Analytical Spectroscopy Review

Transcription

1 Analytical Spectroscopy Review λ = wavelength ν = frequency V = velocity = ν x λ = x 10 8 m/sec = c (in a vacuum) ν is determined by source and does not change as wave propogates, but V can change as wave passes through matter, it becomes less than c. Therfore, λ also changes in matter. Electromagnetic Spectrum & Quantum transitions Nuclear Inner electron Valence electron Molecular vibrations Molecular rotations Nuclear spin

2 Superposition of waves Principle of superposition when two or more wave disturbances occupy the same space at the same time, the resultant disturbance can be predicted by the sum of the individual disturbances. Electric Field representation y = A sin (2πνt + φ) = A sin (ωt + φ) A = amplitude ω = angular frequency Constructive interference Y total n = A sin( ω t + φ ) i= 1 i i i Destructive interference Wavelength Selection Separation of EM radiation into it s constituent frequency components (or colours). In analytical spectroscopy, we may have wavelength selection devices BEFORE sample or AFTER sample, depending on the type of spectroscopy.

3 Basic Optical Schemes Light source 6 sample 6 wavelength selector 6 detector Absorption Excited sample 6 wavelength selector 6 detector Emission some luminescence, atomic emission, Laser or other excitation light source Light induced Emission fluorescence, phosphorecence, Raman Excitation light source 6 sample 6 wavelength selector 6 detector

4 Components of Optical Spectrometers Light Sources Incandescent - made of resistive materials that are heated electrically Arc - an electrical discharge is maintained through a gas or metal vapour. (continuous and line sources). Continuous source - outputs a continuous intensity of radiation as a function of wavelength. Extensively used in molecular spectroscopy. examples: deuterium lamp (UV) tungsten filament (VIS) Nernst Glower (IR) Xe arc (UV-VIS) Glow bar Deuterium Filament Arc Arc with parabolic reflector Spectral Irradiance Curves-Xe Typical spectral irradiance of 150 W Xe Lamp.

5 Spectral Irradiance Curves- Deuterium Fig. 3 Typical spectral irradiance of 1000 W QTH Lamp. Spectral irradiance of various Deuterium Lamps. Spectral Irradiance Curves-Quartz Tungsten Halogen (QTH) Typical spectral irradiance of 1000 W QTH Lamp.

6 Line sources - have radiation at discrete quantized energies. Mercury lamps - many discrete lines in UV and visible Hollow cathode - for atomic spectroscopy Lasers - a special light source in a class of it s own, many different types and wavelengths Fig. 1 Typical spectral irradiance of 200 W Hg Lamp. Stimulated and Spontaneous Emission M* 6 M + hν Emission of photon from excited state species is random in time and direction hν +M* 6 M + 2 hν Emission of photon from excited state species is induced by first photon. Both photons that leave have the same direction, phase and frequency coherent radiation!

7 Blackbodies Blackbody radiator absorbs all EM radiation without reflection (ieblack) but is in thermal equilibrium with surroundings it s rate of absorption is equal to rate of emission. Planck s Law (flux distribution) Φ b λ 2πhc = 5 λ 2 e 1 hc / λkt 1 Wien s Displacement Law (max λ) λ max hc nm K = = 5k T λ in nm, T in kelvins 6 Total radiation of black body Φ total = 0 σ = 5.67x10 2π k Φ λdλ = T c h 8 Wm 2 5 K = σt 4 Stefan-Boltzmann constant Kirchoffs Law Real bodies are not "black", they do not absorb radiation with 100% efficiency at all wavelengths. Kirchoff's law states that if an object absorbs radiation with efficiency, ε λ,, then it emits radiation with a fractional efficiency ε λ of the blackbody emission at temperature, T. Φ λ (T) = ε λ (T) Φ λb (T) The emission spectrum is known if we know the absorption spectrum and T

8 Solar Radiation Spectra Solar radiation flux at sea level is less than at the top of the atmosphere due to reflection by clouds, aerosols and absorption features by O 2, O 3, H 2 O and other trace gases. The earth as a blackbody radiator

