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Introduction to optical spectroscopy

Introduction to optical spectroscopy. Chemistry 243. Fundamentals of electromagnetic radiation. (s -1 ). Electromagnetic spectrum. Low Energy. High Energy. http://www.yorku.ca/eye/spectrum.gif. Terminology. Spectroscopy is the study of the interaction of light and matter

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Introduction to optical spectroscopy

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  1. Introduction to optical spectroscopy Chemistry 243

  2. Fundamentals of electromagnetic radiation (s-1)

  3. Electromagnetic spectrum Low Energy High Energy http://www.yorku.ca/eye/spectrum.gif

  4. Terminology • Spectroscopy is the study of the interaction of light and matter • NMR or X-Ray spectroscopy; spectroscopist • Spectrometry is the establishment of the pattern of interaction (as a function of energy) of light with particular forms of matter • Mass spectrometry (MS); spectrometrist • Spectrophotometry is the quantitative study of the interaction of light with matter • UV-Visible spectrophotometry • (I’ve never heard anyone called a spectrophotometrist)

  5. What chemical and/or material properties can we measure using spectral methods? • Broad and powerful applications • Elemental composition (often metals; CHNO) • Identity of a pure substance (what is it?) • Components of a mixture (purity?) • Amount of a substance in a mixture (how much?) • Bulk/major component, minor component, trace component, ultra-trace component • Surface composition • Material property (stress/strain, polymer cross-linking, change of state, temperature) • Reaction rate, mechanism, products

  6. What properties of incident or generated light can we measure? • Absorption • Fluorescence (fast) & Phosphorescence (slow) • Thermal Emission • Chemiluminescence • Scattering • Refraction or Refractive Index • Polarization, Phase • Interference/Diffraction • Coherence • Chemistry consequent to the above

  7. What atomic/molecular properties affect or are affected by light? • Rotation (typically refers to a molecule) • Vibration (typically refers to a molecule) • Electronic Excitation (atomic or molecular) • Ionization (loss of electron to yield a cation) • Combinations of the above: • Rotation-vibration (infrared/Raman) • Rotational, vibrational, electron excitation (UV-Vis) • Ionization with UV absorbance (strong excitation)

  8. The properties you want to studyhelp to select a suitable wavelength High Energy Low Energy

  9. Why wavenumber? • The energy difference between two wavenumbers is the same regardless of spectral region or λ • Wavelength is not proportional to energy; it has a reciprocal relation to energy, so: • The energy difference between two wavelengths (in nm or angstroms) varies as a function of spectral region.

  10. Selecting the right optical method

  11. Plasma, flame, or chemical Focus Detection Emission Excitation Source Sorting of Energy, Space, and Time Chemiluminescence is emission caused by a chemical reaction. Fluorescence is emission caused by excitation Computer control enhances and optimizes the info extracted from each instrument component.

  12. Light Source Focus Specimen Focus Energy, Space, and Time Sorting Detection Absorption Transmission and/or Reflection can also occur Nearly linear light path geometry for multi-wavelength, simultaneous light detection Absorbance Wavelength (λ) Relaxation is non-radiative; sample warms up a bit via vibration and rotation

  13. Fluorescence (fast) & Phosphorescence (slow) Light Source (Laser) Focus Specimen Typical geometry 90°, but angle variable May include energy sorting Energy, Space, and Time Sorting Focus Detection Emission Power Radiative

  14. Raman Scattering Light Source Laser Focus Specimen Typical geometry 90°, but angle variable Energy, Space, and Time Sorting Focus Detection Same geometrical layout as fluorescence and phosphorescence, … But what happens is not the same as absorption or emission

  15. Raman Scattering Elastic scattering: Eex = Eout Inelastic scattering: Ein < Eout and Ein > Eout Eexcitation virtual state virtual state -E Eex +E

  16. Different classes of optical spectroscopy Absorbance (UV/Vis or IR) Emission Lamps, LEDs Flame, plasma, chemistry Raman scattering Fluorescence/ Phosphoresence Lamps, LEDs, lasers lasers

