Marsha Lester of the University of Pennsylvania is supported by the Chemical Structures, Dynamics and Mechanisms program to carry out a series of fundamental experimental studies of chemical reaction dynamics, many of which are based on chemical systems of significance in the atmosphere. The experiments utilize state-of-the-art spectroscopic and dynamical methods, but with the unique approach of initiating the studies from stabilized intermediates that lie along the reaction coordinate. Specifically, the planned studies focus on weakly bound species produced by association of OH and CN radicals with molecular partners on reactive potential energy surfaces. In each of these studies, fundamental or overtone excitation of the CN radical or hydride stretch of the intermediate will provide sufficient energy to initiate dynamics on a reactive potential energy surface. Experimental observables include spectroscopic parameters that characterize the reaction intermediates in high detail, for example molecular structures, vibrational frequencies, and thresholds for dissociation and/or reaction. In addition, dynamical information on dissociation and/or reaction dynamics will be obtained from the internal and/or kinetic energy release to products, yielding bond energies and enthalpies of formation for the intermediates. Experimental outcomes will be compared with theoretical calculations of the analogous properties derived from ab initio potential energy surfaces.
In this project, young scientists at the undergraduate, graduate, and postdoctoral levels will participate in forefront scientific research, developing skills, experience, and confidence required to move into the scientific and technical workforce. The participants -- many of whom are women -- use technologically advanced laser equipment as they work on basic research projects, which are also relevant to broader issues in atmospheric chemistry.
Ozonolysis has long been known to be an important mechanism in the tropospheric oxidation of alkenes originating from biogenic and anthropogenic sources. Yet there is still much unknown about this class of reactions, in particular the atmospheric fate of the carbonyl oxide species, known as Criegee intermediates, produced in the reaction. This laboratory has obtained the characteristic UV absorption spectra associated with the COO functional group of several prototypical Criegee intermediates under jet-cooled conditions, starting with the simplest CH2OO and expanding to alkyl-substituted Criegee intermediates with methyl, dimethyl, and ethyl substituents. The remarkably strong UV absorption of the Criegee intermediates also induces dissociation of the carbonyl oxides along the O-O bond. The atmospheric lifetimes of the Criegee intermediates arising from solar irradiation have been estimated from the laboratory spectral data. The initial efforts of the program focused on spectroscopic characterization of the intermolecular energy levels of the cyano radical with rare gas and molecular hydrogen partners. The program has broader impacts in training of young scientists at the graduate and postdoctoral level in forefront scientific research with potential impact in atmospheric chemistry. The results of the research have been disseminated broadly in peer-reviewed publications, at local, national and international scientific meetings, and through seminars at research universities and undergraduate institutions.