Homework Due by 5PM September 20 (next class) Does everyone have a topic that has been approved by the faculty?

Transcription

1 Howdy Folks. Homework Due by 5PM September 20 (next class) 5-Problems Every Week due 1 week later. Does everyone have a topic that has been approved by the faculty? Practice your presentation as I will be a stickler for this talk. Keep it simple, on point, audience friendly and remember more is not better. Focus on minimizing content, explaining concepts well and preparing slides that are audience friendly that can be seen easily.

2 Biophysical Spectroscopy

3 Where And When Outline 1. Sept. 10/13 Scattering and Absorption Spectroscopy 2. Sept 17/20 UV-Vis and thermal melting of isolated DNA 3. Sept 24 Fluorescence and Circular Dichroism 4. Sept 27 Applications of Infrared Spectroscopy 5. Oct 1/4 Viscosity, Diffusion and Transport 6. Final Week Presentation Approach will be to couple some theory to applications via problem solving or some complementary laboratory exercise.

4 What is Spectroscopy? --- makes use of the absorption, emission, or scattering of electromagnetic radiation by atoms or molecules (or atomic or molecular ions) to study various physical and chemical properties of atoms and molecules. 3-Main Ways That Light Interacts With Matter 1. Absorption of Radiation 2. Emission of Radiation 3. Scattering of Radiation

5 Effects of the Interaction of Light With Matter 1. Scattering--light interacts with matter (no absorption) creating an oscillating dipole that emits electromagnetic radiation (per Maxwell s Equations). Refraction, diffraction, Rayleigh, Mie, Raman scattering, can be described as scattering processes. 2. Absorption Processes 1. UV-Vis Spectroscopy: absorption of photon from ground to excited electronic states. 2. Infrared Spectroscopy: absorption of photon from ground to excited vibrational state (Raman too can give vibrational information). 3. Microwave Spectroscopy: absorption of photon from ground to excited rotational state 3. Emission: 1. Fluorescence--emission of photon from an excited singlet state to ground state 2. Phosphorence--emission of a photon from an excited triplet state to the ground state.

6 The Jablonski Diagram Absorptive and Emissive Processes and Their Depiction in a Jablonski Diagram First electronic excited singlet state Ground electronic state

7 Electromagnetic Radiation and Molecular Processes Wavelength λ Wavenumber ν

8 Properties of Electromagnetic Radiation

9 Biologically Useful Spectroscopic Regions Wavelength Wavenumber Energy Region Techniques ϒ-Ray Mossbauer 10-8 X-Ray X-ray diff Vacuum UV Electronic 3 x 10-5 Near UV Electronic C---C Bond 6 X 10-5 Visible Electronic RT at 300K 10-2 Infrared Vibrational 10-1 Far IR Vibrational 10 0 Microwave EPR 10 Radio NMR

10 Basic Properties of Light The wavelength, λ, is the crest-to-crest-distance. The frequency ν, is the number of times per second that a crest passes a given point on the x-axis. c is the speed of light (In vacuum = 2.99 x 10 8 m s -1 ). In any other medium the speed of light is c/n, where n is the refractive index of that medium. ---When light passes from one medium to another of different refractive index, the frequency is unchanged, but the wavelength is changed to accommodate the different wave velocity.

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12 Some Properties of Light Frequency (υ) and wavelength (λ) are related by the speed of light. c = νλ where c = 2.99 x 10 8 m s -1 ν is the frequency in Hz (s -1 ) and λ is the wavelength of light. Frequency (υ) and wavelength (λ) can be related to units of energy (J) of a photon through Plank s equation, where h is Plank s constant = 6.63 x J s E = hν = h c λ

13 Frequency, Wavelength, Energy and Wavenumber. Because expression of frequency can be very large number chemists and physicists will specify the properties of a wave in reciprocal wavelength or wave-numbers with units of cm λ = ν c = wavenumber E = hν = h c λ Energy of a single photon is proportional to wavenumber E = N A hν = N A h c λ = N A h ν Energy of a mole of photons, E is called an einstein

14 Symbolizing Electromagnetic Radiation c = λv E x = E 0 cos 2π( x λ νt) A traveling wave Since a sample is located at some fixed position and not moving then the sample s point of view only an oscillating electric field disturbance is present. E E x = E 0 cos(2πνt) E x = E 0 cos(ωt) Oscillating electric field Two-headed vector oscillating at a frequency ν = c/λ

15 Light Scattering

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