Hsu Ching-Yi, Huang Hong-Yi, Lin King-Chuen
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan.
J Chem Phys. 2005 Oct 1;123(13):134312. doi: 10.1063/1.2047570.
By using cavity ring-down absorption spectroscopy technique, we have observed the channel of Br2 molecular elimination following photodissociation of CF2Br2 at 248 nm. A tunable laser beam, which is crossed perpendicular to the photolyzing laser beam in a ring-down cell, is used to probe the Br2 fragment in the B 3Piou+-X1Sigmag+ transition. The vibrational population is obtained in a nascent state, despite ring-down time as long as 500-1000 ns. The population ratio of Br2(v=1)/Br2(v=0) is determined to be 0.4+/-0.2, slightly larger than the value of 0.22 evaluated by Boltzmann distribution at room temperature. The quantum yield of the Br2 elimination reaction is also measured to be 0.04+/-0.01. This work provides direct evidence to support molecular elimination occurring in the CF2Br2 photodissociation and proposes a plausible pathway with the aid of ab initio potential-energy calculations. CF2Br2 is excited probably to the 1B1 and 3B2 states at 248 nm. As the C-Br bond is elongated upon excitation, the coupling of the 1A'(1B1) state to the high vibrational levels of the ground state X 1A'(1A1) may be enhanced to facilitate the process of internal conversion. After transition, the highly vibrationally excited CF2Br2 feasibly surpasses a transition barrier prior to decomposition. According to the ab initio calculations, the transition state structure tends to correlate with the intermediate state CF2Br+Br(CF2Br...Br) and the products CF2+Br2. A sequential photodissociation pathway is thus favored. That is, a single C-Br bond breaks, and then the free-Br atom moves to form a Br-Br bond, followed by the Br2 elimination. The formed Br-Br bond distance in the transition state tends to approach equilibrium such that the Br2 fragment may be populated in cold vibrational distribution. Observation of a small vibrational population ratio of Br2(v=1)Br2(v=0) agrees with the proposed mechanism.
通过使用腔衰荡吸收光谱技术，我们观测到了CF2Br2在248 nm光解离后Br2分子的消除通道。在衰荡池中，一束可调谐激光束与光解激光束垂直交叉，用于探测B 3Piou+-X1Sigmag+跃迁中的Br2碎片。尽管衰荡时间长达500 - 1000 ns，但仍能获得初生态的振动布居。Br2(v = 1)/Br2(v = 0)的布居比确定为0.4±0.2，略大于室温下玻尔兹曼分布所计算的0.22值。Br2消除反应的量子产率也测得为0.04±0.01。这项工作提供了直接证据支持CF2Br2光解离过程中发生分子消除，并借助从头算势能计算提出了一个合理的途径。CF2Br2在248 nm处可能被激发到1B1和3B2态。由于激发时C - Br键伸长，1A'(1B1)态与基态X 1A'(1A1)的高振动态之间的耦合可能增强，从而促进内转换过程。跃迁后，高度振动态激发的CF2Br2在分解之前可能越过一个跃迁势垒。根据从头算计算，过渡态结构倾向于与中间态CF2Br+Br(CF2Br...Br)和产物CF2+Br2相关。因此，一种连续光解离途径是有利的。也就是说，单个C - Br键断裂，然后自由Br原子移动形成Br - Br键，接着是Br2消除。过渡态中形成的Br - Br键距离趋于接近平衡，使得Br2碎片可能以冷振动分布形式存在。观测到的Br2(v = 1)/Br2(v = 0)较小的振动布居比与所提出的机制相符。
