Gonzo Spectroscopy Group: Research

The common theme in our experiments is the measurement of linear and nonlinear spectra of well defined molecules in homogeneous or nearly homogeneous environments. Instead of a detailed account of current and future research projects (which would take many pages), four typical projects are listed below.

1. Studies of the photoisomerization of linear polyene molecules substituted into single crystals and cooled to liquid helium temperatures
   The dynamics of these processes are studied by a variety of methods including measuring fluorescence decays and photochemical hole burning studies. Mapping out photoisomerization behavior as a function of the initial molecular conformation, excitation energy and lattice temperature, determines the fundamental mechanism for polyene photoisomerization in a molecular cavity. This is key to understanding a range of important photochemistry including vision, vitamin D photochemistry, and energy transduction in halobacterium halobium.

2. Measurement of excitation profiles for multiphoton ionization of molecules seeded into rare gas expansions
   To be able to study isolated, non-fluorescent molecules we have developed a spectrometer which measures the number of ions produced by 1-color or 2-color multiphoton ionization as a function of laser wavelength. Combining the powerful techniques of seeded beam laser spectroscopy and time-of-flight mass spectrometry as solved the problem of measuring high resolution optical spectra for isolated large organic molecules. In the near future we plan on adding the capability for laser desorption in the beam source which will open the possibility of studying very high molecular weight species.

3. Measurement of the wavelength dependence of nonlinear optical response for well defined conjugated systems
   Initial studies focus on measuring 2-photon fluorescence excitation spectra and the wavelength dependence of third harmonic generation for well understood linear polyenes in environments where the optical spectra are highly structured. At present, the connection between molecular electronic structure and nonlinear optical response is only poorly understood. These studies will be the first at a level of resolution where the contributions of the various electronic states can be unambiguously established. It is also anticipated that this will provide new spectroscopic techniques that will be complementary to more traditional methods.

4. Use of the resolution enhancement afforded by photochemical hole burning to determine the effects of externally applied electric fields on molecular electronic structure
   In preliminary studies we have shown that this can lead to an extremely detailed characterization of molecular electric fields. This kind of information is of special importance for understanding charge separation in condensed phase systems as, for example, in photosynthesis. Effort is divided between very detailed investigations of relatively simple systems (for example, linear polyenes in n-alkane crystals) to develop the fundamental concepts and the application of these concepts to more complicated systems (for example, myoglobin substituted with protoporphyrin IX).