Adiabatic potential energy surfaces of the three lowest lying singlet states, X 1 A′ 2 1 A′, and 1 1 A′′, of N 2 O have been computed as a function of the R N 2 -O bond distance and the Jacobi angle. The calculations are performed using the complete-active-space self-consistent field (CASSCF) and the multireference configuration interaction (MRCI) electronic structure methods. It is shown that there is a wide avoided crossing between the ground, X 1 A′, and lowest excited, 2 1 A′, electronic state. This avoided crossing is thought to give rise to a seam of conical intersection at other N-N separations. Both excited state surfaces display important conical intersections at linear geometries. The transition dipole moment surfaces for the two excitation processes (2 1 A′ r X 1 A′ and 1 1 A′′ r X 1 A′) are also presented. These calculations provide the basic data needed to compute the dynamics of the N 2 O + hν f N 2 + O( 1 D) photodissociation process for photon frequencies in the range 5.2 eV (240 nm) to 7.3 eV (170 nm).

In this paper, which is the first of a series devoted to some aspects of the title reaction, we present the theoretical results concerning the topology of the lowest potential energy surface of triplet multiplicity. Results at the ab initio CASPT2 level with a complete active space self-consistent field (CASSCF) reference wave function built on a D95 Dunning basis set and involving 10 electrons in 10 orbitals are reported. This triplet surface is expected to play an important role in this reaction since experimental results show HCN(XΣ+) as the main product, and this product can only be obtained via an HCNO(3A) hypersurface. We find that the channel leading to the formation of HCN(X1Σ+) + O(3P) is the most likely product channel, in agreement with available experimental findings. To a lesser extent, we can also observe the formation of the product channels HCO(XA‘) + N(4S), NCO(X2Π) + H(2S), and CO(X1Σ+) + NH(XΣ-). Conversely, the formation of CN(X2Σ+) + OH(X2Π) is highly unlikely because of the existence of high potential energy barriers along this reaction path.

Vibrationally-rotationally inelastic cross sections have been computed for H+SiO collisions over the energy range 200–9600 K. The calculations use a previously calculated ab initio potential energy surface which was fitted to an analytic form [P. Jimeno, M. D. Gray, and G. G. Balint-Kurti, J. Chem. Phys. 111, 4966 (1999)]. Potential optimized discrete variable representation techniques were used to compute vibrational matrix elements of this potential and the infinite order sudden method was then used to compute the required cross sections. The potential is very anisotropic and complicated. This is mirrored in the form of the vibrational matrix elements and in their behavior as a function of the scattering coordinate and of the Jacobi angle. The properties of the potential are markedly different from those of the He+SiO system which has previously been used to model the behavior of both H+SiO and H2+SiO in interstellar and stellar media. Selected vibrationally elastic and vibrationally inelastic cross sections are presented and discussed. Tables of cross sections covering a wide range of vibrational and rotational quantum numbers, as well as a large energy range, are made available through the journal’s EPAPS service. The inelastic cross sections computed here are needed in the modeling of circumstellar SiO maser radiation. Significant structure is observed in the computed inelastic cross sections. This structure is of a complicated nature and might well give rise to the vibrational-rotational quantum state population inversions which in turn leads to the observed circumstellar maser action.

Ab initio calculations of two-dimensional cuts through the full six-dimensional potential energy surfaces of the ground (X ~1A 0 ) and first excited (A ˜1A 00 ) states of HN 3 are reported. Specifically, we have investigated the variation of the potential energy surfaces with respect to simultaneous changes of the H-NNN and HN-NN bond lengths, and to changes of the H-NNN bond length and cN-N-N angle. These pairs of co-ordinates were chosen in the light of the deduced importance of each of these motions in interpreting the experimentally observed ultraviolet photochemistry of this molecule. Two-dimensional quantum mechanical wavepacket calculations of the photodissociation dynamics have been carried out using each of these two-dimensional surfaces. These offer strong support to our earlier interpretations (Cook et al., Phys. Chem. Chem. Phys., 1999, 1, 45) regarding the form of the vibrational energy disposal in the N 3 (X ~) products arising via H-NNN bond fission on the A ˜state potential energy surface. They also allow investigation of factors influencing the relative branching into the competing H-NNN and HN-NN bond fission channels on the excited state potential energy surface, and predict relative branching ratios for these two channels, as a function of excitation energy, in reasonable accord with the limited available experimental data.

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