1. Introduction to optical spectroscopy Chemistry 243

2. Fundamentals of electromagnetic radiation (s -1 )

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

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

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

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

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

15. Raman Scattering Elastic scattering: E ex = E out Inelastic scattering: E in E out E excitation E ex +E-E virtual state

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

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 vaporize sample 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 Skoog, Fig. 6-22 High E = Low λ = High T Wien’s Law b is Wein’s displacement constant

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

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” 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 An LED is a semiconductor which emits electroluminescence

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 L ight A mplification by S timulated E mission of R adiation Intense light source Narrow bandwidth (small range λ < 0.01 nm) Coherent light (in phase)

26. Lasers L ight A mplification by S timulated E mission of R adiation Pumping Spontaneous Emission Stimulated Emission Population Inversion

27. Laser design Lasing medium is often: a crystal, like ruby a dye solution a gas or plasma A photon cascade! 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. Population inversion When the population of excited state species is greater than ground state, an incoming photon will lead to more stimulated emission instead of absorption. Inverted population Normal population distribution P excited > P ground P excited < P ground Skoog, Fig. 7-6

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

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

34. Continuous wave laser sources Nd 3+ :Yttrium aluminum garnet (YAG: Y 3 Al 5 O 12 ) 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 wave laser 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 99.9% reflective 99% reflective

36. Continuous wave laser 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 wave laser 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, E g Skoog, Figs. 7-8 & 7-9. Resonant Cavity emits At 975 nm

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 = y max – also called the amplitude of y ν = frequency in s -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. A B A+B B is in phase with A Interference – magnitudes add or subtract

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

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

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 Skoog, Fig 6-2. c vacuum = 2.99792 x 10 8 m ● s -1

48. Index of Refraction Refractive index is measure of how much light is slowed: Refractive index is wavelength- and temperature- dependent for many materials: 1.46Quartz 1.49Toluene 1.43Hexadecane 1.58Glass (light flint) 1.33Water 1.00Vacuum (air) n @ 589.3 nmMaterial Wavelength-dependence of n SiO2 http://www.rp-photonics.com/refractive_index.html

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

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

51. Reflection Amount of loss at a reflection increases with refractive index mismatch. For right angle light entrance into a medium: Reflective loss is angle-dependent Fresnel equations (which we will skip) Most important case is: total internal reflection

52. Total internal reflection Light incident upon a material of lesser refractive index is bent away from the normal so that the exit angle is greater than then incident angle. At the critical incident angle, the exit angle is 90° - beam does not exit Angles larger than the critical incident angle lead to total internal refection (TIR) Medium 1 Medium 2 n entry n exit Modified from Skoog n entry > n exit θ 2 > θ 1 When this is true, θ 1 = critical entry angle for TIR

53. Total internal reflection TIR fluorescence microscopy: http://hyperphysics.phy-astr.gsu.edu/Hbase/phyopt/totint.html http://www.olympusmicro.com/primer/techniques/fluorescence/tirf/tirfintro.html When this is true, θ 1 = critical entry angle for TIR. If θ 1 > θ critical result is TIR. Evanescent wave samples a very narrow slice of the sample very near to the dielectric interface Typically ~200 nm

54. Total internal reflection TIR fluorescence microscopy: When this is true, θ 1 = critical entry angle for TIR. If θ 1 > θ critical result is TIR. http://hyperphysics.phy-astr.gsu.edu/Hbase/phyopt/totint.html http://www.olympusmicro.com/primer/techniques/fluorescence/tirf/tirfintro.html Good for studying adhered cells; low background

55. Fiber optics Extruded strands of glass or plastic that guide light via total internal reflection. Core has higher refractive index than cladding. Flexible Material choice allows transmission in UV, visible, or IR Skoog, Fig 7-39. Follows all the rules of Snell’s Law

56. Scattering Raman scattering Inelastic scattering offset from by frequency of molecular vibrations Rayleigh scattering Molecules or aggregates smaller than Intensity ~ 1/ 4 Mie scattering Particles large (or comparable) to Used for particle sizing

57. Essential optical elements Lenses Mirrors Prisms Filters Gratings

58. Basic optical components Mirrors Reflection Concave mirror is converging Convex mirror is diverging Prisms Refraction Snell’s Law

