Daud Mohammad Noh, Balint-Kurti Gabriel G, Brown Alex
School of Chemistry, University of Bristol, Bristol BS8 1TS, UK.
J Chem Phys. 2005 Feb 1;122(5):54305. doi: 10.1063/1.1830436.
Adiabatic potential energy surfaces for the six lowest singlet electronic states of N(2)O (X (1)A('), 2 (1)A('), 3 (1)A('), 1 (1)A("), 2 (1)A(") and 3 (1)A(")) have been computed using an ab initio multireference configuration interaction (MRCI) method and a large orbital basis set (aug-cc-pVQZ). The potential energy surfaces display several symmetry related and some nonsymmetry related conical intersections. Total photodissociation cross sections and product rotational state distributions have been calculated for the first ultraviolet absorption band of the system using the adiabatic ab initio potential energy and transition dipole moment surfaces corresponding to the lowest three excited electronic states. In the Franck-Condon region the potential energy curves corresponding to these three states lie very close in energy and they all contribute to the absorption cross section in the first ultraviolet band. The total angular momentum is treated correctly in both the initial and final states. The total photodissociation spectra and product rotational distributions are determined for N(2)O initially in its ground vibrational state (0,0,0) and in the vibrationally excited (0,1,0) (bending) state. The resulting total absorption spectra are in good quantitative agreement with the experimental results over the region of the first ultraviolet absorption band, from 150 to 220 nm. All of the lowest three electronically excited states [(1)Sigma(-)(1 (1)A(")), (1)Delta(2 (1)A(')), and (1)Delta(2 (1)A("))] have zero transition dipole moments from the ground state [(1)Sigma(+)(1 (1)A('))] in its equilibrium linear configuration. The absorption becomes possible only through the bending motion of the molecule. The (1)Delta(2 (1)A('))<--X (1)Sigma(+)((1)A(')) absorption dominates the absorption cross section with absorption to the other two electronic states contributing to the shape and diffuse structure of the band. It is suggested that absorption to the bound (1)Delta(2 (1)A(")) state makes an important contribution to the experimentally observed diffuse structure in the first ultraviolet absorption band. The predicted product rotational quantum state distribution at 203 nm agrees well with experimental observations.
使用从头算多参考组态相互作用（MRCI）方法和大轨道基组（aug-cc-pVQZ）计算了N₂O的六个最低单重态电子态（X¹A′、2¹A′、3¹A′、1¹A″、2¹A″和3¹A″）的绝热势能面。势能面显示出几个与对称性相关以及一些与非对称性相关的锥形交叉点。利用对应于最低三个激发电子态的绝热从头算势能面和跃迁偶极矩面，计算了该系统第一紫外吸收带的总光解离截面和产物转动态分布。在弗兰克-康登区域，对应于这三个态的势能曲线在能量上非常接近，它们都对第一紫外波段的吸收截面有贡献。总角动量在初始态和终态中都得到了正确处理。确定了初始处于基振动态（0,0,0）和振动态激发（0,1,0）（弯曲）态的N₂O的总光解离光谱和产物转动分布。所得的总吸收光谱在第一紫外吸收带区域（150至220 nm）与实验结果在定量上有很好的一致性。最低的三个电子激发态[¹Σ⁻(1¹A″)、¹Δ(2¹A′)和¹Δ(2¹A″)]在其平衡线性构型下从基态[¹Σ⁺(1¹A′)]的跃迁偶极矩均为零。吸收仅通过分子的弯曲运动才成为可能。¹Δ(2¹A′)←X¹Σ⁺(¹A′)吸收主导吸收截面，而向其他两个电子态的吸收则对谱带的形状和漫射结构有贡献。有人认为，向束缚态¹Δ(2¹A″)的吸收对实验观察到的第一紫外吸收带中的漫射结构有重要贡献。在203 nm处预测的产物转动态量子态分布与实验观测结果吻合良好。
