Ultrafast Spectroscopy

Laser source for ultafast spectroscopy in DeCamp Lab.
2D spectroscopy: emission frequency plotted against the excitation frequency allows to read energy transfer from cross peaks.
Two-dimensional infrared spectra of Rh(CO)_2C_5H_7O_2 (RDC) in hexane acquired by averaging 100000 laser shots (DeCamp Lab).
Different amide bands, which provide nearly full coverage of protein backbone dynamics, can be probed by 2D spectroscopy.

Ultrafast spectroscopy refers to pump-probe experiments using picosecond or femtosecond lasers to get time-resolved spectral information. This is not necessarily better than "normal" frequency-domain spectroscopy but does have some important advantages. Most importantly, time-domain spectroscopy is the only method by which one can see the thermodynamically irreversible flow of energy through a system. Time-domain spectroscopy also gives researchers a time-domain point of view of the world giving new insights.

Higher-order time-domain spectroscopy can be used to greatly simplify spectra and obtain novel information about the structure of molecules. For example, echo techniques can be used to remove inhomogeneous distributions of line positions to reveal the "true" spectral line

Traditionally ultrafast spectroscopy was restricted to gas phase reactions, where molecules are considered to be isolated. It is therefore easier to distinguish spectral features and energy transfer mechanisms than in condensed phase systems. These experiments have lead to an ever increasing knowledge of gas phase reactions resulting in quantum control of reactions in the gas phase. Since the development of reliable solid-state ultrafast lasers over 10 years ago, ultrafast spectroscopy is not limited to gas phase reactions. The ability to chose pulsewidth, wavelength, or amplify the traditional Ti:S output means ever more increasing applications in condensed phase systems as well.

The ultrafast spectroscopy methods are becoming increasingly important in the study of biological systems. Globular proteins, for example, are nearly solid density, and yet to function they must often be flexible. Essentially, many biological molecules are distinguished by the very fact that they are not neatly categorized as solids or liquids, but are rather something in between.

Although X-ray crystallography is the most widely used tool for investigating the structure of proteins and of other biomolecules, it generates a static snapshot. Nuclear magnetic resonance (NMR) spectroscopy can be used to observe dynamic motions, but NMR is a relatively slow technique and cannot resolve dynamic information below the millisecond regime. Whereas this is suitable for investigations of many biological processes, it cannot answer questions, for example, about the early stages of protein folding or unfolding. Optical spectroscopies offer much higher time resolution but provide only low-resolution structural information. The structural investigation of ultrafast biological processes can be attacked using two-dimensional optical methods.

The backbone of any ultrafast spectroscopy lab is the laser source. The DeCamp lab has a 1kHz Spectra-Physics Spitfire XP amplifier seeded by a KMLabs Oscillator. The laser generates 3mJ, sub-40 fs laser pulses for ultrafast spectroscopy. The laser output is currently being upgraded to 7+mJ at 1kHz by the instillation of a cyro cooled multipass power amplfier. Using modest focusing, this laser will be able to generated fields greater than 1017W/cm2 and with a power amplifier its output optical pulse energy will increase by a factor of three.

Research Projects:

  • molecular dynamics of biomolecules in solutions.
Selected Publications: 

M. F. DeCamp, L. DeFlores, K. C. Jones, and A. Tokmakoff, Single-shot two-dimensional infrared spectrometer, Optics Express 15, 233 (2007). [URL]