Picosecond Laser Flash Photolysis

Ultrafast spectroscopy is an interdisciplinary area of research that spans various disciplines in chemistry and provides essential information on the mechanistic and kinetic details of chemical events that occur in the timescale of 10 femtoseconds to 100 picoseconds. The chemical events in these experiments are initiated by an ultrafast laser pulse (pump) and the photophysical and photochemical events are probed by another ultrafast laser probe pulse.

A mode-locked, Q-switched Continuum YG-501 DP Nd:YAG laser (pulse width ~18ps) is the workhorse of our picosecond pump-probe experiments. The fundamental output (at 1064 nm) can be doubled (532 nm), tripled (355 nm) or quadrupled (266 nm) to provide the excitation (pump) wavelength. The white continuum picosecond probe pulse is generated by passing the fundamental output through a D2O/H2O solution. An optical delay rail employed to control the delay time of the probe pulse enables detection of transients at desired time intervals after the sample excitation. The output of the probe pulse is fed to a spectrograph (HR-320, ISDA Instruments, Inc.) via fiber-optic cables and is analyzed with a dual diode array detector (Princeton Instruments, Inc.) interfaced with a computer.

In contrast to most picosecond laser flash photolysis systems, the pump and the probe beams are at right angles to each other with sample solution contained in a 1 cm cuvette. This geometry not only simplifies the use of the apparatus but also reliability of the absorbance values and spectral resolution. The spectral region covers the range of 400-980 nm with a detection limit of 0.005 absorbance unit. Time-resolved absorption spectra can be recorded from 0 picoseconds to 8 nanoseconds at delay intervals of 1 picosecond. The cross section of these spectra at any given wavelength can be analyzed to obtain the kinetic information of the transients. The methodology of picosecond laser flash photolysis is useful in characterizing singlet and triplet excited states, excited state energy transfer, inter- and intramolecular electron transfer reactions, charge transfer complexation and radical recombination in aqueous and nonaqueous solvents as well as in heterogeneous systems such as colloidal suspensions and thin films.

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

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Last Modified: 02/02/2006