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Lecture 3 OPTICAL SPECTROSCOPY AND INSTRUMENTATION

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GENERAL DESIGN OF OPTICAL INSTRUMENTS

PHENOMENA USED FOR OPTICAL INSTRUMENTS

Absorption

Flourescence

Phosphorescence

Scaterring

Emission

Chemiluminescence

In all cases, RESPONSE IS PROPORTIONAL TO CONCENTRATION OF ANALYTE.


Many OPTICAL INSTRUMENTS share similar design:

‐ stable radiation source

‐ transparent sample holder

‐ wavelength selector

‐ radiation detector

‐ signal processor and read out


RADIATION SOURCES

An ideal radiation source for spectroscopy should have the following characteristics:

The source must emit radiation over the entire wavelength range to be studied.

The intensity of radiation over the entire wavelength range must be high enough so that extensive amplification of the signal from the detector can be avoided.

The intensity of the source should not vary significantly at different wavelengths.

The intensity of the source should not fluctuate over long time intervals.

The intensity of the source should not fluctuate over short time intervals. Short time fluctuation in source intensity is called “flicker”.

RADIATION SOURCES

RADIATION SOURCES


An ideal radiation source for spectroscopy should have the following characteristics: must emit radiation over the entire wavelength range to be studied.

the intensity of radiation over the entire wavelength range must be high enough so that extensive amplification of the signal from the detector can be avoided.

the intensity of the source should not vary significantly at different wavelengths.

the intensity of the source should not fluctuate over long time intervals.

the intensity of the source should not fluctuate over short time intervals. Short time fluctuation in source intensity is called “flicker”.

Continuum sources produce broad range of 's (often blackbody)

Heated solid (Globar, nichrome wire) (1‐40 m)

Tungsten lamp (300‐3000 nm)

Quartz Tungsten Halogen (QTH) lamp (200‐3000 nm)

high temperature (3500 K)

Evaporation W(s) W(g)

W(g) + I2 (g) WI2 (g)

Redeposition WI2 (g)+ W(s) W(s) + I2 (g)


D2 lamp or Hg/Xe arc‐lamp ‐ (160‐400 nm)

electronic excitation

D2 + Eelectrical D2* D(KE1 ) + D(KE2 ) + h KE1 + KE2 + h = Eelectrical ‐ BDE • bond dissociation energy

LINE SOURCES

Line Sources produce few discrete wavelengths

D2 lamp or Hg/Xe arc‐lamp (>400 nm)

Atomic emission hollow cathode lamp

(a) electron bombardment of cathode

(b) sputtering of cathode atoms

(c) emission from electronically excited cathode atoms



Electrodeless Discharge Lamps (EDL)

Ar ions created by RF energy

ions collide with gaseous metal atoms which then

emit excite

LASERS

LASERS

Light Amplification by Stimulated Emission of Radiation

(a) pumping of excited state (b) stimulated emission to produce emission (Fig. 7‐5)




POPULATION INVERSION and AMPLIFICATION


• Need population inversion for lasing

• Can not produce population inversion in 2‐level system (stimulated emission becomes increasingly dominant). Need 3‐ or 4‐ levels


• Lasing medium can be solid (Nd:YAG, semiconductor diode laser AlGaAs), gas (noble gas Ar+, He/Ne, CO2, N2) or liquid (dye)


Advantages of LASERS

Intense

Monochromatic (line sources)

Pulsed (10‐15 to 10‐6 s) or continous wave (cw)

Coherent

Small beam divergence

SOME EXAMPLES: RUBY Lasers: primarily Al2O3 with roughly 0.05% Cr

+3 distributed among the Al+3 lattice site. Cr+3 is the active lasing material.

Nd : YAG lasers

Neodymium in a host crystal of yttrium aluminum garnet

HeNe Lasers

ARGON LASERS

DYE LASERS

Tunable over a range of 20 to 50 nm.

Lasing materials are solutions of organic materials.

SEMICONDUCTOR LASERS

WAVELENGTH SELECTORS


Wavelength selectors are important instrumental components that are used to obtain a certain wavelength or a narrow band of wavelengths.

Three types of wavelength selectors can be described:

FILTERS

PRISMS

GRATINGS

FILTERS

Filters are wavelength selectors that usually allow the passage of a band of wavelengths and can be divided into three main categories:

Absorption Filters

This type of filters absorbs most incident wavelengths and transmitting a band of wavelengths. Sometimes, they are called transmission filters.

