Theoretical Simulations of Sum-frequency Generation spectroscopy

Open Access
Weiss, Philip Andrew
Graduate Program:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 11, 2014
Committee Members:
  • Lasse Jensen, Thesis Advisor
  • Sum-frequency generation
  • theory
  • plasmonics
Often times, it’s difficult to distinguish the bulk material from a surface or an interface of two or more materials. Compared to the bulk, the response of the material is much smaller at an interface. In order to elucidate these systems, a technique is needed to distinguish interfaces from the bulk. One such technique is known as sum-frequency generation (SFG). In this vibrational spectroscopy, a visible and an infrared photon probe the target molecule, and scatters a photon that is sum of the two incident photons. SFG has garnered much attention for its selectivity to surfaces and interfaces due to zero response for systems with inversion symmetry. This interface specificity allows for the determination of molecular orientation at these interfaces separately from the bulk. In this thesis, we explore various computational methods to model SFG using first principles. Several different types of systems are modeled, and shown to agree quite well with experimental results. These methods were developed with the motivation of being accurate by using a quantum mechanical description of the target molecule that is cheap enough to calculate within the limits of density functional theory. Firstly, this work looks at the SFG of large polymeric crystalline cellulose. By only considering a small portion of the polymer, many features of the experimental spectrum match the theory. Secondly, a new theory for doubly-resonant SFG is shown to give good agreement with experiments, and predicts the orientation of a molecule on a surface. Lastly, a method for modeling the SFG of molecules on metal nanoparticles is shown using a discrete interaction model/quantum mechanics (DIM/QM). DIM/QM has been previously been developed for surface-enhanced Raman spectroscopy, and here we extend it to SFG. All these methods provide a toolbox for scientists for understanding their chemical systems.