200:(hence the name, "Fourier-transform spectroscopy"). The raw data is sometimes called an "interferogram". Because of the existing computer equipment requirements, and the ability of light to analyze very small amounts of substance, it is often beneficial to automate many aspects of the sample preparation. The sample can be better preserved and the results are much easier to replicate. Both of these benefits are important, for instance, in testing situations that may later involve legal action, such as those involving drug specimens.
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1008:), a microwave pulse (EPR) or a radio frequency pulse (NMR) in a strong ambient magnetic field is used as the energizing event. This turns the magnetic particles at an angle to the ambient field, resulting in gyration. The gyrating spins then induce a periodic current in a detector coil. Each spin exhibits a characteristic frequency of gyration (relative to the field strength) which reveals information about the analyte.
1816:
1092:, thus precisely offset the Fellgett's advantage. For line emission sources the situation is even worse and there is a distinct `multiplex disadvantage' as the shot noise from a strong emission component will overwhelm the fainter components of the spectrum. Shot noise is the main reason Fourier-transform spectrometry was never popular for ultraviolet (UV) and visible spectra.
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Some stationary forms retain the
Fellgett multiplex advantage, and their use in the spectral region where detector noise limits apply is similar to the scanning forms of the FTS. In the photon-noise limited region, the application of stationary interferometers is dictated by specific consideration for the spectral region and the application.
1015:, the energizing event is the injection of the charged sample into the strong electromagnetic field of a cyclotron. These particles travel in circles, inducing a current in a fixed coil on one point in their circle. Each traveling particle exhibits a characteristic cyclotron frequency-field ratio revealing the masses in the sample.
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In general, the goal of absorption spectroscopy is to measure how well a sample absorbs or transmits light at each different wavelength. Although absorption spectroscopy and emission spectroscopy are different in principle, they are closely related in practice; any technique for emission spectroscopy
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In addition to the scanning forms of
Fourier-transform spectrometers, there are a number of stationary or self-scanned forms. While the analysis of the interferometric output is similar to that of the typical scanning interferometer, significant differences apply, as shown in the published analyses.
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A pulsed
Fourier-transform spectrometer does not employ transmittance techniques. In the most general description of pulsed FT spectrometry, a sample is exposed to an energizing event which causes a periodic response. The frequency of the periodic response, as governed by the field conditions in
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One of the most important advantages of
Fourier-transform spectroscopy was shown by P. B. Fellgett, an early advocate of the method. The Fellgett advantage, also known as the multiplex principle, states that when obtaining a spectrum when measurement noise is dominated by detector noise (which is
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the light at a certain wavelength (the un-blocked wavelength is set by a knob on the monochromator). Then the intensity of this remaining (single-wavelength) light is measured. The measured intensity directly indicates how much light is emitted at that wavelength. By varying the monochromator's
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of the light to be measured at each different time delay setting, effectively converting the time domain into a spatial coordinate. By making measurements of the signal at many discrete positions of the movable mirror, the spectrum can be reconstructed using a
Fourier transform of the temporal
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of the light. Michelson spectrographs are capable of very high spectral resolution observations of very bright sources. The
Michelson or Fourier-transform spectrograph was popular for infra-red applications at a time when infra-red astronomy only had single-pixel detectors. Imaging Michelson
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Fourier-transform spectroscopy is a less intuitive way to get the same information. Rather than allowing only one wavelength at a time to pass through to the detector, this technique lets through a beam containing many different wavelengths of light at once, and measures the
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is measured (this is called the "sample spectrum"). The sample will absorb some of the light, causing the spectra to be different. The ratio of the "sample spectrum" to the "background spectrum" is directly related to the sample's absorption spectrum.