Absorption filters are cheap and can be as simple as colored glasses or plastics.

They transmit a band of wavelengths with an effective bandwidth (the effective band width is the width of the band at half height) in the range from 30‐250 nm.

Their transmittance is usually low where only about 10‐20% of incident beam is transmitted.

IDEAL OUTPUT

Absorption filter ‐ colored glass or dye between two glass plates


TWO FILTERS CAN PRODUCE A NARROWER BAND


FILTERS

CUT‐OFF FILTERS.In this type of filters, transmittance of about 100% is observed for a portion of the visible spectrum which rapidly decreases to zero over the remainder of the spectrum (the red curve).

INTERFERENCE FILTERS

• These filters are sometimes called Fabry‐Perot filters and are dependent on the concept of light interference.

• An interference filter is composed of a transparent dielectric, like calcium fluoride, sandwiched between two semitransparent metallic films.

• The array is further sandwiched between two glass plates to protect the filter.

• The thickness of the dielectric is carefully controlled as it is this factor which defines the resulting wavelength.

• Incident polychromatic radiation hits the filter at right angles and the transmitted beam will have a very narrow bandwidth.

INTERFERENCE FILTER

To understand how an interference filter can transmit a narrow band of wavelengths, let us assume that the incident beam of radiation hits the surface of

the filter at some angle, less than 90o. Look at the following schematic:

The incident beam (green arrows, x and y) hits surface a of the metallic film where ax and y are partially transmitted and reach surface b of the metallic film. The two beams are partially reflected (red arrows) and hit surface a where they are also partially reflected to surface b. If the distance traveled by the two beams inside the dielectric is the same and correspond to some wavelengths, these wavelengths are transmitted when the two beams and reflected beams hit the b surface. This is attributed to constructive interference between the specified wavelengths of the two beams. The same process occurs for the other beams hitting surfaces a and b.

Now, for constructive interference to take place for beams hitting the surface at right angles the distance traveled by the beam should be twice the thickness of the dielectric (t).

Therefore, we can write:

nl' = 2t cosq

Where, l' is the wavelength in the dielectric and n is an integer called the order. At right angles, q approaches zero and cosq approaches unity. Hence:

nl' = 2t

The wavelength in air l = l'h where h is the refractive index of the dielectric. The final relation can be written as:

l= 2th/n

the wavelength which can be obtained from an interference filter is a function of the thickness of the dielectric and in fact can be predetermined by this factor. The effective band width of an interference filter is much better (narrower) than that of an absorption filter.

INTERFERENCE WEDGES

a wedge machined dielectric can be used. The dielectric in this case has different thicknesses and thus can transmit a wide range of wavelengths accordingly. The figure below is a schematic of a wedge interference filter:

INTERFERENCE WEDGES

Usually, the wedge size ranges from 5 – 20 cm. If a fixed slit is used and a stepping motor or manual adjustment of the wedge along a linear path (wide double headed arrow) was done, different wavelengths can be selected. Interference wedges with wavelength outputs in the entire range from 400‐700 nm are commercially available.

II. PRISMS

A prism is a wavelengths selector that depends on the dispersion ability of the incident radiation by the prism material. Dispersion, as discussed earlier, is the variation of refractive index with wavelength, or frequency. Since a beam of a polychromatic light is composed of several wavelengths, the dispersion of these wavelengths will be different when they are transmitted through the prism. One can see the following dispersion pattern for white light:

PRISMS

Two common types of prisms: Cornu Prism: It is a 60o prism which is made either from glass or quartz. When quartz is used, two 30o prisms (one should be left handed and the other is right handed) are cemented together in order to get the 60o prism. This is necessary since natural quartz is optically active and will rotate light either to right or left hand. Cementing the left and right handed prisms will correct for light rotation and will transmit the beam in a straight direction. Littrow Prism: A littrow prism is a 30o prism which uses the same face for input and dispersed radiation. The beam is reflected at the face perpendicular to base, due to presence of a fixed mirror. A littrow prism would be used when a few optical components are required.

III. GRATINGS

A grating is an optically flat polished surface that has dense parallel grooves.

Two types of gratings are usually encountered, transmission and reflection (diffraction) gratings.

Transmission gratings are seldom used in spectroscopic instruments and almost all gratings which are used in conventional spectroscopic instruments are of the reflection type.

