).

The Huggins band of ozone: A theoretical analysis

The Huggins band of ozone is investigated by means of dynamics calculations using a new (diabatic) potential energy surface for the 3 (1)A'(1B2) state. The good overall agreement of the calculated spectrum of vibrational energies and intensities with the experimental spectrum, especially at low to intermediate excitation energies, is considered as evidence that the Huggins band is due to the two C(s) potential wells of the 1B2 state rather than the single C2v well of the 2 (1)A'(1A1) state. The vibrational assignment of the "cold bands," based on the nodal structure of wave functions, on the whole supports the most recent experimental assignment [J. Chem. Phys. 115, 9311 (2001)]. The quantum mechanical spectrum is analyzed in terms of classical periodic orbits and the structure of the classical phase space.

The Huggins band of ozone: Unambiguous electronic and vibrational assignment

The Huggins band of ozone is investigated by means of exact dynamics calculations using a new (diabatic) potential energy surface for the (1)B(2) state. The remarkable agreement with the measured spectrum strongly suggests that the Huggins band is due to the two C(s) potential wells of the (1)B(2) state. The vibrational assignment, based on the nodal structure of wave functions, supports the most recent experimental assignment.