The Basic Physics of Spectroscopy

8/4/2019 The Basic Physics of Spectroscopy

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The Basic Physics of

Infrared Emission Spectroscopy

William Maddams

University of Southampton

Until recently Chief Vibrational Spectroscopist, BP Research, Sunbury on

Thames, UK.

The analytical potential of infrared emission spectroscopy as a characterisationaltechnique rests upon the fundamental fact that if a body emits radiation of one or morewavelengths under given conditions it will also absorb those wavelengths under thesame conditions. This tenet requires justification; fortunately, the required reasoning

is quite simple.

Consider a body in equilibrium with its surroundings in a uniform temperatureenclosure, exchanging energy only by radiation. Under these conditions it must absorbexactly the same energy, in any time interval, as it emits, otherwise its temperaturewill rise or fall. Furthermore, it must absorb exactly the same wavelength as it emits,otherwise the character of the radiation within the enclosure would alter with time,and would depend on the contents of the enclosure and not simply on its temperature.More generally, it may be shown that in non-equilibrium conditions, which oftenoccur in practice, the absorptions and emission processes, at one or more wavelengths

will still occur, but not necessarily at the same temperature.

Bunsen and Kirchhoff, some one hundred and fifty years ago, devised an elegant butsimple experiment that demonstrates the correctness of what was deduced above bysimple reasoning. Their equipment is shown schematically in Figure 1.


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F is a sodium flame, such as is obtained byfeeding sodium chloride into a Bunsen burner.When this is viewed through the prism

spectroscopes A and B the well known brightyellow doublet, with lines and 5890 and5896angstroms, is seen. When the strongwhite light source [WLS], shining through theflame and in line with A, is switched on, acontinuous spectrum with two dark lines at5890 and 5896A is observed through A.However, spectrometer B, at right angles tothe line of WLS/F/A, still shows that yellowsodium doublet.

The explanation is that the flame has absorbed the wavelengths 5890 and 5896Afrom the white light source and re-radiated it in all directions, and the amount fallingon the entrance slit of spectrometer A is so small that two dark lines appear against theotherwise continuous background. This, incidentally is the cause of the variousFraunhofer lines, characteristic for a number of elements, that appear in the solarspectrum. When the flame, with the white light illumination passing through it in thedirection WLS/F/A, is viewed at right angles via spectrometer B., the two sodiumyellow lines are still present at unchanged intensity.

One clear consequence of the relation between emission and absorption is that awholly transparent medium cannot emit. This may appear contrary to everydayexperience, but it has been demonstrated by Professor R.W. Wood, who devised manybeautiful and clever experiments in physical optics. He used a rod of very pure fusedquartz, free from small particles of foreign matter. When this was heated in a Bunsenflame there was only a very small amount of light emission. However, when it washeated to a higher temperature in an oxy-coal gas flame it became vividlyincandescent. The bluish luminosity faded when the rod was removed from the flame

but, on cooling, it did not pass through the usual stages of yellow and red heat. Theexplanation for the bluish luminosity is that, at ambient temperatures, quartz has anabsorption band well into the ultraviolet spectral regions. This moves to longerwavelengths with increasing temperature and, with an oxy-coal gas flame reaches theviolet end of the visible spectrum. Hence, as there is absorption, emission is alsopossible. The absorption moves back into the ultraviolet region as the sample cools, soit cannot pass through the yellow and red heat stages.


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Any absorption spectrum displays the peaks in terms of loss of intensity from theincident intensity, which is set at 100%, so the spectrum has percentage transmissionas its intensity scale. The emission spectrum represents an increase in intensity from abaseline of zero intensity. It follows that the emission spectrum will be the mirrorimage of the absorption spectrum as is shown in Figure 2, where the intensity of

emission scale has been adjusted, in arbitrary units so that the peak heights in the twospectra are equal.

Figure 2. Infrared absorption and emission spectra

The difference between the two spectra is that the absorption intensity has beenmeasured in terms of a ratio whereas the emission intensity is a straight measurementand will depend upon several factors, most notably the temperature of the emitting


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body. It is therefore highly desirable that the emission intensity should also beexpressed as a suitable ratio. A convenient way for doing this emerges when theemission process is considered in terms of some simple mathematics. The first step isto quantify the term emission. A convenient unit is the total energy emitted per squarecentimetre per second, for a particular temperature. This is termed THE TOTAL

EMISSIVE POWER, denoted by the symbole.

