Ultrafast intramolecular charge transfer in transition metal mixed valence complexes. We are currently studying transition metal mixed valence complexes of the form shown to the left using the cyanide stretching frequencies as reporters of the molecule in the ground and electronic excited states. Recently we have shown that the photoinduced Fe→Pt electron transfer is accompanied by back electron transfer on a 110 fs timescale with a high degree (n>6) of vibrational excitation along the bridging cyano ligands. Our third and fifth order experiments provide a detailed picture of non-equilibrium inter and intra-molecular vibrational relaxation following the ultrafast back electron transfer and reveal how the nuclear coordinates are coherently coupled to the charge transfer process.Quantitative vibrational spectroscopy of the nitrosyl stretching vibration in Fe(II) and Fe(III) complexes: probing structural dynamics in model systems and active sites of proteins. The nitrosyl stretching vibration is an extremely sensitive probe of its local environment.
We are pursuing 2D IR studies on the nitrosyl stretching vibration in sodium nitroprusside (SNP) to understand the details of the changing local solvent environment around the Fe-NO bond. We have used picosecond transient infrared spectroscopy of SNP in methanol to simultaneously detect and characterize photoinduced linkage isomerism and photodissociation of the metal – NO bond. Following our work on model inorganic compounds, we are studying the conformational dynamics of pH sensitive Nitrophorin proteins. We use 2D IR spectroscopy to map the structural heterogeneity around the heme-nitrosyl pocket as a function of pH to shed light on the structure-function relationship in this protein family.
Development of novel femtosecond multidimensional spectroscopies. In an effort to directly correlate electronic and vibrational motion, we are developing new third (S(3)) and fifth (S(5)) order nonlinear spectroscopies. These techniques employ a sequence of femtosecond optical and IR pulses to interrogate the sample of interest. We have used these techniques to map out non-equilibrium vibrational phase and amplitude relaxation among coupled vibrational modes following ultrafast electron transfer.
In a parallel effort, we are developing a tunable broadband IR source spanning an octave of frequencies in the mid IR (2.5- 8.5 microns). We have shown that the IR source is stable and has significant energy per pulse at 1 kHz. Our goal is to use this source in multidimensional IR spectroscopy to coherently excite and probe a broad range of frequencies in complex molecular systems.
Ultrafast X-ray absorption spectroscopy: visualizing charge transfer processes in solution with high spatial and temporal resolution. Ultrafast X-ray absorption spectroscopy is an ideal tool for understanding local time-dependent phenomena accompanying chemical processes in solution as it provides element-specific information about electronic and structural rearrangements with sub-angstrom resolution. We are currently pursuing transient X-ray experiments at the Ru L-edge. The experiments are complemented with TD-DFT simulations of core level excitations revealing a detailed atomic-level picture of photoinduced charge transfer processes in solution. The X-ray experiments are performed at the Advanced Light Source located at Lawrence Berkeley National Laboratory.