Vibrational sum-frequency-generation (VSFG) spectroscopy is a versatile technique for probing molecu-lar organization at interfaces. There is a growing recognition that dynamic phenomena, such as reorien-tation and intermolecular vibrational coupling, can also influence VSFG spectra. The silica/liquid acetonitrile interface is a useful system for exploring these effects in more detail. The organization of ace-tonitrile at this interface has been well studied, and is known to resemble that of a supported lipid bilayer in many ways. Here isotopic dilution is used to explore the influence of resonant intermolecular coupling of methyl symmetric stretches on the VSFG spectroscopy of this system. VSFG spectra in the methyl stretching region at this interface show a blue shift, a decrease in linewidth, and a higher-than -expected intensity upon dilution in deuterated acetonitrile. We demonstrate that resonant coupling influences VSFG spectral shifts through the infrared transition. Using molecular simulations, we show that our experimental observations are consistent with resonant coupling between methyl transition dipoles being a significant, but not the dominant, contribution to the observed spectral shift upon isotopic dilution. Furthermore, our molecular simulations demonstrate that resonant coupling accounts partially for changes in linewidth and intensity. These classical molecular dynamics simulations also elucidate the behavior of the isotropic Raman spectrum in the bulk liquid upon isotopic dilution. We further simulate the resonant coupling among cyano stretches in this system. These simulations match the experimental shift due to the Raman non-coincidence effect shift of the CN stretch in the bulk liquid, but suggest that the corresponding resonant-coupling-induced shift in the VSFG spectrum at the silica interface is mini-mal. Our experimental and simulation results indicate that proximity to an interface can cause substan-tial changes in the resonant coupling of vibrations in a liquid.(c) 2023 Elsevier B.V. All rights reserved.

Two-photon absorption (2PA) spectroscopy in the range from 7 to 10 eV provides new insight on the electronic structure of liquid water. Continuous 2PA spectra are obtained via the pump-probe technique, using broadband probe pulses to record the absorption at many wavelengths simultaneously. A preresonance enhancement of the absolute 2PA cross section is observed when the pump-photon energy increases from 4.6 to 6.2 eV. The absorption cross section also depends on the relative polarization of the pump and probe photons. The variation of the polarization ratio across the spectrum reveals a detailed picture of the 2PA and indicates that at least four different transitions play a role below 10 eV. Theoretical polarization ratios for the isolated molecule illustrate the value of the experimental polarization measurement in deciphering the 2PA spectrum and provide the framework for a simple simulation of the liquid spectrum. A more comprehensive model goes beyond the isolated molecule picture and connects the 2PA spectrum with previous one-photon absorption, photoelectron, and x-ray absorption spectroscopy measurements of liquid water. Previously unresolved, overlapping transitions are assigned for the first time. Finally, the electronic character of the vertical excited states is related to the energy-dependent ionization mechanism of liquid water.

Previous experiments and simulations have shown that acetonitrile organizes into a lipid-like bilayer at the liquid/silica interface. Recent simulations have further suggested that this bilayer structure persists in mixtures of acetonitrile with water, even at low acetonitrile concentrations. This behavior is indicative of microscopic phase separation of these liquids near silica interfaces and may have important ramifications for the use of acetonitrile in chromatography and heterogeneous catalysis. To explore this phenomenon, we have used vibrational sum-frequency-generation spectroscopy to probe acetonitrile/water mixtures at a silica interface. Our spectra provide evidence that acetonitrile partitions to the hydrated silica interface even when the mole fraction of acetonitrile is as low as 10%. A blue shift is observed in the spectrum of the methyl symmetric stretch upon increasing water mole fraction, in agreement with vibrational spectra of bulk mixtures. Line shape analysis suggests that acetonitrile may exist in the form of bilayer patches at high water mole fractions.

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.

