6. Really Basic Optics -15000 -10000 -5000 0 5000 10000 15000 0 0.2 0.4 0.6 0.8 1 1.2 Tim e (s) Am plitude Sample Sample Prep Instrument Instrument Out put Signal (Data) Select light Sample interaction source select Turn off/diminish intensity detect Polychromatic light Selected light Turn on different wavelength
Transcript
Slide 1
6. Really Basic Optics Sample Prep Instrument Out put Signal
(Data) Select light Sample interaction source select Turn
off/diminish intensity detect Polychromatic lightSelected light
Turn on different wavelength
Slide 2
Really Basic Optics Key definitions Phase angle Atomic lines vs
molecular bands Atomic Line widths (effective; natural) Doppler
broadening Molecular bands Continuum sources Blackbody radiators
Coherent vs incoherent radiation
Slide 3
6. Really Basic Optics y Sin=opp/hyp A 90o phase angle /2
radian phase angle /2 3 /2 22
Slide 4
Emission of Photons Electromagnetic radiation is emitted when
electrons relax from excited states. A photon of the energy
equivalent to the difference in electronic states Is emitted e E hi
E lo Frequency 1/s
Slide 5
Really Basic Optics Key definitions Phase angle Atomic lines vs
molecular bands Atomic Line widths (effective; natural) Doppler
broadening Molecular bands Continuum sources Blackbody radiators
Coherent vs incoherent radiation
Slide 6
Slide 7
Theoretical width of an atomic spectral line
Slide 8
Line broadens due 1.Uncertainty 2.Doppler effect 3.Pressure
4.Electric and magnetic fields Lifetime of an excited state is
typically 1x10 -8 s Natural Line Widths frequency
Slide 9
Example: 253.7 nm Typical natural line widths are 10 -5 nm
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Line broadens due 1.Uncertainty 2.Doppler effect 3.Pressure
4.Electric and magnetic fields
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Line broadens due 1.Uncertainty 2.Doppler effect 3.Pressure
4.Electric and magnetic fields The lifetime of a spectral event is
1x10 -8 s When an excited state atom is hit with another high
energy atom energy is transferred which changes the energy of the
excited state and, hence, the energy of the photon emitted. This
results in linewidth broadening. The broadening is Lorentzian in
shape. FWHM = full width half maximum o is the peak center in
frequency units We use pressure broadening On purpose to get a
large Line width in AA for some Forms of background correction
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Line spectra occur when radiating species are atomic particles
which Experience no near neighbor interactions Overlapping line
spectra lead to band emission Line broadens due 1.Uncertainty
2.Doppler effect 3.Pressure 4.Electric and magnetic fields Line
events Can lie on top Of band events
Slide 13
Continuum emission an extreme example of electric and magnetic
effects on broadening of multiple wavelengths High temperature
solids emit Black Body Radiation many over lapping line and band
emissions influenced by near neighbors
Slide 14
Wiens Law Stefan-Boltzmann Law = Energy density of radiation h=
Plancks constant C= speed of light k= Boltzmann constant
T=Temperature in Kelvin = frequency 1.As (until effect of exp takes
over) 2.As T ,exp, Plancks Blackbody Law
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Really Basic Optics Key definitions Phase angle Atomic lines vs
molecular bands Atomic Line widths (effective; natural) Doppler
broadening Molecular bands Continuum sources Blackbody radiators
Coherent vs incoherent radiation
Slide 16
A B The Multitude of emitters, even if they emit The same
frequency, do not emit at the Same time Incoherent radiation
Frequency, , is the Same but wave from particle B lags behind A by
the Phase angle
Slide 17
Begin Using Constructive and Destructive Interference patterns
based on phase lag By manipulating the path length can cause an
originally coherent beam (all in phase, same frequency) to come out
of phase can accomplish Many of the tasks we need to control light
for our instruments Constructive/Destructive interference 1. Laser
2. FT instrument 3. Can be used to obtain information about
distances 4. Interference filter. 5. Can be used to select
wavelengths END: Key Definitions
Slide 18
More Intense Radiation can be obtained by Coherent Radiation
Lasers Beam exiting the cavity is in phase (Coherent) and therefore
enhanced In amplitude
Slide 19
Argument on the size of signals that follows is from Atkins,
Phys. Chem. p. 459, 6 th Ed Photons can stimulate Emission just as
much As they can stimulate Absorption (idea behind LASERs
Stimulated Emission) * o Stimulated Emission The rate of stimulated
event is described by : Is the energy density of radiation already
present at the frequency of the transition B = empirical constant
known as the Einstein coefficient for stimulated absorption or
emission N* and N o are the populations of upper state and lower
states Where w =rate of stimulated emission or absorption The more