The groove density can be as low as 80 to several thousand (6000) lines/mm. Two common types of reflection gratings can be identified: Echellette Gratings: Typical echellette gratings contain from 300 to 2000 lines/mm but an average line density of about 1200 to 1400 lines/mm is most common. The echellette grating uses the long face for dispersion of radiation. It is the grating of choice for molecular spectroscopic instruments. As will be shortly explained, and in contrast to prisms, gratings usually have linear dispersion of radiation. Echelle Gratings: These have relatively coarse grooves (~80‐300 lines/mm). They use the short face for dispersion of radiation and are characterized by very high dispersion ability.

Gratings can be made concave which have the advantage of excluding collimation mirrors, resulting in simpler instrumental designs.

An important category of gratings is the one manufactured by laser technology.

The thus made gratings are called holographic gratings and are characterized by a high degree of perfection and very high line density (up to 6000 lines/mm), in addition to larger size.

They have become a low cost product as well.

Gratings, other than holographic, are made by mechanical ruling of hard surfaces using a mechanical device like a diamond tool.

The first grating to be made takes too much time and effort and is called a master grating.

Replica gratings are made from the master by a liquid resin casting process.

The liquid resin, upon solidification, takes the exact shape of the grooves and is then coated with a reflective metal like aluminum, gold or platinum.

Replica gratings are easy to make and thus after the master is made, the process of production is extremely simple and fast.

DISPERSIONS BY GRATING

We can visualize what is going on when radiation hits the surface of a grating. Our discussion will be focused on echellete gratings but conclusions are fully applicable to all reflection gratings as well.

AC and AD are normals to beams 1’ and 2’. When beam 1 hits the surface of the grating at point A, beam 2 still needs a distance CB to hit the grating surface at B. For reflected beams 1’ and 2’ to show constructive interference, the distance CB+BD should be integer number of wavelengths. Therefore, we can write: nl = CB + BD (1) where n is an integer called order of radiation. Angle 1AX + XAC = 900 Angle BAC + XAC = 90o Therefore, 1AX = BAC = i (incidence angle) Therefore, CB = AB sin i (2) In the same manner: 1'AX + XAD = 90o BAD + XAD = 90o Therefore, 1'AX = BAD = r BD = AB sin r (3) AB is the spacing between two consecutive blazes = d, combination between (2) and (3) and substitution in (1) gives: nl = d sin i + d sin r nl = d(sin i +sin r)

MONOCHROMATORS

A monochromator is the part of instrument responsible for producing monochromatic radiation. It is an essential component of any spectroscopic instrument and is composed of a prism or grating, as the l selector, in addition to focusing elements; like mirrors or lenses. All these components are contained in a box that has an entrance and an exit slit. Two common types of monochromators can be described:

Czerney-Turner Grating Monochromator

Bunsen Prism Monochromators. This type of monochromators uses a prism as the dispersion element in addition to two focusing lenses and two slits.

BANDWIDTH AND EFFECTIVE BANDWITH

Assume that just three monochromatic lines of equal intensity enter the entrance slit in a monochromator with entrance and exit slits of the same size.

The grating setting will allow one of these to be focused on the exit slit and its image will fill the exit slit.

If the distance, on the focal plane, between them is equal: l2 – l1 = l3 – l2, the change of the monochromator setting from l1 to l2 or l3 will result in the image of l1 being totally removed from the exit slit and substituted by the image of one of the other wavelengths.

The bandwidth is defined as the span of wavelengths of the monochromator that is needed to move the image of the entrance slit (the three wavelengths) across the exit slit. For our case, this span is equal to l3 – l1. The effective bandwidth is defined as the span (range) of wavelengths that fills the exit slit at a given wavelength setting. The effective bandwidth is thus related to the exit slit width and both are related to reciprocal linear dispersion by the relation: D‐1 = Dl/Dy where Dl is the range of wavelength exiting the monochromator and Dy is the distance on the focal plane. When Dy approximates the slit width (w), Dl becomes the effective bandwidth (Dleff). Dleff = w D‐1

• Interference (Fabry‐Perot) filter ‐ two thin sheets of metal sandwiched between glass plates, separated by transparent material


• Interference for transmitted wave through 1st layer and reflected from 2nd layer


Constructive interference when


MONOCHROMATORS

Entrance slit

Collimating Lens

Dispersion Element

Focusing Length or Mirror

Exit Slit

Short wavelengths refracted more!

