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INSTRUMENTS FOR OPTICAL SPECTROSCOPY

Spectroscopic methods are based upon the phenomena of emission, absorption, fluores-cence, or ~cattering. While the instruments for each dill'er somewhat in configuration, their basic components are remarkably simi-lar. Furthermore, the general properties of their components are the same regardless of

whether they are applied to the ultraviolet, visible, or infrared portion of the Spectrum.l

Spectroscopic instruments contain five components, including: (1) a stable source of radiant energy; (2) a device that permits em-ployment of a restricted wavelength region;

(3) a transparent container for holding the sample; (4) a radiation detector or transducer that cOnverts radiant energy to a usable signal (usually electrical); and (5) a signal processor and readout. Figure 5-1 shows the arrangement of these components for the four common types of spectroscopic measure-ments :mentioned earlier. As can be seen in the figure, the configuration of components (4) and (5) is the same for each type of instruinent.

Emission spectroscopy ditTers from the other three types in the respect that no exter-nal radiation source is required; the sample itself ~ the emitter. Here, the sample con-tainer; is an arc, a spark, or a flame, which both holds the sample and causes it to emit characteristic radiation.

Absorption as well as fluorescence and scattering spectroscopy require an external source of radiant energy. In the former. the beam from the source passes through the sample after leaving the wavelength selector.

In the latter two, the source iDduces the sample, held in a container, to emit char-acteristic fluorescent or scattered radiation, which is measured at an angle (usually 90 deg) with respect to the source.

Figure 5-2 summarizes the characteristics of the first four components shown in Figure 5-1. It is clear that instrument components

IFor ~ more complcae discuuion of the components of optical instruments, see: R. P. Bauman, Absorption Spec.

lrOICOpy. New York: Wiley. 1962, Chapten 2 and 3; E. J.

Meehan, inTfflItiM 011 AlI4lytlcGlClwmlltry, eds. I. M.

Koltholl' and P. J. Elvin •. New York: Intcncicncc, 1964, Part I, yo\. 5, Chapter 55; and Pltyslc4l Mtfltob ofCIwm-IItry. eds. A. Wciubcracr and B. W. Rouiter. New York:

Wiley·lntcnciencc, 1912, Part IIIB, Yo\. I, Chapters 1-5.

COMPONENTS OF INSTRUMENTS FOR OPTICAL SPECTROSCOPY

Ie)

FI~URE 6-1 Components for various types of instruments for

~Ptlcal spectroscopy: (a) emission spectroscopy, (b) absorption pectroscopy, and (c) fluorescence and scattering Spectroscopy.

differ in detail, depending upon the wave-length region within which they are to be used. Their des~gn also depends on the

pri-~ use of the Instrument; that is, whether il

~ to be e~ployed for qualitative or quantita-tive ~nalyslS and whether it is to be applied to atolDJC or molecular spectroscopy. Never-thele;u, the general function and performance r~~lrements of each type of component are slml~ar, .regardless of wavelength region and apphcatlon.

~n order to be suitable for spectroscopic stud-I~, a s.ource must generate a beam of radia-tion With sufficient power for ready detecradia-tion and measurement. In addition, its output should be stabl~. Typically, the radiant power o~ a source vanes exponentially with the elec-tncal power supplied to it. Thus, a regulated power supply. is often needed to provide the required stability. Alternatively, in some

in-struments. the output of the source is split into. a reference beam and a sample beam.

The first passes directly to a transduc:er while the second interacts with the sample and is then focused on a matched transducer. (Some instruments employ a single transducer which is irradiated alternately by the sample and reference beams.) The ratio of the outputs of the two transducers serves as the analYtical parameter. The effect of fluctuations in the source output is largely canceled by. this means.

Both continuous and line sources are used in optical spectroscopy. The former ;finds wide application in molecular absorption methods. The latter is employed in ftqores-cence and atomic absorption spectroscopy.