  17. Classes of light sources

  18. Light sources:Common examples • Blackbody radiation • Light emitting diode (LEDs) • Arc lamp/hollow cathode lamp • Lasers • Solid-state • Gas/excimer • Dye laser • Thermal excitation • Combinations (laser to vaporizesample leading to thermal emission)

  19. Continuum spectra and blackbody radiation • A solid is heated to incandescence • It emits thermal blackbody radiation in a continuum of wavelengths Wien’s Law High E = Low λ = High T b is Wein’s displacement constant Skoog, Fig. 6-22

  20. Continuum spectra and blackbody radiation T ≈ 1200° C T ≈ 1473 K http://en.wikipedia.org/wiki/Image:Blackbody-lg.png http://en.wikipedia.org/wiki/Black_body

  21. Continuum sources • Common sources • Deuterium lamp (common Ultraviolet source) • Ar, Xe, or Hg lamps (UV-vis) • Not always continuous; spectral structure possible http://www1.union.edu/newmanj/lasers/Light%20Production/LampSpectra.gif http://creativelightingllc.info/450px-Deuterium_lamp_1.png

  22. Light emitting diodes (LEDs) • First practical visible region LED invented by Nick Holonyak in 1962 (GE; UIUC since 1963) • “Father of the light-emitting-diode” An LED is a semiconductor which emits electroluminescence http://en.wikipedia.org/wiki/Nick_Holonyak http://upload.wikimedia.org/wikipedia/commons/7/7c/PnJunction-LED-E.PNG http://www.pti-nj.com/images/TimeMasterLED/LED-spectra_remade.gif

  23. Light emitting diodes (LEDs) • Cheap, low energy, long-lasting, small, fast • Commonly used in display screens, stoplights, circuit boards as state indicators • Lots of colors • Infrared LEDs used in remote controls http://en.wikipedia.org/wiki/File:Verschiedene_LEDs.jpg

  24. Line (emission) sources • Continuous wave • Hollow cathode discharge lamp • Microwave discharge • Flames and argon plasmas • Pulsed • Pulsed hollow cathode • Spark discharge • All these are non-laser A line source is a light source that emits at a narrow wavelength called an emission “line”

  25. Lasers LightAmplification by StimulatedEmission ofRadiation • Intense light source • Narrow bandwidth (small range λ < 0.01 nm) • Coherent light (in phase)

  26. Lasers LightAmplification by StimulatedEmission ofRadiation • Pumping • Spontaneous Emission • Stimulated Emission • Population Inversion

  27. Laser design A photon cascade! • Lasing medium is often: • a crystal, like ruby • a dye solution • a gas or plasma Skoog, Fig. 7-4

  28. Pumping • Generation of excited electronic states by thermal, optical, or chemical means. Skoog, Fig. 7-5

  29. Spontaneous emission or relaxation • Random in time • No directionality • Monochromatic (same λ), but incoherent (not in phase) • Solid vs. dashed line – 2 different photons Skoog, Fig. 7-5

  30. Stimulated emission • The excited state is struck by photons of precisely the same energy causing immediate relaxation • Emission is COHERENT • Emitted photons travel in same direction • Emitted photons are precisely in phase Skoog, Fig. 7-5

  31. When the population of excited state species is greater than ground state, an incoming photon will lead to more stimulated emission instead of absorption. Population inversion Normal population distribution Pexcited < Pground Inverted population Pexcited > Pground Skoog, Fig. 7-6

  32. 3- and 4-state lasers • Population inversion easier in 4-state system Things stack up here. Population inversion easily achieved. Population relatively low down here Skoog, Fig. 7-7

  33. Laser design A photon cascade! • Lasing medium is often: • a crystal, like ruby • a dye solution • a gas or plasma Skoog, Fig. 7-4

  34. Continuous wavelaser sources • Nd3+:Yttrium aluminum garnet (YAG: Y3Al5O12) • Solid state • 1064 nm, 532 nm, 355 nm, 266 nm • The GTE Sylvania Model 605, uses a Nd-YAG laser rod set in a "double elliptical“ reflector, is pumped by two 500-W incandescent lamps, and is limited to a low order mode by an aperture in the laser cavity.