Optical spectroscopy has been used to probe the interfacial organization and dynamics of trimethylacetonitrile (TMACN). Molecular orientation at the silica/liquid, silica/vapor and liquid/vapor interfaces of TMACN has been studied using vibrational sum-frequency generation (VSFG) spectroscopy. These studies reveal that TMACN exhibits appreciable organization at each of these interfaces, despite the bulky nature of its tert-butyl group. VSFG spectra measured from the silica/liquid interface suggest that TMACN does not form the sort of well-organized bilayer that has been observed previously for acetonitrile and propionitrile. Optical Kerr effect studies of TMACN confined in porous silica glasses demonstrate that this, liquid forms a dynamically inhibited surface layer that is roughly one molecule thick, which is consistent with the organizational model suggested by the VSFG data.

Vibrational sum-frequency generation (VSFG) has become a widely used technique for studying molecular orientation and intermolecular structure at interfaces. Due to the interfacial and vibrational selectivity of this technique, it can be used to probe molecular monolayers that are buried within bulk materials. Here, we present a critical review and examination of the assumptions that are generally used in the interpretation of VSFG spectra. This review focuses on three different aspects of VSFG spectroscopy. First, we examine the effect of dynamics (both reorientation and energy transfer) on the interpretation of VSFG spectra. Second, we consider whether (and under what circumstances) VSFG spectra cannot be interpreted solely in terms of the spectral properties of individual molecules. Third, we consider whether VSFG spectra obtained in different modes (frequency-domain, time-domain and broadband) should provide identical information. We conclude with a discussion of the challenges and opportunities provided by the revised viewpoint of VSFG that we present.

The ICN photodissociation reaction is the prototype system for understanding energy disposal and curve crossing in small molecule bond-breaking. The wide knowledge base on this reaction in the gas phase makes it an excellent test case to explore and understand the influence of a liquid solvent on the photo-induced reaction dynamics. Molecular dynamics simulations that include surface-hopping have addressed numerous aspects of how the solvent should influence non-adiabatic transitions and energy flow and ultimately determine product branching for this reaction system. In this paper, we report femtosecond transient absorption work directly combined with new molecular dynamics simulations that make direct connection with the spectroscopic observables. The full spectral evolution after initiating ICN photodissociation at 266 nm in water and ethanol is recorded with unprecedented time resolution, fast enough to see the nascent products emerge before interacting with the solvent cage. Use of a 266 nm pump maximizes the probability of subsequent caging on the upper diabat while launching large rotational energy release for trajectories emerging on the lower diabat. The 2D dataset yields a map of the different products and how they interconvert. In particular, information on the branching ratio and spectral evolution of the product bands is revealed as the products relax their electronic and rotational degrees of freedom. An evolution from rotationally hot gas-phase like CN (sharp band, at 390 nm) to equilibrated and solvated CN radicals (broad, at 326 nm in water and 415 nm in ethanol) is clearly observed in both solvents, and signals assignable to I* are also captured. The non-adiabatic molecular dynamics simulations focus on identifying when trajectories curve cross, filtering the trajectory ensemble into spectroscopically distinct sub-populations and analyzing the rotational energy for the CN product population. The experimental results, taken together with the MD simulations, establish the initial surface crossing probability and suggest multiple passes through the curve crossing region determine the final product yields and provide a source of freshly torqued CN radicals that continues to top up the population of rotationally hot photoproduct over the first few picoseconds.

Typical femtosecond pulse compression of deep ultraviolet radiation consists of prism or diffraction grating pair chirp compensation but, both techniques introduce higher-order dispersion, spatial-spectral beam distortion and poor transmission. While negatively chirped dielectric mirrors have been used to compress near infrared and visible pulses to <10 fs, there has been no extension of this technique below 300 nm. We demonstrate the use of Gires-Tournois interferometer (GTI) negative dispersion multilayer dielectric mirrors designed for pulse compression in the deep ultraviolet region. GTI mirror designs are more robust than chirped mirrors and, can provide sufficient bandwidth for the compression of sub-30-fs pulses in the UV wavelength range. Compression of a 5 nm (FWHM) pulse centered between 266 and 271 nm to 30 fs has been achieved with less pulse broadening due to high-order dispersion and no noticeable spatial deformation, thereby improving the resolution of ultrafast techniques used to study problems such as fast photochemical reaction dynamics.