Li Guosheng, Parr Jessica, Fedorov Igor, Reisler Hanna
Department of Chemistry, University of Southern California, Los Angeles, CA 90089-0482, USA.
Phys Chem Chem Phys. 2006 Jul 7;8(25):2915-24. doi: 10.1039/b603107b. Epub 2006 Apr 13.
The state-to-state predissociation dynamics of the HCl-acetylene dimer were studied following excitation in the asymmetric C-H (asym-CH) stretch and the HCl stretch. Velocity map imaging (VMI) and resonance enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Different vibrational predissociation mechanisms were observed for the two excited vibrational levels. Following excitation in the of the asym-CH stretch fundamental, HCl fragments in upsilon = 0 and j = 4-7 were observed and no HCl in upsilon = 1 was detected. The fragments' center-of-mass (c.m.) translational energy distributions were derived from images of HCl (j = 4-7), and were converted to rotational state distributions of the acetylene co-fragment by assuming that acetylene is generated with one quantum of C-C stretch (nu(2)) excitation. The acetylene pair-correlated rotational state distributions agree with the predictions of the statistical phase space theory, restricted to acetylene fragments in 1nu(2). It is concluded that the predissociation mechanism is dominated by the initial coupling of the asym-CH vibration to a combination of C-C stretch and bending modes in the acetylene moiety. Vibrational energy redistribution (IVR) between acetylene bending and the intermolecular dimer modes leads to predissociation that preserves the C-C stretch excitation in the acetylene product while distributing the rest of the available energy statistically. The predissociation mechanism following excitation in the Q band of the dimer's HCl stretch fundamental was quite different. HCl (upsilon = 0) rotational states up to j = 8 were observed. The rovibrational state distributions in the acetylene co-fragment derived from HCl (j = 6-8) images were non-statistical with one or two quanta in acetylene bending vibrational excitation. From the observation that all the HCl(j) translational energy distributions were similar, it is proposed that there exists a constraint on conversion of linear to angular momentum during predissociation. A dimer dissociation energy of D(0) = 700 +/- 10 cm(-1) was derived.
在非对称C-H（asym-CH）伸缩振动和HCl伸缩振动激发后，研究了HCl-乙炔二聚体的态-态预解离动力学。利用速度映射成像（VMI）和共振增强多光子电离（REMPI）来确定成对相关产物的能量分布。对于两个激发振动态，观察到了不同的振动预解离机制。在asym-CH伸缩振动基频激发后，观察到了υ = 0且j = 4 - 7的HCl碎片，未检测到υ = 1的HCl。通过假设乙炔是在一个C-C伸缩（ν(2)）激发量子下产生的，从HCl（j = 4 - 7）的图像中导出了碎片的质心（c.m.）平动能量分布，并将其转换为乙炔共碎片的转动状态分布。乙炔成对相关的转动状态分布与统计相空间理论的预测一致，该理论仅限于1ν(2)中的乙炔碎片。得出的结论是，预解离机制主要由asym-CH振动与乙炔部分中C-C伸缩和弯曲模式的组合的初始耦合主导。乙炔弯曲与分子间二聚体模式之间的振动能量重新分布（IVR）导致预解离，该预解离在乙炔产物中保留了C-C伸缩激发，同时将其余可用能量进行统计分布。在二聚体HCl伸缩振动基频的Q带激发后的预解离机制则大不相同。观察到了高达j = 8的HCl（υ = 0）转动状态。从HCl（j = 6 - 8）图像导出的乙炔共碎片中的振转状态分布是非统计的，乙炔弯曲振动激发有一个或两个量子。从所有HCl(j)平动能量分布相似的观察结果出发，提出在预解离过程中存在线性动量到角动量转换的限制。得出二聚体解离能为D(0) = 700 +/- 10 cm(-1)。
