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

Ultrafast 2D IR Vibrational Echo Spectroscopy

Figure 19 is a schematic representation of the experiment and the pulse sequence. There are three excitation pulses, ~60 fs in duration produced with a Ti:Sapphire regenerative amplifier pumped optical parametric amplifier (OPA) (see below). The pulse sequence is shown at the bottom of the figure. The OPA is tuned to the frequency of the vibrational mode under investigation. The IR pulses have sufficient bandwidth to span the spectral region of interest. The times between pulses 1 and 2 and between pulses 2 and 3 are called t, and T_w, respectively. The vibrational echo signal radiates from the sample at a time £t after the third pulse in a unique direction. The vibrational echo signals are recorded by scanning t at fixed T_w. The signal is spatially and temporally overlapped with a local oscillator for heterodyne detection, and the combined pulse is dispersed by a monochromator onto an IR array detector. Heterodyne detection provides both amplitude and phase information. If the vibrational echo is detected as it emerges from the sample, there is no phase information. The experiment produces a 2D spectrum in the frequency domain. The measurements are made in the time domain. To go from the time domain to the frequency domain requires two Fourier transforms. To perform a Fourier transform requires both amplitude and phase information. The local oscillator is another IR fs pulse. When it overlaps with the vibrational echo wave packet, interference occurs. As shown below, when the time t is scanned, the vibrational echo pulses moves in time across the fixed local oscillator pulse. The result is a temporal interferogram that provides the necessary phase information.

As shown in figure 19, the combined vibrational echo/local oscillator pulse is passed through a spectrometer and recorded by an IR array detector. The array detector allows us to measure 32 wavelengths simultaneously, reducing the data acquisition time by a factor of 32. Taking the spectrum of the heterodyne-detected vibrational echo signal performs one of the two Fourier transforms and gives the wm axis (m for monochromator) in the 2D IR spectra. When t is scanned, a temporal interferogram is obtained at each wm. The temporal interferograms are numerically Fourier transformed to give the other axis, the wt axis. 2D IR spectra are obtained for a range of T_ws.

Figure 20 shows a very simple example of the nature of the data. The upper left portion of the figure shows the spectrum of the hydroxyl stretching mode of phenol-OD. The H in the OH hydroxyl group has been replaced by a D. The IR absorption spectrum shows a single peak. The structure of the molecule is shown as an inset. As outlined to the right of the spectrum, when t is scanned, the time of the output of the vibrational echo electric field, which is the signal (S), changes relative to the fixed in time of the local oscillator electric field (L). L is much larger than S. The detector measures the absolute value squared of the sum of the electric fields. This gives the three terms. L2 is constant in time. S2 is very small and not detected relative to the much larger cross term, 2LS(t). As t is scanned, the temporal interferogram is created. A portion of an interferogram is shown in the bottom left side of the figure. There is one such interferogram for each monochromator frequency, wm. wm is the vertical axis of the 2D IR spectrum shown in the lower right portion of the figure. This axis is obtained from the frequency measurements performed by the monochromator. The horizontal axis, the wt axis, is obtained by numerical Fourier transformation of interferograms like that shown in the figure. There is one interferogram at each wm where there is signal.

The 2D IR vibrational spectrum at the time T_w = 16 ps has two peaks. The peak on the diagonal (the dashed line) is positive going (red). The peak off-diagonal is negative going (blue). The wt axis is the frequency of the first interaction of the molecules with the radiation field (first pulse). This first interaction is represented by the dashed arrow on the left side of the energy level diagram that is to the right of the spectrum in figure 20. The first pulse makes a coherent superposition state of the vibrational ground state (0) and the first vibrationally excited state, (1). The second pulse, represented by the solid arrow in the energy level diagram transfers the vibrational coherence to populations in the 0 and 1 levels. Phase information is stored in the populations. The third pulse between 0 and 1 (dashed arrow between 0 and 1) again produces a coherent superposition state (a coherence) which gives rise to the vibrational echo emission (wavy arrow) at the frequency the molecules interact with the third pulse. The wm axis is the axis of the vibrational echo emission. The 0-1 vibrational peak is on the diagonal because the first interaction of the molecules with the first radiation field (w_t axis) is at the same frequency as the last interaction and vibrational echo emission (w_m axis).

