P. F. Bernath- Infrared emission spectroscopy

8/2/2019 P. F. Bernath- Infrared emission spectroscopy

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6 Infrared emission spectroscopy

P. F. Bernath

Department of Chemistry, University of Waterloo, Waterloo, ON, Canada

N2L 3G1

1 Introduction

Infrared spectroscopy has been traditionally carried out mainly in absorption. Thevirtues of infrared emission spectroscopy have been largely overlooked, but therehas been a recent surge of interest. The modern infrared Fourier transformspectrometer has made emission measurements much easier. This article is anattempt to provide a comprehensive view of the technique and builds on a shorter

Chemical Society review.1

Previous reviews of infrared emission spectroscopy have concentrated on the basicprinciples and on applications in analytical chemistry.2^6 We shall cover some of thisground as well, but the main focus will be in applications in high resolution molecularspectroscopy. We shall not cover the infrared spectra of atoms although excellentinfrared emission spectra have been recorded at Kitt Peak, Orsay and Lund. Thisreview has some overlap with reviews on the infrared spectra of transient molecules,including ions, free radicals and high temperature molecules,7^10 as well as thespectroscopic reviews of Barrow and Crozet.11

For the purposes of this review, ``infrared'' is arbitrarily dened as10^10 000 cm1 (1^1000 mm), to include the far-infrared, mid-infrared andnear-infrared region. The microwave and submillimeter regions are excluded,although laboratory microwave emission spectroscopy has become very popularwith the proliferation of Flygare^Balle spectrometers.12 These instruments detectthe coherent emission of microwave radiation. At slightly higher frequencies radioastronomers detect molecules in molecular clouds by microwave and millimeter waveemission spectroscopy. The eld of astrochemistry13 is built almost entirely oncentimeter wave and millimeter wave emission spectroscopy of species in various

astronomical objects ranging from stellar envelopes to comets.We shall focus our discussion mainly on emission from gases, although heatedsolids14 and liquids15 give useful spectra. The secret to recording useful spectra fromcondensed phases is to work with thin lms or dispersed particles. For thick samples,multiple scattering of infrared photons results in a nearly featureless blackbodyspectrum that depends only on the temperature of the emitter.

DOI: 10.1039/b001200i Annu. Rep. Prog. Chem., Sect. C, 2000, 96, 177^224 177


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This review will cover mainly high resolution spectroscopy. In this context, ``highresolution'' means that the rotational structure is at least partly resolved for agas phase sample. The emission technique also works equally well for large moleculessuch as C60 and C70,

16 and for species ranging from DNA bases17 to polycyclic aro-matic hydrocarbons,18 in which the rotational structure is not resolved. For these

molecules the solids are heated to about 200^300

C and emission from the vapourprovides excellent spectra even in the far-infrared region.18

The attraction of emission spectroscopy is the possibility of an improvedsignal-to-noise ratio compared to absorption spectroscopy. Ideally only photonsemitted by the sample are detected (``zero background''), free from the noise pro-duced by the continuum lamp in an absorption experiment. This improvement insensitivity is particularly useful for the spectroscopy of transient molecules becauseof their intrinsically low concentrations. This potential advantage of emissionspectroscopy for the detection of ions and free radicals is well known is the visible

and near-UV regions. For example, the violet emission from the CH radical (A2D3X 2P) is readily seen by eye in a ame19 but the measurement of the A2

D2X 2P absorption20 is much more difcult. This emission advantage persists intothe infrared region.

The infrared region is unique because all molecules, with the exception ofhomonuclear diatomics, have at least one allowed vibration^rotation transition.While it is true that there are infrared electronic transitions and a few light moleculeshave far-infrared rotational transitions, infrared spectroscopy is nearly synonymouswith vibrational spectroscopy. Moreover, vibrational spectroscopy through the con-

cept of group frequencies also provides chemical information in a way that rotationaland electronic transitions do not.Even the weak electric quadrupole emission transitions of H2 have been seen by

astronomers from molecular clouds experiencing shock waves.21 Infrared emissionspectroscopy has the potential to be a sensitive, universal, molecule-specic monitorof chemical composition.

This review will be organized by sources, ranging from stars to microwave dis-charges, in which emission spectroscopy has been carried out. The coverage ofthe more recent high resolution spectroscopic work aims to be relatively complete.

Older work and various applications in surface science, astronomy, chemicaldynamics or analytical chemistry are more illustrative than complete. But rst afew basic principles and instrumental considerations will be discussed.

2 Basic principles

Emission spectroscopy is based on a few basic equations and principles. The rst isthe interaction of monochromatic radiation with a sample (Fig. 1) (ignoring

scattering and uorescence). The beam can be reected, absorbed or transmitted, so

a r t 1 1

in which a is the absorptance (absorption factor), r is the reectance (reectionfactor) and t is the transmittance (transmission factor) of a body.5,22 Note that

178 Annu. Rep. Prog. Chem., Sect. C, 2000, 96,177^224


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the difference between a reectance (``-ance'') and a reectivity (``-ivity'') is that thelatter applies under some set of standard conditions (e.g., smooth surface, thick

sample) while the former applies to a specic sample. Because we are generally dis-cussing specic objects, we will use the ``-ance'' terms. Factors a, r and t are wave-length dependent numbers between 0 and 1 and there are three simple limits:

a 1Y r t 0 o lkod 2

r 1Y a t 0 o eet mio 3

t 1Y a r 0 o eet window 4

Kirchhoff's law states that the emittance e of a sample is equal to the absorptance a(see above). The monochromatic emittance of a sample is dened as:

L eLff 5

in which L is the radiance (or ``sterance'', in units of watts per steradian per square

meter of source per hertz of spectral bandwidth)22

of a sample, and LBB

is theradiance of a blackbody. Thus LBB is related to the Planck function and e is aproportionality constant between 0 and 1 that converts it into the observed radiance,L. The expression for the radiance of a blackbody is:

Lff 2hn3

c2ehnakT 16

in units of W sr1 m2 s. Because the emittance and absorptance are equal,

e a 7

all of the selection rules and optical properties of any material (gas, liquid or solid)that are generally dened in terms of absorption transfer directly to emission.All of the above equations depend on both frequency (or wavenumber) and tem-

Fig. 1 Light striking an object will be reected, absorbed or transmitted (ignoring uorescenceand scattering).

Annu. Rep. Prog. Chem., Sect. C, 2000, 96, 177^224 179


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perature so Kirchhoff's law can be written as

L~nY T a~nY TLff~nY T 8

in which n is the wavenumber in customary non-SI units of cm1.Kirchhoff's law has been tested experimentally using a hot CO

2gas sample.23 The

absorptance was measured from the transmittance [e.g., eqn. (1) with r 0],

a 1 t 9

and the emittance measured by comparison with a blackbody source at the sametemperature,

e L

Lff10

Direct comparison for different temperatures and concentrations of CO2 in N2showed that a e.

The direct application of Kirchhoff's law allows the determination of sampletemperature24,25 as illustrated in Fig. 2. This gure also illustrates the differencebetween gas phase emission and absorption measurements. In the upper panel,the absorption of a mixture of acetylene, butane and carbon dioxide at 555 K isdisplayed. In the middle panel, the measured radiance of the sample is plotted.At the bottom, the ``normalized radiance'', dened as the radiance divided byabsorptance, is plotted. By Kirchhoff's law this ratio is the blackbody radiance, i.e.,

L

a

L

1 t LffT 11

The radiance of a blackbody at a certain frequency is a function of only thetemperature, which can be adjusted until the calculated Planck function matchesthe observed ``normalized radiance''. The shaded regions mark regions that containno molecular emission (i.e., no information) and are ignored. The message of thisgure is clear: a thermal emission spectrum is nothing more than the ``inverted''

absorption spectrum modulated by the Planck function.The quantitative interpretation of sample emission is, in principle, as simple as

that of absorption. In practice, however, samples rarely have uniform temperaturesand ``self-absorption'' is a problem. Fortunately sample emission increases stronglywith temperature. For example, the total radiance (integrated over all frequencies)of a blackbody is proportional to the fourth power of temperature:

Ltotl

I0

Lff dn sT4apY 12

in which s is the Stefan^Boltzmann constant. (The factor ofp appears because theemission of the blackbody, eqn. (6), is per steradian and the normal version ofthe equation is simply the total emission from a hole.) This means that the highesttemperature object in the eld of view of the spectrometer dominates the appearanceof the spectrum. The general equation for the observed radiance at a given frequency



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(Fig. 3) for the typical case of a gas sample in front of a wall 26 is

L0 Lz0tz0

tz0

0LffTz dt 13

in which L(z0) is the radiance of an object at z0 away from the observer located at 0.Note that Kirchhoff's law gives de dt. The transmittance t(z0) attenuates this

Fig. 2 Spectra for a mixture of acetylene, carbon dioxide and butane in helium at 555 K: (a)absorption in %; (b) radiance; (c) normalized radiance, with a blackbody t for LBB at 565 K,ignoring the shaded regions.25 Reproduced from ref. 25, with permission.



