Monochromators[1].pptx

8/10/2019 Monochromators[1].pptx

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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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