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As mentioned, computer processing is required to turn the raw data (light intensity for each mirror position) into the desired result (light intensity for each wavelength). The processing required turns out to be a common algorithm called the
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independent of the power of radiation incident on the detector), a multiplex spectrometer such as a
Fourier-transform spectrometer will produce a relative improvement in signal-to-noise ratio, compared to an equivalent scanning
277:. Light from the source is split into two beams by a half-silvered mirror, one is reflected off a fixed mirror and one off a movable mirror, which introduces a time delay—the Fourier-transform spectrometer is just a
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The
Fourier-transform spectrometer is just a Michelson interferometer, but one of the two fully reflecting mirrors is movable, allowing a variable delay (in the travel time of the light) to be included in one of the
1044:, where the scattering from a sharp probe-tip is used to perform spectroscopy of samples with nanoscale spatial resolution, a high-power illumination from pulsed infrared lasers makes up for a relatively small
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combination of wavelengths, giving a second data point. This process is repeated many times. Afterwards, a computer takes all this data and works backwards to infer how much light there is at each wavelength.
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Pulsed FT spectrometry gives the advantage of requiring a single, time-dependent measurement which can easily deconvolute a set of similar but distinct signals. The resulting composite signal, is called a
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can also be used for absorption spectroscopy. First, the emission spectrum of a broadband lamp is measured (this is called the "background spectrum"). Second, the emission spectrum of the same lamp
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dominated, the noise will be proportional to the square root of the power, thus for a broad boxcar spectrum (continuous broadband source), the noise is proportional to the square root of
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To be more specific, between the light source and the detector, there is a certain configuration of mirrors that allows some wavelengths to pass through but blocks others (due to
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because typically the signal will decay due to inhomogeneities in sample frequency, or simply unrecoverable loss of signal due to entropic loss of the property being measured.
782:{\displaystyle {\begin{aligned}I(p)&=\int _{0}^{\infty }I(p,{\tilde {\nu }})d{\tilde {\nu }}\\&=\int _{0}^{\infty }I({\tilde {\nu }})\,d{\tilde {\nu }}.\end{aligned}}}
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Accordingly, the technique of "Fourier-transform spectroscopy" can be used both for measuring emission spectra (for example, the emission spectrum of a star),
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of a light source: how much light is emitted at each different wavelength. The most straightforward way to measure a spectrum is to pass the light through a
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into an actual spectrum. The peak at the center is the ZPD position ("zero path difference"): Here, all the light passes through the
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Hegenbarth, R; Steinmann, A; Mastel, S; Amarie, S; Huber, A J; Hillenbrand, R; Sarkisov, S Y; Giessen, H (2014).
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place the sample after the interferometer in the optical path. The total intensity at the detector is
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of light, and the vertical axis represents how much light is emitted by the torch at that wavelength.
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wavelength setting, the full spectrum can be measured. This simple scheme in fact describes how
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An "interferogram" from a
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976:{\displaystyle I({\tilde {\nu }})=4\int _{0}^{\infty }\left\cos(2\pi {\tilde {\nu }}p)\,dp.}
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Pulsed sources allow for the utilization of
Fourier-transform spectroscopy principles in
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is the number of sample points comprising the spectrum. However, if the detector is
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the spectrometer, is indicative of the measured properties of the analyte.
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to be modulated by the sample before the interferometer. In fact, most
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absorption spectra (for example, the absorption spectrum of a liquid).
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is the spectrum to be determined. Note that it is not necessary for
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The Michelson spectrograph is similar to the instrument used in the
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Fourier Transform Spectroscopy Topical Meeting and Tabletop Exhibit
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with a movable mirror. The beams interfere, allowing the temporal
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The method of Fourier-transform spectroscopy can also be used for
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1203:"High-power femtosecond mid-IR sources for s-SNOM applications"
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Internet Journal of Vibrational Spectroscopy – How FTIR works
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79:(NMR) and magnetic resonance spectroscopic imaging (MRSI),
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Digital Array Scanned Interferometer, issued Dec. 11, 1990
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Description of how a Fourier transform spectrometer works
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485:{\displaystyle I(p,{\tilde {\nu }})=I({\tilde {\nu }}),}
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beam intensity. Next, the beam is modified to contain a
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Semiautomated depositor for infrared microspectrometry
139:: The spectrum of light emitted by the blue flame of a
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or not. It can be applied to a variety of types of
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Stationary forms of Fourier-transform spectrometers
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106:is required to turn the raw data into the actual
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94:), including the continuous-wave and the pulsed
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110:, and in many of the cases in optics involving
1191:, 8th ed. Oxford University Press: Oxford, UK.