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radiance from z0 and the integral accounts for the radiance of the interveningradiating elements. The transmittance is given by Beer's law,

tz e

z0

kzH dzH

14

in which k is the absorption coefcient. If the sample is uniform and the temperatureis constant then eqn. (14) becomes,

tz ekz 15

and the general equation, eqn. (13), reduces to:

L0 Lz0 ekz0 Lff1 ekz0 Lz0tz0 L

ff1 tz0 16

This equation has two simple limits: the optically thin case when t & 1 and L &L(z0), and the optically thick case when t & 0 and L L

BB. In the optically thincase the radiance is that of the wall, while in the optically thick case only the frontof the gas sample is seen.

The discussion presented so far is on the macroscopic level. The connection to themicroscopic world of atoms and molecules is through the Einstein equations; in par-ticular, the Einstein A coefcient for emission from level 2 to 1 is:

A21 16p3n3jm21j

2

3he0c317

in which m21 is the transition dipole moment, and e0 is the permittivity of free space.The Einstein A coefcient measures the rate of photon emission from the excited

level of an atom or molecule,

dN2

dt A21N2 18

and has units of s1. The above expression has been integrated over the lineshape of

Fig. 3 Radiance, L, seen by an observer at 0 looking through a gas sample to a back wall[radiance L(z0)] at z0.



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the transition and the more detailed expression27 is

A21n 16p3n3jm21j

2

3he0c3gn n0 19

in which g(n n0) is the normalized lineshape function. The strong cubic frequencydependence of the emission rate means that emission work in the infrared andfar-infrared is more difcult than in the visible and ultraviolet. Moreover, transitiondipole moments for vibration^rotation transitions tend to be smaller than typicalvalues for allowed electronic transitions or typical values of permanent dipolemoments for pure rotational transitions.

3 Methodology

The basic requirement for emission spectroscopy is a source of radiation and adetector. For some experiments, a simple infrared lter between the source anddetector can provide useful results. For example, Chang and Klemperer28 excitedthe (HF)2 molecule in a jet expansion with a tuneable near-infrared laser and moni-tored the HF infrared emission. This type of action spectroscopy obtained byscanning a laser and detecting the total emission is common at shorter wavelengthsbut also works in the infrared. The simple but effective technique of non-dispersive

infrared absorption spectroscopy (NDIR) has an emission analog called ameinfrared emission (FIRE) spectrometry.29 FIRE is a simple lter^detector combi-nation that can be used as a detector in a gas chromatograph.30 In this analyticalapplication the hydrocarbon analyte from a gas chromatograph is burnt in ahydrogen^oxygen ame and hot CO2 emission is detected.

29,30

The basic requirement for any emission experiment is that the source and detectorhave different temperatures. The detector is also an emitter of radiation, and if thesource and detector have the same temperature then there is no net ux of radiationfalling on the detector. Obviously, a room temperature detector such as triglycinesulfate (TGS) cannot be used to monitor a room temperature sample. But aliquid-nitrogen-cooled InSb detector can certainly see a room temperature sample.Generally emission from a detector at 4 K (liquid He) or even 77 K (liquid N 2)can be ignored.

A slightly more sophisticated system for emission spectroscopy is based on the useof a circular or linear variable lter between the source and detector. By rotating thecircular lter the peak transmission of the lter can be changed. Such lters can bepurchased from, for example, Optical Coating Laboratory, Inc. (OCLI) of SantaRosa, CA. The use of a circular variable lter results in a simple, compactspectrometer but with a low resolving power,

R l

Dl

~n

D~n20



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of typically less than 100. Circular variable lters are still used in astronomy and forsimple analytical applications in which their high optical throughput and relativelylow cost are an advantage.

Early high resolution infrared emission measurements were made using classicalgrating spectrographs or spectrometers to disperse the emission. For example,

the Ballik^Ramsay bands of the C2 molecule were discovered31

in 1963 using aninfrared spectrometer constructed by Douglas and Sharma32 at the NationalResearch Council of Canada. The C2 molecule was made by the evaporation ofgraphite from the walls of a carbon tube furnace (King furnace) operated at about2900 C. The emission was detected using a dry-ice-cooled PbS detector. Thevibronic bands of the A 3Sg^X

3Pu electronic transition were seen at3800^7100 cm1.

The performance of any spectrometer can be improved in the thermal infraredregion (nd 3000 cm1) by cooling the entire instrument. Heroic early experiments

were carried out by McDonald and co-workers33

to study the nascent productsof reactions of F atoms with hydrocarbons such as ethylene. The chemiluminescenceemitted by these free radical reactions at low pressures was very weak.

A modern version of a cooled spectrometer was developed originally by Pimenteland then used by Saykally and co-workers34 to study the weak emission fromlaser-excited polycyclic aromatic hydrocarbons (PAHs) in a supersonic free jetexpansion. In this case the entire spectrometer was cooled to 4 K by liquid heliumin order to take advantage of a new ultrasensitive blocked impurity band detector.This detector from Rockwell is based on Si:As and has an internal avalanche process

that results in gain. In other words, it is a solid state photomultiplier that operates inthe infrared.An even more sophisticated instrument can be made using modern infrared array

detectors. The use of a large format array (typically InSb or HgCdTe) with aspectrograph is attractive because of the multiplex advantage. The most sensitiveinfrared instruments are the cryogenic echelle spectrographs that are in use or underconstruction at all major observatories. One example is Phoenix (Fig. 4) at Kitt PeakNational Observatory in Tucson, Arizona.35

Phoenix is cooled to 50 K to eliminate the thermal emission from the spectrograph

and uses an echelle grating in high order to obtain high resolution in a compactinstrument. Order sorting is carried out with cooled infrared lters. Phoenix cur-rently has a resolving power of about 70 000 or a resolution of 0.03 cm1 at2000 cm1. The use of a 1024 1024 InSb array allows coverage of the1800^10 000 cm1 region.

Phoenix is calculated to have a sensitivity advantage of nearly 100 over a con-ventional Fourier transform spectrometer. This high sensitivity originates from sev-eral factors including a very restricted spectral bandpass, cryogenic cooling andimproved detector performance. The most important factor is that infrared array

detectors are fundamentally different from conventional single element detectors.Because infrared arrays are integrating detectors, very low light levels can behandled. If the main noise source is read-out noise then the signal-to-noise ratiogrows linearly with time. This is in contrast to a conventional single element detector



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where the signal-to-noise ratio grows with the square root of time. The performanceof Phoenix was tested by recording emission spectra of the NH free radical using amicrowave discharge source.36

Most infrared emission measurements are made with Fourier transformspectrometers (FTSs). A typical experimental arrangement37 is illustrated in Fig.

5. Light is admitted through the ``emission port'', which is located near the internalsources used for absorption spectroscopy. Fourier transform spectroscopy hasbecome ``conventional'' so no detailed descriptions are needed.

Infrared emission spectroscopy requires simply that the internal glowbar source bereplaced by the source of interest. The Fourier transform interferometer is bestviewed as a modulator that shifts infrared frequencies into audio frequencies at

Fig. 4 Phoenix cryogenic echelle spectrograph of Hinkle et al.35 at Kitt Peak National

Observatory, Tucson, AZ, USA.



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the detector. For example, the 0^10 000 cm1 spectral region is mapped into the0^10 000 Hz frequency range on the detector if the moving mirror changes theoptical path difference at 1 cm s1. Unusual infrared emission measurements aresometimes made by using a ``second'' modulation of the source.

The most common of the double modulation experiments is time-resolved Fouriertransform spectroscopy (TRFTS).38,39 A conceptually simple approach to TRFTS isthe step-scan method using a periodic infrared source. If the ``moving'' mirror of an

FTS is stopped then the infrared signal can be recorded as a function of time.The moving mirror is then sent to the next sampling position and another infraredemission decay is recorded. By accumulating a set of emission decays, each recordedat the usual sampling position of the interferogram, a set of time-resolved spectra areobtained.

