Microscopic and Macroscopic Insights into Second Order Nonlinear Optics through Computational and Experimental Routes
Creators: Parshotam, Shyam
Publisher: name: University of Alberta Library
Year: 2023

<p>The challenge with obtaining physical insights into physiochemical phenomena is connecting the macroscopic observables with the microscopic processes in a system. In second-order nonlinear optical spectroscopy, experimental techniques such as vibrational sum frequency generation (vSFG) and second harmonic generation (SHG) give a macroscopic picture of environmentally relevant interfaces such as the silica/water interface. Theory-based computations of the microscopic properties can provide valuable insights into the underlying characteristics giving rise to the macroscopic observations. However, when utilizing experimental and computational approaches, the challenge is that factors inherent to the system or measurement can influence the final results. Hence, this thesis focuses on understanding the influence of the excitation wavelength of light in computations of hyperpolarizabilities, the Fermi resonance in vSFG measurements, and the macroscopic and microscopic silica response in SHG studies.<br>
    On a microscopic scale, the first molecular hyperpolarizability strongly correlates to the intramolecular charge transfer in donor-acceptor systems. While numerous studies have highlighted the importance of modifying the system and solvation in computations, studying the excitation wavelength dependence is challenging due to the limited number of experimental studies. As such, by utilizing the charge transfer-dependent hyperpolarizability trend of the para, meta, and ortho nitroaniline isomers (pNA&gt;oNA&gt;mNA), the influence of the excitation wavelength can be elucidated. In non-resonant regions, with reference to CCSD/aug-cc-pVDZ and experimental studies, we find that some computational approaches do not consistently reproduce the nitroanilines’ trend at specific excitation wavelengths. For example, range-separated hybrid functionals require optimal gap tuning to reproduce the trend. In resonant regions, we find that damped response theory predicts that the trend is maintained at the two-photon absorption but breaks down near the one-photon pole. Compared to the two-state model, the undermined charge transfer characteristics at the one-photon pole can be due to the influence of other electronic states. Furthermore, the utility of optimal gap tuning of range-separated hybrid functionals is also dependent on the excitation wavelength.<br>
On a macroscopic scale, vibrational sum frequency generation (vSFG) spectroscopy can provide information on the local hydrogen-bonding environment at the environmentally relevant silica/water interface. However, the inter and intramolecular vibrational coupling in water makes it difficult to decipher the environment from vibrational spectroscopy. As such, vSFG, electrokinetic measurements, and the maximum entropy method are used on isotopically diluted water (HOD) at the silica/aqueous interface to reveal the effects of vibrational coupling at the silica/water interface. In the Stern layer spectra, the frequency centers between H2O and HOD shift at pH 2, signifying the presence of intermolecular coupling in the former that can be related to the double-donor hydrogen-bonded structure of water. Furthermore, the presence of intramolecular coupling in H2O distorts the spectral response, as there are apparent differences in the evolution of the Stern layer. This behavior is further supported by the evolution of the HOD Stern layer spectra closely matching the pKa of the out-of-plane silanols predicted by previous ab initio molecular dynamic simulations.<br>
Connecting the dots between the macroscopic observables to microscopic properties is a valuable approach to rationalizing experimental measurements. The challenge with second harmonic generation measurements at the silica/water interface is the contribution of both silica and water. As such, the silica contribution can be elucidated by combining SHG with previously measured zeta potential and vibrational sum frequency generation (vSFG) intensity measurements under conditions that promote charge inversion. The zeta potential and the vSFG measurements follow a similar trend in these conditions, but SHG yields the opposite behaviour. This observation shows that silica exhibits a pH-dependent response to the SHG measurement, and interference between the silica and water responses can either be constructive or destructive depending on the net orientation of water molecules in the aqueous phase. On a microscopic level, the molecular hyperpolarizability of neutral and deprotonated silica clusters is computed with density functional theory (CAM-B3LYP/6-31+G(d,p)). We reveal a significant increase in hyperpolarizabilities upon the deprotonation of silanol sites. This change suggests that one potential source of the silica contribution to the SHG measurement is the deprotonation of surface silanol sites.</p>