1、Designation: E 573 01 (Reapproved 2007)Standard Practices forInternal Reflection Spectroscopy1This standard is issued under the fixed designation E 573; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A n
2、umber in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 These practices provide general recommendations cov-ering the various techniques commonly used in obtaininginternal reflection spectra.
3、2,3Discussion is limited to theinfrared region of the electromagnetic spectrum and includes asummary of fundamental theory, a description of parametersthat determine the results obtained, instrumentation mostwidely used, practical guidelines for sampling and obtaininguseful spectra, and interpretati
4、on features specific for internalreflection.2. Referenced Documents2.1 ASTM Standards:2E 131 Terminology Relating to Molecular SpectroscopyE 168 Practices for General Techniques of Infrared Quanti-tative AnalysisE 284 Terminology of Appearance3. Terminology3.1 Definitions of Terms and SymbolsFor def
5、initions ofterms and symbols, refer to Terminologies E 131 and E 284,and to Appendix X1.4. Significance and Use4.1 These practices provide general guidelines for the goodpractice of internal reflection infrared spectroscopy.5. Theory5.1 In his studies of total reflection at the interface betweentwo
6、media of different refractive indices, Newton (1)4discov-ered that light extends into the rarer medium beyond thereflecting surface (see Fig. 1). In internal reflection spectros-copy, IRS, this phenomenon is applied to obtain absorptionspectra by measuring the interaction of the penetrating radia-ti
7、on with an external medium, which will be called the sample(2,3). Theoretical explanation for the interaction mechanismsfor both absorbing and nonabsorbing samples is provided bySnells law, the Fresnel equations (4), and the Maxwellrelationships (5).NOTE 1To provide a basic understanding of internal
8、 reflection phe-nomena applied to spectroscopy, a brief description of the theory appearsin Appendix X2. For a detailed theoretical discussion of the subject, see(4).6. Parameters of Reflectance Measurements6.1 Practical application of IRS depends on many preciselycontrolled variables. Since an unde
9、rstanding of these variablesis necessary for proper utilization of the technique, descriptionsof essential parameters are presented.6.2 Angle of Incidence, uWhen u is greater than thecritical angle, uc, total internal reflection occurs at the interfacebetween the sample and the internal reflection e
10、lement, IRE.When u is appreciably greater than uc, the reflection spectramost closely resemble transmission spectra. When u is lessthan uc, radiation is both refracted and internally reflected,generally leading to spectral distortions. u should be selectedfar enough away from the average critical an
11、gle of thesampleIRE combination that the change of ucthrough theregion of changing index (which is related to the presence ofthe absorption band of the sample) has a minimal effect on theshape of the internal reflection band. Increasing u decreases thenumber of reflections, and reduces penetration.
12、In practice,there is some angular spread in a focused beam. For instru-ments that utilize f4.5 optics in the sample compartment, thereis a beam spread of 6 5, but the beam spread in the IRE issmaller because of its refractive index. The value will increaseas lower f-number optics are utilized. This
13、beam spreadproduces a corresponding distribution of effective paths andeffective depth of penetrations.6.3 Number of Reflections, NN is an important factor indetermining the sensitivity of the IRE. Where multiple reflec-tions are employed, internal reflection occurs a number of1These practices are u
14、nder the jurisdiction of ASTM Committee E13 onMolecular Spectroscopy and Separation Science and are the direct responsibility ofSubcommittee E13.03 on Infrared and Near Infrared Spectroscopy.Current edition approved March 1, 2007. Published March 2007. Originallyap-proved in 1976. Last previous edit
15、ion approved in 2001 as E 573 01.2Internal Reflection Spectroscopy, IRS, is the accepted nomenclature for thetechnique described in these practices. Other terms are sometimes used whichinclude: Attenuated Total Reflection, ATR; Frustrated Total Reflection, FTR;Multiple Internal Reflection, MIR; and
16、other less commonly used terms. In olderliterature, one may find references to Frustrated Total Internal Reflection, FTIR.This should not be confused with Fourier Transform Infrared Spectroscopy FT-IR.3Other terms sometimes used for referring to the internal reflection element are:ATR crystal, MIR p
17、late, or sample plate.4The boldface numbers in parentheses refer to the list of references at the end ofthese practices.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.times along the length of the IRE depending on its length, l,thic