Fig. 5 Typical long wavelength infrared chemiluminescence emission experiment using aFourier transform spectrometer equipped with a copper-doped Ge detector.37 Reproducedfrom ref. 37, with permission.



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Continuously scanning FTSs can also record time-resolved infrared spectra. Inthis case, the fringes of the internal He^Ne laser are used (with appropriate timedelays) to trigger the infrared emission source and the sampling of the emissionsignal. In this way, an interferogram at a specic time delay (or series of delays)from the excitation pulse is recorded.

Time-resolved infrared emission spectra are very useful in the study of reactiondynamics and collisional relaxation. In Fig. 6 the time-resolved OH emission from

the O HCl reaction is displayed.

38

As time progresses the highly excited OH mol-ecules produced by the chemical reaction are relaxed by collisions. The developmentof schemes for various time-resolved FTS work continues with, for example, therecent development of the ``event-locked'' method by Weidner and Peale.40

Double modulation techniques are attractive in the emission spectroscopy of tran-sient molecules because they discriminate against the more abundant precursormolecules. The selectivity of velocity,41,42 concentration43 and Zeemanmodulation44,45 have all been demonstrated with FTSs. Unfortunately they generallydo not also offer an increase in sensitivity and work most easily with step-scan instru-

ments so they have not been widely adopted for high resolution work.An important consideration in emission work is the `contrast'' between the sampleand background. This is particularly important in remote sensing of gases and in thefar-infrared region. If the eld-of-view of the spectrometer includes a gaseous sampleagainst a background at a certain temperature, then the lines of the sample willappear in absorption or emission depending on the sample temperature. If the sampleis warmer than the background then the lines will appear in emission, while if it iscooler then the lines will appear in absorption. If the sample and background havethe same temperature then the sample lines will disappear, i.e., there is no contrast.

This spectral line reversal technique has been used to determine ametemperature.19 A ame is viewed against the continuum of a lamp and a metal atomsuch as Na is added to give a bright emission line. As the lamp lament is increasedin temperature from below the ame temperature, the atomic emission line will dis-appear when the ame and lament temperatures are equal. Filament temperaturesare then easily determined with an optical pyrometer.

Fig. 6 The time-resolved Fourier transform emission spectrum of the reaction of O with HCl.The highly excited OH molecules are quenched by collisions with HCl and this causes theOH vibration^rotation line intensities to decrease with time.38



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The problem of contrast is particularly severe in the far-infrared region and thiscauses problems for both absorption and emission measurements. In the far-infraredsources operate usually in the Rayleigh^Jeans limit with hn`` kT, so the blackbodyradiance reduces to46

Lff 2n2

kTc2

2kTl2

X 21

Because the radiance is only linearly proportional to temperature, the contrastbetween a heated sample and the surroundings is much less than in the visible.Indeed, usually the spectrometer itself contributes a major portion of the ``signal''in a far-infrared emission experiment.

4 Chemiluminescence

To a chemist and spectroscopist, the light emitted from a chemical reaction has anundeniable fascination. Unfortunately chemiluminescence tends to be relativelyweak and not very common. Chemiluminescence is closely related to thespectroscopy of ames and to the excitation of molecules by energy transfer frommetastable species. These sources of infrared emission will be discussed separatelybelow.

A Nobel prize has been awarded to J. Polanyi (shared with Y. T. Lee and D.Herschbach) for the study of reaction dynamics by infrared chemiluminescence.47

This work started in 1958 with the observation of HCl emission from the

r glP 3 rgl gl 22

reaction by Cashion and Polanyi.48 A commercial infrared spectrometer with anNaCl prism and a thermocouple detector was used. A similar chemiluminescentreaction (H2 Cl2 ame) was later used by Clayton et al.

49 at Penn State witha high resolution spectrometer. Infrared chemiluminescence in the style of Polanyiis still carried in a number of groups such as those of Setser and Leone. Butkovskayaand Setser50 studied the dynamics of the

yr rf 3 rPy f 23

reaction, for example, while Klaassen et al.51 recorded the rst high resolutionspectrum of HOI from the

gPrSs y 3 rys gPrR 24

chemical reaction.Much of the chemical dynamics work is now carried out by time-resolved Fourier

transform spectroscopy. Recent examples include the work of the Sloan laboratoryat 1 ms time resolution for reactions of H atoms with uorochlorocarbons.52 Witha step-scan FTS system even 10 ns time resolution is possible.53^55



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Chemiluminescence has also proved to be very useful in studies devoted to newspectroscopy (rather than dynamics) of molecules. At long wavelengths, a remark-able high resolution spectrum of FO was recorded by Hammer et al.37 from theenergetic

p yQ 3 py yP 25

reaction. The F atoms were made in a microwave discharge of F2 (Fig. 7). FO emi-ssion up to the 837 vibration^rotation band could be assigned near 1000 cm1 (Fig.8). The analogous OH (or OD) reaction56^58

r yQ 3 ry yP 26

gives an exceptionally ne infrared emission spectrum with OH populated up to u 9. Interestingly this same reaction (26) is responsible for atmospheric nightglow (seebelow) and astronomers have detected emission from u 10.58

Infrared electronic emissions are also possible and the work of E. Fink andco-workers is particularly noteworthy. Most ``normal'' stable main group moleculesdo not have low-lying electronic states, with the exception of O2 and NO: O2 has a p

2

conguration that leads to a X 3Sg ground state with a1Dg and b

1Sg states at 7882and 13121 cm1, respectively; NO has a regular X 2P ground state with two spincomponents, 2P1/2 and

2P3/2, split by 123 cm1. Closed-shell main group molecules

Fig. 7 Chemiluminescence reactor used in the F O3 infrared emission experiment to makeFO.37 The F atoms are created in a microwave discharge of F2 in He. Reproduced from ref.

37, with permission.



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generally have excited electronic states that give rise to UV spectra. By virtue of theirunpaired electrons, free radicals often have low-lying electronic states. Most freeradicals are very reactive molecules but there are a few ``stable'' free radicals suchas O2 and NO.

Fink and co-workers have recorded the infrared electronic spectra of a number offorbidden transitions of free radicals with the same p1 or p2 congurations asNO or O2. The NH, PH, AsH, SbH and BiH family have p

2 congurations andBeutel et al. have measured the a 1D^X 3S infrared transitions of PH,59 AsH60

and SbH.61 These molecules were made by the reaction of H atoms with heatedelemental solids. [BiH was also made in a similar fashion but emission was excitedby energy transfer from O2(

1D), see below.] Although the a 1D^X 3S transitionis forbidden by normal electric dipole selection rules, it is allowed by the magneticdipole transition moment. Excellent high resolution spectra were obtained with ahigh purity Ge detector.

The TeF, TeCl, TeBr and TeI molecules are isovalent with OH and have inverted X2P ground states from a p3 conguration. The spin^orbit coupling constants for TeFand TeCl, however, are about 4000 cm1, as compared to 139 cm1 for OH.Ziebarth et al.62 measured the magnetic dipole emission X 2P1/2 3 X

2P3/2 betweenthe two spin components for TeF and TeCl at high resolution. The excited TeF andTeCl molecules were made by the reaction of a TeH/TeH2 mixture with F2 or Cl2.Low resolution infrared spectra for TeF, TeCl, TeBr and TeI are also known.63

Another interesting example from the Fink group is the infrared electronictransitions of BiP, BiAs and BiSb.64 These diatomics are isovalent with N2 and haveX 1S ground states. Unlike N2, however, the

3S and 5S states that also arise from

Fig. 8 Chemiluminescent vibration^rotation emission from FO created in the F O3reaction.37 The marks at the top indicate the vibrational band origins. Reproduced from ref.37, with permission.



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the lowest energy 4S 4S atomic asymptote are low-lying. The a 3S1 3 X1S

electronic transitions of BiP, BiAs and BiSb were detected in the 7000^10 000 cm1

region through chemiluminescence from the reaction of P, As and Sb atoms withBi

xvapour.

Very recently we have adapted the Broida oven ow reactor to study the infraredchemiluminescence of the classic metal plus oxidizer reactions.65 In particular,the reactions

g xPy 3 gy xP 27

xPy 3 y xP 28

yield excellent near-infrared electronic spectra. The A 1S 3 X 1S and AH 1P3 X1S transitions of SrO are displayed in (Fig. 9).

Fig. 9 Chemiluminescence from SrO from the Sr N2O reaction. The bands are mainly due tothe AH 1P 3 X 1S electronic transition and some isolated Sr atomic lines can also be seen.



