Adding a dimension to the infrared spectra of interfaces using heterodyne detected 2D sum-frequency generation (HD 2D SFG) spectroscopy

Abstract

In the last ten years, two-dimensional infrared spectroscopy has become an important technique for studying molecular structures and dynamics. We report the implementation of heterodyne detected two-dimensional sum-frequency generation (HD 2D SFG) spectroscopy, which is the analog of 2D infrared (2D IR) spectroscopy, but is selective to noncentrosymmetric systems such as interfaces. We implement the technique using mid-IR pulse shaping, which enables rapid scanning, phase cycling, and automatic phasing. Absorptive spectra are obtained, that have the highest frequency resolution possible, from which we extract the rephasing and nonrephasing signals that are sometimes preferred. Using this technique, we measure the vibrational mode of CO adsorbed on a polycrystalline Pt surface. The 2D spectrum reveals a significant inhomogenous contribution to the spectral line shape, which is quantified by simulations. This observation indicates that the surface conformation and environment of CO molecules is more complicated than the simple "atop" configuration assumed in previous work. Our method can be straightforwardly incorporated into many existing SFG spectrometers. The technique enables one to quantify inhomogeneity, vibrational couplings, spectral diffusion, chemical exchange, and many other properties analogous to 2D IR spectroscopy, but specifically for interfaces.

Fig. 1.

Schematics of the molecular system, pulse sequences and spectra of linear SFG, 2D IR and HD 2D SFG spectroscopies. (A) The regions that signal is generated from for each technique. The pulse sequence and schematic spectrum, respectively, for (B, C) linear (1D) SFG, (D, E) 2D IR and (F, G) HD 2D SFG spectroscopy. E_n are mid-IR pulses, E_vis is the visible upconversion pulse, and E_LO is the local oscillator pulse. The coherences used to generate the spectra are shown dashed. In the 2D spectra, the phases of the peaks are labeled “+” and “−“.

Fig. 2.

Linear and HD 2D SFG scans of CO on polycrystalline Pt. (A) The real, imaginary and absolute SFG spectra. (B) Fittings to the imaginary 1D SFG spectrum. (C) Pump-probe spectrum. (D) The time evolution of the fundamental peak at 2,090 cm^-1 with 20 fs and (inset) 10 fs time steps

Fig. 3.

Experimental and simulated HD 2D SFG spectra of a CO monolayer on a polycrystalline Pt surface. (A–C) Experimental absorptive, rephasing, and nonrephasing spectra with (D–F) their corresponding simulations.

Fig. 4.

Simulated HD 2D SFG spectra. Simulated result using visible upconversion pulse approximated as E_vis = e^iωt-t/T for (A) T = 600 fs (spectrum is identical to Fig. 3B) and (B) T = ∞ so that the spectrum is an exact Fourier transform of the molecular response. (C) Simulated experimental spectrum when the local oscillator is a linear SFG spectrum rather than an independent local oscillator.