Figure 5-la lists the most widely used spectroscopic sources.

-

^{.nm VACUV UV ,IVISIBLE NEAR'IR IR • , FARIR}

Spectral region

(.1 Sources

~ I

limp Xenon limp

'I H20rD2 1

Tungsten lamp lamp

Continuous _r

Nern.t glower IZrD2 +Y20,)

Nichrome wire (Ni +Cr) ,

GlobIr (SiC!

Di1continuou. {

Hollow cathode

, lamps

lb) W_ength

:~

'-.,

^.

selectors prism Fused .11..,.orqUlrtz prosm

GllSlprism

.

Continuous

I I i~~

^{KBr pritm}

3000 Iineslmm Grltings with various number of lineslmm 50lineslmm

I Interference wedges Interforence filters

OilContinuous { GllSlabsorption

filters

(e) M.terill. for LiF

cell •• windows, Fused silica orqUIrtz

&lenses

Corexglass!

, Silicate gtlSl

NaCI K8r

TIBr-TII

ldl TWItducers

{

Photornu tiplier

Photon Phototube

detectors

, 7:otocefl Silicon diode

Semiconductor

Thermocouple (volts) or Bolometer (ohms'

{

^{I IGol.y pneumltic cell I}

Heat

I Pyrcetectric cell (capocitonce) detectors

Continuous Sources of Ultraviohtt.

Visible, and Near-Infrared Radiat)on

Continuous sources provide radiation whose power does not change sharply among adja-cent wavelengths.

Hydrogen or Deuterium Lamps. As noted in Chapter 4 (p. 110~ a continuous spectrum in .the ultraviolet region is conveniently produced by the electrical excitation of hydrogen or deuterium at low pressure.

Two types of hydrogen lamps are en-countered. The high-voltage variety employs potentials of 2000 to 6000 V to cause a discharge between aluminum electrodes;

water cooling of the lamp is required if-high radiation intensities are to be produee<i. In low-voltage lamps. an arc is formed between a heated, oxide-coated filament and a rhetal electrode. The heated filament provides 'elec-trons to maintain a dc current when a voltage of about 40 V is applied; a regulated power supply (p. 53) is required for constant intensities.

FIGURE 5-2 Components and materials for spectroscopic instruments ..(Adapt~ ~rom a figure by Professor A. R. Armstrong, College of William and Mary. With permission.)

An important feature of hydrogen discharge lamps is the shape of the aperture between the two electrodes, which constricts the dis-charge to a narrow path. As a consequence.

an intense ball of radiation about 1 to 1.5 mm in diameter is produced. Replacement of hydrogen by deuterium results in a somewhat larger light ball.

Both high- and low-voltage lamps produce a continuous spectrum in the region of 160 to 375 nm. Quartz windows must be employed in the tubes, since glass absorbs strongly in this wavelength region.

Tungsten Filament Lamps. The most common source of visible and near-infrared radiation is the tungsten filament lamp. The energy distribution of this source approxi-mates that of a black body and is thus temperature dependent. Figure 4-13 illus-trates the behavior of the tungsten filament lamp at 3OOOoK.In most absorption instru-ments, the operating filament temperature is about 2870oK; the bulk of the energy is thus .emitted in the infrared region. A tungsten filament lamp is useful for the wavelength region between 320 and 2500 nm.

In the visible region. the energy output of a tungsten lamp varies approximately as the fourth power of the operating voltage. As a consequence. close voltage control is required for a stable radiation source. Constant volt-age transformers or electronic voltvolt-age regu-lators are often employed for this purpose. As an alternative, the lamp can be operated from a 6-V storage battery, which provides a remarkably stable voltage source if it is main-tained in good condition.

Xenon Arc Lamps. This lamp produces intense radiation by the passage of current through an atmosphere of xenon. The spec-trum is continuous over the range between about 25;0 and 600 nm, with the peak inten-sity occurring at about 500 nm (see Figure 4-13, p. 110). In some instruments, the lamp is operated intermittently by regular discharges from a capacitor; high intensities are obtained.

