Article pubs.acs.org/JPCB
Reorientation-Induced Spectral Diffusion in Vibrational SumFrequency-Generation Spectroscopy Christopher A. Rivera,† Amanda J. Souna,† John. S. Bender,† Katherine Manfred,† and John T. Fourkas*,†,‡,§,∥,⊥ †
Department of Chemistry & Biochemistry, ‡Institute for Physical Science and Technology, §Maryland NanoCenter, ∥Center for Nanophysics and Advanced Materials, ⊥Chemical Physics Program, University of Maryland, College Park, MD 20742 S Supporting Information *
ABSTRACT: There is a growing appreciation that dynamic processes play an important role in determining the line shape in surface-selective, nonlinear spectroscopies such as vibrational sum-frequency-generation (VSFG). Here we analyze the influence that reorientation can have on VSFG spectra when the vibrational transition frequency is a function of orientation. Under these circumstances, reorientation-induced spectral diffusion (RISD) causes the underlying spectral line shape to become time dependent. Unlike previously reported mechanisms through which reorientation can contribute to the VSFG signal, RISD influences the line shape regardless of the degree of polarization of the Raman transition that is probed. We assess the impact of RISD on VSFG spectra using a model system of liquid acetonitrile at a silica interface. Comparison of delay-time-dependent VSFG spectra with simulations that employ static line shapes suggests that RISD contributes substantially to the spectra, particularly at delay times that are comparable to or greater than the probe pulse duration. The observed behavior is in qualitative agreement with a two-state RISD model that uses orientational distributions determined from previous molecular dynamics simulations.
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INTRODUCTION Vibrational sum-frequency-generation (VSFG) spectroscopy has become a widely used tool for studying molecular organization at interfaces.1−9 VSFG can take advantage of the symmetry properties of the second-order nonlinear optical susceptibility, χ(2), to accomplish selective vibrational probing of molecules at an interface between two isotropic media. The polarization dependence of the VSFG signal for different vibrational modes further gives direct insight into the interfacial orientations of the functional groups that are probed. Although VSFG is typically used to study the average molecular orientation at interfaces, there is a growing appreciation that this technique can also provide information on dynamics.9 For instance, in 2001 Wei and Shen showed that reorientation can influence the relative VSFG intensity of the free OH stretch at the water liquid/vapor interface under different polarization conditions.10 This effect has since been observed in other systems,11−14 analyzed theoretically,9,11 and quantified using molecular dynamics (MD) simulations.15 Higher-order, surface-selective techniques have also been devised as a more direct means of studying orientational dynamics at interfaces.16−22 A broadband implementation of VSFG spectroscopy can be described in terms of a vibrational coherence that is created with a high-bandwidth infrared (IR) pulse and is later probed with a shorter-wavelength, narrowband pulse via a Raman transition.9,11,15 The IR pulse preferentially creates vibrational coherences in a subset of the interfacial molecules, based on the © 2013 American Chemical Society
molecular orientations. Previous work on the effects of reorientation in VSFG spectroscopy has focused on how the orientational evolution of the distribution of molecules in which vibrational coherences have been created influences the signal by changing the sensitivity of the probing step.10,11,15 The effect of reorientation on other molecular properties has not been considered in these treatments. A vibrational mode of a high-symmetry functional group, such as a freely rotating methyl group, has a Raman tensor that can be described in terms of an isotropic portion and an anisotropic (depolarized) portion.23 The isotropic portion of the polarizability is independent of the orientation of the functional group, and so is not influenced by any reorientation that occurs following creation of a vibrational coherence with the IR pulse. Thus, in the absence of other orientationdependent effects, only the depolarized component of the Raman tensor plays a role in coupling orientational dynamics to the VSFG line shape.11,15 For a symmetric methyl stretch, the isotropic portion of the polarizability is typically significantly larger than the depolarized portion,24−27 and so even rapid reorientation has been found to have a relatively small effect on the VSFG spectrum.15 Special Issue: Michael D. Fayer Festschrift Received: September 4, 2013 Revised: October 2, 2013 Published: October 2, 2013 15875
dx.doi.org/10.1021/jp408877a | J. Phys. Chem. B 2013, 117, 15875−15885
The Journal of Physical Chemistry B
Article
delay relative to the IR pulse. To ensure transform-limited pulses at the sample, a spatial-light-modulator-based pulse shaper (femtoJock, BioPhotonic Solutions) is inserted between the oscillator and amplifier. The MIIPS algorithm38 is employed to determine and implement the optimum phase mask. In the counterpropagating experimental geometry used, the probe beam is directed to the sample at an angle of 64.5° to the surface normal, whereas the IR pump is incident at −54°. The VSFG signal is well-separated from both the probe and IR beams (and their reflections), appearing at −35° to the surface normal. For this study, we are concerned primarily with the SSP polarization combination, where the letters correspond to the polarizations of the signal, probe, and IR beams, respectively. At the sample, the probe pulse energy was 11 μJ, and the IR pulse energy was 8 μJ. The spectra of the probe pulse and the VSFG signal were measured using a spectrometer (Acton SP300i) with a thermoelectrically cooled, 2D CCD array (Spec-10:100, Roper Scientific). To increase the signal-to-noise ratio, only the illuminated rows of pixels were binned to produce the experimental spectrum. The sample consisted of 99+% spectroscopic grade acetonitrile (ACROS) and was held in an IR-grade quartz cell with a 1 mm path length (Hellma). The cell was rinsed, ovendried, and oxygen-plasma cleaned immediately prior to use. The SFG spectrum of gold under PPP polarization conditions, which is proportional to the spectrum of the IR pulse,39 was used as a reference. The VSFG signal was divided by the gold SFG spectrum to obtain the corrected line shape. No other processing was performed on the data reported here, and in particular the data were not smoothed in any manner. VSFG spectra were acquired for 4 min for IR/probe delays of 3 ps and less and for 8 min otherwise. Each spectrum was collected several times to verify reproducibility. We additionally varied the intensities of both the IR and probe beams over a substantial range to ensure that the shapes of the measured spectra were not power dependent. IR/probe delays (as measured between the intensity maxima of the pulses) ranging between 0 and 3.5 ps, with a 0.167 ps step size, were used to collect VSFG spectra that sampled different portions of the vibrational free-induction decay (FID). Time zero was determined carefully by maximization of the gold SFG signal using the full bandwidth of the probe pulse (which had a temporal fwhm of
