Ion imaging studies of angular correlations in unimolecular photodissociation.

摘要

A combination of 1 + 1′ REMPI probing and ion imaging detection was implemented for measuring photofragment rotational orientation and alignment in unimolecular photodissociation. μ- v-j angular correlations were measured for molecules with different sizes and structure. Our goal was to detect preferential photofragment trajectories for large molecules with special chiral structures, possessing the possibility for unidirectional rotation of their fragments.; An ion imaging apparatus was constructed and tested by measuring angular alignment in the 650 nm photodissociation of 2-chloro-2-nitrosopropane. The study of the NO product trajectories confirmed the observations of previous TOF-MS studies about preferential v ⊥ j alignment for the velocity and the angular momentum of the NO photofragment with only a very small correlation between v and the transition dipole μ of the parent molecule. The ion images were analyzed with a specially developed numerical method for extracting angular correlations in photodissociation experiments.; The method was also successfully used for analyzing the results of subsequent experiments for 355 nm NO₂ photodissociation. Here for the first time we observed NO rotational orientation using circularly polarized probe light and a special detection polarization geometry. The NO₂ photodissociation is an example of the creation of “false” two-dimensional dynamical chirality in the plane defined by v and μ. The orientation of j with respect to v × μ was expected to originate from the specific reaction mechanism dynamics. The experimental results and semiclassical modeling of the nonaxial fragment recoil, including the parent rotation contribution, revealed that detecting preferential NO orientation is possible even for purely statistical dissociation due to the different ejection directions of the product molecules with opposite senses of rotation.; The first of their kind measurements of true dynamical chirality were performed in the 650 nm photodissociation of menthone oxime nitroso chloride (MONC) and 2-chloro-2-nitroso-6,6-dimethylbicyclo[3.1.1]heptane (CNMH). These molecules have structures specifically selected for the possibility of generating preferential senses of helical trajectories of NO fragments. The MONC and CNMH molecules showed different behavior due mainly to the different rigidity of their molecular skeleton. The average values of the bipolar moment $b00$(11), describing the pure orientational v- j correlation, were effectively zero for MONC, and −0.022 and +0.011 for the (1S)- and (1R)-CNMH enantiomers, respectively (with an error of ±0.009). The small magnitude of the v-j orientation was attributed to the preferential v ⊥ j alignment for the NO photofragment. The experimental results for the two CNMH enantiomers are the first demonstration of how structural chirality can lead to dynamical chirality in the NO photofragment trajectories.