7.1
Ch. 7 Components of Optical Instruments
• Optical Methods
Absorption
Fluorescence
Phosphorescence
Scattering
Emission
Chemiluminescence
• Typical Components
1. Stable Source of Radiation
2. Sample Cell (transparent)
3. Detector (convert to signal)
4. Signal Processor & Read out
by Prof. Myeong Hee Moon
7.2
Components of Optical Spectroscopy
Absorption h
source lamp
of heated solid
h
sample
holder
h
wavelength
selector
h
sample
holder
I
photoelectric
transducer
h
wavelength
selector
I
photoelectric
transducer
h
h
wavelength
selector
h
I
photoelectric
transducer
emission & chemiluminescence
by Prof. Myeong Hee Moon
signal processor
and readout
fluorescence
phosphorescence
source lamp
of heated solid
source lamp
of heated solid
signal processor
and readout
signal processor
and readout
7.3
7B. Sources of Radiation
Continuum
Line
by Prof. Myeong Hee Moon
7.4
7B-1. Continuum Sources
For absorption & fluorescence Spectroscopy
UV: D2, Ar, Xe, Hg filled arc lamps
VIS:
w filament
IR:
inert solids
7B-2. Line Sources
hollow cathode lamp
For atomic absorption spec. :
atomic & molecular fluorescence
Raman spectroscopy
by Prof. Myeong Hee Moon
7.5
7B-3. Laser Sources
LASER: 1960s
Light Amplification by Stimulated Emission of Radiation
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
by Prof. Myeong Hee Moon
7.6
7B-3. Laser Sources
1. Mechanism of Laser action
Pumping – spontaneous – stimulated – absorption
emission emission
a. pumping :
laser excitation by electrical discharge, passage of electric
currents to an intense radiant source
10-13~10-15 s life time
by Prof. Myeong Hee Moon
7.7
7B-3. Laser Sources
b. spontaneous emission :
random process, incoherent radiation but monochromatic
by Prof. Myeong Hee Moon
7.8
7B-3. Laser Sources
c. stimulated emission :
same direction with phase – coherent emission
d. absorption :
by Prof. Myeong Hee Moon
7.9
7B-3. Laser Sources
2.Light amplification
number of photons
by stimulated
emission
>
number of photons
Lost by
absorption
Population inversion
3. three- and four-level laser systems
Population inversion
favored in 4-level
system
by Prof. Myeong Hee Moon
7.10
7B-3. Laser Sources
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 states
Neodymium in yttrium aluminum garnet, 4-level
give very high power at 1064nm
possible to double at
532nm
used for pumping tunable dye lasers
by Prof. Myeong Hee Moon
7.11
7B-3. Laser Sources
• gas lasers :
-He/Ne (632.8nm): most common, reliable
low power consumption
-Ar ion (514nm-green, 488.0nm-blue) – 4-level
Ar+ : by e or RF discharge
4P 4S : fluorescence & Raman spec.
-N2 (337.1nm) : pulsed mode by spark
used for exciting fluorescence
pumping dye lasers (CO2– 10.6m)
-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.
by Prof. Myeong Hee Moon
7.12
7B-3. Laser Sources
• Dye lasers :
With organic compound fluorescing UV, VIS, IR
4-level system
Tunable 20~50 nm
Band width : ~ 1/100 nm
Tuning : replace non-transmitting mirror
with monochromator with reflection grating
Wavelength selection --- rotate grating
by Prof. Myeong Hee Moon
7.13
7B-3. Laser Sources
• Semiconductor diode lasers : new type, nearly monochromatic
For conductors: band-gap energy is so small that electrons easily
Promoted to conduction band
band gap
E g  h
for semiconductor

