Presentation on theme: "The learning objectives for this course are: (1) Critically consume scientific literature and talks in the area of analytical spectroscopy. Pose meaningful."— Presentation transcript:
The learning objectives for this course are: (1) Critically consume scientific literature and talks in the area of analytical spectroscopy. Pose meaningful questions and present significant comments while exploring a new topic. (2) Identify appropriate spectroscopic techniques for the analysis of any sample. Recognize the strengths and limitations of each technique. (3) Formulate a novel research project addressing an important unanswered question by exploiting analytical spectroscopic methods. Recognize the critical early work and identify current state-of-the-art work in the chosen area.
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Why is Analytical Spectroscopy Important?
Spectroscopy EM Radiation Sources No radiation: Excitation by collisions or chemical reactions can initiate photon emission. Continuum Source: Emit radiation over a broad wavelength range (e.g. incandescent lamps) Line Source: Emit radiation at discrete wavelengths (e.g. Hg arc lamp, laser). Image source: Tungsten Halogen Lamp Mercury Argon Lamp
Interaction between EM Radiation and the Sample absorbradiationradiationlessabsorptionemitradiationradiationlessemission emissionabsorptionphotoluminescence inelastic excitation or deactivation
Atomic vs. Molecular Spectroscopy Atomic Spectroscopy Example (Cl 2 ): Molecular Spectroscopy (CH 3 CH 2 OH): Image source:
Wavelength Selection before Detection Must separate analyte optical signal from a majority of the potentially interfering optical signals. - absorption filters - absorption filters - interference filters - interference filters - spatial dispersion - spatial dispersion - interferometry - interferometry Image source:
Are you getting the concept? The two transmission profiles below are for filters sold by Melles-Griot. Which filter would you buy to block = 15,800 cm -1 light? (a)(b)
Radiant Power Monitors (a.k.a. Detectors) Detectors convert EM radiation into an electrical signal or another physical quantity that can easily be converted into an electrical signal. Thermal Detectors: convert IR radiation into current or voltage Photon Detectors: convert UV and visible photons into current Multichannel Detectors: convert UV and visible photons into charge Skoog and Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.
Review: Are you getting the concept?
Calculate the energy of (a) a 5.30 Å X-ray photon (in eVs) and (b) a 530-nm photon of visible radiation (in kJ/mole).
Are you getting the concept? Sketch the sum wavefunction of the red and blue waves.
Are you getting the concept? If the average irradiance from the Sun impinging normally on a surface just outside the Earths atmosphere is 1400 W/m 2, what is the resulting pressure (assuming complete absorption)? How does this pressure compare with atmospheric pressure (~ 10 5 N/m 2 )? Reminder: 1 W = 1 J/s = 1 N·m/s = 1 kg·m 2 /s 3
Are you getting the concept? Many streetlights are sodium discharge lamps. The emitted orange light is due to the sodium D-line transition: What is the energy level spacing (in eV) for the 3p 3s transition? Reminder: 1eV = 1.6 x J and h = 6.63 x Js
EM Radiation Sources 1. Fundamentals of EM Radiation 2. Light Sources 3. Lasers
Optical Source Characteristics Ingle and Crouch, Spectrochemical Analysis Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.
Continuum Source Line Source Continuum + Line Source Ingle and Crouch, Spectrochemical Analysis
Incandescent Lamp 1. Glass bulb (or "envelope") 2. Low pressure inert gas 3. Tungsten filament 4. Contact wire (goes to foot) 5. Contact wire (goes to base) 6. Support wires 7. Glass mount/support 8. Base contact wire 9. Screw threads 10. Insulation 11. Electrical foot contact
Black-body Radiation In an ideal black body: ( ) = 1, ( ) = 0, T( ) = 0 ( ) = 1, ( ) = 0, T( ) = 0 Because a black body is at thermal equilibrium, emission must equal absorption. Thus, black bodies are perfect absorbers and the most efficient emitters possible. There are no ideal black bodies.
Spectral Distribution of Emission is Characteristic of the Temperature of the Blackbody As T increases, max decreases. Donald McQuarrie, Quantum Chemistry, University Science Books, Mill Valley, CA,
Rayleigh – Jeans Law Spectral Radiance Spectral Radiance (Jm -3 s -1 ) (Jm -3 s -1 ) The Ultraviolet Catastrophe Approximate Blackbody Expressions Wiens Law
Resolved (inadvertently) in 1900 by Max Planck. Assumed atoms could only absorb or emit discrete amounts of energy. Plancks Radiation Law: Donald McQuarrie, Quantum Chemistry, University Science Books, Mill Valley, CA, Spectral Energy Density Spectral Energy Density (Jcm -3 Hz -1 ) (Jcm -3 Hz -1 )
Wiens Displacement Law Eugene Hecht, Optics, Differentiate Plancks Law with respect to and set equal to zero to find m (wavelength of maximum irradiance): Stefan-Boltzman Law M b = T 4 = W·cm -2 · K -4 = W·cm -2 · K -4 Integrate Plancks Law to find the total emittance of a black body:
Are you getting the concept? Suppose that we measure the emitted exitance from a small hole in a furnace to be 22.8 W/cm 2. Compute the internal temperature of the furnace. Recall: = W·cm -2 · K -4
Non-Ideal Sources – Gray Bodies Spectral radiance Spectral radiance of a true black body
Spectral Emissivity, : Ratio of the spectral radiance of a true source to that of a black body Accounts for < 1 Eugene Hecht, Optics, Addison-Wesley, Reading, MA, = ( ) = ( ) Corrections for Non-Ideal Sources
T w ( ) is the transmission factor of the source envelope Corrections for Non-Ideal Sources
Color Temperature (T) T in is an adjustable parameter T is the temperature that the atoms experience The color temperature is the temperature of a black body which most closely matches the lamp's perceived color
Are you getting the concept? Calculate the spectral radiance of a tungsten lamp at 500 nm with a color temperature of 2700 K, = 0.40, and T = 0.92 in J/m 3 s. Recall: k = 1.38 x J · K -1 and h = 6.63 x Js and h = 6.63 x Js