Sum-frequency generation vibrational spectroscopy (SFG-VS) can provide detailed information and understanding of the molecular composition, interactions, and orientational and conformational structure of surfaces and interfaces through quantitative measurement and analysis. In this review, we present the current status of and discuss important recent developments in the measurement of intrinsic SFG spectral lineshapes and formulations for polarization measurements and orientational analysis of SFG-VS spectra. The focus of this review is to present a coherent description of SFG-VS and discuss the main concepts and issues that can help advance this technique as a quantitative analytical research tool for revealing the chemistry and physics of complex molecular surfaces and interfaces.

The ability to achieve sub-wavenumber resolution (0.6 cm(-1)) and a large signal-to-noise ratio in high-resolution broadband sum-frequency generation vibrational spectroscopy (HR-BB-SFG-VS) allows for the detailed SFG spectral lineshapes to be used in the unambiguous determination of fine spectral features. Changes in the structural spectroscopic phase in SFG-VS as a function of beam polarization and experimental geometry proved to be instrumental in the identification of an unexpected 2.78 ± 0.07 cm(-1) spectral splitting for the two methyl groups at the vapor/dimethyl sulfoxide (DMSO, (CH(3))(2)SO) liquid interface as well as in the determination of their orientational angles.

The lack of understanding of the temporal effects and the restricted ability to control experimental conditions in order to obtain intrinsic spectral lineshapes in surface sum-frequency generation vibrational spectroscopy (SFG-VS) have limited its applications in surface and interfacial studies. The emergence of high-resolution broadband sum-frequency generation vibrational spectroscopy (HR-BB-SFG-VS) with sub-wavenumber resolution [Velarde et al., J. Chem. Phys., 2011, 135, 241102] offers new opportunities for obtaining and understanding the spectral lineshapes and temporal effects in SFG-VS. Particularly, the high accuracy of the HR-BB-SFG-VS experimental lineshape provides detailed information on the complex coherent vibrational dynamics through direct spectral measurements. Here we present a unified formalism for the theoretical and experimental routes for obtaining an accurate lineshape of the SFG response. Then, we present a detailed analysis of a cholesterol monolayer at the air/water interface with higher and lower resolution SFG spectra along with their temporal response. With higher spectral resolution and accurate vibrational spectral lineshapes, it is shown that the parameters of the experimental SFG spectra can be used both to understand and to quantitatively reproduce the temporal effects in lower resolution SFG measurements. This perspective provides not only a unified picture but also a novel experimental approach to measuring and understanding the frequency-domain and time-domain SFG response of a complex molecular interface.

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ABSTRACT:The photoelectron spectrum of the oxyallyl (OXA) radical anion has been measured. The radical anion has been generated in the reaction of the atomic oxygen radical anion (O •-) with acetone. Three low-lying electronic states of OXA have been observed in the spectrum. Electronic structure calculations have been performed for the triplet states ( 3 B 2 and 3 B 1 ) of OXA and the ground doublet state ( 2 A 2 ) of the radical anion using density functional theory (DFT). Spectral simulations have been carried out for the triplet states based on the results of the DFT calculations. The simulation identifies a vibrational progression of the CCC bending mode of the 3 B 2 state of OXA in the lower electron binding energy (eBE) portion of the spectrum. On top of the 3 B 2 feature, however, the experimental spectrum exhibits additional photoelectron peaks whose angular distribution is distinct from that for the vibronic peaks of the 3 B 2 state. Complete active space selfconsistent field (CASSCF) method and second-order perturbation theory based on the CASSCF wave function (CASPT2) have been employed to study the lowest singlet state ( 1 A 1 ) of OXA. The simulation based on the results of these electronic structure calculations establishes that the overlapping peaks represent the vibrational ground level of the 1 A 1 state and its vibrational progression of the CO stretching mode. The 1 A 1 state is the lowest electronic state of OXA, and the electron affinity (EA) of OXA is 1.940 ( 0.010 eV. The 3 B 2 state is the first excited state with an electronic term energy of 55 ( 2 meV. The widths of the vibronic peaks of theX 1 A 1 state are much broader than those of theã 3 B 2 state, implying that the 1 A 1 state is indeed a transition state. The CASSCF and CASPT2 calculations suggest that the 1 A 1 state is at a potential maximum along the nuclear coordinate representing disrotatory motion of the two methylene groups, which leads to three-membered-ring formation, i.e., cyclopropanone. The simulation ofb 3 B 1 OXA reproduces the higher eBE portion of the spectrum very well. The term energy of the 3 B 1 state is 0.883 ( 0.012 eV. Photoelectron spectroscopic measurements have also been conducted for the other ion products of the O •-reaction with acetone. The photoelectron imaging spectrum of the acetylcarbene (AC) radical anion exhibits a broad, structureless feature, which is assigned to theX 3 A 00 state of AC. The ground ( 2 A 00 ) and first excited ( 2 A 0 ) states of the 1-methylvinoxy (1-MVO) radical have been observed in the photoelectron spectrum of the 1-MVO ion, and their vibronic structure has been analyzed.