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5 Excitation by energy transfer from metastables

Excitation can be delivered to a molecule by a chemical reaction or by energy transferfrom another excited molecule. Both of these processes occur in ames so that thedistinction that we have made between the three emission sources ^

chemiluminescence, excitation by energy transfer and ames ^ is somewhat articial.Popular metastables include the rare gases, O2(

1D), NF(1D) and active nitrogen. Ofthe rare gases, He is particularly popular because the metastable 23S state has19.8 eV of available energy. Active nitrogen is made by passing N2 gas through dis-charge and is a complex mixture of N and N2 in both ground and excited states.O2(

1D) and NF(1D) are unique in that both species can be made in relatively highconcentrations by purely chemical means. This is attractive for the pumping ofchemical lasers such as the COIL (chemical oxygen^iodine laser) system.66 O2(

1D)atoms are produced by the reaction of Cl2 gas with basic hydrogen peroxide,

glP rPyPl Puyrl 3 yPID Pugll PrPy 29

Energy transfer to I2 causes dissociation and lasing on the I2P1/2 3

2P3/2 transition.NF(1D) can be formed by the thermolysis of the explosive gas uorine azide67 at1000 K,

pxQ 3 xpID xP 30

The chemical production of O2 and NF metastables is clearly not for the fainthearted! For laboratory purposes an electrical or microwave discharge is generallyused to make metastable atoms and molecules.

The COIL laser, in fact, was associated with a mysterious red and infrared emi-ssion that turned out to be CuCl2. Traces of chlorine reacted with heated coppertubing to produce CuCl2, which was excited by energy transfer from the O2(

1D)molecule.68,69 Ultimately this spectroscopic mystery led to the high resolution analy-sis of the linear CuCl2 molecule.

70 Both visible and infrared electronic transitions of

CuCl2 were analyzed using a variety of sources.The group of Fink has made extensive use of the O2(1D) metastable to record

spectra of an amazing number of main group molecules. The main types of maingroup free radicals that give rise to infrared electronic transitions have already beenmentioned in the section on chemiluminescence. For diatomic molecules they gen-erally have p1(2Pr), p

2(3S, 1D, 1S) or p3(2Pi) congurations as exemplied byCH, NH and OH free radicals, respectively. In the CH family, the forbidden X 2P3/23 X 2P1/2 transitions

71,72 were detected for PbF, PbCl, PbBr and PbI. These mol-ecules were all made by the reaction of Pb vapour with halogens and then excited

by O2(

1

D) metastables. The high quality of the spectra is illustrated with PbF (Fig.10). For the NH family, the a 1D 3 X 3S transitions were analyzed for AsI,73

SbF,74 SbCl,74 SbBr,74 SbI,74 BiCl,75 BiBr75 and BiI.75 In addition the X23S1

3 X13S0 ne structure transition of BiH

76 (as well as for BiF,77 BiCl,78 BiBr78

and BiI78) was also measured. The X 2P1/2 3 X2P3/2 transition

79 of TeH andTeD was seen near 4000 cm1.



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The O2 molecule has a similar energy level pattern to the NH family and has beenstudied extensively by magnetic dipole and collision-induced80 emission. The O2

a

1

Dg^X

3

S

g transition occurs near 8000 cm

1

, so the

1

D metastable carries about1 eV of energy. The O2 b1Sg^a

1Dg electric quadrupole transition81 is seen near

5000 cm1 (Noxon band82). The a 1D^X 3S or b 1S^ X 3S infrared electronictransitions were detected for the isovalent SO,83 S2,

84,85 SeO,86,87 SeS,88 Se2,89 TeO,90

TeS,90 TeSe91,92 and Te293 molecules, all excited by O2(

1D).The BiO molecule has a similar electronic structure to the isovalent NO. Extensive

infrared electronic transitions of BiO have been studied94 including the X 2P3/2 3 X2P1/2 transition

95,96 near 7000 cm1. This transition displays a remarkable hypernestructure (Fig. 11). In the N2 family, the bands analogous to the Vergard^Kaplansystem of N

2are shifted from the near-UV region into the near-infrared for the

heavier members. The a 3Su^X1Sg transitions of Sb2

97 and Bi298 are found near

9000 and 5000 cm1, respectively. Both ground and excited states correlate toground state N(4S) atoms, and there is a large change in bond length and vibrationalfrequency. Analogous transitions have been seen for BiN,99 BiP100 and BiAs.100 Allof the above emission spectra were induced by energy transfer from O2 metastables.

The best known main group polyatomic that has an infrared electronic spectrum isHO2. The bent hydroperoxyl free radical has the A

2AH^ ~ 2AHH electronic transitionnear 7000 cm1. Early high resolution emission measurements were made by Tuckettet al.101 using a SISAM spectrometer. The denitive analysis, however, is the recentwork of Fink and Ramsay.102 Analogous transitions of HS2,

103 HSe2 and HTe279

have also been seen by Fink and co-workers.Another new free radical, BiOH, with an infrared electronic spectrum near

6000 cm1 was discovered by Fink et al.104 BiOH forms when H2O is added tothe reaction of Bi vapour and metastable O2. BiOH is similar to BiF (rather than

Fig. 10 Fourier transform emission of the X 2P3/2 3 X2P1/2 transition of PbF.

72 Reproducedfrom ref. 72, with permission.



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the isovalent HNO molecule) in electronic structure. BiOH, however, is bent so the X3S state of BiF splits into ~1

1AH, ~21AHH and ~3

1AH electronic states. The observedinfrared electronic transitions thus correspond to the ``ne'' structure transitions ~23 ~1 and ~3 3 ~1.

Fig. 11 High resolution emission spectrum of the X22P3/2 3 X1

2P1/2 transition of BiO. Thelower panel displays the impressive Bi hyperne structure in the rotational lines.94 Reproduced

with permission, from ref. 94.



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Active nitrogen also gives interesting new spectra. For example, when Vilesov etal.105 added Xe to a nitrogen discharge, a new band appeared very near to theforbidden 2Po 3 2Do emission of the atom near 9600 cm1 (the 2Po, 2Do and4So states arise from the 2p3 conguration of N). They assigned the structure toa bound^bound transition of the Xe:N excimer molecule.105 Active nitrogen canalso be used to excite the vibration^rotation emission of stable molecules106 such

as CO2.He metastables are commonly used to create ions by Penning ionization. Forexample, the A 2P3 X 2S near-infrared electronic transition of CSwas measuredby Horani and Vervloet.107 One of the main problems with using He metastables in aowing afterglow is that their concentration is low and thus the emission of theproduct ions or molecules tends to be weak. A solution to this difculty was foundby Vervloet, who adopted the Engelking supersonic corona discharge108 (see below)as the source of He metastables rather than the customary DC discharge. TheEngelking source is a prolic generator of metastable He atoms and provided a much

brighter source of CS

than usual in a owing afterglow. This same technique wasapplied by Huber and Vervloet109 to record the emission spectrum of the b 3P3 a 3S transition of NO near 6000 cm1.

Another powerful source of rare gas metastables is the novel apparatus originallybuilt by Cossart110 (Fig. 12). This source is often called the ``Cossart source'',but this name is not unambiguous because Cossart has created a number of novel

Fig. 12 Cossart source110 for the Penning excitation of N2 (in this case) with metastable raregases. Reproduced from ref. 110b, with permission.



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and useful sources of molecules. For example, the Cossart source was one of thesources used by Dabrowski et al. to record infrared emission experiments ofArH,111^114 ArD, KrH115^117 and KrD, XeH118 and XeD. These rare gas hydridesare examples of what Herzberg has called ``Rydberg molecules'' as distinct fromRydberg states in molecules.119 Rydberg molecules have no stable ground states

and are based on putting an electron in a Rydberg orbital of a bound ion core.For example, ArH has no chemically bound ground state but ArH is a deeply boundion. By placing an electron in a Rydberg orbital built on the ArH ion core, a seriesof bound excited states are obtained. The excimer molecule ArF (of laser fame)is another example of a Rydberg molecule. Rydberg^Rydberg transitions ofArH are readily detected in the infrared region. Indeed, ArH is so easy to makeeven as an impurity that we generally use Ne gas for infrared emission work witha hollow cathode lamp. The rare gas hydride molecules were made in the Cossartsource by reacting H2 (or D2) with a ow of metastable rare gases.