18、kness, t, and on the angle of incidence, u, of the radiantbeam.NOTE 2The length of an IRE is defined as the distance between thecenters of the entrance and exit apertures.6.3.1 Absorption occurs with each reflection (see Fig. 2),giving rise to an absorption spectrum, the intensity of whichdepends on
19、 N. For single-pass IREs, N can be calculated usingthe following relationship:N 5SltDcot u (1)For double-pass IREs:N 5 2SltDcot u (2)Many single-pass IREs employ approximately 25 reflec-tions.NOTE 3N must be an odd integer for IREs in the shape of a trapezoid,and an even integer for IREs in the shap
20、e of a parallelogram.6.4 Relative Refractive Index, n21, of the Sample, n2, andIRE, n1;(n21=n2/n1)Refractive index matching controls thespectral contrast. If the indexes of the sample and the IREapproach each other, band distortions can occur. Therefore, it isnecessary to select an IRE with a refrac
21、tive index considerablygreater than the mean index of the sample.6.4.1 The refractive index of a material undergoes abruptchanges in the region of an absorption band. Fig. 3 (6) showsthe change in refractive index of a sample across an absorptionband as a function of wavelength. When an IRE of index
22、 nAisselected, there may be a point at which the index of the sampleis greater than that of the IRE. At this wavelength, there is nou at which total internal reflection can take place, and nearly allof the energy passes into the sample. The absorption bandresulting in this case will be broadened tow
23、ard longer wave-lengths, and hence appear distorted. When an IRE of index nBis selected, there is no point at which the index of the sampleexceeds it. On the long wavelength side, however, the refrac-tive indexes approach each other. This results in an absorptionband that is less distorted, but that
24、 is still broadened on the longwavelength side. With an IRE of index nC, a considerablyhigher refractive index than that of the sample, the indexvariation of the sample causes no obvious distortion of theabsorption band.6.5 Depth of Penetration, dpThe distance into the rarermedium at which the ampli
25、tude of the penetrating radiationfalls to e1of its value at the surface is a function of thewavelength of the radiation, the refractive indexes of both theIRE and the sample, and the angle of incidence of the radiationat the interface.6.5.1 The depth of penetration, dp, can be calculated asfollows:d
26、p5l12 p sin2u2n212!(3)where: l15ln15 wavelength of radiation in the IRE.The depth of penetration increases as the angle of incidencedecreases, and becomes infinitely large as u approaches thecritical angle (see Figs. 4 and 5) (7).6.6 Effective Path Length, deThe effective pathlength, orrelative effe
27、ctive thickness, de, for the beam for each reflectionis defined by Harrick (4) in detail, and is different for-polarized than for -polarized radiation. For bulk materi-als, when u = 45, de=12 de, and the average effectivethickness is about equal to the penetration depth, dp. For largerangles, deis s
28、maller than dpand for smaller angles, deis largerthan dp. The total effective pathlength is equal to N times theeffective pathlength, de. An example of the effect of u on N deis shown in Fig. 6.6.7 Absorption Coeffcient, aAs in transmission spectros-copy, the absorptivity of a material affects the f
29、raction of theNOTE 1The ray penetrates a fraction of a wavelength (dp) beyond thereflecting surface into the rarer medium of refractive index n2(thesample), and there is a certain displacement (D) upon reflection. u is theangle of incidence of the ray in the denser medium, of refractive index, n1,at
30、 the interface between the two media.FIG. 1 Schematic Representation of Path of a Ray of Light forTotal Internal ReflectionFIG. 2 Multiple Internal Reflection EffectSolid LineRefractive index of sample.Dotted LineAbsorption band of sample.Dashed LinesRefractive indices of reflector plates.FIG. 3 Ref
31、ractive Index Versus WavelengthE 573 01 (2007)2incident radiation that is absorbed, and hence the spectralcontrast. The internal reflectance of bulk materials and thinfilms, for small abosrptivities, is as follows:R 5 1 2ade(4)The reflectance for N reflections is:RN5 1 2ade!N(5)6.7.1 If adeuc. The F
32、resnel reflectionequations become:r5cos u2i sin2u2n212!cos u1i sin2u2n212!(X2.8)r5n212cos u2i sin2u2n212!n212cos u1i sin2u2n212!(X2.9)When n21is real (both media nonabsorbing), |r|=|r|=1,and internal reflection is total for uc# u = 90.X2.2 Absorbing Rarer MediumX2.2.1 When the rarer medium is absorb
33、ing, its complexrefractive indexn25 n21 1 ik2! (X2.10)replaces n2in the Fresnel Eq X2.8 and Eq X2.9 (Note X2.1).The attenuation index, k, is related to the absorption coeffi-cient, a, and the absorptivity, a, of the Bouguer-Beer law by:nk5aco/4pn (X2.11)P/Po5 e2ab5 102abc(X2.12)a5M a c (X2.13)Here,