59. Filters Absorption filters Cheap, visible region; colored glass Cutoff filters – long-pass short-pass Interference filters Skoog, Fig. 7-12 θ is usually zero so, cos θ = 1. Also, m is usually 1 d = thickness of dielectric layer n = refractive index of dielectric medium m = integer ʹ = wavelength in the dielectric material

60. Interference filters Almost monochromatic Skoog, Fig. 7-13 Bandwidth of a filter is width at half-height (aka full-width @ half-max)

61. Diffraction of coherent radiation: Interference at work Consequence of interference Skoog, Figs. 6-7, 6-8 constructive destructive constructive destructive d = distance from slit B to C Distance x to y is one λ (m is the order of interference) m is: 0 for E 1 for D m used here, text uses n

62. Diffraction of coherent radiation: Interference at work Consequence of interference (m is the order of interference) m used here, text uses n You can now determine the wavelength of light based on things that are easy to measure! Skoog, Fig. 6-8

63. Monochromators Used to spatially separate different wavelengths of light: prisms, gratings Czerny-Turner grating monochromatorBunsen prism monochromator Skoog, Fig. 7-18

64. Gratings and monochromators Reflection + diffraction: echellette-type grating Skoog, Fig. 7-21 The condition for constructive interference. The m = 1 line is most intense. The surface contains “grooves” or “blazes”. Take a look at Example 7-1, Page 184.

65. Monochromators Used to spatially separate different wavelengths of light: prisms, gratings Czerny-Turner grating monochromator Skoog, Fig. 7-18

66. Useful metrics for monochromators Dispersion (page 185); high dispersion is good Integration of at constant i gives the angular dispersion: Linear dispersion, D, is the variation of λ along the focal plane position, y: Reciprocal linear dispersion, D -1 : r = angle of reflection d = distance between blazes More useful, results in D -1 in nm per mm or similar A measure of the ability to separate wavelengths

67. Useful metrics for monochromators—continued Resolving power (R; unitless) Limit of monochromator’s ability to distinguish between adjacent wavelengths. Light gathering power (f-number, F; unitless) Collection efficiency—improve for maximizing S/N Efficiency scales as the inverse square of F

68. Complications with monochromators Overlap of orders m = 1, = 600 nm and m = 2,  = 300 nm spatially overlap You can get ’s mixed up if light source contains many wavelengths Additional wavelength selection often needed Filter, prism, detector λ selection device, digital analysis after data collection, background subtraction Might need to use a different light source if your wavelength of interest is not “clean”

69. Slit width and spectral resolution of a spectrometer Tradeoff exists between sensitivity and resolution High intensity = high sensitivity (low noise) Two basic concepts: If you make the entrance slit width too big, you let in a lot of light (that’s good – high intensity), but it can be multi- wavelength; a large section of light dispersed in  is let in Good light intensity, poor spectral resolution If you make the entrance slit width too small, you let in less light (less intensity), but its range is smaller Poor light intensity, good spectral resolution Entrance slit (creates image) and exit slit (output filter) Usually the same width Optimal slit width based upon grating dispersion

70. Slit width Skoog, Fig. 7-24 For just passing 2

71. Skoog, Fig. 7-25 P total If spectral bandwidth is  /2, good spectral resolution Both entrance and exit slits narrowed from top to bottom Slit width

72. Skoog, Fig. 7-26 Slit width Watch the effect of adjusting the slit width and the resultant spectral bandwidth on the following data sets of benzene vapor.

73. Optical Photodetectors A.Photomultiplier tube (PMT) B.CdS photoconductivity C.GaAs photovoltaic cell D.CdSe photoconductivity cell E.Se/SeO photovoltaic cell F.Si photodiode G.PbS photoconducitivity H.Thermocouple I.Golay cell These generally make current or voltage when light hits them. More sensitive Less sensitive

74. Ideal photodetector (photon transducer) High sensitivity High S/N Fast response time Signal directly proportional to # of photons detected Zero dark current The blank is zero Or, equivalently,

75. High sensitivity High S/N Fast response time Signal directly proportional to # of photons detected Zero dark current The blank is zero Ideal photodetector (photon transducer) High sensitivity High S/N Fast response time Signal directly proportional to # of photons Zero dark current Reality Intrudes Here’s what really happens: Signal is Function of λ Constant dark current term (non-zero)

76. Three main photodetector types Photon transducers (directly “count” photons) Photomultiplier tubes (PMTs) Charge transfer devices Charge injection devices (CID) Charge coupled devices (CCD) Thermal transducers Photons strike the transducer Temp increases Temp increase increases conductivity  Current or voltage are measured

77. Vacuum phototube Cathode is coated with photo-emissive material Emitted electrons are collected anode. # of electrons is directly proportional to # of photons. Current is easy to amplify. Usually have small dark current. Operate at ~ 90V bias Not so portable Skoog, Fig. 7-29.