9 Lasers Light Amplification by Stimulated Emission of Radiation Lasing medium ~100% ~95-99% Power source Properties of Lasers Monochromatic - single wavelength Coherent - all photons appear to originate from a single point, electrical fields are all in phase. Bright spectral irradiance (intensity per unit wavelength) is much higher than conventional sources Focussing can be focussed to extremely small spots (λ/2) Pumping and Population Inversions Pumping an external source provides energy to the lasing medium in order to achieve a "population inversion". A population inversion is necessary for a laser to operate. Pumps may include electrical discharge, optical energy from a flashlamp, or another laser (pump laser). Population inversion there more molecules or atoms are in the upper state (S 2,) than in the lower state (S 1 ) of the quantum transition. N 2 > N 1 x x x x x x x x x x x x S 2 - State 2, E 2 - Energy 2 N 2 - # of molecules Lasing transition S 2 S 1 E = E 2 E 1 = hν = hc/λ N 2 > N 1 x x x x x x x x S 1 - State 1, E 1 - Energy 1 N 1 - # of molecules

10 2-Level System x x x x x x x x x x x x S 2, E 2, N 2 σ 1 2 cross section of absorption σ 2 1 cross section of stimulated emission x x x x x x x x S 1, E 1, N 1 Each absorption event causes a decrease in the radiant flux of photons, Φ, inside the lasing cavity. Each stimulated emission event causes an increase in Φ. From spectroscopic theory, it is known that σ 1 2 = σ 2 1. The change in radiant flux, dφ, inside a laser cavity along the axis starting from an arbitrary point, and travelling a distance dz is given by... dφ = + Φ N 2 σ 2 1 dz - Φ N 1 σ 1 2 dz = + Φ (N 2 -N 1 ) σ dz. For lasing action to occur, the radiant flux of photons inside the cavity must be amplified..ie- must be > 0. Therefore, (N 2 -N 1 ) > 0 OR N 2 > N 1 for lasing action. If N 2 > N 1, dφ > 0, population inversion exists, SE dominates, lasing action occurs. If N 2 = N 1, dφ = 0, no change in Φ (but considering other losses, NO lasing) If N 2 < N 1, dφ < 0, Φ decreases with z, absorption dominates,...ie- Beers Law. Boltzmann Considerations N N j 0 P = P j 0 E exp kt Boltzmann Equation N j = number of molecules or atoms in state j N 0 = number of molecules or atoms in state 0 (ground state for example) P j = multiplicity of state j (2 electrons per state X number of levels with energy j) P 0 = multiplicity of state 0. DE = energy difference between state j and 0 (E j > E 0 ). k = Boltzmann constant (1.28 x J K -1 ) T = temperature (Kelvin). For 2 states with equal multiplicity, N j / N 0 < 1.0 at all times since T > 0K, and E > 0. The best we can do is to heat a system to high T or to minimize the difference in energy between two states, but even if we do this, N j /N 0 only approaches 1.0, but is always less than 1. Therefore we cannot achieve a population inversion by heating a laser system...we must achieve a population inversion in some other way.

11 3-Level and 4-Level Laser Systems E pump S 2, E 2 fast non radiative transition S 1, E 1 laser transition 3-Level System If σ 0->2 > σ 0->1, and if the 2 1 nonradiative transition is fast, then we can achieve a population inversion between state 1 and 0 by optically pumping at the E 2 E 0 wavelength. S 0, E 0 4-Level System Since the laser transition in a 4-level system does not end up in the ground state, the population in the lowest state of the laser transition, S 1 is very low. Thus it is even easier to obtain a population inversion, N 2 > N 1 in this type of system. 4-level systems are common. The first demonstrated laser, a ruby laser, was based upon a 3-level system E pump S 3, E 3 fast non-radiative transition S 2, E 2 laser transition S 1, E 1 S 0, E 0 fast transition Types of Lasers Gas Lasers He/Ne a mixture of neutral gases in the laser cavity. the laser active gas is Ne. He gas is used for collisional pumping...see energy diagram. Lines at 632.8, 612.0, 594.1, nm. The red laser at 632.8nm is most common and most intense. He/Cd economical continuous wave laser source with lines at 442 and 325 nm. Cadmium is the active medium. heat is applied to a reservoir of Cd metal in order to vaporize the metal. Ar+ - used extensively in Raman and laser induced fluorescence spectroscopy. Many laser lines available, most intense are at 488 and 514 nm. Solid State Lasers - active medium is a host atom embedded in a solid state material. Ruby Cr III in Al 2 O 3. pumped with a flash lamp; 3-level system. Nd:YAG Nd +3 in Yttrium Aluminum Garnet nm IR laser. Very high power. these can be frequency doubled (532 nm) or tripled (355 nm). Used for commercial applications. Dye Lasers active lasing medium is a dye or mixture of dyes dissolved ain a solvent that is pumped through the lasing cavity. these lasers can be "tuned" to lase over a broad range of λ's. They are often pumped with a primary laser.

