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Chapter 24 Introduction to Spectrochemical Methods. Contents in Chapter 24. Properties of Electromagnetic Radiation 1) Wave properties 2) Wave-Particle Duality 2. Interaction of Radiation and Matter 1) Electromagnetic Spectrum 2) Types of Quantum Transition
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Contents in Chapter 24 • Properties of Electromagnetic Radiation 1) Wave properties 2) Wave-Particle Duality 2. Interaction of Radiation and Matter 1) Electromagnetic Spectrum 2) Types of Quantum Transition 3) Spectroscopies without Energy Exchange 4) Spectroscopies Involving Energy Exchange 3. Absorption of Radiation 1) The Absorption Process 2) Absorption Spectra 4. Emission of Electromagnetic Radiation
Properties of Electromagnetic Radiation 1) Wave properties (a) Plane-polarized electromagnetc radiation (b) 2D representation of electric factor
2) Wave-Particle Duality λ: wavelength ν: frequency v: light speed c: 3x108 m/s (in vacuum) n: refractive index In vacuum: n= 1 : wavenumber (cm–1) ΔE: energy gap h: Plank’s constant, 6.626x10–34J·s Light measurement: Power (P): The flux of energy per unit time. Intensity (I) -The flux of energy per unit time per area.
Continued λ change between different medium, ν remains constant *Speed of light = c/n, n (usually n > 1) is the refractive index of the medium
2. Interaction of Radiation and Matter 1) Electromagnetic Spectrum
Continued Vacuum ultraviolet (VUV): 120–180 nm Ultraviolet (UV): 180–380 nm Visible: 380–780 nm Near infrared regions (NIR): 0.78–2.5 μm Mid infrared: 2.5–50 m Far infrared (FIR): 50–1000 m.
4) Spectroscopies Involving Energy Exchange (1) Classification
(2) Glossary for Spectroscopies Involving Energy Exchange i) Optical spectroscopy (Involving Energy Exchange): Methods based on the absorption, emission, luminescence of electromagnetic radiation that is proportional to the amount of analyte in the sample. ii) Absorption spectroscopy: Measuring the quantized energy absorbed by atoms/molecules. iii) Emission spectroscopy: Exciting atom by heat (thermal), then, the emitted quantized energy from excited state to ground states is measured.
iv) Photoluminescence: Exciting atom/molecule by light, then, the emitted quantized energy is measured. • Fluorescence: The ground state with the same spin as excited state. • Phosphorescence: The ground state with the opposite spin as excited state. v) Chemoluminescence (chemiluminescence): The luminescence (emission light) is the result of a chemical reaction.
(3) Energy transition process illustrate i) Absorption process
3. Absorption of Radiation 1) The Absorption Process i) Transmittance and Absorbance
ii) Beer’s Law C Po P b ***** C: Analyt’s concentration b: Light path length Beer’s Law: A = abC a: absorptivity, unit is of cm–1conc–1. Analyte in molar concentration: A = bC : molar absorptivity, unit is of cm–1M–1 • Beer’s law is the linear relationship between a sample’s absorbance and concentration. • Values for a or depend on the wavelength of electromagnetic radiation. • Wavelengths corresponding to maxima absorbance in the spectra called λmax.
Example: The following data was obtained from an optical absorption instrument with a cell path length 1 cm. (a) Find the molar absorptivity coefficient. (b) Determine the concentration of an unknown solution that has an absorbance of 1.52. Solution: Y = 201.85X = bC (a) =201.85 cm–1M–1 (b) C= 0.0075 M
iii) Applying Beer’s Law to Mixtures The absorbance at a specific wavelength for a mixture of n components, Am, is given as: Two component mixture for example:
iv) Limitations to Beer’s Law Linear range * Beer’s law is valid only at low concentrations. Generally, < 0.01 M
(cont’d) i) Fundamental Limitations: At higher concentrations: (1) The individual particles of analyte no longer behave independently (recalled “activity”) of one another resulting in changing the value of . (2) Since absorptivity depend on the sample’s refractive index, when the refractive index varies with the analyte’s concentration, the values of will change.