110^118

6 Engelking corona-excited supersonic jet source

Droege and Engelking108 developed a simple but remarkably effective source for theemission spectroscopy of jet-cooled free radicals (Fig. 13). The source is particularly

effective for the production of small non-metal free radicals. The corona excitationoccurs on the high pressure side of the nozzle and the resulting plasma is rapidlyexpanded into vacuum. The high voltage, low current corona discharge is an efcientsource of electronically excited free radicals, which are rotationally cooled by jetexpansion. The main difculties with the source are that large molecules are notalways cooled and that the source is prone to various oscillatory electrical modes.108

Fig. 13 Schematic of the tip of the Engelking corona-excited supersonic jet expansion source.The vacuum pump, typically a Roots blower, is grounded to complete the circuit.108

Reproduced from ref. 108b, with permission.



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Vervloet has adapted this source for infrared emission spectroscopy ofelectronically excited N2 and NO. Vervloet and co-workers

120,121 studied the a1PgâH

1Su, w1Duâ

1Pg and CHH5PuÂH

5Sg (Herman) infrared bands of N2.The b 4S 3 a 4P system of NO was analyzed by Huber and Vervloet.122 High-lRydberg transitions (e.g., 5g^4f) of NO were also reported.123,124

A completely unrelated method of recording emission spectra of cold molecules isto irradiate liquid and gaseous He with a proton beam. Tokaryk et al.125 sawnear-infrared emission from the d 3Su^c

3Sg electronic transition of He2. He2is another example of a Rydberg molecule.

7 Magnetically conned plasma (Penning) sources

The application of a magnetic eld to a DC electrical discharge forces the ions andelectrons into circular orbits. The plasma is thus conned to a smaller volumeand a brighter source results. The use of a magnetic eld in this way was rstadvocated by Penning and Penning-type pressure gauges based on this principleare used still. The magnetic eld allows the Penning source to operate over a verywide pressure range. Cossart has used a number of Penning-type sources126^128

for emission spectroscopy including the application of a Penning-type dischargeto a jet expansion source.126 Penning-type sources are good sources of molecularions.

Bernard et al.129 used a Penning source to study the A 2P3X 2S transition ofN2 in the near-infrared. Highly excited infrared electronic transitions of CO (E1P3B 1S and C 1S3B 1S) were also investigated in this way.130,131

8 Flames

The infrared spectroscopy of ames has a long history, starting in 1890 with Julius, asdiscussed by Gaydon.19 Hydrocarbon ames provide hot infrared emission spectraof H2O, CO2, CN, C2, CO and OH.

For the purpose of this review the term `àme'' refers mainly to hydrocarbon plusoxygen (or air) ames. Infrared spectra of many other types of ames such as acety-lene plus nitrous oxide132 or C2F4 plus oxygen

133 have been recorded mainly to studythe combustion process (rather than spectroscopy). Worden et al.134 recorded theinfrared emission spectrum of a forest re from an airplane at high resolution. Theydetected emission features due to several molecules, including hot H2O. Most of

the molecules detected (NH3, CO, CH3OH, etc.), however, appeared in absorptionagainst the continuum emitted by the re.Early use of ames for high resolution infrared spectroscopy includes the work on

the A 2P3X 2S transition of CN by Bacis et al.,135 and the Phillips (A 1Pu^X1Sg)

136 and Ballik^Ramsay (b 3Sgâ3Pu)

137 systems of C2. C2 was producedin an oxyacetylene torch and CN in a similar nitrous oxideâcetylene ame.



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The detection of vibration^rotation spectra is also possible. Indeed, the infraredemission spectra of hydrocarbon ames are dominated by the main combustionproducts CO2 and H2O. A low pressure (20 mbar) methane plus oxygen amewas used to study CO2 and CO emission in the 1800^5000 cm

1 region.138,139

Numerous highly excited energy levels could be assigned. An oxyacetylene torch

was also used to record the emission spectrum of hot water140,141

and the OH142

free radical from about 3000 to 10 000 cm1. The corresponding ODvibration^rotation bands were made in a D2 O2 ame.

143 The Meinel systemof OH covers nearly the entire near-infrared region with bands in the Du 1, 2and 3 vibrational sequences.

The major advantage of ames is that they are a bright source of hot moleculeswith temperatures ranging up to 3000 K. However, they have a number of draw-backs including large pressure-broadened linewidths (typically *0.1 cm1) if theyare operated at atmospheric pressure. Because CO2 and H2O so dominate the emi-

ssion spectra, recording data for other species can be difcult. More recentspectroscopic work with ames (e.g., for H2O) has been at reduced total pressuresto reduce the pressure-broadened linewidths.144

Finally, ames have been used to measure the rotational emission spectrum ofOH145 in the far-infrared between 50 and 375 cm1. The rotational emissionspectrum of hot H2O has also been measured at somewhat higher wavenumbers.

144

9 Pyrotechnic and propellant combustion

The visible spectra of reworks are well known but infrared emission spectra ofvarious solid propellants, ares and other pyrotechnic materials have also beenrecorded. The goal of this work is an understanding of the combustion processand the measurement of the temperature. Infrared emission spectra of the combus-tion of boron-containing,146 silicon-containing147 and magnesium-containing148

pyrotechnics have been recorded by FTS emission spectroscopy at low resolution.Various combustion products ranging from HCl to BO could be identied depending

on the composition of the are.

10 Shock tubes

Another interesting source for infrared emission is the shock tube. The shock com-pression of an inert gas seeded with 1^10% or so of a precursor molecule results

in temperatures of 1000^4000 K, depending on the experimental conditions. Theheating is of short duration (*5 ms) so a rapid-scan infrared spectrometer isrequired to make measurements. The collision-induced quadrupole emission fromH2 was measured near 4000 cm

1 using a shock tube.149,150 Emission spectra ofshock-heated small hydrocarbons were recorded by Stephens and Bauer.151 A smallHgCdTe infrared array detector has been used with a spectrograph to measure



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the emission from shock-heated NO.152 All of these measurements were at low res-olution, but a modern time-resolved FTS or a spectrograph like Phoenix35 wouldgive much improved spectra.

11 Positive column of a DC discharge

In a typical direct current (DC) discharge, most of the cell is lled by the positivecolumn. The positive column is infrequently used for infrared emission spectroscopybecause the molecular glow is much less bright than from the plasma inside a hollowcathode. The positive column is characterized by a nearly constant electric eld and a

modest voltage drop, leading to a relatively extended diffuse glow.153

By moving theelectrodes to the side of the cell (Fig. 14), only the positive column emission is sentinto the spectrometer.154 In addition, by injecting the carrier gas near the electrodesand the precursor molecule directly into the positive column, a steady dischargecan be obtained. Although a hollow cathode discharge is brighter than the positivecolumn, the cathode surface is not very tolerant of hydrocarbon ``impurities''.The larger discharge volume and the more stable operation in the presence of CH4,etc., make the positive column an attractive emission source for someapplications.154

Tokaryk and Civis

155,156

built a cooled hollow cathode similar to those used byOka and co-workers for the laser spectroscopy of ions. They were able to recordclean emission spectra of the ~ 3Pg 3 a

3Pu transition of12C3 and

13C3. Amethane^helium discharge was used to create a remarkably extensive series of bandsin the Du2 0 sequence. A similar source provided ArH

in emission, although inthis case a hollow cathode gives a better signal.157

Fig. 14 Schematic of the positive column discharge cell used to measure the vibration^rotationemission of CD.154 Reproduced with permission, from ref. 154.



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In Orsay, France, a DC discharge through pure NO gave the spectra of severalinfrared electronic transitions of NO (M 2SÊ 2S, D 2SÂ 2S, E 2S^D2S and E 2SÂ 2S).158,159 In addition, the vibration^rotation spectra of NO (Fig.15) could be recorded up to u 22 in the Du 3 sequence.160,161 The highly excitedvibration^rotation bands of CO and CO2 were also recorded from the positivecolumn of a DC discharge.162^167 In this case the CO2 was diluted with He and/orN2 to mimic the conditions found in a CO2 laser plasma. Time-resolved FTS spectraof CO emission have also been carried out to study energy transfer.168

12 Hollow cathode discharge

There are a large number of different types of hollow cathode discharges. Forexample, Herzberg et al.169 built a hollow cathode that is cooled by liquid nitrogenin order to study the H2 and H3 molecules (Fig. 16). In this source it is possibleto view either the hollow cathode or the positive column.169,170 This was very usefulbecause H3 was found only in the cathode but H2 appeared in both the cathode

and positive column regions. Bacis

171

has another design for a liquid-nitrogen-cooledcathode but more typically water cooling is used.An uncooled hollow cathode can run very hot and Davis et al. exploited this to

record new infrared spectra of the Phillips (A 1Pu^X1Sg)

172 and Ballik^Ramsay(b 3Sgâ

3Pu)173 systems of C2. In this case, the C2 was made largely by evaporation

of the red hot cathode rather than by sputtering from the surface. Ferguson et al.174

Fig. 15 Vibration^rotation emission of NO excited in the positive column of a DC dis-charge.161 The 12^9 band head is visible near 4830 cm1. Reproduced from ref. 161, withpermission.