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163:, an instrument that blocks all of the light
1374:Vibrational spectroscopy of linear molecules
1166:, Cambridge University Press: Cambridge, UK.
295:instruments, which are easier to construct.
37:are collected based on measurements of the
1369:Nuclear resonance vibrational spectroscopy
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1032:Nanoscale spectroscopy with pulsed sources
362:{\displaystyle {\tilde {\nu }}=1/\lambda }
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1742:Inelastic electron tunneling spectroscopy
1422:Resonance-enhanced multiphoton ionization
1123:Time stretch dispersive Fourier transform
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1510:Extended X-ray absorption fine structure
1164:Principles of Nuclear Magnetic Resonance
1133:Infrared spectroscopy of metal carbonyls
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1038:scanning near-field optical microscopy
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1187:Peter Atkins, Julio De Paula. 2006.
232:", a common technique in chemistry.
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1013:Fourier-transform mass spectrometry
33:is a measurement technique whereby
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1732:Dual-polarization interferometry
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204:Measuring an absorption spectrum
1747:Scanning tunneling spectroscopy
1722:Circular dichroism spectroscopy
1717:Acoustic resonance spectroscopy
151:One of the most basic tasks in
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1364:Vibrational circular dichroism
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1474:Cavity ring-down spectroscopy
1379:Thermal infrared spectroscopy
1227:10.1088/2040-8978/16/9/094003
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143:. The horizontal axis is the
1608:Inelastic neutron scattering
1113:Forensic polymer engineering
1040:techniques. Particularly in
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275:Michelson–Morley experiment
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1754:Photoacoustic spectroscopy
1696:Time-resolved spectroscopy
1118:Nuclear magnetic resonance
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1000:In magnetic spectroscopy (
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1764:Pump–probe spectroscopy
1653:Ferromagnetic resonance
1445:Laser-induced breakdown
1162:Antoine Abragam. 1968.
299:Extracting the spectrum
226:absorption spectroscopy
155:is to characterize the
122:Conceptual introduction
116:Wiener–Khinchin theorem
85:electron spin resonance
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1019:Free induction decay
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818:{\displaystyle I(p)}
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1395:Ultraviolet–visible
1219:2014JOpt...16i4003H
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1108:Forensic chemistry
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1765:
1762:
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1757:
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1752:
1748:
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1699:
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1443:
1441:
1438:
1434:
1431:
1430:
1429:
1426:
1423:
1420:
1418:
1417:Near-infrared
1415:
1413:
1410:
1406:
1403:
1402:
1401:
1398:
1396:
1393:
1392:
1390:
1386:
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1268:
1265:
1264:
1251:
1244:
1236:
1232:
1228:
1224:
1220:
1216:
1213:(9): 094003.