PRISMS: (many older instruments)

TYPICAL PRISM MONOCHROMATOR:


• Extra pathlength traveled by wave 2 must be n for constructive interference

DIFFRACTION GRATING (most modern instruments)

EXAMPLE:


GRATING MONOCHROMATOR

(1) Spectral purity scattered or stray light in exit beam

Use entrance and exit windows, dust and light‐tight housing, coat interior with light absorbing paint

(2) Dispersion ability to separate small wavelength differences Linear dispersion or reciprocal linear dispersion ‐ variation in l across the focal plane

QUALITY OF MONOCHROMATORS

(3) Light Gathering ‐ light collection efficiency

QUALITY OF MONOCHROMATORS

(4) Spectral bandwidth ‐ range of wavelengths exiting the monochromator

Related to dispersion and entrance/exit slit widths


• Effect of slit width



Complete resolution of two features only possible when slit is adjusted to produce effective bandwidth half (or less) of difference between 's

SAMPLE CELL

Sample Containers and Optics:

• Cuvettes

• Lenses

• Prisms, gratings, filters

Made of suitable material (see table 7‐2):

Glass 400‐3000 nm (vis‐near IR)

Silica/quartz 200‐3000 nm (UV‐near IR)

NaCl 200‐15,000 nm (UV‐far IR)


TRANSDUCERS

PROPERTIES OF AN IDEAL TRANSDUCER

High sensitivity: The transducer should be capable of detecting very small signals Signal to noise ratio (S/N): A high signal to noise ratio is an important characteristic of a good transducer Constant response: When radiation of different wavelengths but of the same intensity are measured, the transducer should give a constant response Fast response: A short response time is essential especially for scanning instruments. Zero dark current: In absence of illumination, the detector output should read zero Zero drift: If radiation of constant intensity hits the transducer, signal should be constant with time Signal (S) should be proportional to intensity of incident radiation

PROPERTIES OF IDEAL TRANSDUCER

Signal (S) should be proportional to intensity of incident radiation (I) S = kI However, in practice, a fixed value (called dark current, Kd) is usually added to signal S = kI + kd We will concentrate our discussion to transducers in the UV‐Vis range which are referred to as photon transducers.

PHOTON TRANSDUCERS

Photovoltaic or Barrier Cells These are simple transducers that operate in the visible region (350-750 nm) with maximum sensitivity at about 550 nm. The cell is composed of a copper or iron base on which a selenium semiconducting layer is deposited. The surface of semiconductor is coated with a thin semitransparent film of a metal like silver or gold. The whole array is covered with a glass plate to protect the array. The copper base and silver thin film are the two electrodes of the cell.

PHOTON TRANSDUCERS

Electrons, from selenium, are released due to breakdown of covalent bonds as a result of incident radiation and thus an equivalent number of holes is formed. The electrons migrate towards the metallic film while holes move towards the copper base. Electrons move through the external circuit towards the base and thus a current can be measured, which is dependent on the intensity of incident radiation.

Barrier cells are simple, rugged, and cheap. They have the extraordinarily important advantage that they do not require an external power supply, which make them the transducers of choice for portable instruments and remote applications. However, they have some important drawbacks including low sensitivity except for intense radiation, they suffer from fatigue (signal decreases with time although the intensity is constant). They have low resistance which makes amplification of the signal difficult to achieve.

VACUUM PHOTOTUBES

A photo tube transducer is one of the most common and wide spreading transducers that are formed from an evacuated glass or quartz envelope that houses a semicylindrical cathode and a wire anode assembly. The cathode surface is coated with a layer of a photoemissive materials like Na/K/Cs/Sb but other formulations exist which have various sensitivities and wider wavelength ranges.

VACUUM PHOTOTUBES

The voltage difference between the cathode and the anode is usually maintained at about 90 V. The incident beam hitting the cathode surface is proportional to radiation intensity.

This detector has better sensitivities than the barrier cell and does not show fatigue.

The detector is good for the general detection of radiation intensity n the UV‐Vis region and is used in most low cost instruments.

The transducer is also rugged and reliable.

However, a small dark current is always available.

PHOTOMULTIPLIER TUBES

A photomultiplier tube (PMT) is one of the most sensitive transducers which can measure radiant powers of very low intensities. The operational mechanism of the PMT is similar to the vacuum phototube described above but with extra electrodes (dynodes: same surface composition as cathode) for signal amplification.