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used a water-cooled steel cathode to record new spectra of the A

2

Pu^X

2

S

g(Meinel) system of N2

. The Meinel system of N2 is commonly seen in the spectra

of aurora (see below). A composite wall (SiC/Cu) hollow cathode171 was usedto record the rst spectra of SiC175,176 (d 1S^b 1P, A 3P^X 3P), SiC is isovalentwith C2 but was produced by sputtering rather than evaporation.

Infrared electronic transitions of Rydberg molecules such as He2,177 XeH178 and

H3169 can also be seen in hollow cathodes. Rydberg transitions (up to 6 h35 g)

of H2 and D2 are also prominent179^182 and the ionization potential of D2 was deter-

mined by extrapolation180 to be 124 745.353 cm1.Transition-metal-containing molecules also commonly possess infrared electronic

transitions. The presence of an open d shell leads to a large number of low-lyingelectronic states that are conveniently studied by infrared emission. Metalmonoxides are made by adding a trace of O2 to the carrier gas (Ne or Ar, commonly)and the metal is sputtered from the cathode surface, e.g., for HfO,183 NiO,184 PtO,185

CoO,186 CuO,187 and AgO.188 For metal mononitrides (ScN,189 YN,190 HfN,191

Fig. 16 Liquid-nitrogen-cooled hollow cathode that was used by Herzberg et al.169 to study aninfrared electronic transition of the Rydberg molecule, H3. Notice that the positive column(anode glow) could also be observed for comparison purposes. Ions and Rydberg moleculesare more abundant in the cathode region.



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RuN192 and OsN193) or monohydrides (ScH,194 YH,195 LaH196 and PtH197) a traceof N2 or H2 is effective, but monohalides are better made in other ways (furnacesand microwave discharges) because the hollow cathode discharge is usually unstable.All of the metal mononitrides listed above are new molecules and a typical spectrum

is illustrated for YN190 in Fig. 17.Vibration^rotation emission spectra are more difcult to measure than electronic

transitions because they are generally weaker and are at lower wavenumbers. Indeed,no emission spectrum of a transient molecule has been detected using a hollowcathode lamp below the InSb cut-off of 1800 cm1. The protonated rare gases,NeH,198 ArH,199,200 KrH,200 and XeH,201 however, give excellent spectra froma discharge of a rare gas plus a trace of H2. HeH

is too weak and is best detected bylaser spectroscopy. The vibration^rotation emission spectrum for CuH202 wasinadvertently detected during the course of experiments on NeH and remains

the only metal hydride detected in this way. Finally, a special high current hollowcathode discharge was made to detect the H3

and D3 ions in emission.203^206 This

work used the clever trick of pressure labelling to distinguish H3 emission from

that of H2. Experimentally it was found that H3 lines were prominent at 50 Torr

of total pressure but nearly absent at 8 Torr. The interfering H2 lines, on the otherhand, tended to decrease in intensity as the pressure increased. H3

emission canbe seen in the spectra of gas giant planets as discussed below.

13 Radio-frequency and microwave discharges

Radio-frequency (RF) and microwave discharges are similar (but not identical)sources of excited molecules. RF discharges generally operate at 27 MHz andmicrowave discharges at 2450 MHz to avoid interference with communications

Fig. 17 The 1^1 band of the A 1S^X 1S electronic transition of YN.190



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users. The main advantage of these sources is that they can operate with electrodesexternal to the discharge tube. Although they can therefore work with very ``dirty''precursor molecules such as hydrocarbons or metal halides, the build-up of depositsinside the tube does change the power coupling into the discharge cell. By switchingthe microwave source on and off, time-resolved spectra can be recorded.207

The groups of Guelachvili and Vervloet are particularly fond of RF discharges. Aschematic of the reactor of Chollet et al.208 is provided in Fig. 18 and some spectrafrom this source in Fig. 19. An RF discharge was used to record the vibronic bandsof the A 2P^ ~ 2S transition209 of C2H and the c

3P^b 3S transition210 of

CO. Vibration^rotation emission spectra of HNO(n1),

211

NH2(2n2),

212

HNSi(n1),213,214 HCN(n1),

215 HNC(n1),215 NH,216 SiH217,218 and SH219 were all

recorded with an RF plasma reactor (Fig. 18). The HNSi is a new transient moleculerelated to HCN, but with the H bonded to the N atom, and was recorded along withthe A 2P^X 2S spectrum of SiN220 from a SiH4 plus N2 mixture.

A microwave discharge tube has also provided spectra of SH221 and NH36/ND222

as well as the BH,223 PH222,224 and CH225,226 molecules. Very recently the spectrumof SeH was recorded227 and this molecule was the last main group monohydrideto be detected by vibration^rotation spectroscopy. The NH work includes a direct

comparison of the performance of the cryogenic echelle spectrograph, Phoenix, witha Fourier transform spectrometer.36

There is another common RF discharge called the inductively coupled plasma(ICP) source that is used largely for analytical chemistry. The ICP is generallyoperated at atmospheric pressure with a ow of argon gas. The discharge operatesat 27 MHz with 1^2 kW of power to achieve plasma temperatures of about

Fig. 18 The 3 m long radio-frequency plasma reactor described by Chollet et al.208 withWhite-type mirrors to collect more emission. Note that a simple loop of wire around a quartztube, as in the work of Vervloet or in the ICP source, also makes a satisfactory RF emissioncell. Reproduced from ref. 208, with permission.



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5000^6000 K.228 The goal is to dissociate all molecules into atoms, which are thenmonitored by emission spectroscopy or mass spectrometry. The ICP plasma, unlike``normal'' RF and microwave discharges that are operated at low pressure and*100 W of power, is a prolic source of ions. The ICP has, however, been occasion-ally used for molecular spectroscopy. The infrared emission spectra of OH229 andOD230 have been recorded with an ICP.

The microwave discharge is a particularly useful source for the infrared emission

of non-metal diatomics and a few triatomics. The precursor molecules are usuallyintroduced as gases. Molecules studied in this way include BN,231 BF,232 C2,233^236

C3,237 CN,238 CP,239,240 SiF,241,242 NO,243^245 N2,

246^250 P2251,252 and O2.

253 Of par-ticular note are the Rydberg^Rydberg transitions of NO and SiF, and the discoveryof two new infrared electronic transitions236 of C2 (B

1DgÂ1Pu, BH

1SgÂ1Pu)

and a triplet^triplet transition237 of C3.The electronic emission spectra of metal oxides, suldes and halides are readily

produced. The secret is to nd a relatively volatile precursor molecule (e.g.,organometallics or halides) to aid in the introduction of the metal into the plasma.

For example, FeO can be made by reacting a mixture of ferrocene, argon and oxygenin the discharge region.254 Early work in this area was carried out, for example, byMerer and co-workers. Molecules studied in this way are TiO,255 VO,256 CrO,257

NbO,258 AlO,259 ZrS,260 HfS,261 TaS,262 TiCl,263 ZrCl264,265 and YI.266 Aninteresting example is the discovery of the bH 3Pâ 3D system of ZrS by Jonssonand Lindgren.260 The ZrS emission was detected in the 7400^9700 cm1 region from

Fig. 19 A time series of spectra recorded by Chollet et al.208 with various gas mixtures.Carbon-containing impurities in the cell are responsible for CO and HCN. Reproduced fromref. 208, with permission.



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the microwave discharge of a mixture of ZrCl4 and sulfur powders without a carriergas. These new bands of ZrS matched absorption spectra of an unknown moleculein the S-type star, R And.

14 Laser-excited infrared emission

With the exception of some vibration^rotation emission work on acetylene267 andthe HF dimer,28 almost all work has been on laser-excited electronic transitions.The technique was pioneered by Verge s and Amiot, who, along with the Lyon groupin France, continue to dominate the eld. The typical experimental apparatus usedby Amiot268 for Rb2 is displayed in Fig. 20. Metal dimers are made in a heat pipeoven and a laser beam is focussed through a hole in a mirror. The mirror is usedto send the laser-induced uorescence into a high resolution Fourier transformspectrometer. To access highly excited states two lasers can be used. Verge s et al.269

have written a recent review of the technique with examples.