  35. Continuous wavelaser sources • Helium-Neon (HeNe) • Gas, but emission comes from generated plasma (very excited state atoms) • 632.8 nm, 612 nm, 603 nm, and 543.5 nm; 1.15 & 3.39 μm • Emission lines all the way out to 100 μm 99% reflective 99.9% reflective

  36. Continuous wavelaser sources • Ar+ • Gas laser, but emission comes from ions • Uses lots of electrical power to generate ions • 351.1 nm, 363.8 nm, 454.6 nm, 457.9 nm, 465.8 nm, 476.5 nm, 488.0 nm, 496.5 nm, 501.7 nm, 514.5 nm, 528.7 nm, 1092.3 n Coherent Innova 90 Up to 5 W of output! ~100x my laser pointer

  37. Other continuous wavelaser sources • Cu vapor • 520 nm • HeCd • 440 nm, 325 nm • Dye lasers

  38. Pulsed lasers sources • Nd:YAG • Solid state • Often nanosecond pulses • 1064 nm, 532 nm, 355 nm • Ti:sapphire • Solid state—often pumped by Nd:YAG • Tunable output aroudn 800-1200 nm • Produces femtosecond pulses • Nitrogen • Gas • 337 nm • Excimer lasers (gas mixtures; excited state is stable) • Tunable dye lasers (λ is selective within limits)

  39. Laser diodes • Used in CD and DVD players (not very strong) • Wavelengths now available from IR to near UV regions Band gap energy, Eg Resonant Cavity emits At 975 nm Skoog, Figs. 7-8 & 7-9.

  40. Tip going forward Keep your variables straight v for velocity or  for frequency Microsoft equation editor gives: I will use m for integer, textbook uses n Easy to get mixed up with refractive index, n

  41. Properties of electromagnetic radiation • Transmission • Refraction • Reflection • Scattering • Optical Components • Interference • Diffraction

  42. Properties of electromagnetic radiation y = magnitude of the electric field at time t A = ymax–also called the amplitude of y ν = frequency ins -1 (cycles per second) φ = phase angle (an offset relative to a reference sine wave) ω = angular velocity in radians/sec (a handy definition) Recall: π radians = 180 degrees

  43. Interference – magnitudes add or subtract A B A+B B is in phase with A

  44. Interference – magnitudes add or subtract A B A+B B is 180 degrees (πradians) shifted from A

  45. Interference – magnitudes add or subtract A B A+B B is 90 degrees (π/ 2 radians) shifted from A

  46. Interference between waves of different frequency Wave 1 + 2 Skoog, Fig. 6-5

  47. Transmission through materials • Compared to vacuum, the velocity of light is reduced when propagating through materials that have polarizable electrons. • Wavelength also decreases • All electrons are polarizable to some extent cvacuum = 2.99792 x 108 m● s-1 Skoog, Fig 6-2.

  48. Wavelength-dependence of nSiO2 Material n @ 589.3 nm Vacuum (air) 1.00 Water 1.33 Hexadecane 1.43 Quartz 1.46 Toluene 1.49 Glass (light flint) 1.58 Index of Refraction • Refractive index is measure of how much light is slowed: • Refractive index is wavelength- and temperature-dependent for many materials: http://www.rp-photonics.com/refractive_index.html

  49. Refraction • Snell’s law: • Oil immersion lenses for high magnification microscopy Medium 1 Medium 2 Skoog, Fig. 6-10 Here, n2 > n1 Velocities, not frequencies

  50. For your information … Book Error on page 141, equation 6-12: This is correct: Snell’s Law of Refraction

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