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Spectroscopy with Polarized Light (I)

Spectroscopy with Polarized Light (I). Overview 0. Various Spectroscopic Transitions 1. Absorption Spectroscopy with unpolarized light 2. Absorption Spectroscopy with polarized light. Jun. 27, 2006. Various Spectroscopic Transitions. Blue: One-photon absorption.

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Spectroscopy with Polarized Light (I)

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  1. Spectroscopy with Polarized Light (I) Overview 0. Various Spectroscopic Transitions 1. Absorption Spectroscopy with unpolarized light 2.Absorption Spectroscopy with polarized light Jun. 27, 2006

  2. Various Spectroscopic Transitions Blue: One-photon absorption

  3. FIG. A representation of the quantization of the energy of different types of motion.

  4. Interaction of Light with Matter • Semiclassical Approach • Quantum Approach The probability for light absorption or emission (stimulated) per unit time (assuming linear polarized light & randomly oriented molecules) Dipole operator Radiation density No information about spontaneous emission Remedied by

  5. Absorption Spectroscopy with Unpolarized Light • A Simplified System Absorbance of the sample due to true absorption FIG Schematic of absorbance measurement.

  6. True Absorption: 不希望被偵測到! Click (Emitted isotropically) Preventing it from striking the detector (let the solid angle small) Dissipated via a j, b, h, or i Heat Fluorescence Phosphorescence Not True Absorption: 希望被偵測到! Can be effectively eliminated by a ref. measurement ??? Being detected if large particles are absent (frequency changes are so small) (also compensated by a ref. measurement) Emitted photon is deflected by a large angle (inelastic) (does not perturb absorbance measurements)

  7. Beer’s Law Application: 儀器分析實驗 名稱：盤尼西林G之濃度分析 儀器：紫外光／可見光光譜儀 應用：定量分析 http://www.chem.vt.edu/chem-ed/spec/beerslaw.html

  8. Absorption Spectroscopy with Polarized Light E.g., crystals  For anisotropic samples, the measurement result depends on the orientation of the linearly polarized light  Dichroism: Unequal absorption of two orthogonal linearly polarized light • Experimental Parameters • The uniaxial sample (two equivalent axes & one unique one) • Forpartially oriented samples: Incident linearly polarized light with polarization direction parallel the unique axis Perpendicular Unique axis

  9. Degree of anisotropy • Potential artifacts • Sample birefringence will present for measurement of weak linear dichroism Charge distribution In an electric field Amplitude Application: Kornfield, J. A. and G. G. Fuller, “Infrared dichrioms measurement of molecular relaxation in binary blend melt rheology”Macromolecules22, 1334 (1989). Short-chain dynamics

  10. Molecular Fluorescence Spectroscopy • Optical emission from excited molecules (excited with light) • Advantage: Greater sensitivity (zero background) achievable compared to absorption measurements (excited with light) Translational Rotational Vibrational Electric FIG Transitions between molecular electronic energy levels Back

  11. Spectroscopy with Polarized Light (II) Overview 1. Polarized Emission Spectroscopy  Fluorescence Polarization Meas. 2.Raman Spectroscopy July 11, 2006

  12. LAST TIME: Beer’s Law & Potential artifacts THIS WEEK: Fluorescence & Scattering Phosphorescence Fluorescence

  13. Polarized Emission Spectroscopy b i • Fluorescence (Phosphorescence): Two states of the same (different) spin multiplicities. s1 j & h (nonradiative decay; heat) Competing modes of relaxation Fluorescence s0 • Providing a map of the • vibrational levels of the • ground state FIG Various spectroscopic transitions

  14. Fluorescence Polarization Measurements Yield Info on the Orientations of Molecules • The polarization of light emitted by a fluorescent species depends on the orientation of the emission dipole • Applications: Rotational diffusion rates, viscosity etc can be obtained FIG Geometry for detection of polarized light

  15. : dipole • Scattering of Polarized Light by a Molecules • The dipole does not radiate in the direction of its axis A Review of Basic Optics

  16. Raman Spectroscopy d • . ● e, f There is an energy transfer between the molecules and the radiation field e (Stokes Raman scattering) (Anti-Stokes Raman scattering) f

  17. (ease of displacing electrons) e- E e- e- Induced dipole moment: • A Change in Polarizability of the Molecules upon Interaction with the Incident Light (Raman Scattering) + + e- e- e- e- e- e- moving with the oscillating electric vector e- Light emission (scattering) e- e- + e- Polarizability fluctuates with time  Oscillating e- e- e- e- e- e- + e- e- e- e- e- e-

  18. The Depolarization Ratio Two limiting cases:

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