(cont’d) ii) Chemical Limitations Deviations from Beer’s law also occur when the analyte dissociates, associates, or reacts with a solvent to produce a product having a different absorption spectrum from the analyte. Example: HIn = H+ + In- color 1 color 2 The above reaction causes the color to be pH dependent (indicators for instance). Thus, must buffer our solution to a constant pH to eliminate pH related chemical deviations.
(cont’d) iii) Instrumental limitation (1) Beer’s law is followed only with truly monochromatic, the polychromatic radiation cause deviations from Beer’s law. (2) Stray radiation(any radiation reaching the detector that does not follow the optical path from the source to the detector) cause deviations from Beer’s law.
2) Absorption Spectra • Atomic Absorption (line spectra) • When a atom absorbs specific quantized UV/Vis radiation, it undergoes a change in its valence electron configuration: * Transitions between two different orbital are termed electronic transitions. e h For example, Na consists of a few, discrete absorption lines corresponding to transitions between 3s→3p, 3s→4p etc.
Molecular Absorption (band spectra) { Vibration level * Molecular absorptions spectra are generally broad band (band spectra) because vibrational and rotational levels are "superimposed" on the electronic levels. Electronic Excited { Vibration level Electronic Excited { Vibration level Electronic Ground state
Example of UV-Visible absorption spectra Gaseous phase Nonpolar solvent Analyte: 1,2,4.5-tetrazine Polar solvent
1) Emission Spectra 4. Emission of Electromagnetic Radiation Emission spectrum of a brine sample with an oxyhydrogen flame
Atomic Fluorescence • Radiant emission from atoms that have been excited by absorption of electromagnetic radiation. * Resonance fluorescence: fluorescence emission at a wavelength that is identical with the excitation wavelength.
3) Molecular Fluorescence i) Energy Level Diagram
A* A ii) More Illustration a) Life time Lifetime of an analyte in the excited state (A*): • ~10–5–104 s for electronic excited states • ~10–15 s for vibrational excited states. b) Relaxation types of excited state (1) Nonradiative relaxation, e.g., vibrational deactivation, excess energy is released to solvent molecules: A* → A + heat (2) Released as a photon of electromagnetic radiation: A*→ A + h c) Strokes shift: Difference in wavelengths of incident and emitted radiation.
Continued d) Vibration Deactivation versus Internal Conversion (1) Vibrational deactivation (relaxation): A nonradiative relaxation when a excited molecule nonradiatively loses vibrationalenergy in a same electronic level, lifetime is rapid (10–13 to 10–11 s). (2) Internal conversion: A nonradiative relaxation in which the analyte moves from a higher electronic level to a lower electronic level.
e) Fluorescence versus phosphorescence S0 S1 T1 (1) Fluorescence: Emission of a photon when the analyte returns to a lower-energy state with the same spin as the higher energy state, i.e., S1→S0, in which the electron life time in the excited state is ~10–5–10–8 s. (2) Phosphorescence: Emission of a photon when the analyte returns to a lower-energy state with the opposite spin as the higher-energy, i.e., T1→S0, in which the electron life time in the excited state is ~10–4–104 s.
f) Fluorescence intensity equation*** (1) Fluorescence is generally observed with molecules where the lowest energy absorption is a π→π* transition, and those chromophores are called fluors or fluorephores. (2) For low concentrations of the fluorescing species, where εbC is less than 0.01, the intensity of fluorescence (If) is expressed as: If= 2.303kΦfP0εbC C: analyt’s concentration b: light path length ε: molar absorptivity k: efficiency constant of collecting and detecting the emission P0: excitation incident power Φf : number of photons emitted/number of photons absorbed (quantum yield).
Homework (Due 2014/4/10) Skoog 9th edition, Chapter 24 Questions and Problems 24-5 24-6 (a) (b) 24-9 24-23 End of Chapter 24
