Department of Chemistry
Research in the Douberly group employs infrared laser spectroscopy to study neutral and ionic molecular assemblies isolated in ultra-low temperature (0.4 Kelvin) helium nanodroplets. Liquid helium droplets are formed by the condensation of gaseous helium in a cryogenic nozzle expansion. The droplets are approximately 10 nm in diameter, and molecular assemblies are formed within each droplet by the sequential “pick up” of individual atoms or molecules. The interaction between the molecular solute and the helium solvent is extremely weak due to the quantum nature of helium at 0.4 Kelvin. As a result, helium droplets provide a unique environment to probe the structural and dynamical properties of the isolated species with a variety of emerging methods in high resolution laser spectroscopy.
The Douberly group is using this methodology to address a diverse set of fundamental problems in chemical physics. The general strategy of their research effort is to boil down critically important mesoscale and bulk phenomena to the cluster limit, and probe with high resolution and precision the fundamental molecular physics that underpin the larger scale phenomena. For example, they are investigating the mechanisms associated with several key elementary reactions in atmospheric and combustion chemistry. Helium mediated, low temperature reactions involving hydrocarbon radicals and molecular oxygen are probed with infrared laser spectroscopy. These measurements identify the structural configuration of key intermediates along the reaction path, along with the associated product branching ratios. One of the major impacts of this work is that these studies provide important benchmarks for theoretical studies, in which the ultimate goal is to establish a predictive combustion modeling capability that allows for the design and optimization of next generation engine technologies. Several reactions have been probed that involve the hydroxyl radical (OH) and other small atmospherically relevant species such as water, oxygen, and ozone, which are critically important to our understanding of the atmospheric ozone balance and tropospheric atmospheric chemistry in general. Once again, these measurements will provide a basis upon which predictive atmospheric chemistry models are developed.
The low temperature and rapid cooling provided by helium droplets results in a perfectly suited medium to bring otherwise transient reactants together in a way such that they are stabilized in high energy metastable configurations. The products that result from vibrational or electronic excitation of the metastable reactants can be interrogated with infrared or electronic spectroscopy. Multi-laser schemes allow for a systematic study of how product branching ratios change as vibrational energy is placed into different modes of the reacting system. A long term goal of the Douberly group’s work is to develop helium nanodroplet isolation into a general technique for studying the laser driven chemistry of highly reactive species near absolute zero.
Experiments are also underway in the Douberly lab in which small water clusters and mixed acid-water clusters are assembled in the low temperature liquid helium environment. Here they use infrared laser spectroscopy to probe the evolution of the spectral signatures associated with the formation and trapping of metastable, non-equilibrium cluster geometries, and ultimately the onset of acid ionization, which is a fundamental issue underlying a range of bulk phenomena associated with biochemistry. Furthermore, they employ this methodology to trap model biomolecule systems and to investigate their three dimensional structure, preferred conformations, rearrangements upon solvation, and thermochemistry at a high level of detail. Indeed, the correlation between the structure of biological macromolecules and their function is well recognized, and the development of high resolution structural probes is essential if we are to achieve a microscopic understanding of this relationship. This is another important fundamental area that they foresee contributing significantly to in the coming years.