34、cois the velocity of light in vacuo, and n its frequency.M is the natural logarithm of 10, M = 2.303; b is samplethickness, and c is the concentration of the absorbing species inthe sample.6The Symbols provided in Appendix X1 are not to be considered standardnomenclature. These are under advisement
35、by Subcommittee E13.04 on Nomen-clature and must be further approved by ASTM Committee E-13 on MolecularSpectroscopy.NOTE 1Reflectance versus angle of incidence for an interface be-tween media with indices, n1= 4 and n2= 1.33, for light polarizedperpendicular, R, and parallel, R, to plane of inciden
36、ce for externalreflection (solid lines) and internal reflection (dashed lines). uc, uB, and upare the critical, Brewsters, and principal angles, respectively.FIG. X2.2 Reflectance Versus Angle of IncidenceE 573 01 (2007)14NOTE X2.1The complex refractive index is written n2= n2+ ik2byIUPAC, and k2is
37、called the absorption index.X2.2.2 Internal reflection is affected by an absorbing rarermedium as illustrated in Fig. X2.3. For radiation incidentbetween u = 0 and uup, internal reflectance is ratherinsensitive to absorption coefficient, until it becomes verylarge. For angles of incidence greater th
38、an the critical angle,however, internal reflectance can be highly sensitive to theabsorption coefficient, and the parallel component of polariza-tion is more sensitive than the perpendicular.X2.3 Attenuated Total ReflectionX2.3.1 Maxwells equations predict the evanescent wavethat extends into the me
39、dium of lower refractive index, beyondthe reflecting interface. The frequency of this wave is that ofthe incident radiation, and its amplitude diminishes exponen-tially with distance from the interface. It is possible to couplewith this evanescent wave and extract energy from it, therebymaking the r
40、eflection less than total. The strength of thecoupling depends (in part) on the amplitude (electric fieldstrength) of the evanescent wave. Frustrated total reflectionoccurs when the coupled medium does not absorb the energy,but conducts it away from the interface. Attenuated totalreflection occurs w
41、hen the coupled medium absorbs the energyextracted from the evanescent wave.X2.3.2 Attenuated total reflection is observed when theangle of incidence is maintained greater than the critical anglewhile wavelength is scanned across an absorption band. Theamount by which internal reflection is diminish
42、ed from beingtotal, because of absorption of energy from the evanescentwave, that is, the reflectance loss per reflection, is the absorp-tion parameter, a:a 5 1 2 R (X2.14)The absorption parameter is greater near the critical anglethan at larger angles, and is also greater for -polarizationthan for
43、-polarization.X2.3.3 The relationship between attenuated internal reflec-tance and the absorption coefficient of Beers law can beexpressed in simplified form if absorption is small, for ex-ample, ab 0.1. Then Beers law can be approximated by:P/Po 1 2ab (X2.15)where ab is the fraction absorbed for tr
44、ansmission through asample of thickness, b. The corresponding quantity for internalreflection is the absorption parameter, so that the internalreflectance of a single reflection can be expressed by:R 5 1 2 a 5 1 2ade(X2.16)Here deis an effective pathlength, or effective thickness of athin film, and
45、is defined by:de5 a/a (X2.17)X3. INTERNAL REFLECTION ELEMENTSX3.1 Various transparent optical elements used in internalreflection spectroscopy for establishing the conditionnecessaryto obtain the internal reflection spectra of materials are shownin Fig. X3.1.NOTE 1Internal reflectance at an interfac
46、e versus angle of incidenceat l = 0.4 m for n21= 0.333 and various values of absorption coefficienta2. Note that the curves tend to resemble those for external reflectionwhen a2becomes high.FIG. X2.3 Internal Reflectance at an Interface Versus Angle ofIncidenceE 573 01 (2007)15REFERENCES(1) Newton,
47、Opticks II, Book 8, 1917 p. 97.(2) Fahrenfort, J., “Attenuated Total ReflectanceA New Principle forProduction of Useful Spectra of Organic Compounds,” MolecularSpectroscopy, 1962, p. 701.(3) Harrick, N. J., Discussion of December 1959, p. B.D.-4, followingpaper presented by Eischens, R. P., “Infrare
48、d Methods Applied toSurface Phenomena in Semiconductor Surfaces,” (Proceedings ofSecond Conference), Pergamon Press, London, 1960, p. 56.(4) Mirabella, F. M., and Harrick, N. J., Internal Reflection SpectroscopyReview and Supplement, Harrick Scientific Corp., Ossining, NY, 1985.(5) Born, M., and Wol
49、f, E., Principles of Optics, 2nd ed., Pergamon Press,NY, 1964.(6) Wilks Scientific Corp., “Internal Reflection Spectroscopy,” 1965, p. 1.(7) Gilby, A. C., Cassels, J., and Wilks, P. A., Jr.,“ Internal ReflectionSpectroscopy III, Microsampling,” Applied Spectroscopy, Vol 24, No.5, 1970.(8) Paralusz, C. M., “Internal Reflection Spectroscopy Applied to theAnalysis of Adhesive Tapes,” Journal of Colloid and InterfaceScience, Vol 47, No. 3, 1974, pp. 719746.(9) Wolfe, W. L., Ballard, S. S., and McCarthy, K. A.,“ Refractive
