In this project, funded by the Experimental Physical Chemistry Program of the Chemistry Division, Professor Jingsong Zhang of the University of California at Riverside will investigate the photochemistry of prototypical polyatomic free radicals in molecular beams. High resolution high-n Rydberg atom time-of-flight spectrometry (HRTOF) will be utilized to study spectroscopy and state-resolved photodissociation dynamics of reactive intermediates important in combustion and atmospheric chemistry. The systems to be studied include vinyl radical and vinylidene, formyl radical, and HOCO radical.
The scientific issues to be examined include: (a) open-shell electronic states and potential energy surfaces, and their nonadiabatic interactions and influences on photodissociation dynamics; (b) detailed mechanisms involving complex nuclear rearrangements and competitive dissociation pathways of free radicals; and (c) free radical electronic structures, thermodynamics, and energetics. The research will also provide high-quality benchmarks for the theoretical chemistry and physics communities.
A diversified group of graduate and undergraduate students at the University of California at Riverside will participate in this research. They will acquire knowledge and skills through the multi-disciplinary research activities. Faculty members and undergraduate students from California State University and local community colleges will participate in the research during the summer.
The spectroscopy, photochemistry, and decomposition of several polyatomic free radicals that are important in hydrocarbon combustion have been investigated. The systems include benzyl radical, phenyl radical, and allyl radical, which are aromatic and conjugate free radicals. These systems are common in that there is strong absorption in the UV region from 220-270 nm due to the excitation of the conjugate p system. Upon the UV excitation, these radicals dissociate via internal conversion of the electronically excited states to the ground electronic state, followed by unimolecular decomposition of the hot radical on the ground electronic state. For the benzyl radical, the co-fragment of the H-atom elimination channel was identified to be fulvenallene experimentally. For the phenyl radical, the lowest energy product channel, H + benzyne, was also experimentally confirmed. The studies on the benzyl and phenyl radicals have provided more mechanistic insight into the decomposition of aromatic free radicals. Laser spectroscopy and detection of atmospheric reactive free radicals have been carried out using cavity ring-down spectroscopy (CRDS). Two versions of the instrument for ambient detection and spectroscopy of the peroxy radicals have been developed. The instruments are based on the peroxy radical chemical amplification (PERCA) method combined with CRDS. The detection sensitivity as high as 4 ppt/10s has been demonstrated. Real time ambient measurements of the peroxy radicals have been carried out. The CRDS technique was also utilized to monitor sulfur dioxide (SO2), a major air pollutant that can contribute to the production of particulate sulfate and increase the acidity in the environment. The thermal decomposition of a potential oxygenated fuel component tert-amyl methyl ether (TAME) has been investigated. In the pyrolysis of TAME, molecular elimination of methanol to form iso-amylenes are the dominant process for temperatures ≥ 600K, and at higher temperatures, ≥ 950 K, secondary pyrolysis of the product olefins and homolysis of TAME itself can take place. These results indicate that TAME could be a modestly good antiknock additive in the gasoline. This project helps to train total ten graduate students in the PI's group. Four of them have been awarded Ph.D degrees during this funding period. Among these ten graduate students, four were women students and two were underrepresented minority. Several undergraduate students have been hosted in the PI's group to participate the NSF project. Especially, underrepresented minority undergraduate students from the local community were recruited into the research project to carry out summer research. The PI's lab has also hosted two visiting professors from the California State University for collaborative research. The PI has participated in many outreach activities to the local high school students in Southern California.
