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C) Echelle Monochromators
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`
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2-D distribution of light and detectionusing an array of transducers (for later)
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MONOCHROMATOR
SLITS
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Slits = hole in the wall Control the entrance of light into and out from the
monochromator. They control quality!
Entranceslits control theintensityof light entering the
monochromator and help control the range of wavelengths of
light that strike the grating
Less important than exit slits
Exit slights help select the range of wavelengthsthat exit the
monochromator and strike the detector
More important than entrance slits
Can be:
Fixed (just a slot)
Adjustable in width(effective bandwidth and intensity)
Adjustable in height(intensity of light)
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Monochromator Slits
Good slits
Two pieces of metal to give sharp edges
Parallel to one another
Spacing can be adjusted in some models
Entrance slit
Serves as a radiation source
Focusing on the slit plane
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Effect of Slit Width on Resolution
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Calculating slit width
Effective bandwidth(Dleff) and D-1
D-1= Dl/Dy When Dy = w = (slit width) D-1= Dleff /w
Example Recpiprocal linear dispersion = 1.2nm/mm Sodium lines at 589.0 nm and 589.6 nm Required slit width?
Dleff = (589.6-589.0) = 0.3 nm W = 0.3 nm/(1.2 nm/mm) = 0.25 mm Practically, narrower than the theoretical values is necessary
to achieve a desired resolution
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FIGURE 7-25The effect of the slit width on spectra. The entrance slit is illuminated with A" A"and A 3 only. Entrance and exit slits are identical. Plots on the right show changes in emitted
power as the setting of monochromator is varied.
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Choice of slit widths
Variable slits for effectivebandwidth
Narrow spectrum
Minimal slit width Bet decrease in the radiant
power
Quantitative analysis
Wider slit width
for moreradiant power
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Effect of bandwidthon spectral
detail for benzene vapor
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SAMPLE HOLDERS
(CELLS)
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Sample Holders (Cells)
Must:
contain the sample without chemical interaction be more-or-lesstransparent to the wavelengths of light in use
be readily cleaned for reuse
be designed for the specific instrument of interest.
Examples
quartz is good from about 190-3000 nm glass is a less expensive alternative from about 300-900 nm
NaCl and KBr are good to much higher wavelengths (IR range)
Cells can be constructed to:
transmit light absorbed at 180 degrees to the incident light
allow emitted light to exit at 90 degrees from the incident light
contain gases (lower concentrations) and have long path lengths (1.0
and 10.0 cm cells are most common)
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Sample Containers
The cells or cuvettes that hold the samples mustbe made of material that is transparent to radiationin the spectral region of interest. Quartz or fusedsilica is required for work in the ultraviolet region
(below 350 nm), both of these substances aretransparent in the visible region. Silicate glassescan be employed in the region between 350 and
2000 nm. Plastic containers can be used in thevisible region. Crystalline NaCl is the mostcommon cell windows in the i.r region.
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Absorbance: usually in a matched pair!
Fluorescence, Phosphorescence, Chemiluminescence
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Different Shapes and Sizes of Cells
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RADIATIONTRANSDUCERS
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RADIATIONTRANSDUCERS
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Radiation Transducers
IntroductionThe detectors for early spectroscopic
instruments were the human eye or a
photographic plate or film. Now a days
more modern detectors are in use that
convert radiant energy into electrical signal.
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properties of the Ideal Transducer
The ideal transducer would have a high sensitivity, ahigh signal-to-noise ratio, and a constant response
over a considerable range of wavelengths. In
addition, it would exhibit a fast response time and a
zero output signal in the absence of illumination,Finally, the electrical signal produced by the ideal
transducer would be directly proportional to the
radiant powerP.
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the relative spectral response of
the various kinds of transducers that are useful for UV,
visible, and IR spectroscopy.
