optical methods typical components - 연세대학교...
TRANSCRIPT
7.1
by Prof. Myeong Hee Moon
Ch. 7 Components of Optical Instruments
• Optical Methods
AbsorptionFluorescencePhosphorescenceScatteringEmissionChemiluminescence
• Typical Components
1. Stable Source of Radiation2. Sample Cell (transparent)3. Detector (convert to signal)4. Signal Processor & Read out
7.2
by Prof. Myeong Hee Moon
Components of Optical Spectroscopy
source lampof heated solid
sample holder
wavelengthselector
photoelectrictransducer
signal processorand readout
h h h I
sample holder
wavelengthselector
photoelectrictransducer
signal processorand readout
h h I
source lampof heated solid
h
Absorption
fluorescencephosphorescence
source lampof heated solid
wavelengthselector
photoelectrictransducer
signal processorand readout
h h I
emission & chemiluminescence
7.3
by Prof. Myeong Hee Moon
7B. Sources of Radiation
Continuum
Line
7.4
by Prof. Myeong Hee Moon
7B-1. Continuum Sources
For absorption & fluorescence Spectroscopy
UV:
VIS:
IR:
7B-2. Line Sources
For atomic absorption spec. : atomic & molecular fluorescenceRaman spectroscopy
D2, Ar, Xe, Hg filled arc lamps
w filament
inert solids
hollow cathode lamp
7.5
by Prof. Myeong Hee Moon
7B-3. Laser Sources
LASER: 1960s
Extremely high intensity narrow band width (<0.01nm)Become very important tool in UV, VIS, IR region
Typical source: Rubby (GaAs): organic dye: Ar or Kr
Light Amplification by Stimulated Emission of Radiation
7.6
by Prof. Myeong Hee Moon
a. pumping :
10-13~10-15 s life time
laser excitation by electrical discharge, passage of electric currents to an intense radiant source
1. Mechanism of Laser actionPumping – spontaneous – stimulated – absorption
emission emission
7B-3. Laser Sources
7.7
by Prof. Myeong Hee Moon
b. spontaneous emission : random process, incoherent radiation but monochromatic
7B-3. Laser Sources
7.8
by Prof. Myeong Hee Moon
c. stimulated emission :
d. absorption :
same direction with phase – coherent emission
7B-3. Laser Sources
7.9
by Prof. Myeong Hee Moon
2.Light amplification
>number of photons
Lost by absorption
number of photons by stimulated
emission
Population inversion
Population inversionfavored in 4-level
system
3. three- and four-level laser systems
7B-3. Laser Sources
7.10
by Prof. Myeong Hee Moon
4. Useful Lasers
• solid state lasers : 1st type & wide usage, 3-level device- ruby (Al2O3 + 0.05% Cr(III) doped in Al(III) lattice)
Cr(III) : active lasing material, 694 nm
- Nd : YAG, most common in solid statesNeodymium in yttrium aluminum garnet, 4-level
give very high power at 1064nmpossible to double at 532nm
used for pumping tunable dye lasers
7B-3. Laser Sources
7.11
by Prof. Myeong Hee Moon
• gas lasers :
-He/Ne (632.8nm): most common, reliablelow power consumption
-Ar ion (514nm-green, 488.0nm-blue) – 4-levelAr+ : by e or RF discharge
4P 4S : fluorescence & Raman spec.-N2 (337.1nm) : pulsed mode by spark
used for exciting fluorescencepumping dye lasers (CO2– 10.6m)
-Excimer lasers (XeF: 351nm, KrF: 248nm, ArF: 193nm)
gaseous mixtures of He, F2, and one of (Ar, Kr, Xe) to form (ArF*, KrF*, XeF* --- excimers) by I (current)
Excimers: stable only at excited states.