Consider, once again, a body at equilibrium in a constant temperature enclosure. Theenergy emitted per square centimetre per second in all directions between

wavelength and +d , at a given temperature, is ed , where e is the emissivepower of the surface for that particular wavelength and temperature.

Let the energy falling on unit area per second within the same wavelength limits bedQ. This depends only on the temperature of the enclosure. Also, let the fraction of

the incident energy absorbed between the wavelengths and +d be a, an entityusually termed the ABSORPTIVE POWER of the surface. Hence, the total energy

absorbed by the unit area per second is adQ.

As the temperature of the body is constant, the rate of emission of energy must equal

the rate of absorption. Thus, ed = adQ or

e dQa

Since d is a chosen, fixed interval and dQ depends only on the temperature of theenclosure, the right hand side of the equation must be constant so

e constanta

This equation is generally applicable and, in particular, to a black body, which isdefined as a perfectly absorbing, perfectly emitting body. If, for this black body, the

emissive and absorptive powers are denoted by E and A respectively, then

e Ea

Then, because absorptive power is defined as a fractional quantity, A = 1, giving

e E


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a

Hence, at any given temperature, the ratio of the emissive power of any body to itsabsorbing power for that wavelength at that temperature is constant, and is equal tothe emissive power of a black body at that temperature. This is Kirchhoff's Law, and

provides the basis for infrared emission spectroscopy. Since E is a clearly definedentity, it is convenient for practical purposes to express the results of experimental

measurements as the ratio e/E. This is known as the EMITTANCE, and spectra area plot of emittance as a function of wavelength. It is useful to note, at this juncturethat, for experimental purposes, a roughened surface coated with soot provides a goodapproximation to a black body.

As noted above, emissive power is temperature dependent, increasing with increasing

temperature. However, emittance is independent of temperature, because e and

E are always measured at the same temperature. If a black body at temperature T issurrounded by a perfectly black surface at temperature To, the latter being greater thanT, the gain of energy of the black body is given by the equation E = s (To

4 - T4 ).

This is Stefan's Law, which he deduced experimentally, and it was later given atheoretical proof by Boltzmann, on thermodynamic grounds.

The practical implications of this equation are that the emission from a body at ahigher temperature than its surrounding increases rapidly as the temperaturedifference between them increases. This is important because the emitted energy level

is relatively small but, on the other hand, the chemical stability of the sample beingheated may determine the maximum temperature to which it may be heated to obtainan infrared emission spectrum. Additionally, it is evident that if the sample is atambient temperature, but the radiation detector is cooled it may be possible tomeasure a spectrum, a fact which has practical implications.

A black body, by definition, can only absorb and emit. With real life samples someenergy transfer by reflection will always occur. The degree of specular reflection asthe emitted radiation leaves the sample, is a function of the refractive index of thesample, and at normal incidence and a refractive index of 1.5, which is typical for

many organic compounds, it amounts to about 4%. This is not significant but its effectis evident, as a secondary effect, in distorting the shapes of stronger emission peaks.This is because the refractive index of the sample changes in the vicinity of thesepeaks, falling on the high frequency side of the maximum and rising on the lowfrequency side, to as much as 2.5. The consequential effect is a small reduction in thereflectance on the high frequency edge of the peak and a rather larger increase on thelow frequency side. This, in turn, produced a degree of distortion in the emission


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spectrum, but the effect is unlikely to be large enough to lead to a misinterpretation ofthe measured emission spectrum.

To conclude, a few brief comments on the measurement of infrared emission spectrawill complement and supplement the background information given above. The

T4dependence of the emission intensity shows that a doubling of the difference in thesample and detector temperatures should lead to a sixteen-fold increase in intensity. Inpractice, the maximum usuable temperature may well be determined by the samplestability. Fortunately, the sensitivity of modern Fourier Transform spectrometers issuch that emission intensity is seldom the limiting factor. Furthermore, instrumentswith cooled detectors are available, so that it may be possible to operate with theomitting sample at ambient temperature, given that the spectrometer design is suchthat radiation emitted by surfaces in the instrument itself does not reach the detector.

The usual practice is to remove the normal spectrometer radiation source and replace

it by the heated sample. This can take a number of forms. Solid films are convenientlydealt with mounted inside a heated liquid cell, whose transparent windows have a loeemissivity. Alternatively, a solid sample may be placed in contact with a polishedmetal surface and this is effective given that the sample is not too thick and there is asignificant temperature gradient from the back to the front of the film. Such samplesmay occur in practice, in the form of thin polymer films on aluminium cans. When theemission spectrum of the sample has been recorded and stored in the computermemory, the sample is replaced by a black body at the same temperature. Its emissionspectrum is measured and the ratio of the emission curves of the sample and the blackbody gives the required emission spectrum in units of emittance.