12 Wavelength selectors filters, prisms, gratings and monochromators these devices are used in all types of optical spectroscopy in order to select a specific range of wavelengths (bandwidth) from a much broader band of light.\ polychromatic wavelength monochromatic radiation selector radiation Interference filters through constructive interference, only a a narrow band of light is allowed to pass while the rest of the light is rejected (reflected). Interference filter glass dielectric glass silver films (semitransparent) Long Pass colored filters

13 Interference Filters Interference filterscont'd

14 Interference filters cont'd 2 Monochromators

15 Monochromators prism vs. grating dispersion Monochromator Function

16 Gratings λ =?? θ = 45 o C Grating # Intended Use Lines/m m Grating Selection Spectral Range Blaze Wavelength Best Efficiency (>30%) 1 UV nm 300 nm nm 2 UV/VIS nm 400 nm nm 3 VIS/Color nm 500 nm nm 4 NIR nm 750 nm nm 5 UV/VIS nm holographic/uv nm 6 NIR nm* 750 nm nm 7 UV/VIS nm* holographic/uv nm 8 UV nm holographic/uv nm 9 VIS/NIR nm* holographic/vis nm 10 UV/VIS nm* holographic/uv nm 11 UV/VIS nm* holographic/vis nm 12 UV/VIS nm* holographic/vis nm* 13 UV/VIS/NIR nm** 500 nm nm 14 NIR nm 1000 nm nm

17 Grating example Grating Efficiency

18 Grating Monochmomator performance Slits

19 resolution Photon Detectors

20 Phototube Photomultiplier tube (PMT)

21 Photodiodes Fiber Optics

22 Photodiode Array CCD Signal Collection

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24 Jablonski Energy Level Diagram

25 relaxation mechanisms beers law

26 derivation of beers law deviations from beers law

27 deviations from beers law 2 ε Absorbance 1

28 Absorbance 2 absorbance 3

29 absorbance instruments absorbance instruments 2

30 luminescence fluorescence

31 fluorescence 2 Fluorescence3

32 Excitation, Fluorescence and Phosphorescence of Phenanthrene excitation fluorescence phosphorescence Schematic of a generic fluorescence instrument

33 Filter Fluorometer Emission filter Excitation filter Spectrofluorometer OR PDA or CCD multichannel detector

34 Excitation and Fluorescence Spectra of Anthracene Excitation Spectra similar to absorbance spectra. Obtained by measuring luminescence at a fixed wavelength while scanning the excitation monochromator wavelength. Fluorescence (emission) Spectra obtained by holding excitation wavelength constant while scanning the emission monochromator Factors affecting fluorescence

35 FAF2 Fluoresc vs abs

36 Phosphorescence Phos instrument

37 Chemiluminescence 1 Chemilum 2

38 Quiz 5 Quizzes worth 6%, best 3 out of 5...2% each. For those who need a final quiz, it will be held on Monday April 4. Two times, 2 versions... QUIZ 5A - 12:30-1:00 QUIZ 5B 2:30 3:00 Please meet at my office (PS 301) 5 minutes before quiz time, the location to write will be announced at that time. Topic Luminescence & Atomic Spectroscopy. For those who do well, the quiz could be worth 4%, 2% applied to quiz mark, 2% applied to your last test mark (if you do better on the quiz than your test. AS1

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