Toward in Situ Measurement of the Density of Liquid Benzene Using Optical Kerr Effect Spectroscopy

Intramolecular Vibration Energy-Transfer-Driven Cascaded Stimulated Raman Scattering and Four-Wave Mixing in Benzene Series

Cascaded stimulated Raman scattering (SRS) of benzene, bromobenzene, chlorobenzene, ethylbenzene, and toluene was investigated by a pulsed Nd:YAG laser with 532 nm wavelength. The results showed that the third-order Stokes SRS of the ring skeleton vibration (CC at 3006 cm^-1) accompanied by another higher-frequency Stokes SRS of the CH stretching vibration (at 3066 cm^-1), which arose only when the third-order Stokes SRS of the ring skeleton was produced, can be attributed to the vibration energy transfer between vibration energy levels of CC and CH. The Stokes and anti-Stokes SRS rings, which originated from the intramolecular energy-transfer-enhanced four-wave mixing (FWM) processes, can be observed only in the forward direction along different angles apart from the pump beam direction. The phenomenon also existed in other derivatives of benzene. We propose the intramolecular energy-transfer-enhanced SRS for the first time, which can be used for a broadband Raman laser.

Low-Frequency (Gigahertz to Terahertz) Depolarized Raman Scattering Off n-Alkanes, Cycloalkanes, and Six-Membered Rings: A Physical Interpretation

Molecular liquids have long been known to undergo various distinct intermolecular motions, from fast librations and cage-rattling oscillations to slow orientational and translational diffusion. However, their resultant gigahertz to terahertz spectra are far from simple, appearing as broad shapeless bands that span many orders of magnitude of frequency, making meaningful interpretation troublesome. Ad hoc spectral line shape fitting has become a notoriously fine art in the field; a unified approach to handling such spectra is long overdue. Here we apply ultrafast optical Kerr-effect (OKE) spectroscopy to study the intermolecular dynamics of room-temperature n-alkanes, cycloalkanes, and six-carbon rings, as well as liquid methane and propane. This work provides stress tests and converges upon an experimentally robust model across simple molecular series and range of temperatures, providing a blueprint for the interpretation of the dynamics of van der Waals liquids. This will enable the interpretation of low-frequency spectra of more complex liquids.

The polarizability response of a glass-forming liquid reveals intrabasin motion and interbasin transitions on a potential energy landscape

Potential energy landscape (PEL) concepts have been useful in conceptualizing the effects of intermolecular interactions on dynamic and thermodynamic properties of liquids and glasses. "Basins", or regions of reduced potential energy associated with locally preferred molecular packing are important PEL features. The molecular configurations at the bottom of these basins are referred to as inherent structures (ISs). Experimental methods for directly characterizing PEL features such as these are rare, largely relegating PEL concepts to theory and simulation studies, and impeding their exploration in real systems. Recently, we showed that quasielastic neutron scattering (QENS) data from propylene carbonate (PC) exhibit signatures of picosecond timescale motion that are consistent with intrabasin motion and interbasin transitions [Cicerone et al., J. Chem. Phys., 2017, 146, 054502]. Here we present optically-heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy studies on PC. The data exhibit signatures of motion within and transitions between basins that agree quantitatively with and extend the QENS results. We show that the librational component of the OKE response corresponds to intrabasin dynamics, and the enigmatic intermediate OKE response corresponds to interbasin transition events. The OKE data extend the measurement range of these parameters and reveal their utility in characterizing PEL features of real systems.

Empirical Analysis of Optical Kerr Effect Spectra: A Case for Constraint

Ultrafast optical Kerr effect (OKE) spectroscopy is a widely used method for studying the depolarized, Raman-active intermolecular dynamics of liquids. Through appropriate manipulation of OKE data, it is possible to determine the reduced spectral density (RSD), which is the Bose-Einstein-corrected, low-frequency Raman spectrum with the contribution of diffusive reorientation removed. OKE RSDs for van der Waals liquids can often be fit well to an empirical function that is the sum of a Bucaro-Litovitz function and an antisymmetrized Gaussian (AG). Although these functions are not directly representative of specific intermolecular dynamics, the AG fit parameters can provide useful insights into the microscopic properties of liquids. Here we show that fits using the AG function are typically not well-determined, and that equally good results can be obtained with a wide range of fitting parameters. We propose the use of a physically motivated constraint on the amplitude of the AG function, and demonstrate that this constraint leads to more intuitive trends in the fit parameters for temperature-dependent RSDs in 1,3,5-trifluorobenzene and hexafluorobenzene.