Surface sum-frequency vibrational spectroscopy of nonpolar media

Abstract

Sum-frequency generation spectroscopy is surface specific only if the bulk contribution to the signal is negligible. Negligible bulk contribution is, however, not necessarily true, even for media with inversion symmetry. The inevitable challenge is to find the surface spectrum in the presence of bulk contribution, part of which has been believed to be inseparable from the surface contribution. Here, we show that, for nonpolar media, it is possible to separately deduce surface and bulk spectra from combined phase-sensitive sum-frequency vibrational spectroscopic measurements in reflection and transmission. The study of benzene interfaces is presented as an example.sum-frequency spectroscopy | surface structure | bulk contribution | electric-quadrupole contribution | nonpolar media S um-frequency vibrational spectroscopy (SFVS) has been established as a powerful and versatile tool for studies of surfaces and interfaces of media with inversion symmetry (1-6). It is based on the idea that under the electric-dipole (ED) approximation, the bulk response for SF generation vanishes, but the surface response is necessarily nonvanishing because of the broken inversion symmetry at the surface. However, beyond the ED approximation, the bulk response is generally nonvanishing (7,8). In using SFVS as a surface probe, one must always worry whether the bulk response is negligible or can be distinguished from the surface response. In many publications on SFVS studies of an interface, however, the bulk response is simply ignored. A few papers have appeared to theoretically and experimentally describe how the surface and bulk responses may or may not be separately deduced from measurement (9-18),* , † but they do not seem to have clarified the situation. The conclusion from a more rigorous theory is that part of the bulk response is intrinsically inseparable from the surface response if surface and bulk resonances are not clearly different (19). However, as we shall show in this paper, separate deduction of surface and bulk spectra is possible for nonpolar media from properly designed reflection and transmission SFVS measurements. Using benzene as a test case, we present, to our knowledge, the first real surface spectrum of a neat liquid. The results also provide guidelines on when bulk contribution may be significant in applications of SFVS to surface studies.We begin with a brief review on the various physical mechanisms that contribute to SF generation (SFG) from an interfacial system (20). We consider a system formed by two semi-infinite isotropic media. SFG from the system in transmission or reflection measures a corresponding effective surface nonlinear susceptibility χ ! ð2Þ S,eff , which has contributions from four different physical origins: (i) An ED contribution from the interfacial layer due to broken inversion symmetry. (ii) An electric-quadrupole (EQ) contribution from the rapid field variation at the interface. (iii) An EQ contribution from the bulk. (iv) An EQ contribution from the bulk intrinsically inseparable fr...

Sum frequency generation vibrational spectroscopy (SFG-VS) for complex molecular surfaces and interfaces: Spectral lineshape measurement and analysis plus some controversial issues

“…For the water bending mode at the air/water interface, calculation indicates that the bulk quadrupole contributions are 8.7 times of that for the surface dipole contribution [163]. In spirit, these results seem to be in drastic disagreement with the studies by Shen et al as described above [161]. It is known that there are ambiguities in the surface nonlinear susceptibility calculations [160], particularly the calculated quadrupole contributions are known to be coordinate-dependent.…”

“…SFG-VS can only be surface specific when the bulk contribution to the signal is negligible. How much contribution is to the total SHG or SFG-VS signal from the multipolar responses of the centrosymmetric bulk other than the dipolar contribution of the surface/interface has been a long-standing issue in SHG and SFG-VS studies [12,[154][155][156][157][158][159][160][161]. A systematic theoretical treatment of the dipolar and quadrupolar contribution to the surface SFG-VS can be found in the recent monograph on SFG-VS by Shen [6].…”

“…Accordingly, the total effective susceptibility of the surface of a centrosymmetric medium is [6,161], Here (2) SS χ includes the interfacial electric dipole (ED) term (2) Sd χ , the electric quadrupole (EQ) term…”

“…In one of the recent studies by Shen et al, C-H stretching SFG-VS spectra in both the reflection and the transmission geometries of the nonpolar benzene liquid surface was carefully measured and compared [161]. What was found was that the…”

Selective Probing of Thin-Film Interfaces Using Internal Reflection Sum-Frequency Spectroscopy

“…as has been demonstrated in several experiments. [52][53][54][55][56][57][58] Selectively probing the interface at z = d necessitates the visible and infrared beams traversing the polymer film, so quadrupolar contributions may be significant. However, in cases where measurements of sufficiently thick films is possible in both internal and external reflection geometries, the spectra may be compared to those obtained for thin films.…”