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Ultrafast Vibrational Spectroscopy


2D IR spectroscopy

Two-dimensional infrared (2D IR) spectroscopy is a tool we use to study transient molecular structure and dynamics in solution. As a vibrational spectroscopy, it directly interrogates the vibrations of chemical bonds and how the vibrations interact with one another. 2D IR spectroscopy spreads a vibrational spectrum over two frequency axes, allowing you to reveal structural and kinetic correlations. Crosspeaks in the spectrum encode the couplings and orientation between vibrations. Modeling this spectrum reveals a structure in terms of connectivity, distance, or orientation between chemical bonds. Since the measurement is made with a picosecond or faster shutter speed, it captures information on molecular structure in solution on a time scale fast compared to most dynamics.

“H2O-2Dspectra"



Ultrafast IR light sources and detection
   

We are developing broadband IR light sources that can be used for ultrafast spectroscopy across the entire vibrational infrared spectrum. Our primary effort is directed at developing a laser plasma source for femtosecond mid-IR pulses with bandwidth spanning the entire vibrational IR spectrum, and developing the technology to amplify and integrate these broadband IR pulses into 2D IR spectrometers. Additionally, we explore new methods in optical parametric amplification for generating and amplifying IR pulses, and novel techniques for compressing mid-IR pulses to their transform limit.

“Interfautocor”



Fluorescence-encoded IR spectroscopy
   

We are developing new methods of performing coherent Fourier transform IR spectroscopy with high sensitivity using fluorescence detection. Fluorescence-encoded IR (FEIR) vibrational spectroscopy is a mixed IR-visible technique that measures the modulation of visible-excited fluorescence induced by a sequence of mid-infrared pulses resonant with vibrations on the electronic ground state. Utilizing the background-free advantage of fluorescence detection, our objective is to increase the sensitivity from the 100 mM level by many orders of magnitude, thereby opening up new avenues for infrared microscopy and fluctuation correlation techniques.

Our development of FEIR spectroscopy is currently directed along two paths. One approach is focused on implementing fluorescence encoding for the ultrafast multidimensional IR spectroscopies that we currently use in all of our projects. A second track seeks to push the sensitivity of fluorescence encoding down to the 1–100 molecule level using high repetition rate laser excitation with single-photon counting. In this regime, it is possible to characterize equilibrium molecular fluctuations similar to Fluorescence Correlation Spectroscopy (FCS). With FEIR fluctuation measurements, we hope to gain a new tool for studying chemical kinetics in a structurally specific way without perturbing the system's natural equilibrium state.

“fluorescence”



Ultrafast 2D IR microscopy
   

We have developed a 2D IR microscope that combines the micron-scale spatial-resolution of IR microscopy with the bond-specific molecular information and ultrafast dynamics of 2D IR spectroscopy. This instrument allows us to map heterogeneous environments through correlations in time, frequency, and space. Vibrational dynamics such as spectral-diffusion rates or excited-state lifetimes of a known species can be used as a concentration-independent contrast agent to map micron-length variations in chemical environments.

“microscopy”



Transient 2D IR spectroscopy
   

Combining 2D IR with a fast phototriggered process allows us to follow non-equilibrium dynamics of chemical reactions and biophysical processes. 2D IR acts as the camera by which to follow the structural changes of the system with time. We currently use a nanosecond laser-induced temperature-jump (T-jump) of up to 20° to track dynamics and kinetics on 10 nanosecond to 50 millisecond time scales. This has been applied to protein unfolding, thermal dissociation of protein dimers, DNA duplex melting, proton transfer in DNA tautomerism, and lipid gel-to-fluid phase transitions.




T-jump