perturbing photons the greater the Stimulated emission Light
Amplification by Stimulated Emission of Radiation
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can be described by the Planck equation for black body
radiation at some T If the populations of * and o are the same the
net absorption is zero as a photon is Absorbed and one is emitted
In order to measure absorption it is required that the Rate of
stimulated absorption is greater than the Rate of stimulated
emission frequency
Slide 21
Need to get a larger population in the excited state Compared
to the ground state (population inversion) Degeneracies of the
different energy levels Special types of materials have larger
excited state degeneracies Which allow for the formation of the
excited state population inversion Serves to trap electrons in the
excited State, which allows for a population inversion pumpE
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Multiple directions, Multiple phase lags Stimulated emission
1.Single phase 2.Along same path =Constructive Interference
Coherent radiation Incoherent radiation Radiation not along the
Path is lost mirror Constructive/Destructive interference 1. Laser
2. FT instrument 3. Can be used to select wavelengths 4.Can be used
to obtain information about distances 5.Holographic Interference
filter.
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FTIR Instrument Constructive/Destructive interference 1. Laser
2. FT instrument 3. Can be used to select wavelengths 4.Can be used
to obtain information about distances 5.Holographic Interference
filter.
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Time Domain: 2 frequencies 1 beat cycle
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Moving mirror IR source Beam splitter Fixed mirror B C A
detector Constructive interference occurs when
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-2 0 +1
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INTERFEROGRAMS Remember that: Frequency of light
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An interferometer detects a periodic wave with a frequency of
1000 Hz when moving at a velocity of 1 mm/s. What is the frequency
of light impinging on the detector?
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No need to SELECT Wavelength by using Mirror, fiber optics,
Gratings, etc.
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FOURIER TRANSFORMS Advantages 1.Jaquinot or through-put little
photon loss; little loss of source intensity 2.Large number of
wavelengths allows for ensemble averaging (waveform averaging) 3.
This leads to Fellget or multiplex advantage multiple spectra in
little time implies?
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DIFFRACTION Huygens principle = individual propagating waves
combine to form a new wave front Can get coherent radiation if the
slit is narrow enough. Coherent = all in one phase
Constructive/Destructive interference 1. Laser 2. FT instrument 3.
Can be used to select wavelengths 4.Can be used to obtain
information about distances 5.Holographic Interference filter.
Slide 32
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June 19, 2008, Iowa Flood Katrina Levee break
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Fraunhaufer diffraction at a single slit
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From which we conclude C B F F D E d W L O
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The complete equation for a slit is Width of the line depends
upon The slit width!! Therefore resolution depends On slit width
Also see This spectra leak of Our hard won intensity B D E d W L
b=W/2
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The base (I =0) occurs whenever sin =0 Which occurs when The
smaller the Slit width the Smaller The line width, Which leads To
greater Spectral Resolution Remember R is Inversely proportional To
the width of The Gaussian base
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SLIT IMAGE 1 2 3 Image 1 2 3 4 5 Position number Slit When edge
AB atDetector Sees Position 10% power Position 250% power Position
3100% power Position 450% power Position 50 % power Detector
output: Triangle results when Effective bandpass = image To resolve
two images that are apart requires Implies want a narrower slit A
B
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Essentially, Narrow slit widths Are generally better
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GRATINGS GratingsGroves/mm UV/Vis300/2000 IR10/20 Points:
1.Master grating formed by diamond tip under ground 1.Or more
recently formed from holographic processes 2.Copy gratings formed
from resins
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q q + -
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EXAMPLE Calculate for a grating which has i=45 2000 groves per
mm 1)Get d 2) Use grating equation to solve for
Slide 44
Multiple wavelengths Are observed At a single angle Of
reflection!! You get light of 674.9 nm ; 1/3; 1/4; 1/5; etc.