REF: W. Maddams.Internet J. Vib. Spec.[www.ijvs.com] 5, 2, 2 (2001)

3. Recording emission Spectra

The Editor

Research grade FTIR is usually have an emission port (although this may be occupied

by an FT Raman accessory if you are lucky enough to have one). Removing some sortof plug or cover and usually swinging away a mirror inside can access the port .However, there is a snag - you need a collimated beam as you will see in Figure 1.Bill suggested replacing the source by the emitting sample but this is usually verydifficult to do. Illuminating the interferometer through the emission port is opticallyidentical to replacing the source. Where do you go from here?


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Figure 1.

If you are wealthy enough to have an emission accessory or can beg, steal of borrowone to try the technique all is well, but most of you just want to try the method to seeif there is anything in it for you. So just as most of us paint our houses because wecannot afford to have someone do it for us. I suggest a little bit of "do-it-yourself". Tomake a cheap, functional emission accessory - see Figure 2 - you need two mirrors,

ideally M, they should be an off-axis paraboloid but a front surface aluminisedshaving mirror quality spherical will do just fine. M2 is plane and must again be frontsurface aluminised. Window glass quality is fine.


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Figure 2.

Cheap optics are not difficult to buy but if you have trouble contactComar Instruments on +44 (0)1223 245470, ask for Mr Marsh. Either ofthe two gentlemen of this name will sort you out with advice and mirrorsand not charge an arm and a leg.

To support the mirror and sample, I recommend you find a piece of steel (3mm thickis fine but thicker is no problem). Most workshops can help with this. Find a lab jack

and for safety screw the bottom to a wooden board and the top to the steel sheet. Ifyou find the latter is beyond you use contact adhesive. Now - engineering supplierssell magnetic bases for "dial test indicators" and "marking out" accessories. I drawone in Figure 3. Buy two of them and mount the mirrors as I indicate. The whole set-up shouldn't cost more than $100.


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Figure 3.

If your instrument has a laser spot on axis lining up the accessory is easy but if notyou will have to be a little cleverer. To do so - place M1 by eye, similarly M2. Coverthe instrument detector, switch on the source and put a strip of "magic tape" or tracingpaper at the normal transmission sample point. Operate the instrument as atransmission machine and mark the patch of light on the tape or paper with a pencil.Using a small torch bulb find a position where the light from the bulb goes throughyour home built accessory and the interferometer and illuminates centre of the markon the white screen. Now adjust all the mirrors to get a good optical path and note theposition of the bulb filament. This will be your sample point. Uncover the detector,put a beaker containing hot water at the sample point and carefully tweak the mirrorsto give you maximum energy. You are now ready to proceed except for one littleproblem - moist air. The beam splitter in your instrument will suffer if it is exposed tothe atmosphere . However, a few minutes exposure will cause no damage. Findsome thin polythene sheet and some masking tape and cover your accessory carefully

so that it is as sealed as you can make it. Now purge your instrument and theaccessory will be purged as well.

In the tropics be VERY careful. IF the atmosphere is hot and humidand you do not have air conditioning DO NOT open the interferometer atall.


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What to do if you have no emission part or the one you have is occupied? There is analternative method which will fit into the transmission sample area. As you will see inFigure 4, the beam passing through an interferometer goes forwards and backwardsi.e. 50% passes through the device and 50% returns to the source. Although not asefficient as replacing the IR source by your sample, you can exploit this property of

interferometers.

Figure 4.

In Figure 5 I show you how. The emitted radiation passes into the interferometer andthe returned radiation goes to the detector.


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Figure 5.

Obviously, half the exit J-stop is used to illuminate and half to view theinterferometer. There are two problems with the method and both are experimentaland simple to solve.

a) Mirror M2 must not cast a shadow due to its thickness so it must have a bevellededge and

b) The mirror has to be very accurately positioned and angled.

The problem is that interferometer have a real entrance J-stop (piece of metal with ahole in it) but a virtual exit J-stop i.e. the exit J-stop is the reverse projected image ofthe detector - not a great help when you are trying to set up a do-it-yourself accessory.Proceed as follows.

Set the sample stand at the focus of the instrument. Put a strip of magic tape or tracingpaper over the cell holder as suggested above and carefully mark with a pen, theposition of the illuminated patch. Using a piece of steel as a base as above andmagnetic bases, move the mirror M2 until it cuts of the patch. Illuminate M1 with

the light from the interferometer , adjust M1and locate the emission sample point.