Continuous Sources of Infr ••.••• Rlldlation

The common infrared source is an inert solid heated electrically to temperatures between lSOO and 2OQOoK. Continuous radiation approximating that of a black body results (see Figure 4-13). The maximum radiant intensity at these temperatures occurs be-tween 1.7 and 2#Ll11 (6000 to SOOOem -I). At longer wavelengths, the intensity falls off co~-tinuously until it is about I

%

of the maxI-mum at 151lm (667 em-I). On the short wavelength side, the decrease is much more rapid and a similar reduction in intensity is_{, I} observed at about 111m (10,000 em- ).

TIle Nerast Glower. The Nemst glower is composed of rare earth oxides formed into a cylinder having a diameter of I to 2 mm and a length of perhaps 20 Mm. Platinum leads are sealed to the ends of the cylinder to permit passage of current. This device has a large negative temperature coefficient of elec-trical resistance, and it must be heated ex-ternally to a dull red heat before

a

sufficient current passes to maintain the desired temperature. Because the resistance decreases with increasing temperature, the source cir-cuit must be designed to limit the current;

othe(Wise the glower rapidly becomes so hot that it is destroyed.

The Globar Source. A Globar is a silicon carbide rod, usually about 50 mm in length and 5 mm in diameter. It also is electrically heated and has the advantage of a positive coefficient of resistance. On the other hand, water cooling of the electrical contacts is required to prevent arcing. Spectral energies of the Globar and the Nernst glower are com-parable except in the region below 5#Ll11

(2000 em -I), where the Globar provides a significantly greater output.

Inl:andeseeat Wire Source. A source of somewhat lower intensity but longer life than the Globar or Nernst glower is a tightly wound spiral of nichrome wire heated by

pass-age of current. A rhodium wire heater sealed in a ceramic cylinder has similar properties as a source.

~ Nonparallel radiation •

..•.._ .•..,._A'

'..>.-;""'"

Mirror

t----i:

Active I~ing mediu'!'

'X-i- ~

JZ:" ^{t -.} =s --.

Radiation --- ~ . ". transmitting mirror

Pumping

g£~

SOurce ''.i'~lv . .

Line Sources

Sources that emit a few discrete lines find use in atomic absorption spectroscopy, Raman spectroscopy, refractometry, and polarimetry.

Metal Vapor Lamps. Two of the most common line sources are the familiar mercury and sodium vapor lamps. A vapor lamp con-sists of a transparent envelope containing a gaseous element at low pressure. Excitation of the characteristic line spectrum of the element occurs when a potential is applied across a pair of electrodes fixed in the envelope. Con-duction occurs as a consequence of electrons and ions formed by ionization of the metaL Ordinarily, initial heating is required to pro-duce sufficient metal vapor; once started, however, the current is self-sustaining.

The mercury lamp produces a series of lines ranging in wavelength from 254 to 734 nm. With the sodium lamp. the pair of lines at 589.0 and 589.6 nm predominate.

Hollow CathcMle Lamps. Hollow cathode lamps provide line spectra for a large number of elements. Their use has been confined to atomic absorption and atomic fluorescence spectroscopy. Discussion of this type of source is deferred to Chapter II.

FIGURE 5-3 Schematic representation of a typical laser source.

processes with lifetimes in the range of 10-9

to 10-12 s, the detection and determination of extremely small concentrations of species in the atmosphere, and the induction of is0-topically selective reactions.3 In addition, laser sources have become important in several routine analytical methods, including Raman spectroscopy (Chapter 91 emission spectros-copy (Chapter 12), and Fourier transform infrared spectroscopy (Chapter 8).

The term laser is an acronym for light amplification by stimulated emission of radia-tion. As a consequence of their light-amplifying properties, lasers produce spatially narrow, extremely intense beams of radiation.

