(a) Schematic representation of broadband pump-probe SFG spectroscopy of interfacial water at the water-lipid interface. The scheme represents the broadband probe IR pulse convoluted with a narrow band visible pulse to generate the vibrational SFG spectrum of water. The time delay represents the delay between the pump pulse and the probe pair. By scanning the delay and monitoring the modulation in the SFG signal, one can probe the femtosecond vibrational dynamics of interfacial molecules. (b) Energy level representation of the pump-probe SFG process. (c) Cartoon of the dynamics SFG transient showing the modulation of the SFG signal as a function of the pump-probe delay. (d) Cartoon representing the 2D-IR concept; the pump pulse excites vibrational mode and the probe pulse interrogates the mode in order to address the vibrational coupling between these modes. Also shown is a model 2D-IR plot. The one-color IR pump-probe response lies on the diagonal (dotted line) whereas the two-color coupling response is shown at the off-diagonals. The blue response is the bleach signal corresponding to the population excitation from to whereas the red response is the transient absorption of population from to . The redshift of the response with respect to the response corresponds to the anharmonicity of the system.
Scheme for generation of pump and probe IR by difference frequency mixing of amplified 800 nm and doubled idler pulse. The difference frequency mixing in the KTP crystals are a type-II process, with the polarizations of the 800 nm, doubled idler, and the IR pulse being horizontal, vertical, and horizontal, respectively. The home-built pulse shaper disperses the 800 nm beam and the frequency components are imaged by the lens onto the Fourier plane where a mirror and a slit are placed. The slit is adjusted to select a narrow band frequency and the mirror behind the slit causes the selected frequency to retrace the path, back onto the grating and is coupled out.
Instrumentation at sample (see text for details).
Scheme for the detection path, including the galvanometric scanning mirror to spatially separate the pump-on and pump-off SFG spectra. Cylindrical lenses are used to facilitate focusing of the pump reference SFG, pump-on and pump-off SFG beams onto the spectrometer slit. The lenses on the left focus on the vertical plane whereas the lens on the right, close to the spectrometer slit, focuses the pump reference SFG and collimates the pump-on and pump-off SFG beams onto the slit. The focal lengths of the two cylindrical lenses on the left are different since the SFG beam paths are different. The scanning mirror is used only when the data acquisition is in the spectral mode (CCD mode) and not in the photon-counting mode (PMT mode).
Scheme for the electronics in the setup. The green channels are in use only when experiments are done with the PMT. For the chopper output signal, a cheap diode laser is used and is aligned through the same hole as the pump IR beam and is detected by a PD. The signals from the PD and from the function generator are sent to an oscilloscope for tuning the phases.
Image of the CCD camera, corrected for large spikes presumably caused by cosmic radiation. The upper and lower traces depict the broadband SFG signal with and without pump, respectively, and the lower trace shows the spectrum of the pump pulse.
(a) Schematic for the third-order IIV-SFG process generated by overlapping the pump IR and the probe pulse pair at the interface. In this process, only the probe IR is resonant with the fundamental vibrational transition. By scanning the pump pulse in time with respect to the probe pair, one can obtain a cross-correlation trace (b) as a function of the pump-probe delay. The narrow band visible pulse is about 2–3 ps long, so the cross-correlation trace is determined by the temporal convolution of the two IR pulses. These results demonstrate that the typical time resolution of the TR-SFG experiment amounts to . The time resolution of the 2D-SFG experiment is longer and asymmetric around time-zero (Gaussian rise and exponential decay) since the pump IR pulse is shaped to a narrow band by the Fabry–Pérot etalon.
Time-resolved SFG trace. Figure 6(a) shows the vibrational dynamics of the neat water/interface;66 (b) water/lipid interface.67
2D pump-probe SFG plot of dodecanol on subphase.65 The IR spectrum of dodecanol is shown in Fig. 8(a), which the narrow band pump IR interrogates through a frequency scan. The SFG spectrum of dodecanol due to the broadband probe IR is plotted in Fig. 8(b), which indicates only three SFG active modes. Figures 8(c) and 8(d) show a plot of the pump-probe signal as a function of the pump and probe IR frequencies, at - and -polarizations of the pump IR pulse, respectively.
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