PHOTOMULTIPLIER TUBES

When a photon hits the photoemissive cathode surface, electrons are released and are accelerated to the first dynode at a positive potential to cathode (about 90 V). Extra electrons are generated since accelerated electrons from cathode strongly hit the more positive dynode surface. Electrons are further released from this first dynode to the more positive second dynode (90 V more positive than the first dynode) resulting in release of more electrons. This process continues as electrons are accelerated to other more positive dynodes and thus huge amplification of signal results (~106 electrons for each photon).

PHOTOMULTIPLIER TUBES Photomultiplier tubes are limited to measurement of low radiant power radiation since high radiant powers would damage the photoemissive surfaces, due to very high amplification. It is the very high amplification which imposes a relatively important high dark current value of the PMT. Dark current may arise due to electronic components or an increase in the temperature. A release of a single electron from the cathode surface will generate a cascade of electrons from consecutive dynodes. Cooling of the PMT is suggested to increase sensitivity where cooling to ‐30 oC Can practically eliminate dark current. PMTs have excellent sensitivities, fast response time and operational capabilities in both UV and visible regions of the electromagnetic spectrum.

SILICON DIODE TRANSDUCERS A semiconductor material like silicon can be doped by an element of group V (like arsenic and antimony) would have more electrons as a group V atom (5e) replaces a silicon atom (4e). The thus doped semiconductor is called an n-type semiconductor. In contrast, when a group III element (3e) is doped in a silicon matrix, replacement of a silicon atom (4e) with a group III atom (like indium or gallium, 3e) results in the formation of a less electrons semiconductor or a p-type.

SILICON DIODE TRANSDUCERS

A diode is a device that has a greater conductance in one direction than the other. A diode is manufactured by forming adjacent n-type and p-type regions within the same silicon or germanium single crystal. The term pn junction refers to the interface between these two regions.


A diode can be connected to a power supply (a battery) in one of two modes: a. Reverse bias mode: the p-type region is connected to the negative terminal of the battery while the n-type region is connected to the positive terminal. In this case, a depletion layer is formed due to migration of positive holes toward the cathode and migration of electrons in the n-type region to the positive anode. Reverse biased mode is the useful mode which will be used in transducer technology.


b. Forward bias mode: the p-type region is connected to the positive terminal of the battery while the n-type terminal is connected to the negative terminal of the battery. No depletion layer is observed.


A silicon diode transducer consists of a reverse-biased pn junction formed on a silicon crystal. The application of a reverse bias creates a depletion layer that will ultimately result in zero current. When a beam of radiation hits silicon diode, holes and electrons will be formed in the depletion layer thus producing a current proportional to the intensity of incident radiation. Silicon diodes are more sensitive than phototubes but far less sensitive than photomultiplier tubes. They can be used in both UV and visible regions.

MULTICHANNEL PHOTON TRANSDUCERS

The simplest multichannel transducer ever made is the photographic film where the full image can be captured in one shot. However, the time required for handling and developing the film makes it difficult to practically use it in conventional instruments, although it is still in use in some techniques like x‐ray diffraction spectroscopy. There are three major classes of multichannel photon transducers which find important applications and use in spectroscopic instruments.


Photodiode arrays (PDA): these are simply linear arrays of silicon diodes described above. The number of linear diodes used in each photodiode array usually 64 to 4096 with 1024 silicone diodes as the most common. One can imagine the complexity of the electronic circuitry used in such an array as well as the data handling and manipulation requirements. The entrance slit is usually fixed at a size enough to fill the surface area of one silicone diode. The entire spectrum can thus be instantaneously recorded. The arrays are also called diode array detectors (DAD).


Charge Transfer and Charge Coupled Transducers

The photosensitive elements are, in contrary to PDAs, arranged in two dimensions in both charge injection devices (CID) and charge coupled devices (CCD).

Therefore, these are very similar to photographic films. For example, a commercially available transducer is formed from 244 rows with each row containing 388 detector elements.

This will add up to a two dimensional array holding 16672 detector elements (pixels) on a silicon chip that is 6.5 mm by 8.7 mm.


The full description of the system and its mechanism will not be covered here as this is but know that these important transducers function by first collecting the photogenerated behind the scope of this course.

However, we should qualitatively charges in different pixels and then measuring the quantity of the charge accumulated in a brief period.

Measurement is accomplished by transferring the charge from a collection area to a detection area.

general design of optical instruments · general design of optical instruments ... therefore, we...

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