The process of stimulated emission produces a beam of highly monochromatic (band-widths of 0.01 nm or less) and remarkably coherent (p. 99) radiation. Because of these unique properties, lasers have become impor-tant sources for use in the ultraviolet, visible, and infrared regions of the spectrum. A limi-tation of early lasers was that the radiation from a given source was restricted to a rela-tively few discrete wavelengths or lines. Re-cently, however, dye lasers have become available; tuning of these sources provides a Lase,.

The first laser was constructed in 1960.2Since that time, chemists have found numerous useful applications for these sources in high-resolution spectroscopy, kinetic studies of

1For a more complete discussion or lasers. see: B. A.

Lengye~lAsers. New York: Wiley, 1971; S. R. Leone and C. B. Moore,Chemical allll Biological A.pplications of lAsers, eel. C. E. Moore. New York: Academic Press, 1974; and A.nalytical lAser Spectroscopy. eel. N.

Omen-etlo. New York: Wiley, 1979. • For a review or some or Ihese applicalions. see: S. R.

Leone,J. Chem. Educ.,53, 13 (1976~

narrow band of radiation at any chosen wavelength within the range of the source.

Figure 5-3 is a schematic representation showing the components of a typical laser source. The heart of the device is a lasing medium. It may be a solid crystal, such as ruby, a semiconductor such as gallium arse-nide. a solution of an organic dye, or a gas.

The lasing material can be activated or pumped by radiation from an external source so that a few photons of proper energy will trigger the formation of a cascade of photons of the same energy. Pumping can also be carried out by an electrical current or by an electrical discharge. Thus, gas lasers usually do not have the external radiation source shown in Figure 5-3; instead, the power supply is connected to a pair of electrodes contained in a cell filled with the gas.

A laser normally functions as an oscillator, in the sense that the radiation produced from the laser mechanism is caused to pass back and forth through the medium numerous times by means of a pair of mirrors as shown in Figure 5-3. Additional photons are gen-erated with each passage, thus leading to enormous amplification. The repeated pass-age also produces a beam that is highly par-allel because nonparpar-allel radiation escapes from the sides of the medium after being reflected a few times (see Figure 5-3~

In order to obtain a usable laser beam, one of the mirrors is coated with a sufficiently

thin layer of reflecting material so that a frac-tion of the beam is trilnsmitted rather than reflected (see Figure 5-3).

. Laser action can be understood by con-sidering the four processes depicted in Figure 5-4, namely, (a) pumping, (b) spontaneous emission (fluorescence1 (c) stimulated emis-sion, and (d) absorption. For purposes of illustration, we will focus on just .two of several electronic energy levels that wil( exist in the atoms, ions, or molecules making up the laser material; as shown in the figure, the two electronic levels have energies E, and Ex.

Note that the higher electronic state is shown as having several slightly different vibrational energy levels, E" E~, E;, and so forth. We have not shown additional levels for the lower electronic state, although such often exist.

Pumping. Pumping, which is necessary for laser action, is a process by which the active species of a laser is excited by means of an electrical discharge, passage of an electrical current, exposure to an intense radiant source, or interaction with a chemical species.

During pumping, several of the higher elec-tronic and vibrational energy levels of the active species will be populated. In diagram a-I of Figure 5-4, one atom or molecule is shown as being promoted to an energy state E;; the second is excited to the slightly higher vibrational level

E;.

The lifetime of excited vibrational states is brief, however; after 10-13 to 10-14 S, relaxation to the lowest excited vibrational level (E, in diagram a-3) occurs with the production of an undetectable quantity of heat. Some excited electronic states of laser materials have lifetimes con-siderably longer (often I ms or more) than their excited vibrational counterparts; long-lived states are sometimes termed metastable as a consequence.

