High-information infrared spectroscopy of unstable molecules

Vol. 3, No. 5/May 1986/J. Opt. Soc. Am. B 687

High-information infrared spectroscopy of unstablemolecules

P. Chollet, G. Guelachvili, and M. Morillon-Chapey

Laboratoire d'Infrarouge, Associ6 au Centre National de la Recherche Scientifique, Universit6 de Paris-Sud,Bbtiment 350, 91405 Orsay C6dex, France

P. Gressier and J. P. M. Schmitt

Ecole Polytechnique, Route de Saclay, 91120 Palaiseau, France

Received July 30, 1985; accepted November 6, 1985A new experimental setup for the production and the spectroscopic observation of unstable molecules is described.The species are created in a 3-m-long plasma reactor, which permits rf excitation of a large volume of flowing gas.Two complementary spectrometers, a high-resolution diode-laser spectrometer and Fourier-transform spectrome-ter, are optically coupled to the multipass system located inside the reactor. They may work simultaneously.Actual infrared spectra of the radicals SiH and NH and their comparison with the best available laser results arepresented to illustrate the capabilities of the device. Observations of highly excited levels of stable molecules arealso possible. This is demonstrated by SiH4, N2, HCN, and CO spectra and by the observation of lines due tomolecular-hydrogen Rydberg transitions between high-i states of the Rydberg electron.

INTRODUCTION

Unstable molecules have long been a topic of great interest.'Their very nature is of fundamental interest. As intermedi-ate species in chemical reactions their role is essential, andthey are of great importance in many industrial processes.With the advent of new tools such as the laser spectrometerand the Fourier-transform spectrometer, and with recentprogress in both theoretical and numerical techniques, thespectroscopic study of these species is currently blossom-ing.2-20

High concentrations of unstable molecules in the labora-tory are generally difficult to obtain. This is mainly due totheir short lifetime relative to their high chemical reactivity.In an inert-gas matrix of small volume, one can reach 1014molecules cm- 3. This concentration for the gas phase isdecreased by several orders of magnitude. Generally, theunstable species are observed through emission processes inwhich the intensity is proportional to U4. Therefore it isunderstandable that the majority of spectroscopic studieshave been performed in the visible.

However, infrared studies are attractive for several rea-sons. First, the infrared is the spectral range of the vibra-tion-rotation transitions, which occur mainly in the funda-mental electronic X level. The emission contains informa-tion on the population of the upper level, so themeasurement of the global concentration in the X level is adirect possibility. Also, the recent development of tunablelasers in the infrared provides a new and efficient way ofprobing and monitoring gas phases such as plasmas. This ismade possible through accurately calibrated measurementsof spectral lines. Yet another interest in the infrared stemsfrom the fact that it is the range of the transitions betweenhighly excited electronic levels. Often the unstable speciesresult from collisions between electrons and stable mole-

cules that dissociate. The fragments are created with theirexcess energy dissipated by collisional processes or radia-tion. Infrared studies are then helpful in understanding thedissociation, formation, and recombination mechanisms.More generally, infrared research complements that in thevisible and microwave ranges. Finally, as has been clearlydemonstrated during these two past decades by the study ofstable molecules, much more research on unstable speciesmay still be done in the infrared. From this point of view itis worth noting that the total number of diatomic and tri-atomic radicals on which infrared results have been obtainedis currently smaller than the number of elements of theMendeleev table.

The aim of the research reported in this paper was toconstruct an experimental device designed to produce un-stable species efficiently and to observe their spectra, pri-marily in the infrared. A description of the experimentalsetup is given first. In the second part of this paper, somespectra obtained from silane and nitrogen + hydrogen plas-mas are presented to illustrate the actual capabilities of thedevice.

EXPERIMENT

Several guiding assumptions have been present since thebeginning of this research. First it was supposed to bedifficult to construct a satisfactory source of unstable spe-cies. Consequently, it was desired to be able to record theemitted spectrum efficiently and as completely as possible.A Fourier-transform spectrometer fulfills this requirement.

