Transcript 4) Spectroscopies Involving Energy Exchange

Chapter 24
Introduction to Spectrochemical
Methods
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Contents in Chapter 24
1. 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
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1. Properties of Electromagnetic Radiation
1)
Wave properties
(a) Plane-polarized electromagnetc
radiation
(b) 2D representation of electric
factor
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2) Wave-Particle Duality
Wave properties:
v  c/n  λν
ν  1/λ
Particle properties:
(in vacuum)
E  h
 hc/  hc
λ:
ν:
v:
c:
n:
wavelength
frequency
light speed
3x108 m/s (in vacuum)
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.
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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
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2. Interaction of Radiation and Matter
1) Electromagnetic Spectrum
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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.
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2) Types of Quantum Transition
Type of Transition
Nuclear
Inner electron
Part of spectrum
Valence electron
Rotation/Vibration
Rotation
Spin of electrons
UV-vis
Infrared
Microwave
Electron Spin Resonance
Spin of nuclei
Nuclear Spin Resonance
-ray
X-ray
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3) Spectroscopies without Energy Exchange
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4) Spectroscopies Involving Energy Exchange
(1) Classification
Type of Energy
Transfer
Electromagnetic
Spectrum Region
Spectroscopic Technique
Absorption
-ray
Mossbauer spectroscopy
x-ray
x-ray absorption spectroscopy (XAS)
UV/Vis
UV/Vis spectroscopy
Atomic absorption spectroscopy (AAS)
infrared
Infrared spectroscopy (IR)
Raman spectroscopy
Emission (thermal
excitation)
microwave
Microwave spectroscopy
radio waves
nuclear magnetic resonance spectroscopy (NMR)
UV/Vis
Atomic emission spectroscopy (AES)
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Continued
Type of Energy
Transfer
Electromagnetic
Spectrum Region
Spectroscopic Technique
Photoluminescence
x-ray
x-ray fluorescence (XRF)
UV/Vis
Fluorescence spectroscopy
Phosphorescence spectroscopy
Atomic fluorescence spectroscopy (AFS)
Chemiluminescence
UV/Vis
Luminescence spectroscopy
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(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.
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iv) Photoluminescence: Exciting atom/molecule by light, then,
the emitted quantized energy is measured.
a) Fluorescence: The ground state with the same spin as excited
state.
b) 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.
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(3) Energy transition process illustrate
i)
Absorption process
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ii) Emission or Chemoluminescence process
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iii) Photoluminescence process
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3. Absorption of Radiation
1) The Absorption Process
i)
Transmittance and Absorbance
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Continued
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*****
ii) Beer’s Law
Po
C
P
C: Analyt’s concentration
b: Light path length
b
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.
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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.
Concentration
(moles/L)
0.001
0.002
0.005
0.01
Absorbance
0.21
0.39
1.01
2.02
Solution:
Y = 201.85X = bC
(a) =201.85 cm–1M–1
(b) C= 0.0075 M
Beer's Law Plot
Absorbance
2.50
2.00
y = 201.85x
1.50
1.00
0.50
0.00
0
0.002
0.004
0.006
0.008
0.01
0.012
Concentration
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iii) Applying Beer’s Law to Mixtures
The absorbance at a specific wavelength for a mixture of n
components, Am, is given as:
n
n
i 1
i 1
Am   Ai    i bci
Two component mixture for example:
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(cont’d)
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(cont’d)
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iv) Limitations to Beer’s Law
Linear range
* Beer’s law is valid only at low concentrations.
Generally, < 0.01 M
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(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.
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(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+ + Incolor 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.
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(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.
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2) Absorption Spectra
i) Atomic Absorption (line spectra)
When a atom absorbs specific quantized UV/Vis radiation, it
undergoes a change in its valence electron configuration:
h
e
* Transitions between two different
orbital are termed electronic
transitions.
For example, Na consists of a few,
discrete absorption lines corresponding
to transitions between 3s→3p, 3s→4p
etc.
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ii) Molecular Absorption
(band spectra)
* Molecular absorptions
spectra are generally
broad band (band
spectra) because
vibrational and
rotational levels are
"superimposed" on the
electronic levels.
Vibration
level
{
Electronic
Excited
Vibration
level
{
Electronic
Excited
Vibration
level
{
Electronic
Ground state
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Example of UVVisible absorption
spectra
Gaseous phase
Nonpolar solvent
Analyte:
1,2,4.5-tetrazine
Polar solvent
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iii) Visible Spectrum and Complementary Colors
Wavelength of max (nm)
Color Absorbed
Color Remaining
380-420
420-440
Violet
Violet-blue
Green-yellow
Yellow
440-470
470-500
500-520
520-550
Blue
Blue-green
Green
Yellow-green
Orange
Red
Purple
Violet
550-580
580-620
620-680
Yellow
Orange
Red
Violet-blue
Blue
Blue-green
680-780
Purple
Green
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4. Emission of Electromagnetic Radiation
1) Emission Spectra
Emission
spectrum of a
brine sample
with an
oxyhydrogen
flame
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2) 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.
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3) Molecular Fluorescence
i) Energy Level Diagram
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ii) More Illustration
a) Life time
A*
A
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.
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Continued
d) Vibration Deactivation versus Internal Conversion
(1) Vibrational deactivation (relaxation): A nonradiative
relaxation when a excited molecule nonradiatively loses
vibrational energy 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.
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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.
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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).
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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
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