Spontaneous Emission. As was pointed QUt in the discussion of fluorescence, a species in an excited electronic state may lose all or part of its excess energy by spontaneous emission of radiation. This process is depicted in the

diagrams shown in Figure 5-4b. Note that the wavelength of the fluorescent radiation is directly related to the energy difference be-tween the two electronic states, E, - Ex. It is also important to note that the instant at which the photon is emitted, and its direction, will vary from excited electron to excited elec-tron. That is, spontaneous emission is a random process; thus, as shown in Figure 5-4, the fluorescent radiation produced by one of the particles in diagram b-I differs in direc-tion and phase from that produced by the second particle (diagram b-2). Spon.taneous emission, therefore, yields incoherent radiation.

Stimulated Emission. Stimulated emission, which is the basis of laser behavior, is depicted in Figure 5-4c. Here, the' excited laser particles are struck by externally produced photons having precisely the same energies (E,- Ex) as the photons produced by'spontaneous emission. Collisions; of this type cause the excited species to r~lax im-mediately to the lower energy statei and to emit simultaneously a photon of exactly the same energy as the photon that stimulated the process. More important, and more remarkable, the emitted photon is precisely in phase with the photon that triggered the event.

That is, the stimulated emission is totally coherent with the incoming radiation.

. Absorption. The absorption process, which competes with stimulated emission. is depicted in Figure 5-4d. Here, two photons with energies exactly equal to E, - Ex are ab-sorbed to produce the metastable excited state shown in diagram d-3; note that the state shown in diagram d-3 is identical to that attained by pumping diagram a-3.

Population Inversion

a'"

Light Amplifica-tion. In order to have light amplification in a laser, it is necessary that the number of~hotons produced by stimulated emission exceed the number lost by absorption. This condition will prevail only when the number of particles in the higher energy state exceeds the number in the lower; that is, a population inversion from the normal distribution of energy states must exist.

(1)

-_ EM

=+=~.

.,- -r·

E•

Pumping I I

energy I I

-+- +E.

:±: ~

^Heat

---

----~~ ---

__=cvu-FIGURE 5-4 Four processes important in laser action: (a) pumping (excitation by electrical. radiant. or chemical energy). (b) spontaneous emission. (c) stimulated emission. and (d) absorption.

Population inversions are brought about by pumping. Figure 5-5 contrasts the effect of incoming radiation on a noninverted popula-tion with an inverted one.

11Iree- •••• Four-Level LIser Systems. Fig-ure 5-6 shows simplified energy diagrams for the two common types of laser systems. In the three-level system, the transition responsible for laser radiation is between an excited state E, and the ground state Eo; in a four-level system, on the other hand, radiation is gen-erated by a transition from E, to a state Ex that has a greater energy than the ground state. Furthermore, it is necessary that transi-tions between Ex and the ground state be rapid.

The advantage of the four-level system is that the population inversions necessary for laser action are more readily achieved. To understand this, note that at room tempera-ture a large majority of the laser particles will be in the ground-state energy level Eo'in both systems. Sufficient energy must thus be provided to convert more than 50% of the lasing species to the E, level of a three-level

AbIofption "',

rv\ri

_I

V\r

Stimulated / emission

system. In contrast, it is only necessary to pump sufficiently to make the number of par-ticles in the E, energy level exceed the number in Ex of a four-level system. The lifetime of a particle in the Ex state is brief, however, be-cause the transition to Eo is fast; thus, the number in theEx state will generally be negli-gible with respect to Eo and also (with a modest input of pumping energy) with respect to E,. That is, the four-level laser usually achieves a population inversion with a smaller expenditure of pumping energy.

Some Examples of Useful Lasers. The first successful laser, and one that still finds wide-spread use, was a three-level device in which a ruby crystal was the active medium. The ruby

Some Examples of Useful Lasers. The first successful laser, and one that still finds wide-spread use, was a three-level device in which a ruby crystal was the active medium. The ruby

In document COMPLUTENSE DE MADRID UNIVERSIDAD (página 56-60)