Another assumption was that the optimal concentrationof the desired unstable molecules could be achieved onlythrough the methodical process of adjusting all the relevantparameters. For this purpose a diode laser is used in theexperimental setup. It has two main functions. First, it

0740-3224/86/050687-09$02.00 © 1986 Optical Society of America

Chollet et al.

688 J. Opt. Soc. Am. B/Vol. 3, No. 5/May 1986

GAS GASSUPPLY EXHAUST

Fig. 1. The plasma reactor can be simultaneously observed by thediode-laser and Fourier-transform (F.T.S.) spectrometers. Flow-ing gas mixtures are rf excited.

helps in the search for the best conditions for the source byvarying the intensity of one particular transition of the mole-cule of interest. Second, it allows the stability of this sourceto be monitored while the entire spectrum is completelyrecorded with the Fourier-transform spectrometer.

The third assumption is directly concerned with the na-ture of the source. The choice of the low-pressure gas phasewas due to the need for high-resolution spectra. This isquite helpful for the unambiguous identification of the spe-cies and also for the determination of the spectroscopic as-signments. On the other hand, the high resolution permitsthe observation of the multiplet structure of the lines. Thismeans a decrease of the intensity of each transition andconsequently a decrease of the sensitivity of the detection.The choice of a rf-excited plasma system was mainly dictat-ed by the need for a source stable for several hours. The rfability to excite large volumes was another strong argumentin favor of this type of excitation.

The organization of the relative components of the experi-mental setup is schematically given in Fig. 1. The species isproduced in a rf-excited plasma from the initial collisionsbetween electrons and stable molecules, which are providedby a flowing system. The diode-laser and the Fourier-trans-form spectrometers can work simultaneously.

Plasma ReactorThe plasma reactor is a 3-m-long stainless-steel cylinderwith a diameter of 22 cm (Fig. 2). The rf antenna is locatedin its central part. The antenna is made of two 1-m-longcoaxial electrodes having rectangular sections (external di-mensions 13.5 cm X 11 cm). The voltage is applied to theinner electrode. The outer electrode is maintained at earthpotential. This arrangement, together with the metallicnature of the reactor walls, greatly helps to reduce the para-sitic rf emission from inside and outside the laboratory to asatisfactory level. Otherwise, the operation of the two spec-trometers would be impaired. The excited volume is of theorder of 15 liters. The volume expands beyond the ends ofthe antenna for a visually estimated total length of about 1.5in. The gas mixture flows into the reactor in its central part.The gas is evacuated at both ends. Exhausted gas is burned.Until now the flow rate had been limited to about 50 stan-dard cubic centimeters per minute (S.C.C.M.) because ofthe small cross section of the exhaust tube (20 cm2) and of

the flow rate of the primary pumps (30 m3/h). 21 The rfsupply is a Heathkit 2-kW amateur-radio system. Imped-ance matching is obtained with adjustable capacitors.Available power is approximately 1100 W. Only 5% of thispower is reflected back to the rf amplifier when it is operat-ing at about 7 MHz. Actual pressures used in several plas-mas were of the order of 0.1 Torr. Under such conditionsthere is almost no pressure variation along the axis of thereactor, where the flow is both laminar and viscous.

Optical ArrangementThe plasma reactor was optically designed to match theusual 6tendue of the interferometer of the Laboratoire d'In-frarouge.2 2 The excited volume was made as nearly identi-cal as possible to the optical volume of interest. This opti-mizes the efficiency of the excitation, as the rf energy densityis confined to a minimal volume. The optical path can beincreased with a White-type multireflection system. Threespherical mirrors, Ml, M2, and M3, each of focal length 135cm, are located inside the cylinder. Ml and M2 can beadjusted under vacuum. Observation of the inner part of thereactor is permitted through appropriate windows. Themaximum optical path actually obtained with the laserbeam was about 150 m, corresponding to a plasma column50-80 m long. The diode-laser beam may propagate intothe plasma through the multireflection system shown in Fig.2. The plasma emission recorded with the Fourier interfer-ometer is spatially shifted. These two relative beam loca-tions correspond to a vertical shift of the series of images onmirror M3. The diode-laser beam may also probe the activemedium entering the opposite side of M3. In this case thecopper-doped germanium detector is located near Ml andM2, and the total absorbing path is 5.4 m (plasma -3 in). Insuch a configuration the Fourier spectrometer can recordabsorption spectra from the plasma of a white source thattakes the place of the laser-beam detector.

