Photofragment spectroscopy with lasers in the vacuum UV energy region.

用多光子电离法研究了二氧化氮和二硫化碳在308 nm处的双光子解离。碎片的角度分布表现为1 + < β >_2P_2(cos< θ > + < β >_4P_4 （cos< θ >）。由于第一个单光子过程产生的NO_2 （^2B_2）的中间态是解离的，所以四次勒让德多项式的系数< β bbb_4是可观的。然而，由于中间态CS_2（^1B_2）具有较长的荧光寿命，所以CS_2的< β bbb_4系数不明显。这些角分布不仅揭示了NO_2和CS_2的双光子激发态的对称性和寿命，而且还提供了这些分子的双光子激发过程的信息。采用激光照射308 ~ 364 nm，研究了甲基亚硝酸盐在分子束中的光解作用。光刻碎片的平动能量分布表明，ch3o和NO自由基有两种不同的产生途径；一个是单光子解离，另一个是双光子解离。碎片强度的角分布与光解光的电矢量方向正交，表明第一和第二单光子吸收的跃迁矩与断裂的O-N键垂直。利用Rydberg态双光子共振三光子电离制备的甲基碘离子，在335 ~ 403nm激光照射下进行单光子光解离，形成甲基碎片离子。这些碎片离子的平行角分布具有相当小的各向异性参数，表明解离是通过CH_3I^+的A（^2E_<1/2>）态进行的。

Two-photon dissociation of nitrogen dioxide and carbon disulfide at 308 nm has been studied by means of multiphoton ionization of photofragments. The angular distributions of fragments are well represented by a form of 1 + <beta>_2P_2(cos<theta> + <beta>_4P_4 (cos<theta>). Because the intermediate state of NO_2 (^2B_2) generated by the first one-photon process is dissociative, the coefficient <beta>_4 for the fourth degree Legendre polynomial is appreciable. The coefficient <beta>_4 for CS_2 is, however, not appreciable because the intermediate CS_2(^1B_2) state has a long fluorescence Iifetime. The angular distributions not only reveal the symmetry and lifetimes of the two-photon excited states of NO_2 and CS_2 but also information on the two-photon excitation process of these molecules.The photodissociation of methylnitrite in the molecular beam has been studied by laser irradiation at 308-364 nm. The translational energy distribution for the photofragment indicates that there are two distinct paths by which CH_3O and NO radicals are produced; one is a one-photon dissociation, and the other is a two-photon dissociation. The angular distribution of fragment intensities peaks in a direction orthogonal to the elctric vector of the photolysis light indicating that the transition moments for both the first and the second one-photon absorptions lie perpendicular to the O-N bond that breaks.Methyliodide ions prepared by two-photon resonant three-photon ionization via Rydberg states are photodissociated by a one-photon process to result in the formation of methyl fragment ions by laser irradiation at 335-403 nm. The parallel angular distrubutions of these fragment ions have rather small anisotropy parameters, suggesting that dissociation is predissociative via the A(^2E_<1/2>) state of CH_3I^+.