Remember - before you make a measurement, turn off the normal IR source. You willfind the measurements are not difficult to make especially if a cooled detector is usedand the results can be very interesting indeed.

Good Luck


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REF: P.J. Hendra.Internet J. Vib. Spec.[www.ijvs.com] 5, 2, 3 (2001)

4. Some thoughts on Catalysisand the use of IR & Raman

in its study

The Editor

There is absolutely no doubt that vibrational spectroscopy has been a prime analyticaltool in heterogeneous catalysis for half a century but dare I suggest that in a way this

is a little surprising.

In heterogeneous catalysis reactants are passed over a catalytically active surfaceusually but not always heated well above ambient and as if by magic, the desiredproducts are produced. [I will always remember that my organic tutor at Universityback in the 1950's - a man very classical in his views, always used to mutter -" anyfool can shove a gas down a hot tube and get a load of old rubbish out the end"].Modern catalytic systems when operated at precise temperatures under carefullypurified reactants at controlled pressures can produce desired reactions with precisionand at very high throughput. These processes are the heart of the large-scale chemical

industry if only because they are flow systems and not batch procedures.

In investigating these reactions and in devising new systems the nature of thereactants, the products, temperature pressures and flow rates are all easy to controland monitor. What is really hard is to actually see the reaction(s) itself. The reactantssorb to the surface. At reactive sites rearrangements, decomposition or addition occursand the products usually vacate the surface very rapidly. As a consequence, exactly asis the case in non-catalytic processes where the activated complex is completelytransitory, the reacting species at the surface are there in only minute concentrationi.e. they are hard or impossible to study.

There is a large range of methods available for the detailed analysis of surfaces andsome are listed in the table below.

Pressure Range Method Information gained

Several

Atmospheres 10-IR Molecular Structure

Raman Molecular Structure but at


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10 atmos very low pressures needSERS

As above Scanning TunnelingMicroscopy

Shape of molecules onwell defined surfaces

Not too far from 1 atmos

unless surface haschemisorbed species on it

NMR Very detailed molecular

structure

Very low pressures overclean surfaces

Variety of electronspectroscopies andMolecular Spectroscopy

Usually elementedanalysis and/or valencystate. Rarely molecularstructure.

Table 1. Some of the methods available to study surfaces.

If our quest is to analyse the molecular species involved it seems to be absolutelyobvious that the analytical procedure must operate at sensible pressures (from say 20 atmospheres) and representative temperatures (RT 100K) becauseonly then will the reaction under study be typical of what it is that we are hoping toachieve. Under this set of constraints, infrared and Raman spectroscopies appear to beideal because you can heat and pressurise and actually 'look' at the surfaces whilstthey are reacting. However, the view is nave - the concentration of active sites andeven worse the concentration of reacting species on the surface is swamped by theunreacted reactants and generated products. So where do we go from here?

The examination of clean highly characterised surfaces such as single crystals to

which are sorbed sub-monolayers of reactive gases or vapours must obviously be farmore likely to yield results of significance on catalytic processes. The problem is thatby definition there is almost nothing to examine! To give you an idea: assuming ourtechnique examines 1 sqcm of surface, a monolayer of benzene lying flat on thesurface would weigh only 8 micrograms and the thickness of the layer would be ~1angstroms. If we wish to carry out a simple transflectance measurement the samplethickness would be ~2 angstroms i.e. ~10-5 of the conventional infrared pathlength. Ifone planned to try Raman the sample size is absurdly small. So the outlook looksbleak but not disastrously so.

Once the performance of infrared spectrometers had improved sufficiently it wasdemonstrated that it was possible to observe incredibly weak bands. If multi-reflectiontechniques were devised and/or glancing reflection resorted to, the effective pathlength in these cases could be raised. Infrared experimenters as early as the 1970'swere able to study CO over metal crystal surfaces and the field has expanded fromthere very rapidly.


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Raman is obviously not attractive - its lack of sensitivity and the minute samplevolume inevitable with such a thin specimen make the method impracticable.However, the SERS effect can enhance the spectrum by 106or much more infavourable cases and so SERS is a technique with immense potential* success hasalready been achieved - see reference 1.

* Readers should consult the Special Edition on SERS published last year.

