RESEARCH INTERESTS

Current Research

The realm of most traditional areas of chemistry is the Earth, which consists of "only" ~10⁵⁰ molecules. However, there are ~10⁶⁶ molecules in the Milky Way galaxy, which makes the study of Earth's chemistry seem like a small part of the overall picture. Our research is in the emerging field of "astrochemistry," the study of molecules of astronomical importance.

Our primary tool is high-resolution spectroscopy, which has the ability to provide unambiguous and detailed information about the structure and intramolecular dynamics of free molecules. This structural and dynamical information is not only of fundamental interest, but is also valuable in many branches of physical, organic, inorganic, and astrochemistry. In the latter case, spectroscopy serves as our only probe of the physical and chemical conditions of interstellar clouds, which represent the starting material for the formation of stars and planetary systems.

Molecular Ion Spectroscopy. Molecular ions are rare on Earth because they are highly reactive, and consequently they are challenging to study in the laboratory. However, their reactivity makes molecular ions essential for the chemistry at the low densities (~100 cm^-3) and temperatures (~30 K) of interstellar space. In fact, there are ~10,000 times more CH₅⁺ molecules in the Milky Way than there are all molecules on Earth. We are developing cutting-edge laboratory techniques for the laser spectroscopic study of molecular ions in the gas phase. This involves combining supersonic expansions (which produce cold molecular ions) with ion-beam techniques (to separate the ions from the neutrals) and ultra-sensitive spectroscopic methods (which will approach the limit of single-molecule absorption spectroscopy). This powerful combination promises to usher in a new era in molecular ion spectroscopy.

Structure of Carbocations. We are particularly interested in studying the structure and intramolecular dynamics of carbocations such as CH₅⁺ and C₆H₇⁺. These molecules not only play key roles in astrochemistry, but they also serve as reactive intermediates in organic chemistry (S_N1 reactions and electrophilic aromatic substitution reactions). Such molecules are fundamentally interesting because they violate the "rules" of chemical bonding (carbon is only supposed to make four bonds, not five). A close interplay between spectroscopic experiments and theoretical calculations will be needed to achieve a full quantum-mechanical understanding of molecular ions such as these.

Observational Astrochemistry. With laboratory spectra obtained in the gas-phase at low temperatures in hand, we can use the techniques of astronomical spectroscopy to measure the concentrations of molecular ions (and neutral molecules) in the interstellar medium using powerful ground-based and space-based telescopes. The observed concentrations can then be interpreted using models based on chemical kinetics to serve as a remote probe of both the chemical and physical conditions in interstellar clouds. There are myriad unsolved mysteries in this young and highly interdisciplinary field!

Molecular Physics of Hydrogenic Species. Another aspect of our research involves the study of the simplest polyatomic molecule: H₃⁺. In collaboration with a group at the University of Stockholm, we are studying the electron recombination of individual quantum states of H₃⁺, using a supersonic expansion source. In our laboratory, we are also studying the nuclear spin exchange "reaction" between H₃⁺ and H₂. Both of these processes are important in the interstellar medium, and we are performing complementary observations and modeling to utilize the results of these experiments.

Previous Research

The following links contain brief descriptions some of the major laboratory and observational research projects Prof. McCall has been involved in as a graduate student at the University of Chicago and as a Miller Fellow at the University of California at Berkeley. Some of these research topics are still being aggressively pursued.

 

Laboratory Projects - Berkeley	Observational Projects - Berkeley
* Storage-Ring Dissociative Recombination Measurements * Laboratory Spectroscopy of C3 * Investigation of New Raman-shifting Techniques	* H3+ towards Zeta Persei and a High Cosmic-Ray Flux * Diffuse Interstellar Bands * Carbon Chains in the Diffuse Interstellar Medium
Laboratory Projects - Chicago	Observational Projects - Chicago
* Laboratory Spectroscopy of H3+ * Compilation and Analysis of H3+ Spectroscopic Data	* Discovery of H3+ in the Diffuse Interstellar Medium * Survey of H3+ in Dense Interstellar Clouds