Czerny-Turner construction 440.3 220.1 146.8 88 73 All come
through
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Physical Dimensions: 89.1 mm x 63.3 mm x 34.4 mm Weight: 190
grams Detector:Sony ILX511 linear silicon CCD array Detector range:
200-1100 nm Pixels:2048 pixels Pixel size:14 m x 200 m Pixel well
depth:~62,500 electrons Sensitivity: 75 photons/count at 400 nm; 41
photons/count at 600 nm Design: f/4, Symmetrical crossed
Czerny-Turner Focal length: 42 mm input; 68 mm output Entrance
aperture: 5, 10, 25, 50, 100 or 200 m wide slits or fiber (no slit)
Grating options: 14 different gratings, UV through Shortwave NIR
Detector collection lens option: Yes, L2 OFLV filter options:
OFLV-200-850; OFLV-350-1000 Other bench filter options: Longpass
OF-1 filters Collimating and focusing mirrors: Standard or SAG+ UV
enhanced window: Yes, UV2 Fiber optic connector: SMA 905 to 0.22
numerical aperture single-strand optical fiber Spectroscopic
Wavelength range: Grating dependent Optical resolution: ~0.3-10.0
nm FWHM Signal-to-noise ratio: 250:1 (at full signal) A/D
resolution: 12 bit Dark noise: 3.2 RMS counts Dynamic range: 2 x
10^8 (system); 1300:1 for a single acquisition Integration time: 3
ms to 65 seconds Stray light:
PHOTONS AS PARTICLES The photoelectric effect: The experiment:
1. Current, I, flows when E kinetic > E repulsive 2. E repulsive
is proportional to the applied voltage, V 3. Therefore the
photocurrent, I, is proportional to the applied voltage 4. Define V
o as the voltage at which the photocurrent goes to zero = measure
of the maximum kinetic energy of the electrons 5. Vary the
frequency of the photons, measure V o, = E kinetic,max Energy of
Ejected electron Frequency of impinging photon (related to photon
energy) Work function=minimum energy binding an Electron in the
metal
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To convert photons to electrons that we can measure with an
electrical circuit use A metal foil with a low work function
(binding energy of electrons)
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DETECTORS Ideal Properties 1.High sensitivity 2.Large S/N
3.Constant parameters with wavelength Where k is some large
constant k d is the dark current Classes of Detectors Namecomment
Photoemissivesingle photon events Photoconductive (UV, Vis, near
IR) Heataverage photon flux Want low dark current
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1.Capture all simultaneously = multiplex advantage 2. Generally
less sensitive Rock to Get different wavelengths Very sensitive
detector
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Sensitivity of photoemissive Surface is variable Ga/As is a
good one As it is more or less consistent Over the full spectral
range
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Diode array detectors -Great in getting -A spectra all at once!
Background current (Noise) comes from? One major problem -Not very
sensitive -So must be used -With methods in -Which there is a large
-signal
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Photodiodes Photomultiplier tube The AA experiment
Slide 112
Charge-Coupled Device (CCD detectors) 1. Are miniature
therefore do not need to slide the image across a single detector
(can be used in arrays to get a Fellget advantage) 2. Are nearly as
sensitive as a photomultiplier tube +V 3.Apply greater voltage
4.Move charge to gate And Count, 5.move next bin of charge and keep
on counting 6. Difference is charge in One bin 1.Set device to
accumulate charge for some period of time. (increase sensitivity)
2.Charge accumulated near electrode Requires special cooling, Why?
The fluorescence experiment
Slide 113
END 6. Really Basic Optics
Slide 114
Since polarizability of the electrons in the material also
controls the dielectric Constant you can find a form of the C-M
equation with allows you to compute The dielectric constant from
the polarizability of electrons in any atom/bond N = density of
dipoles = polarizability (microscopic (chemical) property) r =
relative dielectric constant Frequency dependent Just as the
refractive index is Typically reported Point of this slide:
polarizability of electrons in a molecule is related to the
Relative dielectric constant
Slide 115
Grating 2 nd order 1 st order Angle of reflection i=45