Fig. 20 The apparatus used by Amiot268 to record the infrared laser-induced uorescence ofK2. The l meter is a wavemeter and F.P. is a Fabry^Perot etalon that are used to monitorthe laser wavelength. Reproduced from ref. 268, with permission.

Annu. Rep. Prog. Chem., Sect. C, 2000, 96, 177^224 20 5


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The heat pipe oven has allowed numerous electronic states of all of the alkalidimers, Li2,

270^274 Na2,275^277 K2,

278^281 Rb2,282^287 andCs2,

288^292 (except the radio-active Fr2), to be studied. The ground states have been followed nearly to dis-sociation and highly reliable dissociation energies have been extracted. Fig. 21shows a typical long vibrational progression in the A 1Su state of

6Li2. In this exper-iment272 the F 1Sg and E

1Sg states are populated by optical^optical doubleresonance spectroscopy. The uorescence could be observed to the last few boundlevels of the A 1Sg state

272 and the dissociation energy De was found to be8517.03 cm1 for the ground state of 6Li2. The technique has also been applied

to the heteronuclear dimers, LiNa,293 NaK294,295 and RbCs.296,297 The problem withusing a heat pipe oven for these molecules is the difference in vapour pressures of thealkali metals. A heat pipe oven can thus make LiNa but LiCs is very difcult.

The original work on FTS detection of laser-induced uorescence was carried outon I2. The B

3P0u^X1Sg transition was excited with an argon ion laser and a long

ground state progression in the visible and near-infrared was seen.298 Similar exper-iments were carried out for Te2,

299 CsH300 and Bi2.301 A classic experiment was car-

ried out by Vervloet302 who made NH2 in an RF discharge and excited variouslevels with a dye laser. The bending levels of NH2 could be assigned up to u2

10 and clearly showed the re-ordering of the K levels at the barrier to linearity.The heat pipe oven was also used in a series of experiments on the alkaline earth

monohalides, CaF,303,304 BaF,305,306 BaCl307^310 and BaI.311 The monohalides weremade by heating a mixture of the metal and the metal dihalide to about 1000 C,

Fig. 21 The laser-induced uorescence of the F 1Sg 3 A1Su and E

1Sg 3 A1Su

transitions of 6Li2.272 Fluorescence to the last few bound vibrational levels of the A state

is seen on the left. Reproduced from ref. 272, with permission.



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e.g.,

f fpP 3 Pfp 31

For CaF, BaF and BaCl, the BH 2D states were located by laser excitation of the C 2P2 X 2S transition and Fourier transform emission spectroscopy of the C 2P 3 BH

2D infrared transitions. For BaF and BaCl extensive measurements were madeon all of the low-lying electronic states.

Laser-induced uorescence can also be measured with the time-resolved FTStechnique. Typically a pulsed ultraviolet laser dissociates a molecule such as HCCHand the infrared emission from a product such as C2H is detected,

312

gPrP 3hn

gPr r 32

These experiments are carried out mainly to investigate photodissociation dynamics

although they could be adapted for high resolution spectroscopy.

15 Furnaces

Like the hollow cathode and arc discharges, the furnace is a traditional source ofmolecular spectra. When Fourier transform spectrometers became available itwas natural to extend emission spectroscopy of electronic transitions into the

infrared. (The arc discharge is difcult to use in the infrared because sourceuctuations are a problem for FTSs.) The early work on the Ballik^Ramsay systemof C2 in a King furnace has already been cited

31 and similar spectra of Si2 (d1Sg^b

1Pu) have been measured.313 Thermal emission spectra of the new molecule

BaLi314,315 were measured through two near-infrared electronic transitions (2)2S^X 2S and (2) 2P^X 2S. A similar heat pipe oven source was used for Bi2

316

and the forbidden BH 2D^X 2S transition of BaH.317 Other molecules studied inthis way are MnH,318 MnCl,319 CrF,320 CrCl,321 CoF,322 ScF,323,324 ScCl,325,326

ScI,327 LaF,328 LaCl,329 LaS,330 TiS,331 CuS332 and AlS.333 Most notable here is

the analysis of the complex B6

P^X6

S

transition of CrF by Wallin et al.320

CrF was made by mixing Cr powder with CrF3 and heating the mixture to 1500 Kin a ceramic furnace.

The most interesting development, however, is the adaptation of thermal emissionfor rotational and vibration^rotation spectroscopy. The method works for stablemolecules such as HF,334,335 HCl,335,336 HBr,337 DCN338 and H2O.

339^347 ForHF and HCl, pure rotational emission was also measured.334,335 The H2O emissionspectra allowed the identication of hot H2O absorption in sunspots (``water onthe Sun'')339,340 (Fig. 22). Water emission spectra have now been assigned from

400 to 6000 cm

1

for pure rotational as well as vibration^rotation emission. Thenumber of known energy levels of water was more than doubled in this work.339^347

The very complex spectra of hot water led to the use of a direct variational predictionof energy levels from a state-of-the-art ab initio surface in order to assign quantumnumbers.340 A King furnace was used for the vibration^rotation emission spectraof CN,348 AlH,349 SH,350 CS,351,352 CuH,353 AgH,353 and AuH.353



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Early work (1989) in vibration^rotation emission spectroscopy below the1800 cm1 cut-off of the InSb detector was carried out on GeS by Uehara et al.,354

in addition to the work on FO.37 This work was followed by the SiS emissionmeasurements by Frum et al.355 in 1990. This spectacular emission measurement(Fig. 23) was recorded inadvertently and convinced the author of this review ofthe power and utility of the technique. Similar work on SiO356 and GeO357 followed.

In the case of SiO,356

the laboratory emission measurements at 1400

C were com-bined with SiO absorption measurements in a sunspot at 3200 K. The laboratorySiO data were found by accident because of the reaction of molten gallium witha mullite (2Al2O3SiO2) ceramic tube.

Vibration^rotation emission spectra of the metal hydrides LiH,358 CaH,359

SrH,360 BaH,361 AlH,362 GaH,363 InH364 and BiH365 were recorded using ceramic(alumina or mullite) furnaces. All of the above molecules were formed by thereaction of molten metals with H2 gas (or D2, for deuterides) at high temperature.Interestingly, the spontaneous reactions of Ca and Sr with H2 did not occur,

360,361

presumably because of a large energy barrier to reaction. An electrical dischargewas then used to promote the reaction (Fig. 24) in the furnace. The pure rotationalemission spectrum of LiH and LiD (Fig. 25) was also recorded in the far-infraredregion.358 Clearly infrared emission spectroscopy has the sensitivity to detect tran-sient molecules even at very long wavelengths. Metal halides also give excellent

Fig. 22 The absorption spectrum of a sunspot (*3000 K) is on top and the laboratory emissionspectrum of hot water (*1800 K) is on the bottom.347 All of the strong lines have been assigned

and the asterisks mark lines seen in the laboratory but not in the sunspot.



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spectra, as illustrated by LiF,366 NaF,367 KF,368 MgF,369 CaF,370 SrF,371 BaF,372

BF,373 AlF,373,374 GaF,375 InF,376,377 TlF,378 NaCl,379 KCl,379,380 AlCl,381 NaBr,382

AlBr,383 LiI,384 CsI385 and BeF2.386 The CsI molecule gave a spectrum near 100 cm1

but only the vibration^rotation band heads could be measured.385 The BeF2antisymmetric stretching mode and associated hot bands near 1500 cm1 allowedan equilibrium Be^F bond length of 1.373 to be determined.386

16 Atmospheric science and remote sensing

Emission spectroscopy is relatively common in atmospheric science but the term hasa peculiar meaning in this area. For example, the infrared radiance of the Earth canbe detected from an aircraft or a satellite viewing in the nadir direction. This is often

Fig. 23 Vibration^rotation thermal emission spectrum of SiS355 observed at 13 mm obtainedfrom a mixture of Si and SiS2 powders at 1300 K.

Fig. 24 Schematic of the furnace used to record the vibration^rotation emission spectrum ofCaH359 by discharging a mixture of Ca vapour and H2.

Annu. Rep. Prog. Chem., Sect. C, 2000, 96, 177^224 20 9


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called an ``emission'' spectrum although the atmospheric molecules of interestappear in absorption against the blackbody radiance of the Earth. This review,however, will adopt the usual denition of emission spectroscopy.