The off-diagonal 1-2 peak in the spectrum is shifted along the wm axis by the vibrational anharmonicity (unequal spacing of the energy levels as shown in the energy level diagram). The first two interactions are the same as discussed above. They produce population in the 1 level. Because the bandwidth of the laser is large, it is greater than the anharmonic shift of the levels. So the third interaction can produce a coherence between the 1 and 2 levels, which is represented by the dashed arrow connecting the 1 and 2 levels. The 1-2 coherence gives rise to vibrational echo emission at the 1-2 transition frequency. Because the first interaction (wt axis) is at the 0-1 transition frequency and the third interaction and vibrational echo emission (wm axis) is at the shifted 1-2 transition frequency, the 1-2 peak appears off-diagonal. The difference between diagonal and off-diagonal peaks is very important. If the first and last interactions are at the same frequency, a peak will be on the diagonal. If the first and last interactions are at different frequencies, a peak will be off-diagonal.
Figure 21 is a simplified schematic of the equipment used to perform the 2D IR vibrational echo experiments. The upper portion of the figure shows the laser equipment. A CW diode pumped neodymium vanadate laser is used to pump a Ti:Sapphire oscillator, which seeds a Ti:Sapphire regenerative amplifier. The regen is pumped with 7 watts by a 1 kHz diode pumped Nd:YLF Q-switched laser. The output of the regen at 800 nm is 2/3 mJ, 35 fs pulses. The 800 nm pulses drive a multi-stage OPA, which produces 3 uJ pulses with 60 fs duration. These are extremely short pulses. At a wavelength of 4 mm, the pulses are only ~4 cycles of light. This number of cycles of light would be the equivalent of an ~6 fs pulse with a wavelength in the middle of the visible spectrum. The lower portion of figure 21 shows the optical setup that does the vibrational echo experiment. The input IR pulse from the OPA is split into 5 beams using beam splitters (BS). Three of them are the three excitation pulses, 1, 2, and 3 in the pulse sequence shown at the bottom of figure 19. A fourth pulse is the local oscillator (see figure 19). The fifth pulse is called the tracer. It is aligned along the path that the vibrational echo will emerge from the sample (see figure 19) and is used only for alignment purposes. It is blocked during the experiments. Pulses 1, 2, and the LO are passed down ultra-precision translation stages labeled 1, 2, and LO. These stages can take 10 nm steps and are exceedingly reproducible in their position.

The ultra-precision is necessary because the vibrational echo is a type of IR holography. The interferogram shown in figure 20 must be accurate and reproducible. The translation stages labeled 3 and tr (tracer) are manual translation stages. Once these timings are set, they do not change. The three excitation beams are focused into the sample using an off-axis parabolic reflector (P1). After passing through the sample the three excitation beams and the vibrational echo beam are collimated by a second off-axis parabolic reflector (P2). The beams other than the vibrational echo are blocked. The LO is split into two beams. One is combined with the vibrational echo. The combined pulses pass through the monochromator and are detected by the upper stripe of the dual stripe IR 32 element array detector. The other portion of the LO passes through the monochromator and is detected by the lower stripe of the array. The signal from the lower stripe is used to normalize the signal on the upper stripe for the IR spectrum and shot-to-shot intensity fluctuations. The signals on the array detector are read out after each laser shot by a computer for processing into the 2D IR vibrational echo spectrum.
 

Ultrafast 2D IR Spectroscopy Recent Publications

311. “Vibrational Echo Correlation Spectroscopy Probes of Hydrogen Bond Dynamics in Water and Methanol,” John B. Asbury, Tobias Steinel, and M. D. Fayer, J. Lumin. 107, 271-286 (2004).

332. “Ultrafast Dynamics of Solute-Solvent Complexation Observed at Thermal Equilibrium in Real Time,” Junrong Zheng, Kyungwon Kwak, John Asbury, Xin Chen, I. Piletic, and M. D. Fayer, Science 309, 1338-1343 (2005).

361. “Ultrafast 2D-IR Vibrational Echo Spectroscopy: A Probe of Molecular Dynamics,” Sungnam Park, Kyungwon Kwak, and M. D. Fayer Laser Phys. Lett. 4, 704-718 (2007).

363. “Frequency-Frequency Correlation Functions and Apodization in 2D-IR Vibrational Echo Spectroscopy, a New Approach,” Kyungwon Kwak, Sungnam Park, Ilya J. Finkelstein, and M. D. Fayer, J. Chem. Phys. 127, 124503 (17 pages) (2007).

374. “Taking Apart 2D-IR Vibrational Echo Spectra: More Information and Elimination of Distortions,” Kyungwon Kwak, Daniel E. Rosenfeld, and M. D. Fayer J. Chem. Phys. 128, 204505 (2008).
 

>> Next: Polarization Selective Pump-Probe

[ Top | Previous: Complex Liquids ]

[ Home | Research Overview | Group Members | Quantum Mechanics Book | Publications ]