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photon transducers
Several types of photon transducers are available, including
(I)photovoltaic cells, in which the radiant energy generates a current at the
interface of a semiconductor layer and a metal; (2)
phototubes, in which radiation causes emission of electrons from a photosensitive
solid surface; (3)photomultiplier tubes,
which contain a photoemissive surface as well as several additional surfaces thatemit a cascade of electrons when struck by electrons from the photosensitive area;
(4)photoconductivity transducers in which absorption of radiation by a semiconductor
produces electrons and holes, thus leading to enhanced conductivity;
(5) silicon photodiodes. in
which photons cause the formation ofelectron-hole pairs and a current across a
reversebiasedpn junction; and ` (6)
charge-transfer transducers, in which the charges developed in a silicon crystal as a
result of absorption of photons are collected and measured.
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a) Photovolatic cell
Structure
metal-semiconductor-metalsandwiches
produce voltage when irradiated
350-750 nm
550 nm maximum response 10-100 microA
Barrier-layer cell
Low-price
Amplification difficulty
Low sensensitivity for weakradiation
Fatigue effect
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What do we want in a transducer?
High sensitivity
High S/NConstant response over many ls (wide range ofwavelength)
Fast response time S = 0 if no light present
S P (where P = radiant power)
Photon transducers: lightelectrical signal
Thermal transducers: response to heat conduction bands (enhance conductivity)
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c) Photomultiplier Tube (PMT)
Extremely sensitive (use for low light applications).
Light strikes photocathode (photons strike emitselectrons); several electrons per photon.
Bias voltage applied (several hundred volts) electrons
form current. Electrons emitted towards a dynode (90 V more positive
than photocathodeelectrons attracted to it).
Electrons hit dynode each electron causes emission ofseveral electrons.
These electrons are accelerated towards dynode #2 (90 V
more positive than dynode # 1) etc.
d) Ph t lti li t b (f d i d d
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d) Photomultiplier tubes (found in more advanced,
scanning UV-VIS and spectroscopic instruments)
Also function based on the photoelectric effect
Additional signal is gained by multiplying the number of electrons produced bythe initial reaction in the detector.
Each electron produces as series of photo-electrons, multiplying its signal. Thus
the name PMT!
Very sensitive to incoming light.
Most sensitive light detector in the UV-VIS range.
VERY rugged. They last a long time.
Sensitive to excessive stray light (room light + powered PMT = DEAD
PMT)
Always used with a scanning or moveable wavelength selector (grating) in amonochromator
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FIGURE7-31 Photomultiplier tube: (a), photograph of a typical commercial tube; (b), cross:
sectional view; (c), electrical diagram illustrating dynode polanzatlon and photocurrent mea
surement. Radiation striking the photosensitive cathode (b) gives nse to photoelectrons by the
hotoelectric effect. Dynode D1 is held at a positive voltage Withrespect to the photocathode.
~Iectrons emitted by the cathode are attracted to the first dynode and accelerated In the fteld.
Each electron striking dynode D1 thus gives rise to two to four secondary electrons. These
are attracted to dynode D2, which is again positive with respect to dynode D1. The resulting
amplification at the anode can be 106 or greater. The exact amplification factor depends on
the number of dynodes and the voltage difference between each. ThiSautomatic Internal
amplification is one of the major advantages of photomultiplier tubes. With modern Instrumentation,the arrival of individual photocurrent pulses can be detected and counted Instead
of being measured as an average current. This technique, called photon counting, IS
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Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis,Saunders College Publishing, Fort Worth, 1992.
819 dynodes (9-10 is most
common).
Gain (m) is # e-emitted per
incident e-(d) to the power
of the # of dynodes (k).
m = dk
e.g. 5 e-emitted / incident e-10
dynodes.m = dk= 5101 x 107
Typical Gain = 104- 107
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e) Silicon Diodes
Constructed of charge depleted and charge rich
regions of silicon (silicon doped with other ions) Light striking the detector causes charge to be
created between the p and n regions.
