Research Topics

• Chemical Exchange Dynamics
• Dynamics of Water
• Proton Transfer Dynamics
• Protein Dynamics and Biological Water
• Complex Liquids

Experimental Methods

• 2D-IR Echo Spectroscopy
• Polarization Selective Pump-Probe
• Optical Kerr Effect Method
• UV-Vis Transient Absorption and Fluorescence

Visible and Ultraviolet Transient Absorption and Fluorescence Measurements

 

Pump-probe experiments in which a particular wavelength is used for the pump and a continuum is used as the probe were performed using a Ti:Sapphire regenerative amplified source and a CCD detection system.  In the experiments described in the Proton Transfer Dynamics in Nanoscopic Water and Bulk Liquids section, the pump beam was centered at 400 nm at 7 μJ per pulse and 65 fs duration with a spot size diameter of 200 μm.  A “white light” continuum, used for the probe, was generated by focusing 1-2 μJ of 800 nm light in a 1 mm cuvette of water.  A half-wave plate/polarizer combination was used to attenuate the 800 nm light to give the most stable white light.  The continuum generated ranged from the near-IR to somewhat beyond 390 nm and was at most 100 nJ integrated over the entire wavelength range.  The white light was separated into two beams with a beam splitter.  One beam was used as the probe and crossed with the pump in the sample, and the other one was used as a reference to monitor the intensity and spectral characteristics of the white light.  The timing between the pump and probe was achieved by passing the pump beam down a high-resolution delay line with ~1 fs resolution.

The probe and reference beams were focused into two optical fibers.  The outputs of the fibers are at the entrance slit of a 0.3 m monochromator.  The dispersed outputs of the two input beams are detected by a 1340 ×100 pixel CCD detector.  The probe and reference produce separately readout stripes on the CCD, which are used to obtain the difference absorption spectrum between pump on and pump off.  The reference spectrum permits correction for variation over time of the white light characteristics.

The transient absorption experiment also measures stimulated emission, which is the equivalent of measuring very fast fluorescence.  For longer time scale fluorescence measurements (>50 ps) time correlated single photon counting was used.  The output of a Ti:Sapphire oscillator, that can be tuned from 710 nm to 950 nm is passed through an acousto-optic single pulse selector that runs at 4 MHz.  The single pulses are doubled for excitation.  The fluorescence is detected through a computer controlled polarizer and a monochromator by a channel plate detector and time correlated single photon counting electronics.  The fluorescence is measured at the magic angle for population relaxation and I_|| and I_^ are measured and combined as shown in figure to obtain the time dependent fluorescence anisotropy.

UV-Vis Transient Absorption and Fluorescence Measurements Recent Publications

352.     “The Deprotonation Dynamics and Stokes Shift of Pyranine (HPTS),” D. B. Spry, A. Goun, and M. D. Fayer J. Phys. Chem. A 111 230-237 (2007).

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