1212:
1208:
1204:
1197:
1190:
1184:
1178:
1172:
1165:
1159:
1155:
1144:
1141:
1139:
1136:
1134:
1131:
1129:
1126:
1124:
1121:
1119:
1116:
1114:
1111:
1109:
1106:
1104:
1101:
1100:
1093:
1091:
1087:
1083:
1079:
1075:
1074:monochromator
1068:
1058:
1049:
1047:
1043:
1039:
1029:
1027:
1016:
1014:
1009:
1007:
1003:
993:
970:
967:
964:
957:
948:
942:
939:
933:
930:
926:
919:
916:
913:
907:
902:
899:
894:
888:
882:
878:
867:
863:
859:
856:
844:
835:
828:
827:
826:
809:
803:
795:
772:
763:
757:
747:
738:
732:
729:
723:
720:
717:
714:
699:
690:
680:
676:
672:
670:
656:
650:
638:
632:
629:
623:
613:
609:
605:
603:
595:
589:
578:
577:
576:
574:
549:
540:
511:
502:
479:
472:
468:
459:
453:
450:
446:
442:
439:
436:
433:
418:
409:
406:
394:
388:
385:
379:
372:
371:
370:
356:
352:
348:
345:
336:
326:
310:
296:
294:
289:
284:
280:
276:
266:
259:
255:
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210:
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186:
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161:monochromator
158:
154:
146:
142:
138:
133:
119:
117:
113:
109:
105:
101:
97:
93:
88:
86:
82:
78:
74:
70:
66:
62:
61:
56:
52:
48:
44:
40:
36:
32:
19:
1843:Spectroscopy
1819:
1807:
1787:(a misnomer)
1773:Applications
1691:Time-stretch
1675:
1582:paramagnetic
1400:Fluorescence
1318:Spectroscopy
1243:
1210:
1206:
1196:
1188:
1183:
1171:
1163:
1158:
1089:
1081:
1077:
1070:
1055:
1035:
1025:
1022:
1010:
999:
990:
791:
494:
302:
272:
260:spectrograph
257:
253:
245:
243:
237:
234:
223:
194:
187:
181:
177:
174:
169:
164:
153:spectroscopy
150:
141:butane torch
99:
95:
89:
75:, FT-NIRS),
60:spectroscopy
58:
30:
29:
1359:Vibrational
293:Fabry–Pérot
47:time-domain
1837:Categories
1565:Two-photon
1467:absorption
1349:Rotational
1150:References
1086:shot-noise
325:wavenumber
145:wavelength
63:including
1643:Terahertz
1624:Radiowave
1522:Mössbauer
1138:nano-FTIR
1042:nano-FTIR
952:~
949:ν
943:π
934:
895:−
873:∞
864:∫
848:~
845:ν
767:~
764:ν
742:~
739:ν
733:π
724:
703:~
700:ν
686:∞
677:∫
660:~
657:ν
642:~
639:ν
619:∞
610:∫
553:~
550:ν
515:~
512:ν
463:~
460:ν
454:π
443:
422:~
419:ν
398:~
395:ν
357:λ
340:~
337:ν
288:coherence
283:coherence
254:Michelson
182:different
51:radiation
43:radiative
39:coherence
1809:Category
1538:Electron
1505:Emission
1455:emission
1412:Vibronic
1235:49192831
1096:See also
1080:, where
157:spectrum
137:spectrum
108:spectrum
1821:Commons
1648:ESR/EPR
1596:Nucleon
1424:(REMPI)
1215:Bibcode
35:spectra
1662:Others
1450:Atomic
1233:
495:where
269:beams.
165:except
1603:Alpha
1572:Auger
1550:X-ray
1517:Gamma
1495:X-ray
1428:Raman
1339:Raman
1334:FT-IR
1231:S2CID
178:total
41:of a
369:is
323:and
170:some
83:and
73:FTIR
1631:NMR
1223:doi
1011:In
1006:NMR
1002:EPR
931:cos
721:cos
440:cos
256:or
246:and
98:or
1839::
1636:2D
1555:UV
1229:.
1221:.
1211:16
1209:.
1205:.
1004:,
825::
118:.
67:,
53:,
1310:e
1303:t
1296:v
1237:.
1225::
1217::
1090:m
1082:m
1078:m
971:.
968:p
965:d
961:)
958:p
940:2
937:(
927:]
923:)
920:0
917:=
914:p
911:(
908:I
903:2
900:1
892:)
889:p
886:(
883:I
879:[
868:0
860:4
857:=
854:)
839:(
836:I
813:)
810:p
807:(
804:I
773:.
758:d
754:]
751:)
748:p
730:2
727:(
718:+
715:1
712:[
709:)
694:(
691:I
681:0
673:=
651:d
648:)
633:,
630:p
627:(
624:I
614:0
606:=
599:)
596:p
593:(
590:I
559:)
544:(
541:I
521:)
506:(
503:I
480:,
477:]
473:)
469:p
451:2
447:(
437:+
434:1
431:[
428:)
413:(
410:I
407:=
404:)
389:,
386:p
383:(
380:I
353:/
349:1
346:=
311:p
71:(
20:)
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