The charge collected is then measured as currentand the array is reset for the next collection
Used most frequently these days in instrumentswhere the grating is fixed in one position and lightstrikes an array of silicon diodes (aka the diode array Can have thousands of diodes on an array
Each diode collects light from a specific wavelength range
The resolution is generally poorer than with a PMT
However, you can scan literally thousands of times aminute since there are NO moving parts! Then, the scans
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Photodiodes
Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis,Saunders College Publishing, Fort Worth, 1992.
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the relative spectral response of
the various kinds of transducers that are useful for UV,
visible, and IR spectroscopy.
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Forward biasing Reverse biasing
High resistant
e-
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MULTICHANNEL PHOTON
TRANSDUCERS
M l i h l h
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Multichannel photon
transducersThe first multichannel detector used in spectroscopy was a
photographic plate or a film strip that was placed along the
length of the focal plane of a spectrometer so that all the lines
in a spectrum could be recorded simultaneously. Photographic
detection is relatively sensitive, with some emulsions thatrespond to as few as 10 to 100 photons. The primary limitation
of this type of detector, however, is the time required to
develop the image of the spectrum and convert the blackening
of the emulsion to radiant intensities. Modern multichanneltransducers 24 consist of an array of small photosensitive
elements arranged either linearly or in a two-dimensional
pattern on a single semiconductor chip.
M lti h l Ph t T d
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Multichannel Photon Transducers
Photographic plate or a film strip
Place along the focal plane of a spectrometer
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Photodiode Arrays
Ph di d A
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Photodiode Arrays
In a PDA, the individual photosensitive elements
are small silicon photodiodes, each of which
consists of a reverse-biasedpn junction
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Photodiode Transducer
A silicon photodiode transducer consists of aReversed Biasedpnjunctionformed on a silicon chip
A photon promotes an electron from the valence bond(filled orbitals) to the conduction bond (unfilledorbitals) creating an electron(-) - hole(+) pair
The concentration of these electron-hole pairs isdependent on the amount of light striking thesemiconductor
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Photodiode Array
Semiconductors (Silicon and Germanium) Group IV elements
Formation of holes (via thermal
agitation/excitation) Doping
n-type: Si (or Ge) doped with group V element(As, Sb) to add electrons.
As:[Ar ]4S23d104p3
p-type: Doped with group III element (In, Ga) toadded holes
In: [Kr ]5S24d105p1
Skoog et al, p43
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FIGURE 7-33A reverse-biased linear diode-array
detector: (a)cross section and (b)top view.
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Photodiode Arrays
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Charge-Transfer Device
Charge Transfer Device (CTD)
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Charge-Transfer Device (CTD)
Important for multichannel detection (i.e., spatial
resolution); 2-dimensional arrays. Sensitivity approaches PMT.
An entire spectrum can be recorded as a
snapshotwithout scanning.
Integrate signal as photon strikes element.
Each pixel: two conductive electrodes over aninsulating material (e.g., SiO2).
Insulator separates electrodes from n-doped silicon.
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Semiconductor capacitor: stores charges that are
formed when photons strike the doped silicon.
105 106 charges/pixel can be stored (gain
approaches gain of PMT).
How is amount of charge measured?
Charge-injection device (CID): voltage change
that occurs from charge moving between
electrodes.Charge-coupled device (CCD): charge is moved
to amplifier.
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PHOTO CONDUCTIVITY
TRANSDUCERS
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Photo conductivity Transducers
The most sensitive transducers for monitoring
radiation 10 the near-infrared region (0.75 to 3
/m) are semiconductors whose resistances
decrease when they
absorb radiation within this range.
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Thermal Transducers
Thermal Transducers are used in infrared
spectroscopy. Phototransducers are not
applicable in infrared because photons inthis region lack the energy to cause
photoemission of electrons. Thermal
transducers are Thermocouples,
Bolometer (thermistor).
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Thermocouples
In its simplest form, a thermocouple consists of a pair
of junctions formed when two pieces of a metal such as
copper are fused to each end of a dissimilar metal such
as constantan as shown in Figure 3-13. A voltage develops
between the two junctions that varies with the
difference in their temperatures.
