The use of light to understand detailed electronic structure and chemical properties of a molecule through light-matter interaction is fundamentally essential to design and analyze any chemical system. Over the past decades, rapid developments on optics and laser techniques improved the detection efficiency of multiphoton processes with more detailed chemical information than one-photon processes. Most notable among the multiphoton processes are two-photon processes which can probe either vibrational or excited state properties of a molecule. This thesis focuses on two two-photon processes: Raman and two-photon absorption. A general overview and applications of these processes is discussed in Chapter 1.First, Raman spectroscopy in conjunction with scanning probe microscopy (SPM) is used to study the electrochemical and catalytic activities of a molecule at the single-molecule or nanoscale spatial resolution. This method is called tip-enhanced Raman spectroscopy (TERS), which combines the sensitivity and rich chemical information of surface-enhanced Raman spectroscopy (SERS) with the nanoscale imaging capability of SPM. The ability to simultaneously obtain spectroscopic information with TERS and surface topography with SPM with nanoscale resolution makes TERS a promising method for investigating single-molecule electron transfer reaction on heterogeneous surfaces. In Chapter 2, the development of EC-TERS on an atomic force microscopy (AFM) platform and a quantitative description of single-molecule TERS cyclic voltammetry is discussed. Chapter 3 covers the use of imaging experiments to investigate the site-dependent local electrochemistry based on the platform detailed in Chapter 2. In Chapter 4, theoretical methods to model oxygen adsorption on metal supported molecular catalysis in TERS are presented. The rest of the thesis covers theoretical modeling of two-photon absorption of organic chromophores using entangled photons. In classical two-photon absorption, molecules are excited by randomly arriving photon pairs. Recently the use of entangled photons that are generated through spontaneous parametric down conversion (SPDC) in two-photon absorption has gained an increasing attention. The entangled photons are strongly correlated in time and give rise to an exceptionally high two-photon cross section. In Chapter 5, a theoretical model based on second-linear response time-dependent density functional theory (SLR-TDDFT) to efficiently describe the interaction between entangled photons and organic chromophores is covered. A quantitative comparison of classical and entangled two-photon absorption cross sections from both theory and experiments is presented in Chapter 6. Taken together, extensive experimental and theoretical efforts to explore molecules towards sub-nm spatial and sub-ps temporal limits offer an exciting new opportunity to further upgrade our understanding of molecular electronic structures.

Creator

• Kang, Gyeongwon

DOI

• https://doi.org/10.21985/n2-h5k5-sx70

Subject

• Chemistry

Language

• en

Alternate Identifier

• etdadmin_upload_828620
• http://dissertations.umi.com/northwestern:15617

Keyword

• Single-molecule Spectroscopy
• Two-photon Absorption
• Raman Spectroscopy
• Electrochemistry
• Spectroelectrochemistry
• Quantum Optics

Date created

• 2021-01-01

Resource type

• Dissertation

Rights statement

• In Copyright