While in principle the frequency-domain and time-domain spectroscopic measurements should generate identical information for a given molecular system, the inhomogeneous character of surface vibrations in sum-frequency generation vibrational spectroscopy (SFG-VS) studies has only been studied with time-domain SFG-VS by mapping the decay of the vibrational polarization using ultrafast lasers, this due to the lack of SFG vibrational spectra with high enough spectral resolution and accurate enough lineshape. Here, with the recently developed high-resolution broadband SFG-VS (HR-BB-SFG-VS) technique, we show that the inhomogeneous lineshape can be obtained in the frequency-domain for the anchoring CN stretch of the 4-n-octyl-4'-cyanobiphenyl (8CB) Langmuir monolayer at the air-water interface, and that an excellent agreement with the time-domain SFG free-induction-decay can be established. We found that the 8CB CN stretch spectrum consists of a single peak centered at 2234.00 ± 0.01 cm(-1) with a total linewidth of 10.9 ± 0.3 cm(-1) at half maximum. The Lorentzian contribution accounts only for 4.7 ± 0.4 cm(-1) to this width and the Gaussian (inhomogeneous) broadening for as much as 8.1 ± 0.2 cm(-1). Polarization analysis of the -CN spectra showed that the -CN group is tilted 57° ± 2° from the surface normal. The large heterogeneity in the -CN spectrum is tentatively attributed to the -CN group interactions with the interfacial water molecules penetrated/accommodated into the 8CB monolayer, a unique phenomenon for the nCB Langmuir monolayers reported previously.

Substantial progress has been made in the quantitative understanding and interpretation of the hydrogen bonding and ordering structure of the air/water interface since the first sum-frequency generation vibrational spectroscopy (SFG-VS) measurement by Q. Du et al. in 1993 (Phys. Rev. Lett. 1993, 70, 2312-2316). However, there are still disagreements and controversies on the consistency between the different experimental measurements, as well as in the theoretical and computational results. One critical problem lies in the lack of consistency between the SFG-VS intensity measurements and the recently developed SFG-VS phase spectra measurements of the neat air/water interface, which has inspired various theoretical efforts. In this report, the reliability of the SFG-VS intensity spectra of the air/neat water interface is to be quantitatively examined, and possible sources of inaccuracies in the SFG-VS phase spectral measurement are to be discussed based on the nonresonant SHG phase measurements. Solid evidence is shown indicating that the SFG-VS intensity spectra from different laboratories are now quantitatively converging and in agreement with each other. However, the possible inaccuracies and inconsistencies in the SFG-VS phase spectra measurements need to be carefully examined against a properly corrected phase standard to take full advantage of this powerful experimental tool.

Electronic structure of the oxyallyl diradical and the anion is investigated using high-level ab initio methods. Converged theoretical estimates of the energy differences between low-lying electronic states of oxyallyl (OXA) as well as detachment energies of the anion are reported. Our best estimates of the adiabatic energy differences between the anion (2)A(2) and the neutral (3)B(2) and (3)B(1) states are 1.94 and 2.73 eV, respectively. The (1)A(1) state lies above (3)B(2) vertically, but geometric relaxation brings it below the triplet. The two-dimensional scan of the singlet (1)A(1) potential energy surface (PES) reveals that there is no minimum corresponding to a singlet diradical structure. Thus, singlet OXA undergoes prompt barrierless ring closure. However, a flat shape of the PES results in the resonance trapping in the Franck-Condon region, giving rise to the experimentally observable features in the photoelectron spectrum. By performing reduced-dimensionality wave packet calculations, we estimated that the wave packet lingers in the Franck-Condon region for about 170 fs, which corresponds to the spectral line broadening of about 200 cm(-1). We also present calculations of the photodetachment spectrum and compare it with experimental data. Our calculations lend strong support to the assignment of the photoelectron spectrum of the OXA anion reported in Ichino et al. (Angew. Chem., Int. Ed. Engl. 2009, 48, 8509).