A well-designed thermocouple transducer is capable
of responding to temperature differences of10-6 K. This difference corresponds to a potential difference
of about 6 to 8V/W.
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Thermocouples
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bolometer
A bolometer is a type of resistance thermometer
constructed of strips of metals, such as
platinum or nickel, or of a semiconductor.
Semiconductor bolometers are often calledthermistors .
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Pyroelectric transducers
Pyroelectric transducers are constructed from single
crystalline wafers ofpyroelectric materials, which
are insulators (dielectric materials) with very special
thermal and electrical properties. Triglycine sulfate(NH2CH2COOH)3 H2SO4(usually deuterated or
with a fraction of the glycines replaced with alanine), is
the most important pyroelectric materialused in the
construction of infrared transducers.
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SIGNAL PROCESSORS
ANDREADOUTS
Signal Processors and Readouts
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Signal Processors and Readouts
The signal processor is ordinarily an electronic
device that amplifies the electrical signal from the
transducer. In addition, it may alter the signal from
dc to ac (or the reverse), change the phase of the
signal, and filter it to remove unwanted
components. Furthermore, the signal processor
may be called upon to perform such mathematical
operations on the signal as differentiation,
integration, or conversion to a logarithm.
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PHOTON COUNTING
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Photon counting
The output from a photomultiplier tube consists of
a pulse of electrons for each photon that reaches the detector
surface. This analog signal is often filtered to remove
undesirable fluctuations due to the random appearance
of photons at the photocathode and measured as a de voltage or
eurrent.
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FIBER OPTICS
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Fiber optics
In the late 1960, analytical instruments began to appear
on the market that contained fiber optics for
transmiting radiation and images from one
component of the instrument to another. Fiber optics
have added a new dimension of utility to optical
instrument designs."
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PROPERTIES OFOPTICAL
FIBERS
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Properties of Optical Fibers
Optical fibers are fine strands of glass or plastic that
transmit radiation for distances of several hundred feet
or more. The diameter of optical fibers ranges from
0.05 pm to as large as 0.6 cm. Where images are to be
transmitted, bundles of fibers, fused at the ends, are
used. A major application of these fiber bundles has
been in medical diagnoses, where their flexibility permits
transmission of images of organs through tortuouspathways to the physician. Fiber optics are used
not only for observation but also for illumination of
objects. In such applications, the ability to illuminate
without heating is often very important.
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Optical Fiber
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FIBER-OPTIC SENSORS
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F iber-optic sensors
Fiber-optic sensors, which are sometimes called
optrodes, consist of a reagent phase immobilized on
the end of a fiber optic. Interaction of the analyte with
the reagent creates a change in absorbance,
reflectance, fluorescence, or luminescence, which is
then transmitted to a detector via the optical fiber.
Fiber optic Sensors are generally simple, inexpensive
devices that are easily miniaturized.
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TYPES OF OPTICAL
INSTRUMENT
Types of Optical Instruments
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Types of Optical Instruments
Spectroscope
Optical instrument used for visual identification of atomic emission lines Colorimeter
Human eye acts as detector for absorption measurements
Photometer Contains a filter, no scanning function
Fluorometer A photometer for fluorescence measurement
Spectrograph Record simultaneously the entire spectrum of a dispersed radiation using
plate or film
Spectrometer Provides information about the intensity of radiaition as a function of
wavelength or frequency More(confusing)
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types of Optical instrument
Spectroscope:an optical instrument used for the
visual identification of atomic emission lines.We use the term
colorimeter: to designate an instrument for absorption
measurements in which the human eye serves as the detector
using one or more color-comparison standards.spectro graph:is similar in construction to the two monochromators shown in
Figure 7-18 except that the sht arrangement is replaced with a
large aperture that holds a detector or transducer that is
continuously exposed tn the entire spectrum of dispersedradiation.spectrometer:is an instrument that provides
information
about the intensity of radiation as a function of wavelength or
frequency.
