Facilities

A. Transient Optical Absorption Spectroscopy (~100 fs and slower)

In this technique, a pulsed laser is used to initiate a photochemical process such as photoisomerization, photodissociation or simple optical excitation by exciting electrons from the ground to various excited states of the molecules.

1. The Ti:sapphire laser system (pulse width: 100 fs, pump probe with delay in the 100 fs to 10 ns time scale; wavelength range: 266 nm - 8 microns)

This system is very versatile in both the wavelength and the pulse width tunability. The laser system is based on the state-of-the-art, all solid-state technology. A Coherent Verdi is used to pump an ultrafast Ti:sapphire oscillator to produce pulses of 100 fs at 82 MHz. The oscillator is amplified by chirped pulse amplification (CPA) in a regenerative amplifier (1 kHz). The amplified pulses (each having ~1 mJ in energy) are used to pump second or third harmonic generators or to pump two Quantronix optical parametric amplifiers to generate widely tunable outputs ranging from 470 nm to 8 microns (with pulse widths of 100 fs or 1.5 ps). Thus, an optical process can be initiated at the desired wavelength from one optical parametric amplifier (OPA) (the "pump" pulse) and the transient optical or infrared absorption can be recorded at the wavelength of interest generated from the second OPA (the "probe" pulse). The time delay between the "pump" pulse and the "probe" pulse is controlled by a computer that drives a step motor. Therefore, the transient absorbance change can be studied with ~100 fs resolution for intermediates with lifetimes as short as 100 fs and as long as 10 ns.

In the ultraviolet, visible, and near IR regions (this is typically the region of interest for electronic excitation), the probe beam can be a white light continuum generated by focusing the ~100 fs pulses into a sapphire/quartz window. In this case, a multichannel detector such as a CCD camera is used to record the transient absorption spectrum as a function of delay time between the pump pulse and the white light continuum probe pulse. This is a very powerful technique to study the transient absorption spectrum as a function of time. Important intermediates and their temporal evolution can be recorded in the 100 fs to 10 ns time domain.

2. Pump-Probe Studies on the Nanosecond Time Scale ( wavelength 230 nm (UV) - 2.5 microm (IR), 7 ns pulse width, delay time between 10 ns - 10 ms)

The Spectra Physics MOPO 730 laser produces pulses of 7 ns duration and 0.20 cm^-1 linewidth. Its usefulness is in its wavelength tunability, which ranges from 230 nm to more than 2000 nm and its narrow linewidth, which enables spectral hole burning and fluorescence line narrowing. The MOPO system, together with a pulsed dye laser system (PDL-1/DCRI) can be used in pump-probe optical studies in many linear and nonlinear time-dependent experiments on the ns, micros, or ms time scale. Using two different colors from the two independent laser systems, one can perform kinetic studies based on pump-probe or identify the molecular structures of transients by using time-resolved resonance Raman spectroscopy. The delay time can be varied electronically from ~10 ns to tens of milliseconds.

In addition, the MOPO 730 laser coupled to our Raman system will allow the determination of the Resonance Raman excitation spectra of any system. This is particularly useful for the studies of nanoparticle phonons as a function of their sizes. The ICCD (gated intensified CCD) can follow optical spectral evolution on a time scale between 5 ns and 80 ms. This has been used by our group to study the spectral diffusion processes in porous silicon. The gating of the ICCD can also help to remove the scattered light from the detector when the fluorescence emission is relatively slow.

3. The Flash Photolysis System (10 ns to tens of ms, excitation wavelength: 266 nm, 355 nm, 532 nm, 560 - 750 nm, probe wavelength: 350nm - 800 nm)

In this system, a 7 ns laser pulse is used to initiate the photochemical process of interest. The probe is from either a 100 W Xenon arc lamp or a pulsed flash lamp. A CCD can record the change in the transient absorption with time or a fast response PMT can be used to monitor the intensity change of the probe at a specific wavelength to determine the decay or rise times of the transients. A fast LeCroy transient digitizer (~2 ns) is used to record the intensity change of the probe after the excitation laser is pulsed. Using this system, transients with lifetimes of ~10 ns to tens of ms can be studied with the sensitivity of a few mOD (thousandths of optical density).

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