Eg
h
by Prof. Myeong Hee Moon
7.14
7B-3. Laser Sources
- Types of Semiconductor diode lasers
GaAlAs(900nm), GaP(550nm), GaN(465nm)
too low energy for spectroscopy
LED :
DBR
(distributed-Bragg-reflector) laser diode
: GaAs pn-junction diode
975 nm (IR)
band width 10-5nm with
grating integrated in
resonant cavity
: Light sources for CD player, bar coded scanner
but only around red (IR region)
can be overcome by frequency doubling
by Prof. Myeong Hee Moon
7.15
7B-3. Laser Sources
A frequency-doubled system for converting 975-nm laser
Output to 490 nm.
by Prof. Myeong Hee Moon
7.16
7C. Wavelength Selectors
For spectroscopic analysis,
narrow band width needed
7C-1. Filters
interference filters, absorption filters (VIS only)
by Prof. Myeong Hee Moon
7.17
7C-2. Monochromators
a) Czerney-Turner grating
b) Bunsen prism type
prism
Dispersion by prism.
a) quartz Cornu types
b) Littrow type
by Prof. Myeong Hee Moon
7.18
7C-2. Monochromators
• 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 grating
Surface coated with Al, Au, or Pt
by Prof. Myeong Hee Moon
7.19
7C-2. Monochromators
Echellette-type
(CB + BD) = n
CB = d Sin i
BD = d Sin r
n = d (Sin i + Sin r)
n=1,2,3…
Concave grating - focusing function added
by Prof. Myeong Hee Moon
7.20
by Prof. Myeong Hee Moon
7.21
7C-2. Monochromators
• Resolving power of monochromator
R

 nN

n: diffraction order
N: number of
grating blazes
103 ~ 104 for typical UV-VIS monochromator
by Prof. Myeong Hee Moon
7.22
7C-2. Monochromators
• Effect of bandwidth
0.5nm bandwidth
2.0nm bandwidth
by Prof. Myeong Hee Moon
1.0nm bandwidth
7.23
7D. Sample Containers
Cells, cuvettes
Fused silica : UV below 350 nm
Silicate glass : 350~2000 nm
Plastic : VIS
NaCl : IR
by Prof. Myeong Hee Moon
7.24
7E. Random Transducers
7E-1. Introduction
photographic 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)
• types of transducers
S = k P + kd
P: radiant power
S: electrical response
kd: dark current
Photon – photoelectric UV, VIS, near-IR : poor cons. response
Heat – IR (lower sensitivity)
vs. wide 
by Prof. Myeong Hee Moon
7.25
7E-2. Photon Transducers
• Photovoltaic or Barrier-Layer cells
E
I (10~100A) at the interface of semiconductor & metal
VIS region
Max sensitivity at 550nm
10% of max at 350, 750nm
Photon hits semiconductor -- covalent bond broken
--- e-holes
--- e moves toward metallic film
current generated
by Prof. Myeong Hee Moon
7.26
7E-2. Photon Transducers
• Vacuum phototubes
Photon
Electrons from cathode
(photoemissive)
photocurrent
Photoemissive surface
: Na, K, Cs, Sb
Individual or multi-alloy
by Prof. Myeong Hee Moon
7.27
7E-2. Photon Transducers
dynode
• Photomultiplier tubes (PMT)
106~107 electrons/photon
Features
1. Sensitive to UV, VIS
2. Fast response
3. Dark current reduced
by cooling (-30oC)
4. Care for exposure to
daylight
by Prof. Myeong Hee Moon
7.28
7E-3. Multi-channel photon transducers
~10 to 100 photons
Based in silicon diode
by Prof. Myeong Hee Moon
# of transducer elements
64~4096, 1024 (common)
Storage cap: 10 pF for each diode
7.29
7E-4 Photoconductivity transducers
Crystalline semiconductors
(sulfides or selenides of : Pb, Cd, Ga, In)
ie. PbS - 0.8~3m
Near-IR (0.75~3m)
Change in electronic conductivity
- Resistance decrease at absorption
7E-5. Thermal transducers
: Small blackbody
: Minute temp rise upon small radiant power
(~1/1000 K)
(10-7 ~ 10-9)
: problem – thermal noise
to reduce, vacuum & beam chopping
by Prof. Myeong Hee Moon
7.30
7E-5. Thermal transducers
• thermocouples
Copper fused to constantan (alloy)
Potential between
two junctions
T: 10-6K ~ 6~8 V/W
by Prof. Myeong Hee Moon
7.31
7E-5. Thermal transducers
• Bolometers
Resistance thermometer (strips of metal Pt, Ni)
Semiconductor – thermistors
Large change in resistance, mid-IR
• Pyroelectric transducers
Single crystalline wafers of pyroelectric materials
- insullators (dielectric material) with
special thermal & electrical properties.
: triglycerin sulfate: (NH2CH2COOH)3H2SO4
: fast response time --- most FT-IR
When electric field applied across dielectric material
electric polarization induced
depending on temperature
Temperature dependent polarizer
by Prof. Myeong Hee Moon
7.32
7F. Signal processors & readouts
Signal processor:
Readout:
amplifier of electrical signal
perform math. operations
- integration, differentiation
d’Arsonval meter
Digital meter
Potentiometer
Recorders
Cathode-ray tubes
• photoncounting
Output from PMT -- a pulse of electrons for each photon
Radiant power is read by the number of pulses / unit time
(rather than average current or potential : analog type)
by Prof. Myeong Hee Moon
7.33
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
by Prof. Myeong Hee Moon
7.34
7I. Principles of FT-Optical measurements
FT-spectroscopy (1950s)
7I-1. Advantage of Fourier Transform
-Throughput adv. (Jaquinot)
high S/N
large power to detector (than in dispersive instr.)
-High resolving power (/)
thus, wavelength reproducibility
-Fast analysis, all signals reach detector at once
i.e. scan IR region 500~5000cm-1
if  = 3cm-1
m=1500
0.5 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
by Prof. Myeong Hee Moon
7.35
7I. Principles of FT-Optical measurements
S Sx

