Ultrafast Nonlinear Infrared Spectroscopy
The Petersen Group develops and utilizes novel infrared (IR) spectroscopies to directly probe ultrafast dynamics at surfaces and in the bulk. Mid-IR spectroscopy directly interrogates the vibrations of the chemical bonds that compose molecules and offers a direct and local probe of molecular structure and nuclear motion. Nonlinear spectroscopy involves processes in which, the sample makes multiple interactions with light (defining the order) leading to the emission of a signal photon. This include time-resolved methods that use multiple laser pulses of ultrafast or femtosecond (10-15 s or fs) duration to study molecular processes and chemical reactions on the natural time-scale of molecular motion. Nonlinear infrared spectroscopy is technology driven, where breakthroughs are made through method development. Each of the orders of spectroscopy has its own advantages as well as technical challenges and complexity that motivate our development of advanced techniques and methods. We are developing new methods and expanding the capabilities of current infrared spectroscopies through the first four orders of nonlinear spectroscopy to solve existing problems and controversies, and discover new phenomena.
Technical Developments
We have made innovations within the first four orders of vibrational spectroscopy. The technical developments include:
1st order (linear) spectroscopy:
– Solvation Shell Spectroscopy
2nd order spectroscopy:
– SFG
– Chiral SFG
– Phase- sensitive SFG
3rd order spectroscopy:
– CIR
– Transient IR
– 2D IR
4th order spectroscopy:
– Transient and 2D SFG
Solvation Shell Spectroscopy
Water is a remarkable solvent that, facilitated by the flexibility of the hydrogen-bonded network, provides an adaptable and stabilizing solvation environment for small molecules and biomolecules alike. The solute-water interactions cause a perturbation of the water structure nearest to the solute molecule. This is defined as the solvation shell, where the water structure is perturbed and distinct from the bulk solvent. The spectrum of the solvation shell is obtained through a very accurate subtraction between the spectra of the solution and that of the solvent. Analysis of the OH stretch of the water molecules in the solvation shell provides insight into the distribution of hydrogen-bond strengths in the solvation shell. This provides information on both the fundamental physical chemistry of solvation, including the hydrophobic effect, and the biochemistry and biophysics of biomolecules. For example, the solvation shell spectrum of an anti-freeze protein, shown to the right, revealed that the protein perturbs the water structure 3 layers away at room temperature thus providing insight into the function of anti-freeze proteins.
Surface-Specific Vibrational Spectroscopy using Sum Frequency Generation (SFG)
Sum-frequency generation (SFG) is a second-order nonlinear process where two photons are incident on a sample, and a photon with the sum of the energies is emitted. The method require broken inversion symmetry, which is found at interfaces thus making SFG a surface specific technique for liquids.
Water at Tunable Interfaces
Aqueous interfaces play defining roles in a wealth of atmospheric, biological, and technical processes. We use self-assembled monolayers to tune the physical and chemical properties of surfaces. Using our technical developments in SFG, we studied the structure, orientation, and dynamics of interfacial water as a function of surface chemistry at hydrophobic (OTS), hydrophilic (PEG) and mixed (OTS/PEG) self-assembled monolayer interfaces. We are currently extending these studies to zwitterionic surfaces and exploring effects of substrates on their surface chemistries.
Chiral SFG of DNA
SFG can probe chirality of buried, solvated biomolecules and the chiral imprint on the surrounding solvent. In Accordingly, we found that the solvation shell of water around DNA form a chiral water super-structure templated by the chiral DNA structure.
Heterodyne-Detected SFG
In conventional SFG, phase information is lost and interferences between different spectral features cause distortions to the observed spectrum. This can be avoided by heterodyne (HD) detection, which present further technical challenges.
With our experimental developments it is now possible to probe both exposed and buried solid-liquid interfaces in different polarization combinations with HD SFG, providing further information about the vibrational modes in the interfacial region.
Ultrafast mid-IR continuum spectroscopy
Ultrafast time-resolved vibrational spectroscopy reveals vibrational dynamics and structural information about bulk systems. However, the method is normally limited to about 10% of the vibrational spectrum in a single experiment. In the Petersen group, we use a continuum IR pulse that spans the entire vibrational spectrum and thus allows for the observation of dynamics and couplings across the very broad spectra and of complex systems. Coupling between different vibrational modes can be observed in a 2D IR experiment, where a pair of pump pulses is used to frequency-resolve the excitation axis to provide a 2D spectrum connecting the excitation frequency to the detection frequency.
Vibrational Energy Pathways Revealed by mid-IR Pump –CIR Probe:
2D IR Spectroscopy
Strongly hydrogen-bonded motifs provide structural stability and can act as proton transfer relays to drive chemical processes in biological and chemical systems. Our mid-IR continuum spectroscopy studies of the cyclic 7-azaindole–acetic acid (7AI–AcOH) heterodimer revealed the vibrational relaxation dynamics and couplings of this complex hydrogen-bonded system.
Proton Transfer Dynamics
Proton transfer through the hydrogen bonds of DNA base pairs is a proposed mechanism for the photostability of DNA. The asymmetric 7AI–AcOH dimer has a similar structure to adenine-thymine base pairs and is believed to undergo double-proton transfer after UV excitation. UV pump – CIR probe spectroscopy of the heterodimer allows the moving protons to be tracked directly during this process.
Petersen Group 