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New Pump-Probe Technique Improves Imaging Spectroscopy

Photonics Spectra
Jan 2005
Richard Gaughan

Pump-probe spectroscopy offers details about photochemical or photophysical reactions in a narrow slice of time, but reconstructing a reaction sequence takes a series of exposures, leading to photofatigue of the sample and other logistic problems. Now scientists at Yokohama National University in Japan and the University of Florida in Gainesville have developed a technique that makes a spectroscopic record of an entire sequence of intermediate steps within a reaction.

The team, led by Jun Takeda, an associate professor in the department of physics at Yokohama National University, uses 100-fs pulses of 800-nm light as the source for both pump and probe beams. A series of Spectra-Physics lasers generate the pulses.

A Tsunami Ti:sapphire laser produces the initial pulse, which is amplified in a Spitfire regenerative amplifier. The resulting train of pulses is split: One portion is directed right to the sample and the other, to a water cell. Interactions within the water cell convert the monochromatic source into a white-light pulse, which is directed to the sample at an angle with respect to the pump. Cylindrical lenses focus both beams so that they strike the sample in overlapping lines.

At a given instant, only one portion of the white-light probe beam will interact with the excited sample. As time passes, the probe beam continues to propagate through the excited sample, but the interaction region has shifted because of the angle between pump and probe beams. Each physical location of the sample has a different pump-probe delay time. Temporal information is spatially encoded.

The probe light transmitted through the sample is fed into the entrance slit of an Acton Research SpectraPro-300i 0.3-m grating monochromator, coupled to a 1340 × 1300-pixel Princeton Instruments UV-coated CCD detector. The CCD records a two-dimensional image, with wavelength on one axis and sample position on the other. But the angled beams have already spatially encoded the temporal information, so one 2-D exposure of the CCD represents the time history of absorption within the sample. With a 20.6° angle, the geometry results in a time resolution of 200 to 300 fs and a total frame exposure of about 5 ps.

To demonstrate the advantages of the system, Takeda's team investigated the spectroscopic evolution of absorbance in beta-carotene. The system accumulated data in 30 to 60 s of exposure to the 1-kHz pump-probe beams. Several sets of data were acquired and linked to generate the absorbance profile as a function of time for 25 ps after exposure to the pump beam. The data compared well to that generated by traditional pump-probe techniques, with the significant difference being that the traditional technique would have required an accumulation time of approximately 10 hours.

Takeda said that the new approach offers benefits in applications involving irreversible photochemical reactions such as photoaddition, photodissociation and photochromism in solid-state materials. Moreover, he believes that the team is just a couple of months away from gathering pump-probe data in nearly single-shot detection with refinements to the setup.

The technique does require a particular sample geometry -- specifically, a 5-mm homogeneous area in the sample -- and precise alignment is crucial for getting a good time resolution. Nevertheless, Takeda summarized the development simply: "We believe that this technique is better than the conventional one."


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