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PRINCIPLES OF FOURIERTRANSFORM OPTICAL
MEASUREMENTS
Fourier Transform (FT)
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( ) The instruments we have been talking about work over the frequency domain (we are
measuring signal vs. frequency or wavelength)
Fourier transform techniques measure signal vs. time and then convert time to
wavelength or frequency FT techniques have much greater resolving power than frequency domain techniques
Fewer mechanical parts
No monochromator
Mathematical deconvolution of the spectrum
FT techniques have higher light throughput because there are fewer opticalcomponents.
Widely used in IR and NMR
Originally developed to separate out weak IR signals from astronomical objects.
An interferometer splits the light beam into two beams and then measures the intensityof recombined beams
The frequency of these beams is related to the frequency of the light that causedthem.
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History
In 1950s, astronomy
Separate weak signals from noise
Late 1960s, FT-NIR & FT-IR
Fourier transform
R l i f FT
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Resolution of FT spectrometer
Two closely spaced lines only separated if one complete"beat" is recorded.
As lines get closer together, dmust increase.
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INHERENT ADVANTAGES OF
FOURIER
TRANSFORM SPECTROMETRY
Ad f FT
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Advantages of FT
Throughput / Jaquinot advantage Few optics and slits
Less dispersion, high intensity
Usually to improve resolution decrease slit width
but less light makes spectrum "noisier" (S/N)
High Resolution Dl/l= 6 ppm
Short time scale Simultaneously measure all spectrum at once saves time
frequency scanning vs. time domain scanning Fellgett or multiplex advantage
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TIME -DOMAIN
SPECTROSCOPY
ti d i t
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time -domain spectroscopy
Conventional spectroscopy can be termed
frequency domain spectroscopy in that radiant
power data are recorded as a function of
frequency or the inversely related wavelength.In contrast, time-domain spectroscopy, which
can be achieved by the Fourier transform, is
concerned with changes in radiant power withtime.
Ti d i
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Time domain spectroscopy
Unfortunately, no detector can respond on 10-14s time scale
Use Michelson interferometerto measure signal
proportional to time varying signal
F d i / i d i
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Freq-domain / time-domain
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ACQUIRING TIME-DOMAIN SPECTRA
WITH A
MICHELSONINTERFEROMETER
d l ti
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modulation
Velocity of movingmirror(MM)
Time to move l/2 cm
Bolometer, pyroelectric,photoconducting IRdetectors can
"seechanges on 10-4s
time scale!
Thi ti d i t i d f
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This time domain spectrum is made of
different wavelengths of light arriving at the
detector at different times.
Mi h l i t f t
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Michelson interferometer
Anal sis of interferogram
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Analysis of interferogram
Computer needed to turncomplex interferogram intospectrum
Figure 7-43
(b) resolved lines (c) unresolved lines
FT
Time -> Frequency
inverse FT Frequency -> Time
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Four ier Transformation of
Interferograms
Interferogram
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Interferogram
retardation d Difference in pathlength
interferogram
Plot signal vs. d cosine wave with frequency proportional to light
frequency but signal varies at much lower frequency
One full cycle when mirror moves distance l/2(round-trip = l)
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resolution
resolution
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resolution
The resolution of a Fourier transformspectrometer can be described in terms of the
difference in wavenumber between two lines
that can be just separated by the instrument.That is,
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Semiconductor Diodes
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Semiconductor Diodes Diode: is a nonlinear device that has greater
conductance in one direction than in another Adjacent n-type and p-type regions
pn junction: the interface between the two regions
This process continues for 9 dynodes
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Result: for each photon that strikes photocathode
~106
107
electrons collected at anode. Is there a drawback? Sensitivity usually limited by
dark current.
Dark current = current generated by thermalemission of electrons in the absence of light.
Thermal emissionreduce by cooling.
Under optimal conditions, PMTs can detect singlephotons.
Only used for low-light applications; it is possibleto fry the photocathode.
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