Many catalysts are highly porous. Compounds such as silica aluminas or zeolites havehuge areas to which molecules can sorb. In microporous materials the holes may be ofmolecular size and limit access to sites nearer the surface, but the surface area of thesematerials can be huge indeed - values of 100 1000m2g-1 are not unusual. Sorbinga monolayer to these materials does provide a decent amount of material and sovibrational spectroscopy started with this type of catalyst system. Pioneers such asNorman Sheppard adsorbed methane to silicas and using infrared were able to show

how the molecules interacted. The selection rules dictate that the bands expected for Iare not the same as for II or III.

----- = surface

Whilst the sorbtion process will alter the strength and hence the 'rigidity' of the CHbands lowering their vibrational frequencies.

This approach is half a century old but still vitally important. The experimentalmethods available have improved but the analysis is based on the same logic. Perhapsthe main limitation with this method is that the predominant species must always be

the support and hence the spectrum of the adsorbate must overlie that of the catalystitself.

Silica based materials absorb strongly in the 1000-1500cm-1region so as a result muchof the 'fingerprint' region is obliterated. Non crystalline and partially crystallineceramics often have very weak Raman spectra so it is obvious that monolayercoverages of good Raman scatters over porous surfaces might well be accessible.


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They are and pioneering data appeared as early as 1970 but the field was dogged byfluorescence. The mere act of baking the catalyst under vacuum to drive off water andcontaminants prior to sorbing the molecule of interest could produce astronomiclevels of fluorescence. The problem was eventually traced to oil and grease in thevacuum system and was tamed but as a field of study, Raman has not been nearly as

significant as infrared.

So - if the study of molecules sorbed directly to surfaces is of considerablefundamental interest, but tricky to do, what other measurements are of value?Catalysts are frequently highly heterogeneous and their preparation may well becomplex and chemically rather obscure e.g. highly significant catalysts such as thoseused in vehicle exhaust systems involve tiny clusters of metal atoms supported on aporous ceramic material. The methods by which these materials are produced are ofvery considerable commercial importance. The best method of production can wellresult in market domination. In general, the ceramic is treated with a solution of themetal(s) as salts or as organometallics.

Once dried and often roasted, the catalyst is then frequently reduced using H2 orcracked ammonia generating the desired clusters of metal atoms. Vibrationalspectroscopy has been remarkably successful in following the reactions involved.Most of the competing surface analytical techniques are relatively unattractive onthese incredibly complex heterogeneous mixtures.

Zeolite and silica alumina catalysts have a variety of acidic sites on their surfaces.They may contain surface OH groups (I) or Lewis acid acceptor sites (II)

D = Donor Molecule

or even Bronsted acid sites allowing protonabstraction III.


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The performance of the catalyst is frequently dominated by the relative and absolutedensity of these sites on the surface. This is another area where vibrationalspectroscopy is unequally valuable. The trick is to use a tell-tale molecule. Ammoniahas always been popular but pyridene can also be used. Let us take pyridene as anexample.

Liquid pyridene shows its ring breathing mode at 990cm-1. Pyridene/water mixturesshow a band near 1000cm-1. The pyridene cation vibrates still faster whilstcoordination of pyridene to a metal pushes the frequency into the 1025cm-1area. Quiteobviously, if one sorbs the pyridene to the catalyst and runs the spectrum, the natureof the species and their relative distribution can quickly be measured. The example Igive is based on Raman bands but infrared is just as useful, but relies on bands atmuch higher frequencies. This type of surface assay is routinely used to characteriseand/or monitor catalysts. You can be even more clever - the actual frequency of thetell-tale molecule vibration is closely related to the strength of the adsorbate -adsorbent interaction. So a careful analysis of the band shape can yield data on thedistribution of activities of a specific type of site e.g. the Lewis acid activity on acracking catalyst. See ref 2.

All surface scientists concern themselves with the adsorption isotherm. The subject iscovered in all classical Physical Chemistry textbooks, but let me over simplify. Inmany examples of porous catalysts, the plot of the amount of material absorbed vs. thepressure over the system is shown below.

Monolayer coverage coincides with the sharp downward bend in the curve. Abovethis level, sorption produced multi-layers and eventually liquid trapped in the poresuntil at Y free liquid wets the catalyst. Surface scientists measure these isotherms andcan make much of their shape. Since vibrational spectroscopy is applied to samples in


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enclosed cells at controlled temperatures, spectra can be recorded at precisely knownpoints on the isotherm. This is particularly and almost unequally valuable in bothfundamental and applied work.

So - vibrational spectroscopy is valuable because of its enormous versatility. Haven't I

heard that before somewhere?

The Basic Physics of Spectroscopy

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