In the Laser Dynamics Laboratory (LDL), state-of-the-art lasers are used in spectroscopic techniques to investigate the dynamics of energy relaxation and transformation, molecular mechanisms of the primary processes of photosynthesis, and charge and energy transport in nanomaterials.

The laser systems and other equipment present at LDL can also be used by researchers interested in studying the detailed changes in molecules or materials following linear or nonlinear laser excitation in the 100 femtosecond time scale or longer. The laser-induced changes can be followed by observing the optical absorption, fluorescence, Raman , or infrared spectra of the transients. The nonlinear properties of materials can be studied applying pico- or femtosecond laser pulses, where the laser power is high and the induced nonlinear signals are intense.

The systems and techniques at LDL may be divided into the following categories:

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

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    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. 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.
  2. 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.

  3. 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.
  4. 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.

  5. 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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    1. Time-resolved Raman Spectroscopy and Fourier Transform Raman Spectroscopy

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    Although transient optical absorption is a powerful technique to study the molecular dynamic processes, it does not provide information regarding the structure of the intermediates. Vibration spectroscopy is a very powerful tool to achieve this goal. This can be studied by either time resolved Raman techniques (TRR) or time-resolved infrared spectroscopy. Even for molecules with very broad electronic transitions, the Raman spectrum, which gives the vibrational frequencies of short-lived transients, can be relatively sharp. Our ps OPO's pumped by the CPA-1000 regenerative amplifier are versatile, tunable Raman excitation sources. The Raman signal is detected by a liquid nitrogen cooled CCD detector which is mounted on a spectrograph. This system provides stray light rejection and thus enables us to detect Raman signals of less than 200 cm-1 from the excitation wavelength. The pump and probe wavelengths can be tuned independently from 240 nm to 1000 nm with energies between a few microJ to 100 microJ. Time delays may be varied from 1 ps to tens of nanoseconds. This picosecond OPA system provides a spectral resolution of ~30 - 40 cm-1. When the nanosecond MOPO 730 and the PDL-1/DCR-1 lasers are used, the time resolution of the time-resolved Raman can be extended to a range of 10 ns to tens of milliseconds. The advantage of resonance Raman spectroscopy is that it enables us to study the changes in the vibration spectra of the chromophores in biological systems without the interference of the more dense vibrations of the surrounding protein residues. This is done by using Raman excitation wavelengths in the absorption region of the chromophore and away from that of the protein residues. For Fourier Transform Raman spectroscopy, there is a Nicolet 860 FT Raman Module which allows us to acquire Stokes and anti-Stokes steady-state Raman spectra with the high sensitivity that is characteristic of interferrometric detection methods. The excitation source is 1064 nm light from a Nd:YAG laser incorporated in the commercial unit. One application of this technique is the measurement of surface enhanced Raman spectra (SERS) of molecules adsorbed on gold nanoparticle aggregates
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  1. Time-resolved IR spectroscopy (TRIR) and Time-resolved Fourier Transform Infrared (TRFTIR) Spectroscopy

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Complimentary to the Raman technique is the time-resolved infrared (TRIR) spectroscopy. In some molecules, fluorescence might be so strong that Raman studies are very difficult or impossible. In addition, changes in specific protein residues during photobiological processes can be followed by TRIR spectroscopy in conjunction with site-directed mutation. There are two ways to obtain a transient IR spectrum. The first one is the two-laser pulse transient optical spectroscopy where an IR pulse from an OPO/OPA is used as the probe. A spectrum is constructed by performing the transient absorption at many different infrared wavelengths. The advantage of this technique is that the time resolution is limited only by the laser pulse width, which is [/collapse collapsed]

  1. Fluorescence Spectroscopy

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Steady-state fluorescence spectra can be obtained with a PTI Model C60 Steady-State Spectrofluorometer using a xenon arc lamp source and a photomultiplier detection system. A PTI Model C-72 Fluorescence Lifetime Spectrometer using a PTI GL-3300 Nitrogen Laser and a GL-302 Tunable Dye Laser can be used to detect fluorescence lifetimes with nanosecond time resolution. Fluorescence decay data can be analyzed with a program written in-house. The program performs a Levenberg-Marquardt analysis to determine the multiexponential contributions of the luminescence rise and decay of each component. The emission is detected using a switcheable analog/photon counting photomultiplier. For routine measurements, we have a Shimadzu RF-5301 PC spectrofluorimeter operating in the wavelength region between 220 and 750 nm. Short excited-state lifetimes can also be measured by use of the Ti:Sapphire OPA pump-probe system by using the probe pulse to stimulate the fluorescence of the excited state. By measuring the intensity of the stimulated emission as a function of delay time between the pump and probe laser pulses, decay times of very short lived excited states can be determined.
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  1. Other Techniques

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We have a fast (microsecond) picometer to measure the time dependence of photocurrents in bR or any other photoelectric material (such as silicon or photoconductive polymers). Nanosecond lasers such as the MOPO 730 can be used to induce the photocurrent. The time resolution is limited by the capacitance of the measuring system. Submicrosecond time resolution can be achieved, using a nanosecond laser as excitation source. In addition, the Beckman DU650 spectrophotometer has a kinetics software to measure slow kinetics, such as enzyme kinetics. The time resolution is seconds or longer. A temperature regulator (NesLab RTE 100) is also attached to the sample compartment so that temperatures ranging from 5 to 70 degrees Celsius are easily achieved. For very low temperature studies, there is a low temperature helium cryostate ( CTI-Cryogenics model 22cp) with temperature range of 10 K to 300 K. From the temperature dependence of the kinetics or fluorescence intensity studies, activation energies can be obtained. Other characterization techniques also available for our use include TEM, AFM, and Micro optical & Raman Spectrometer.
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