78. Photomultiplier tube (PMT) # of electrons is amplified by photoelectric effect upon acceleration towards dynodes Each dynode biased ~ 90V more positive than previous dynode (or cathode) Voltage drop accelerates electrons to dynode cascade Amplification: 10 6 -10 7 electrons per incident photon; electron cascade http://www.nt.ntnu.no/users/floban/KJ%20%203055/PMT.jpg

79. Photomultiplier tube (PMT) Advantages: Very sensitive in UV-Vis region, single photon sensitivity Cooled PMT has very low background (k dark approaches zero) Linear response Fast response Disadvantages Easily damaged by intense (ambient) light Noise is power dependent Single channel: can’t use for imaging

80. Photovoltaic cell Light strikes a semiconductor (Se) and generates electrons and holes Magnitude of current is proportional to # of photons Requires no external power supply! Disadvantages: hard to amplify signal and fatigue (wears out) Useful for portable analyses, field work, outdoor setting Skoog, Fig. 7-28

81. Photodiodes (Silicon 190-1100 nm, InGaAs 900 – 1600 nm) Reverse-biased p-n junction Conductance goes to near zero Photons create electron hole pairs that migrate to opposite contacts and generate current Battery powered Portable applications Are not as prone to some electronic noise sources 60 Hz line noise Skoog, Fig. 7-32

82. Multichannel transducers Allow simultaneous interrogation of multiple wavelengths Imaging Photodiode arrays (1-D array) Charge-transfer devices (2-D array) Charge-injection devices Charge-coupled device (CCD) CMOS

83. Photodiode arrays Each diode has defined spatial address Advantages Multichannel (used for imaging) More robust than PMT Disadvantages Not as sensitive as PMT Slower response time Common in cheaper UV-Vis instruments Often perfectly adequate Skoog, Fig. 7-33

84. Charge transfer devices Converts light into charge Negative-biasing leads to increased capture of holes under pixel electrodes Potential well Photon ejects electron and the device collects and stores charges 10 5 -10 6 charges per pixel Configured as CID or CCD Skoog, Fig. 7-35 Schematic is for CID

85. Charge transfer devices (continued) Charge-injection device (CID)-measures accumulated voltage change (nondestructive read; persistent after read) Measurements can be made while integrating Charge-coupled device (CCD)-moves accumulated charges to amplifier and readout (destructive read; gone after read) Very high sensitivity; 10 4 -10 5 pixels High resolution spectral imaging Complementary metal oxide semiconductor (CMOS) Webcam technology: CHEAP! OK sensitivity, large pixel density

86. CCD (charge coupled device) Pixels read one at a time by sequential transfer of accumulated charge From: “CCD vs. CMOS: Facts and Fiction” by Dave Litwiller, in Photonics Spectra, January 2001

87. CMOS detectors Digital camera and webcam technology Each pixel can be read individually From: “CCD vs. CMOS: Facts and Fiction” by Dave Litwiller, in Photonics Spectra, January 2001 Image from Wikipedia

88. CCD CMOS Essentially serial Each pixel read one at a time by common external circuitry Voltage conversion and buffering Outputs an analog signal Historically gave higher- resolution images Relatively expensive High power consumption Up to 100x more than CMOS Essentially parallel Each pixel has its own red out circuitry “on-chip” Allows amplification and noise correction More susceptible to noise Outputs a digital signal Reduced area for light absorption Relatively inexpensive Highly commercialized fab Runs on less power Requires less “off-chip” circuitry Both approaches exist today

89. Photoconductivity transducers Semiconductors whose resistance decreases when they absorb light Absorption promotes electron to conduction band. Useful in near IR( = 0.75 to 3  m) Cooling allows extension to longer wavelengths by reducing thermal noise

90. Thermal transducers Solution for IR region (low energy photons) Thermocouples Light absorbed heats the junction (two pieces of dissimilar metal) which leads to a change in voltage relative to a reference thermocouple. Bolometer (thermistor) Material changes resistance as a function of temp Pyroelectric devices Temperature-dependent capacitor Change in temperature leads to change in circuit current