An ab initio molecular orbital study of the electronically excited and cationic states of the ozone molecule and a comparison with spectral data

Investigation of 3-fragment photodissociation of O3 at 193.4 and 157.6 nm by coincident measurements

Photodissociation of the ozone molecule at 193.4 nm (6.41 eV) and 157.6 nm (7.87 eV) is studied by fast-beam translational spectroscopy. Coincident detection of the dissociation products allows direct observation of the 3-fragment channel and determination of its kinematic parameters. The results indicate that at each wavelength, 3-fragment dissociation proceeds through synchronous concerted bond breaking, but the energy partitioning among the fragments is different. The branching fraction of the 3-fragment channel increases from 5.2(6)% at 193.4 nm to 26(4)% at 157.6 nm, in agreement with previous studies. It is shown that vibrational excitation of the symmetric stretch mode in O3 molecules created by photodetachment of O(3)(-) anion enhances the absorption efficiency, especially at 193.4 nm, but does not have a strong effect on the 3-fragment dissociation.

Toward the observation of quartet states of the ozone radical cation: insights from coupled cluster theory

Since the discovery of ozone depletion, the doublet electronic states of the ozone radical cation have received much attention in experimental and theoretical investigations, while the low-lying quartet states have not. In the present research, viable pathways to the quartet states from the lowest three triplet states of ozone, (3)A(2), (3)B(2), and (3)B(1), and excitations from the (2)A(1) and (2)B(2) states of the ozone radical cation have been studied in detail. The potential energy surfaces, structural optimizations, and vibrational frequencies for several states of ozone and its radical cation have been thoroughly investigated using the complete active space self-consistent field, unrestricted coupled cluster theory from a restricted open-shell Hartree-Fock reference including all single and double excitations (UCCSD), UCCSD method with the effects of connected triple excitations included perturbatively, and unrestricted coupled cluster including all single, double, and triple excitations with the effects of connected quadruple excitations included perturbatively. These methods used Dunning's correlation-consistent polarized core-valence basis sets, cc-pCVXZ (X = D, T, Q, and 5). The most feasible pathways (symmetry and spin allowed transitions) to the quartet states are (4)A(1)<--(3)A(2), (4)A(2)<--(3)A(2), (4)A(1)<--(3)B(2), (4)A(2)<--(3)B(1), (4)B(2)<--(3)B(1), (4)A(2)<--(1)A(1), (4)B(2)<--(1)A(1), and (4)A(1)<--(1)A(1) with vertical ionization potentials of 12.46, 12.85, 12.82, 12.46, 12.65, 13.43, 13.93, and 14.90 eV, respectively.

Three-body breakup in the dissociative recombination of the covalent triatomic molecular ion O3+

We report the first observation of almost exclusive three-body breakup in the dissociative recombination of a covalent triatomic molecular ion O3+. The three-body channel, constituting about 94% of the total reactivity, has been investigated in detail. The atomic fragments are formed in only the first two electronic states, 3P and 1D, while formation in the 1S state has not been observed. The breakup predominantly proceeds through dissociative states with linear geometry.

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 dynamics of ozone at 193nm: An O(D21) angular momentum polarization study

Polarized laser photolysis, coupled with resonantly enhanced multiphoton ionization detection of O(1D2) and velocity-map ion imaging, has been used to investigate the photodissociation dynamics of ozone at 193 nm. 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(1D2) photofragments, in addition to providing high-resolution information about their speed and angular distributions. Images obtained at the probe laser wavelength of around 205 nm indicate dissociation primarily via the Hartley band, involving absorption to, and diabatic dissociation on, the B 1B2(3 1A1) potential energy surface. Rather different O(1D2) speed and electronic angular momentum spatial distributions are observed at 193 nm, 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 least one other state in the dissociation process. Evidence for a contribution from absorption into the tail of the Hartley band at 193 nm is also presented. A particularly surprising result is the observation of nonzero, albeit small values for all three rank K = 1 orientation moments of the angular momentum distribution. The polarization results obtained at 193 and 205 nm, together with those observed previously at longer wavelengths, are interpreted using an analysis of the long range quadrupole-quadrupole interaction between the O(1D2) and O2(1Deltag) species.

