Femtosecond Technique Probes Photochemistry

Hank Hogan

After seven years of effort, researchers at the University of California, Berkeley, have developed a femtosecond stimulated Raman spectroscopy instrument capable of illuminating chemical reaction mechanisms that take place in less than a picosecond. The group recently used the device to uncover previously hidden details regarding the transformation of the visual pigment rhodopsin from one configuration to another after photon absorption. The results indicate how the biomolecule converts light into other forms of energy and information so efficiently.

According to Richard A. Mathies, a chemistry professor at the university and head of the research team, that’s only the beginning of what can be discovered. The technology expands the ability of scientists to study the mechanisms of all types of light-to-energy and light-to-information transduction systems, he said.

Because mid-infrared Raman spectroscopy probes molecular vibrational states, it provides information about molecular structure. However, spontaneous Raman spectroscopy has a picosecond time resolution, which is too slow for many reactions. It also suffers from a poor signal-to-noise ratio and long data acquisition times. Femtosecond stimulated Raman spectroscopy, in contrast, has a time resolution of better than 100 fs, provides excellent vibrational data and has an acquisition time of only seconds.

In their instrument, the scientists generate three pulses from a Ti:sapphire source. They create the first, which is 30 fs wide, using a nonlinear optical parametric amplifier and adjust the output wavelength as needed to initiate the photochemical reaction being studied. Using a custom grating pair, they produce the second, a 3-ps burst at a wavelength of 800 nm with a spectral width of 0.3 nm. This acts as an energy-defining pump in the stimulated Raman process.

They generate the final pulse by sending light through a sapphire plate, resulting in a compressed Raman probe continuum of 20-fs duration to the red of the pump that stimulates all Raman transitions in the target molecule simultaneously. They control the important spectral distance between pump and probe with a long-pass filter. Mathies said cross correlation between the first and third pulses determines the system’s 50-fs temporal resolution.

To detect the signal that arises when the three pulses hit a solution containing the biomolecule of interest, the researchers use dual photodiode arrays operating in parallel. They measure the probe spectrum alone and with the stimulated Raman superimposed on it to correct for fluctuations in the continuum shape and intensity of the probe from shot to shot.

“A photodiode array is much better than a CCD in this case, since the full-well depth capacity is much better with the diode array and we can integrate much longer on the chip before readout,” Mathies said.

Plans are to examine other biomolecules with the system. Mathies said that the mechanisms behind efficient energy storage and quantum yield in the visual system are of particular interest.