Molecular Spectroscopy

Bruce Weisman and his group investigate the spectroscopy and photophysics of fullerenes and carbon nanotubes. All of these are closed nanoscopic structures formed from carbon atoms. Fullerenes, such as C₆₀, C₇₀, and their chemical derivatives, have unusual molecular properties that cause interesting behaviors following the absorption of light. Time-resolved absorption and emission methods are used to study radiationless decay, photochemical reactions, and energy transfer in fullerenes. Another major research topic is single-walled carbon nanotube spectroscopy. Following the discovery in Weisman's lab of near-infrared nanotube fluorescence, the group has measured and unraveled the absorption and emission spectra of more than 30 semiconducting nanotube species. Follow-up projects include detailed elucidation of nanotube electronic structure, as well as applications in non-invasive biomedical imaging and analytical nanotechnology.

Emission-excitation Raman spectra of carbon nanotubes

The spectra, structure, and kinetics of small free radicals are being studied by Robert Curl and Graham Glass using color center and difference frequency infrared lasers. The purposes of this work are to develop sensitive methods for detecting these radicals and following their concentration in interesting chemical systems such as flames, to obtain and analyze high resolution spectra of these species thereby providing definitive information about their electronic and geometrical structure, and to study the kinetics of their reactions. The principal technique employed is kinetic spectroscopy. With this method, high transient concentrations of the radical species are produced by excimer laser photolysis of a suitable precursor and the resulting transient absorption spectrum is probed with a tunable infrared laser. By scanning the infrared probe laser wavelength while acquiring the infrared detector output a short time after the flash, the high resolution infrared spectrum of the transient free radical can be obtained. By fixing the probe laser wavelength to the peak of an absorption line and then acquiring the output of the infrared detector as a function of time after the flash, reaction rates of the radical can be determined.

Infrared Kinetic Spectroscopy Data

Experimentally, the most challenging aspect of this work is the development of infrared probe lasers which tune smoothly and cover a wide range of infrared wavelengths. In the workhorse probe laser, an AutoscanTM Ti:sapphire laser is mixed with a cw 1.064 μm Nd:YAG in periodically-poled LiNbO₃ to produce IR tunable from somewhat below 2000 cm-1 to over 4000 cm^-1. As radical species become larger, their infrared spectra become morecongested and complex becoming extremely difficult to analyze. In recent work, the radicals are cooled to about 20 K by supersonic jet expansion to depopulate the higher levels simplifying the spectra, making them much easier to analyze.

James Kinsey's research with Carter Kittrell and Bruce Johnson deals with the dynamics of single molecules that have been photoexcited to unstable electronic states. In every elementary chemical reaction, the molecular system passes through a continuously evolving intermediate species that is neither reactant nor product, but the former turning into the latter. The details of these "transition states" control the reactant-product conversion.

Spectroscopy and dynamics of bond photolysis

The separation of the fragments produced in a photodissociation is essentially a "half-collision" or "half reaction." Work in the Kinsey laboratory on photoemission during photo-dissociation gives a spectroscopic means of following the motion of the transient state leading to photochemical fragmentation. This is accomplished by recording the spectrum of light emitted by the excited molecule. Because typical times for photodissociation (~ 10^-14 s) are much shorter than typical radiative life times (~ 10^-8 s), dissociation is a powerful quenching mechanism for fluorescence. There is a tiny but finite photon yield, however, and the spectral characteristics of this radiation are extraordinarily informative about dynamic processes in the excited electronic state and in the Infrared Kinetic Spectroscopy data ground electronic state as well.

In the process of coming apart, the molecule sweeps through infinite displacements in molecular geometry, thus developing the ability to radiate into extremely high vibrational levels of the electronic ground state. The pattern of intensities in these lines is a "footprint" of the dissociation process. The Kinsey group has investigated the photoemission/photodissociation spectra of O₃, CH₃I, C₆H₅I, ClNO and several other molecules. Each of these molecules has provided surprising new insights, and much work remains to be done before they are fully understood. Related theoretical investigations of dynamics on experimentally characterized potential energy surfaces are also carried out using the techniques of time-dependent quantum mechanics.