7B-3. Laser Sources
7.12
by Prof. Myeong Hee Moon
• Dye lasers :
With organic compound fluorescing UV, VIS, IR4-level systemTunable 20~50 nmBand width : ~ 1/100 nm
Tuning : replace non-transmitting mirrorwith monochromator with reflection grating
Wavelength selection --- rotate grating
7B-3. Laser Sources
7.13
by Prof. Myeong Hee Moon
• Semiconductor diode lasers :
hEgh
Egfor semiconductor
band gap
new type, nearly monochromatic
7B-3. Laser Sources
For conductors: band-gap energy is so small that electrons easilyPromoted to conduction band
7.14
by Prof. Myeong Hee Moon
- Types of Semiconductor diode lasers
LED :
DBR
: GaAs pn-junction diode975 nm (IR)band width 10-5nm withgrating integrated in resonant cavity
: Light sources for CD player, bar coded scannerbut only around red (IR region)can be overcome by frequency doubling
GaAlAs(900nm), GaP(550nm), GaN(465nm)too low energy for spectroscopy
(distributed-Bragg-reflector) laser diode
7B-3. Laser Sources
7.15
by Prof. Myeong Hee Moon
7B-3. Laser Sources
A frequency-doubled system for converting 975-nm laserOutput to 490 nm.
7.16
by Prof. Myeong Hee Moon
7C. Wavelength Selectors
For spectroscopic analysis, narrow band width needed
7C-1. Filters
interference filters, absorption filters (VIS only)
7.17
by Prof. Myeong Hee Moon
7C-2. Monochromators
a) Czerney-Turner gratingb) Bunsen prism type
prism
Dispersion by prism. a) quartz Cornu types b) Littrow type
7.18
by Prof. Myeong Hee Moon
• Grating UV, VIS, IR
UV, VIS : 300~2000 grooves/nm(1200~1400)
IR : 10~ 200 (100 common) grooves/nm
From a master – casting to make replica gratingSurface coated with Al, Au, or Pt
7C-2. Monochromators
7.19
by Prof. Myeong Hee Moon
Echellette-type
(CB + BD) = nCB = d Sin iBD = d Sin r
n = d (Sin i + Sin r)
n=1,2,3…
Concave grating - focusing function added
7C-2. Monochromators
7.20
by Prof. Myeong Hee Moon
7.21
by Prof. Myeong Hee Moon
• Resolving power of monochromator
nNR
103 ~ 104 for typical UV-VIS monochromator
n: diffraction orderN: number of
grating blazes
7C-2. Monochromators
7.22
by Prof. Myeong Hee Moon
• Effect of bandwidth 0.5nm bandwidth 1.0nm bandwidth
2.0nm bandwidth
7C-2. Monochromators
7.23
by Prof. Myeong Hee Moon
7D. Sample Containers
Cells, cuvettes
Fused silica : UV below 350 nmSilicate glass : 350~2000 nmPlastic : VISNaCl : IR
7.24
by Prof. Myeong Hee Moon
7E. Random Transducers
7E-1. Introductionphotographic film transducers
Convert radiant E to V or I• Ideal transducers- high sensitivity- high S/N- constant response vs. wide range l- fast response time- zero output signal at no illumination
(dark current) S = k P + kdP: radiant powerS: electrical responsekd: dark current
• types of transducers
Photon – photoelectric UV, VIS, near-IR : poor cons. responseHeat – IR (lower sensitivity) vs. wide
7.25
by Prof. Myeong Hee Moon
7E-2. Photon Transducers
• Photovoltaic or Barrier-Layer cells
E I (10~100A) at the interface of semiconductor & metal
Max sensitivity at 550nm10% of max at 350, 750nm
VIS region
Photon hits semiconductor -- covalent bond broken--- e-holes--- e moves toward metallic film
current generated
7.26
by Prof. Myeong Hee Moon
• Vacuum phototubes
Photon
Electrons from cathode(photoemissive)
photocurrent
Photoemissive surface: Na, K, Cs, SbIndividual or multi-alloy
7E-2. Photon Transducers
7.27
by Prof. Myeong Hee Moon
• Photomultiplier tubes (PMT)
106~107 electrons/photondynode
Features1. Sensitive to UV, VIS2. Fast response3. Dark current reduced
by cooling (-30oC)4. Care for exposure to
daylight
7E-2. Photon Transducers
7.28
by Prof. Myeong Hee Moon
7E-3. Multi-channel photon transducers
# of transducer elements64~4096, 1024 (common)Storage cap: 10 pF for each diode
~10 to 100 photonsBased in silicon diode
7.29
by Prof. Myeong Hee Moon
7E-4 Photoconductivity transducers
Crystalline semiconductors(sulfides or selenides of : Pb, Cd, Ga, In)ie. PbS - 0.8~3m
Near-IR (0.75~3m)Change in electronic conductivity- Resistance decrease at absorption
7E-5. Thermal transducers
: Small blackbody :
: problem – thermal noiseto reduce, vacuum & beam chopping
Minute temp rise upon small radiant power(~1/1000 K) (10-7 ~ 10-9)
7.30
by Prof. Myeong Hee Moon
• thermocouples
Copper fused to constantan (alloy)
Potential between two junctions
T: 10-6K ~ 6~8 V/W
7E-5. Thermal transducers
7.31
by Prof. Myeong Hee Moon
• Bolometers
Resistance thermometer (strips of metal Pt, Ni)Semiconductor – thermistorsLarge change in resistance, mid-IR
• Pyroelectric transducers
: triglycerin sulfate: (NH2CH2COOH)3H2SO4: fast response time --- most FT-IR
When electric field applied across dielectric material
Temperature dependent polarizer
Single crystalline wafers of pyroelectric materials- insullators (dielectric material) with
special thermal & electrical properties.
