Z.-W. Qu

The ultra-violet photodissociation of ozone revisited

Abstract We have performed new electronic structure calculations for the five lowest 1A′ states of ozone using the multi-reference configuration interaction method with an augmented triple zeta valence atomic basis set. Several avoided crossings, which are important for interpreting the Huggins–Hartley band system, are identified and two-dimensional diabatic potential energy surfaces are constructed. It is argued that the Huggins and the Hartley band systems are due to excitation of the same electronic state.

Experimental and theoretical investigation of the reaction NH(X3∑-)+H(2S) → N(4S)+H2(X1∑+g)

The rate coefficient of the reaction NH(XΣ−3)+H(S2)→k1aN(S4)+H2(XΣg+1) is determined in a quasistatic laser-flash photolysis, laser-induced fluorescence system at low pressures (2mbar⩽p⩽10mbar). The NH(X) radicals are produced via the quenching of NH(aΔ1) (obtained by photolyzing HN3) with Xe whereas the H atoms are generated in a H2∕He microwave discharge. The NH(X) concentration profile is measured under pseudo-first-order condition, i.e., in the presence of a large excess of H atoms. The room temperature rate coefficient is determined to be k1a=(1.9±0.5)×1012cm3mol−1s−1. It is found to be independent of the pressure in the range considered in the present experiment. A global potential energy surface for the A″4 state is calculated with the internally contracted multireference configuration interaction method and the augmented correlation consistent polarized valence quadruple zeta atomic basis. The title reaction is investigated by classical trajectory calculations on this surface. The theoretical room...

Experimental and theoretical investigations of the reactions NH(XΣ−3)+D(S2)→ND(XΣ−3)+H(S2) and NH(XΣ−3)+D(S2)→N(S4)+HD(XΣg+1)

The rate coefficient of the reaction NH(XΣ−3)+D(S2)→k1products (1) is determined in a quasistatic laser-flash photolysis, laser-induced fluorescence system at low pressures. The NH(X) radicals are produced by quenching of NH(aΔ1) (obtained in the photolysis of HN3) with Xe and the D atoms are generated in a D2/He microwave discharge. The NH(X) concentration profile is measured in the presence of a large excess of D atoms. The room-temperature rate coefficient is determined to be k1=(3.9±1.5)×1013cm3mol−1s−1. The rate coefficient k1 is the sum of the two rate coefficients, k1a and k1b, which correspond to the reactions NH(XΣ−3)+D(S2)→k1aND(XΣ−3)+H(S2) (1a) and NH(XΣ−3)+D(S2)→k1bN(S4)+HD(XΣg+1) (1b), respectively. The first reaction proceeds via the A″2 ground state of NH2 whereas the second one proceeds in the A″4 state. A global potential energy surface is constructed for the A″2 state using the internally contracted multireference configuration interaction method and the augmented correlation consistent ...

Time-dependent density functional theory study of the electronic excitation spectra of chlorophyllide a and pheophorbide a in solvents.

The electronic excitation spectra of both chlorophyllide a (Chl) and pheophorbide a (Pheo) molecules in solvents have been investigated by using the time-dependent density functional theory (TDDFT) along with the polarizable continuum solvation model (PCM). With increasing Hartree-Fock (HF) exchange percentage in DFT functionals, the predicted HOMO-LUMO gaps increase linearly while the excitation energies increase gradually and even strongly for excited states with partial intramolecular charge-transfer (CT) nature. On the basis of the calculated excitation energies, oscillator strengths and frontier molecular orbital analysis, we provide some new insights into the absorption spectra of Pheo and Chl both in the gas phase and in solutions, especially for the B and higher electronic absorption bands. It is shown that the experimental observed visible Q(y) and Q(x) and ultraviolet B(y) and B(x) bands are all due to singlet (1)(pi,pi*) valence excitations, with the B bands being more strongly red-shifted by solvent effects. Two (1)(pi,pi*) dark states are predicted slightly below (or near) the strong B band for both Chl and Pheo, with one related to the excitation of tetrapyrrole ring and the other related to the excitation of ring I vinyl substituent. The (1)(n,pi*) CT state from the conjugated carbonyl substituent is above B bands and further strongly blue-shifted by solvent effects. The higher eta and N bands are mainly due to (1)(pi,pi*) valence excitations with only partial CT character, which are also red-shifted in solvent.

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.

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.

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.

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.