Threshold photoelectron spectroscopy of ozone

Threshold photoelectron spectrum of ozone is presented for the first time at a resolution of 21-38 meV using synchrotron radiation in the energy region of 12-21 eV. The ionization energies of the first ionized states were determined and an interpretation of the O3 spectrum with respect to its first three ionic states, 1 2A1, 1 2B2, and 1 2A2, is presented. Above 16 eV the enhancement of the intensities of the 2 2B1, 3 2A1, and 4 2B2 band systems due to the contribution of indirect processes was observed, not accessible by conventional photoelectron spectroscopy. It was also resolved and assigned the extensive vibrational structures of ozone. Between 15.5 and 18.5 eV the main band contours are similar to those found in conventional photoelectron spectroscopy, except that our threshold photoelectron spectrum reveals extensive additional vibrational structures. The band 2 2B1 was found to present an irregular vibrational spacing DeltaE, with a minimum value of 80 meV at approximately 16.47 eV.

Infrared spectrum of cyclic ozone: A theoretical investigation

The infrared absorption spectrum of cyclic ozone is calculated by means of a new ab initio potential energy surface, the dipole moment function, and exact quantum mechanical dynamics calculations. Five different isotopomers are considered. The absorption line for excitation of the bending fundamental near 800 cm(-1) is by far the strongest band; all other bands are more than one order of magnitude less intense. This spectral pattern as well as the isotope shifts for the various isotopomers are important for identifying cyclic ozone. Several possibilities for accessing the ring minimum of cyclic ozone are also discussed on the basis of recent electronic structure calculations.

Excited Electronic States of the Cyclic Isomers of O3 and SO2

The low-lying electronic states of O3 and SO2 in their bent and cyclic isomers up to about 10 eV are calculated using the multireference configuration interaction (MRCI) method with a standard Gaussian correlation consistent polarized triple-zeta (cc-pVTZ) basis set. The vertical excitation energies, electron configurations, and oscillator strengths of these states are reported. The molecular orbital structures and excited states of the cyclic isomers are discussed in relation to the bent ones. Coherent anti-Stokes Raman spectroscopy (CARS) schemes for detecting the synthesis of the cyclic isomers are suggested.

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.

The triplet channel in the photodissociation of ozone in the Hartley band: Classical trajectory surface hopping analysis

The triplet channel in the photodissociation of ozone in the Hartley band, O3 + hv-->O(3P) + O2(3sigma), is investigated by means of a classical trajectory surface hopping method using ab initio diabatic potential energy surfaces for the B and the R states. Because of the strong recoil in the R state along the breaking O-O bond, O2(3sigma) is produced with a high rotational energy. The nonadiabatic transition probability depends markedly on the coordinate along the crossing seam. As a consequence a unique correlation is found between the internuclear geometry at the crossing and the final vibrational state of O2(3sigma). The calculated distribution of the translational energy is in good accord with the measured distribution.

High-resolution pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopic study of the two lowest electronic states of the ozone cation O3+

The pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectrum of jet-cooled O3 has been recorded in the range 101,000-104,000 cm(-1). The origins of the X 1A1-->X+ 2A1 and X 1A1-->A+ 2B2 transitions could be determined from the rotational structure of the bands, the photoionization selection rules, the photoionization efficiency curve, and comparison with ab initio calculations. The first adiabatic ionization energy of O3 was measured to be 101,020.5(5) cm(-1) [12.524 95(6) eV] and the energy difference between the X+ 2A1 (0,0,0) and A+ 2B2 (0,0,0) states was determined to be DeltaT0=1089.7(4) cm(-1). Whereas the X-->X+ band consists of an intense and regular progression in the bending (nu2) mode observed up to v2+=4, only the origin of the X-->A+ band was observed. The analysis of the rotational structure in each band led to the derivation of the r0 structure of O3+ in the X+ [C2v,r0=1.25(2) A,alpha0=131.5(9) degrees ] and A+[C2v,r0=1.37(5) A,alpha0=111.3(38) degrees ] states. The appearance of the spectrum, which is regular up to 102,300 cm(-1), changes abruptly at approximately 102,500 cm(-1), a position above which the spectral density increases markedly and the rotational structure of the bands collapses. On the basis of ab initio calculations, this behavior is attributed to the onset of large-amplitude motions spreading through several local minima all the way to large internuclear distances. The ab initio calculations are consistent with earlier results in predicting a seam of conical intersections between the X+ and A+ states approximately 2600 cm(-1) above the cationic ground state and demonstrate the existence of potential minima at large internuclear distances that are connected to the main minima of the X+ and A+ states through low-lying barriers.

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.