Time-resolved Resonance Raman Spectroscopy with a 2.0 MeV Van de Graaff Accelerator

A unique experimental capability for free radical structure and kinetics determination

Pulses of electrons from this accelerator are used to create free radicals in solution, where they are studied by time-resolved resonance Raman spectroscopy. Raman spectroscopy provides vibrational frequencies of the molecules under investigation. Since each molecule has a different characteristic set of frequencies the Raman spectrum can provide structural information about the radical. The time-resolved feature makes Raman spectroscopy valuable as a tool for following reaction rates. This facility is the only one in the world where radiolytic products are studied by Raman spectroscopy.

A schematic of the experiment is shown on the right. The laser light is set to a frequency near an absorption frequency of the radical under study. Some photons collide with molecules in the sample and are scattered out of the laser beam. Most are scattered elastically, that is, they do not exchange energy with the molecule. Sometimes the collision excites a vibration in the molecule. Then the molecule absorbs enough energy from the photon to equal the energy of the vibration, and the slightly less energetic photon emerges with a longer wavelength. This wavelength-shifted light makes up the Raman spectrum of the molecule. A variable delay between the electron pulse and the laser pulse provides time resolution for kinetics measurements.

The characteristics of the Van de Graaff accelerator are
Nominal beam energy: 2.0 MeV
Pulse duration: 100 nanoseconds to 5 microseconds
Pulse frequency: 2 to 60 Hz
Maximum beam current: 1 ampere

The characteristics of the Raman detection system are
Probe laser: excimer-pumped dye laser (335 to 650 nm)
Laser pulse energy: 5 to 15 millijoules
Spectrometer: 0.85 m Czerny-Turner
Diode array detector: OMA with intensified, gated channels spanning approximately 100 Å
Spectral accuracy: <2 cm-1

In this experiment, transients with optical absorbances greater than 1000 M-1 cm-1 can be observed.

Supported by the Division of
Chemical Sciences
Office of
Basic Energy Sciences
at the
U.S. Department of Energy

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Radiation Laboratory
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Tel: (574) 631-6163
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Last Modified: 02/02/2006