The most spectacular infrared emission spectra of our atmosphere are recorded inthe far-infrared region. Typically, a high resolution Fourier transform spectrometerin a balloon records stratospheric spectra by looking out at the limb of the Earth.An example of such a spectrum is presented as Fig. 26. The groups of Carli387^389

in Florence and Traub390 at the Smithsonian Astrophysics Observatory recordfar-infrared spectra of this type. Specic molecules such as OH can be monitored

with a simpler system based on Fabry^Perot etalons, but nothing beats a Fouriertransform spectrometer for wide spectral coverage.389 Mid-infrared emission exper-iments of the limb of the Earth will be made with two satellite instruments,MIPAS391 (Michelson Interferometer for Passive Atmospheric Sounding) andTES392 (Tropospheric Emission Sounder). The difculty with these measurementsis the weak radiance emitted by the atmospheric molecules at 200^300 K. Thegeometry is as drawn in Fig. 3, but the radiance of the end wall is very small becausethe view is to deep space in the limb geometry. The goal in these experiments is tounderstand the chemistry, particularly ozone chemistry, in our upper atmosphere.

Emission measurements of our atmosphere can also be made from the ground witha Fourier transform spectrometer. A typical spectrum (Fig. 27) recorded by Evansand Puckrin shows both emission and absorption lines.393

Highly excited infrared emission from our atmosphere can be recorded fromairglow (nightglow and dayglow) as well as from aurora. Prominent infrared airglowemission394,395 is from the a 1Dg^X

3Sg system of O2 and the Meinel system of OH.

Fig. 25 Pure rotational lines of a mixture of6LiH, 6LiD, 7LiH and 7LiD obtained by detectionof far-infrared thermal emission with a Fourier transform spectrometer.358 Reproduced fromref. 358, with permission.



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Fig. 26 Far-infrared emission spectrum of the stratosphere recorded by viewing the limb of theEarth from a balloon.26 Notice the excellent signal-to-noise ratio for the OH lines obtained witha Fourier transform spectrometer. Reproduced from ref. 26, with permission.

Fig. 27 Low resolution infrared emission spectrum of the zenith sky in winter.393 Most of thelines appear in absorption but some are in emission. Reproduced from ref. 393, with the per-mission of the American Meteorological Society.



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The OH emission is prominent at night from the mesopause region at 85^90 km andthis emission can be used to monitor atmospheric gravity waves.395 Infrared auroralemissions include the Meinel system396 of N2

.The remote sensing of forest res by infrared emission spectroscopy has already

been discussed in the section on ames.134 Other types of infrared emission remote

sensing are also possible. For example, the emission of stack gas has been monitoredat low resolution with a portable Fourier transform spectrometer.397 A number ofrecent emission spectra of volcanic plumes have also been recorded.398,399 Loveet al.399 noted that the ratio of SiF4/SO2 seems to decrease just before a volcaniceruption. There are also military applications for the remote sensing of rocket plumesand other engine exhaust in the infrared.

17 Matrices

Free radicals and other transient molecules can be trapped in rare gas matrices.Upon excitation, generally with a laser, the molecule will sometimes emit radiationin the infrared. Two reviews by Bondybey et al.400,401 have covered both thespectroscopy and photophysics of recent matrix work. Electronic emission of thePhillips and Ballik^Ramsay systems of C2

402 as well as the BN,403 BC404 andCuCl2

405 molecules were seen in the Bondybey group. Vibrational emission fromWO406 and ND407 was measured, and ND displayed nearly unperturbed rotationalstructure.

18 Liquids

Infrared emission spectroscopy of liquids has been carried out for thin lms ofmolten salts.15 For example, Bates and Boyd studied nitrate and chlorate melts.408

Strong bands show some distortions but the use of a thick sample as a referencealleviates this problem.15 Infrared emission of molecules in solution is also possibleand Ogilby and co-workers, for example, have studied O2(

1Dg) emission in varioussolvents.409

19 Surfaces and solids

The presence of thin lm, even monolayer, surfaces can be readily monitored byemission spectroscopy. As already mentioned, the best spectra are obtained withthin samples of a few micrometers in thickness. Some band distortions are often

noticeable due to self-absorption

410

etc., but the use of a thick sample as a referenceeliminates the problem.15 Sullivan et al.411 have reviewed the use of emissionspectroscopy for surface analysis. A wide variety of lms have been studied, includ-ing CO, metal oxides and organic molecules on a variety of substrates includingmetals, non-metals such as Si, metal oxides and zeolites. A typical application wouldbe the study of a chemical reaction412 (e.g., NO NH3 on a catalyst surface, such as



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V2O5). Emission spectroscopy has been used to monitor thin lms formed on siliconwafers as part of the fabrication of semiconductor devices.413 The emission of solidparticles in combustion has proved to be a useful diagnostic.24,25

The spectra of bulk solids are most easily obtained by grinding the sample to apowder and sprinkling it on a heated metal substrate.414,415 Photoluminescence

of direct band gap semiconductors like GaAs is an important tool in solid statephysics.416 Impurities in solids, e.g., ZnO:Cu2 (some of these materials can lase),can also be studied through their emission.417 Time-resolved FTS spectroscopyhas also been applied to study the emission of solids, particularly for solid statelaser media.418 An interesting variation on the technique is the use of a laser to heata thin surface layer of a solid in order to record emission.419

20 Astronomy

Most astronomical infrared spectra appear in absorption against a bright continuumdue to a star or dust. Some sources, however, such as comets, provide spectacularinfrared emission spectra. Solar radiation evaporates the ``icy snowball'' as it nearsthe Sun. The hot evaporated molecules and their photolysis products appear in emi-ssion against the dark sky. Infrared electronic spectra of C2 and CN are readilyseen420 and more recently vibration^rotation lines of many molecules were foundin the bright comets Hale-Bopp and Hyakutake.421

In a recent review Crovisier421 has summarized the species detected throughvibration^rotation transitions so far and they include H2O, CO, CO2, CH4, C2H2,C2H, CH3OH, HCN and OCS. To this list can be added CH.

422

Fig. 28 The spectrum of Jupiter's north tropical zone showing stratospheric emission from then9 band of ethane on the left.

427 Reprinted, with permission, from ref. 427,# 1984 by AnnualReviews www.AnnualReviews.org



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Most of these new molecules were detected with cryogenic echelle spectrographssuch as Phoenix422 and a few observations were made from space with the InfraredSpace Observatory, ISO.423 A particular surprise was the unexpectedly high abun-dance of ethane, C2H6, in Hyakutake.

424

In our solar system, atomic and molecular features in the Sun and sunspots appearin absorption although a few high-l Rydberg transitions of Mg and other atomsappear in emission near 1000 cm1. The lines have very large Zeeman effectsand can be used to study solar magnetic elds.425 Spectra of most of the planets,satellites and asteroids are also absorption spectra, because the features are viewedagainst the planetary blackbody emission. Typical spectra of rocks and gaseous

Fig. 29 The emission of the n2 fundamental band of H3 in the aurora of Uranus and Jupiter.428

Reproduced from ref. 428, with permission.



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CO2 were recorded by the Thermal Emission Spectrometer (TES) on Mars.426 As in

atmospheric science, the word ``emission'' is not always used in the same senseas in this review. Some of the giant planets, however, display some interesting emi-ssion spectra.427 For example, the n9 band of C2H6 appears in emission in the strato-sphere of Jupiter427 (Fig. 28). The most beautiful spectra, however, are those of H3

in the aurora of Jupiter428,429

(Fig. 29). Saturn430

and Uranus431

also display H3

emission. On Jupiter (and to a lesser extent on Saturn and Uranus)431 the H3

is formed by the interaction of the solar wind with the planetary magnetic eldand emission is, therefore, localized near the magnetic poles. H3

has become animportant tool for the study of Jupiter's atmosphere.428

The three most important molecules in astronomy are H2, CO and H2O. Theforbidden H2 vibration^rotation and pure rotational lines can be detected in shockedregions.21,432 H2, OH , H2O and CO emission can be observed in supernovaremnants.432,433 H2 emission is even detectable in starburst and Seyfert galaxies.

434

Star-forming regions in the Orion nebula also show H2,21

H2O435

and CO436

ininfrared emission. Circumstellar envelopes can also show infrared emission as seenfor CO in the carbon star IRC10216437 and H2O in the Mira variable (red giant),R. Cas.438

Planetary nebulae also show very strong emission spectra of mainly atoms and aset of broad features that have been assigned to gaseous PAHs.18,439^441 The ideais that PAHs absorb ultraviolet radiation and after internal conversion emit inthe infrared.439,440 This PAH assignment, however, is not universally accepted.An alternative assignment is a carbonaceous solid441 and even a giant

molecule/small particle is quite possible.

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P. F. Bernath- Infrared emission spectroscopy