N Nx
Average signal
N
Average noise
Increase S/N to 2, require N=4 4 spectra
4x750s=50m
In FT method: measure all at once
decrease multiple measurement time
by Prof. Myeong Hee Moon
7.36
7I-2. Time-Domain Spectroscopy
Conv.method ---- frequency domain spectroscopy
f(t) change in radiant power with time
P (t )  k cos(21t )  k cos(2 2t )
by Prof. Myeong Hee Moon
f()
7.37
7I-3. Michelson Interferometer
Signal modulation : split beam into two beams & recombine
: measure intensity variation
as a function of length
of the path of two beams
difference in the path lengths
: 2(M-F) = 
retardation
by Prof. Myeong Hee Moon
7.38
7I-3. Michelson Interferometer
moving mirror
fixed mirror
path “b”
50% beamsplitter
FTIR apparatus
path “a”
a
b
x
j- stop
detector
sample
by Prof. Myeong Hee Moon
source
7.39
7I-3. Michelson Interferometer
Interferogram:
x=0
x=0
FT
Mirror travel
Single beam
spectrum of air:
100%
H2O
CO2
H2O
400
4000
Frequency, (cm-1)
by Prof. Myeong Hee Moon
7.40
7I. Principles of FT-Optical measurements
Interferrogram: a plot of output power vs. 
; time for mirror to move /2 cm
 M


2
M: moving velocity of mirror
 M 2 M

 /2

2
f  2 M  M   10 10
f 
1

c
: frequency of radiation
c=3x1010cm/s
if v =1.5cm/s
P ( ) 
1
P(  ) cos 2ft
2
amplitude of the
interferrometer signal
by Prof. Myeong Hee Moon
f: frequency of signal
at detector
radiant power
of beam
7.41
7I. Principles of FT-Optical measurements
P ()  B(  ) cos 2ft
P ()  B(  ) cos 2 M t
M 

2t
P ()  B (  ) cos 2

P ()  B1 (  ) cos 21  B2 (  ) cos 22
For continuum source

P ( )   B( ) cos 2 d


B ( )   p ( ) cos 2 d

by Prof. Myeong Hee Moon
7.42
Perkin Elmer Galaxy 2000 FTIR
top of
beamsplitter
by Prof. Myeong Hee Moon
He-Ne
laser