The major focus of our current work concerns the development
and application of femtosecond spectroscopy for the study of the
structure and dynamics in a wide range of materials which include
liquids, supercritical fluids, photodissociative molecules, biologically
important species and wide band gap semiconductors. With our amplified,
high repetition rate, Ti:sapphire based optical parametric amplifier
laser system femtoseond pulses throughout the visible and ultraviolet
are routinely generated. The experimental projects of current interest
include:
1. Dispersed Optical Heterodyne Detected Birefringence and Dichroism.
In this novel electronically nonresonant two-dimensional spectroscopy
we combine elements of both time and frequency domain probes
in order to address questions regarding the nature of condensed
phase and fluid motions on the subpicosecond and picosecond time
scales. The probe must be amplitude modulated in order to address
questions regarding the nature of these intermolecular interactions
(low frequency modes). This is accomplished here with a simple
lens prism combination pulse shaper. These results in simple
liquids at least (CS2, CO2, etc.) are compared with simulations
and instantaneous normal mode calculations.
2. “Designer Pulses”: Phase and Amplitude Control
of Material Responses. The goal here is to acquire additional
control of the ultrafast responses of materials by designing
phase as well as amplitude modulated ultrafast pulses. This is
accomplished in a more sophisticated fashion than just the simple “pulse
shaper” mentioned above via liquid crystal spatial light
modulators. These phase and amplitude “tailored” pulses
can be optimized in order to address questions of liquid structure
and relaxation photochemistry.
3. Ultraviolet Pump-Probe Femtosecond Spectroscopy. Transform
limited pulses as short as 40 fs tunable through out the 245
- 320 nm regime are generated with our current amplified high
repetition rate (100 - 300 KHz) OPA system. These wavelength
are ideal for pump-probe studies of nucleic acids and aromatic
containing proteins. Impulsive pump excitation provides a probe
of the low frequency regime of these macromolecules. In addition
to these biologically relevant chromophores, these ultrafast
UV pulses are ideally suited for studies of the charge carrier
relaxation dynamics of novel wide band gap semiconductors. The
development of such materials is an active area of research at
the Boston University Photonics Center (http://www.bu.edu/photonics).
UV pulses in this wavelength range will be subsequently upconverted
to the 200 - 230 nm region for femtosecond pump-probe studies
of the amide residue itself.
4. Two-photon Resonant Optical Heterodyne Detected Dichroism
and Birefringence: The role of two-photon electronic resonances
in pump-probe spectroscopy. Ultrafast excited state relaxation
processes are often evident in one-photon resonant pump-probe
studies of chromophores in both the gas and condensed phases.
Within the framework of a simple vibronic density matrix picture
the nonstationary excited electronic state responses probed in
two pulse studies decays with a time constant associated with
electronic population decay (T1) or excited state vibrational
coherence decay (T2) issues of inhomogeneous broadening aside.
The electronic or optical dephasing rate to the resonant transition
does not directly govern the time scale of these one-photon resonant
decays. We have just begun some preliminary studies of ultrafast
OHD birefringence and dichroism in order to explore the role
of two-photon resonances in two pulse pump-probe femtosecond
spectroscopy and hence directly observe optical dephasing decays
in solutions and fluids. The goals of these studies are to help
distinguish the multiple time scales anticipated in these responses,
the relative homogeneous and inhomogeneous contributions to these
purely electronic coherence decays and solvation effects on photodissociation
in photodissociative electronic levels.
5. Spontaneous Resonance Raman Studies of Photodissociative
and Biological Chromophores. We have along standing interest
in the use of complementary frequency domain studies of short-lived
and biologically relevant molecules via ultraviolet resonance
Raman spectroscopy. Current experiments, in particular, exploit
the redistribution of resonance emission between resonance Raman
and resonance fluorescence type emissions as a probe of ultrafast
photodissociation, optical dephasing and solvation time scales.
These dynamics are compared with the results of MD simulations
and INM analyses and complement the results of direct time domain
studies. See publications for more details.
