In this award, funded by the Experimental Physical Chemistry Program of the Chemistry Division, Prof. Curt Wittig of the University of Southern California and his postdoctoral, graduate, undergraduate and high school student colleagues will study the dynamics of helium nanodroplets which have absorbed an impurity species. Some of the things to be studied include the presence or absence of vortices in the helium nanodroplet, induced by the pick-up of a lead atom, and the formation of Rydberg droplets -- a lead ion sheathed by the He droplet with an extended Rydberg orbital surrounding the whole. Neither of these species have been observed previously, and as such the experiments are risky. The entire effort benefits from collaboration with theorist Anna Krylov and her students, also at USC.
Helium nanodroplets are an unusual quantum environment.that are seeing increased use as a unique, miniature laboratory for studying the physics and chemistry of atoms and molecules, but these studies require the kind of fundamental experiments outlined in this work to develop the necessary models to describe how matter behaves in these nanoscopic quantum liquids. In addition to the broader scientific impacts of these studies on physics and chemistry, Prof. Wittig will continue his mentoring of a diverse group of students -- whose career stages range from high school through postdoctoral level. In addition, he will continue to develop his unique chemical physics/physical chemistry graduate level curricular materials.
Research Results Display technologies comprise a billion dollar per year industry in which competition is fierce and global. One of the most important light emitting species in this area is an organo-metallic complex based on iridium (Ir(ppy)3 for short), which can be embedded in numerous hosts and activated using electric current. It has already entered the marketplace, albeit in embryonic form, and major applications are planned. For example, imagine a large "screen" that is one mm thick and can be rolled into a cylinder. At the same time, a number of important molecular parameters that are needed for device optimization are unavailable and/or poorly understood. Among these, ionization energy is prominent, as it impacts directly system efficiency. In addition, a detailed understanding of ionization in this system augurs well for "educated guesses" of analogous systems. Our research approached this issue on three fronts: experimental studies of one- and two-photon ionization of (1) gaseous Ir(ppy)3 and (2) Ir(ppy)3 embedded in helium nanodroplets, which provide an efficient means of cooling species in a weakly interacting host, and (3) a detailed theoretical study was carried out. One of the important outcomes of this work is an upper bound to the ionization energy (6.4 eV), with a likely value of ~ 6 eV. This is significantly lower than the only other experimental estimate in the literature: 7.2 eV. This latter number was obtained indirectly subject to a bold assumption; it is off by a large amount. Another aspect of the work dealt with fundamental studies of spin and its properties, and an extensive study of conical intersections and geometric phases has recently appeared as an invited article. Conical intersections are ubiquitous in nature, arising not only in organo-metallic complexes (and limiting their usefulness in devices), but in essentially every area of molecular science. This work includes the molecular case in a broad theoretical framework, promoting deeper understanding and related applications. Toward the end of the grant period, a parallel effort was launched to examine complex changes that occur in amorphous solid water containing embedded CO2. This area is germane to clathrate formation and dissolution, and CO2 sequestration. The experiments used laser excitation and a mass spectrometer arrangement that enabled a large amount of data to be obtained following excitation with a single laser pulse. This obviates the need for complicated deconvolution and/or extensive averaging. A paper describing the work was published last year and the future looks bright. Broader Impact A significant broader impact accomplishment is the development and dissemination of material under the auspices of what I call the "Notes Project." These are not just any set of notes. Following intense preparation, they were printed and bound professionally, resulting in 120 copies. Copies were distributed to interested parties in the US, and the material was mounted on my website as downloadable files. In 2011, C&E News interviewed me on the subject of teaching graduate-level courses on mathematical methods. After this interview appeared in print, the number of downloads skyrocketed. The notes are now expanded considerably. There are ~ 700 pages of the same quality as the original 350 pages. Again, these will be mounted on my website, printed, and distributed at no cost to those receiving copies. Future distribution will be via downloads. The notes are only part of the story. Over the last several years I developed classes that address the needs of entering graduate students with different backgrounds, including interdisciplinary areas, e.g., physics undergraduates entering our graduate chemical physics program, and students whose main emphasis is preparative and materials chemistry. Regarding the latter group, prior to this effort we have never had students from preparative and materials chemistry take chemical physics courses. This new approach has proven to be a "game changer" that serves well the preparation of students who face a changing research landscape. Much of the motivation for putting together this material grew out of the research. For example, topics that arose in our research include: radiationless processes, heat capacity and thermal conductivity in complex systems, phonons such as modes that appear in spectra of crystalline CO2, energy transfer within and between moieties, and photodissociation of gaseous and condensed phase species. These topics are covered in the notes. Where is this headed? Packages will be distributed in 2012 and 2013. Following this, emphasis will be on distillation and organization into modules, and the development of exercises and projects. To combat the widespread availability of solution manuals and worked problems, it is recommended that projects be developed around toy models, with student presentations and, importantly, each student turning in critical reviews of each of the other presentations. I envision continued interplay between research and pedagogy, with progressive enlistment of this strategy in our graduate curriculum.