electric polarization induceddepending on temperature
7E-5. Thermal transducers
7.32
by Prof. Myeong Hee Moon
7F. Signal processors & readouts
Signal processor:
Readout: d’Arsonval meterDigital meterPotentiometerRecordersCathode-ray tubes
• photoncounting
Output from PMT -- a pulse of electrons for each photonRadiant power is read by the number of pulses / unit time
(rather than average current or potential : analog type)
amplifier of electrical signalperform math. operations - integration, differentiation
7.33
by Prof. Myeong Hee Moon
7G. Fiber Optics
Late 1960s
• properties of optical fibers
Glass or plastic fibers (0.05 m id ~ 0.6cm): transmitting radiation (UV, VIS, IR)
Fiber, coating, medium
7.34
by Prof. Myeong Hee Moon
7I. Principles of FT-Optical measurements
FT-spectroscopy (1950s)
7I-1. Advantage of Fourier Transform
-Throughput adv. (Jaquinot) high S/Nlarge power to detector (than in dispersive instr.)
-High resolving power (/)thus, wavelength reproducibility
-Fast analysis, all signals reach detector at oncei.e. scan IR region 500~5000cm-1
if = 3cm-1 m=15000.5 s 750s or 12.5m
for each meas.
Decrease in width induce decrease in S/N(weaker source signal)
But detector noise does not increase
7.35
by Prof. Myeong Hee Moon
NN
S
N
S
x
xAverage signal
Average noise
Increase S/N to 2, require N=4 4 spectra4x750s=50m
In FT method: measure all at oncedecrease multiple measurement time
7I. Principles of FT-Optical measurements
7.36
by Prof. Myeong Hee Moon
7I-2. Time-Domain Spectroscopy
Conv.method ---- frequency domain spectroscopy f()f(t) change in radiant power with time
)2cos()2cos()( 21 tktktP
7.37
by Prof. Myeong Hee Moon
7I-3. Michelson Interferometer
Signal modulation : split beam into two beams & recombine
: measure intensity variationas a function of lengthof the path of two beams
difference in the path lengths
retardation: 2(M-F) =
7.38
by Prof. Myeong Hee Moon
path “a”
pat
h “
b”
ab
moving mirror
fixed mirror
detector
50% beamsplitter
x
sourcesample
FTIR apparatus
j- stop
7I-3. Michelson Interferometer
7.39
by Prof. Myeong Hee Moon
Mirror travel
Frequency, (cm-1)
x = 0
x = 0
4004000
Interferogram:
Single beamspectrum of air:
FT
100%
H2O H2OCO2
7I-3. Michelson Interferometer
7.40
by Prof. Myeong Hee Moon
Interferrogram: a plot of output power vs. ; time for mirror to move /2 cm
2
M M: moving velocity of mirror
10M
M
MM
102
2
2
/2
1
cf
f
: frequency of radiationc=3x1010cm/s
ftPP 2cos)(2
1)(
amplitude of theinterferrometer signal
radiant powerof beam
f: frequency of signalat detector
if v =1.5cm/s
7I. Principles of FT-Optical measurements
7.41
by Prof. Myeong Hee Moon
t2M
2211 2cos)(2cos)()( BBP
dpB
dBP
2cos)()(
2cos)()(
For continuum source
7I. Principles of FT-Optical measurements
ftBP 2cos)()(
2cos)()( BP
tBP M2cos)()(
7.42
by Prof. Myeong Hee Moon
He-Ne lasertop of
beamsplitter
Perkin Elmer Galaxy 2000 FTIR