Recording ProceduresUse of a diode-laser spectrometer2 3 was found to be quitehelpful. It complements the Fourier-transform interferom-eter almost perfectly. This spectrometer provides immedi-ate information on the efficiency of the production of theunstable species in the plasma. Adjustments of variousparameters such as pressure, rf excitation power, impedancematching, flow rate, and gas-mixture ratios were then much

c L

Pump Ga supply Pump

Fig. 2. Plasma reactor. The diode-laser beam and the plasmaemission are represented by dotted-dashed and solid lines, respectively.

Chollet et al.


easier to perform. Parameters were adjusted to improveintensity of a transition of the molecules under study. Inthe case of SiH, which is barely observable in the infrared,this procedure saved a significant amount of time. Thewave-number tunability and the narrow profile of the laseremission were particularly appropriate for the investigationof the spectra composed of numerous and narrow transi-tions. Modulating the frequency of the diode emission istrivial to do. This permits first- and second-derivativetypes of detection and provides a higher sensitivity. It isthen also easy to servo the laser emission frequency to anygiven observable transition.2 4 25 Thus the diode laser is cer-tainly an efficient tool for the convenient investigation, con-trol, and monitoring of excited volumes such as a plasma.

Whatever the recording mode may be (fast scanning orstep by step), a long time is required to generate a high-information spectrum with a Fourier-transform interferom-eter. On the other hand, once the spectrum is obtained, it isunambiguous and correctly calibrated. This is another as-pect of the complementariness of the diode and Fouriertechniques. The diode-laser spectrometer takes great ad-vantage of accurate preliminary knowledge of the spectrallandscape provided by adequate reference spectra.2 6 This-facilitates the simple control of the competition among sev-eral possible modes and the easy spectral location of thediode emission. To use reference spectra, a 1.5-m-long cellis inserted between the diode-laser spectrometer and theplasma reactor. Finer interpolation between the calibratinglines in the diode-laser spectra is made possible with aFabry-Perot talon with a fringe spacing equal to 50 X 10-3cm-1

.

RESULTS

Possible results are illustrated by low- and high-resolutioninfrared spectra of a SiH 4 plasma and a N2 + H2 plasma.These spectra were recorded in the frame of spectroscopicstudies of the radicals SiH (Ref. 27) and NH.28 These twoneutral unstable species have different electronic funda-mental levels, 2 and 3, respectively, which characterizemany radicals. Both species are used in industrial process-es, namely, the deposition of amorphous silicon and thenitriding of metals, respectively.2 9 30

Silane PlasmaThe first spectroscopic check performed on the reactor wasto examine the breaking efficiency for silane molecules.This was done by measuring the intensity variations of asilane transition located at 1855.591 cm-' with the diode-laser beam frequency locked 31 onto this transition, whichbelongs to the 2 + 4 band in interaction with 2 2 and 2 4.

32

The concentration variation is illustrated by the lower traceof Fig. 3, which corresponds to three different periods sepa-rated by times at which the plasma is switched on (t = 0) andoff (t = 10 sec). During this time the flow rate remainedconstant.

The dramatic effect of frequency locking the laser is evi-dent from a comparison of the two traces of Fig. 3. They arerecorded under the same conditions, except that for theupper trace the laser is tuned to the same silane line positionand then is left unlocked. When the plasma is switched on,the signal of the lower trace decreases monotonically. In

contrast, the upper trace shows erratic fluctuations. Thesefluctuations are due to the rf parasitic flux, which was in-tense in the present case and rendered all measurementsimpossible.33 When the plasma is switched off, the signal ofthe lower trace again varies regularly, and the unlockeddiode emission frequency returns to its initial value andoscillates around the silane line, which is swept four times onthe figure.

The interpretation2 5 of the lower trace of Fig. 3 is thatabout 90% of SiH 4 molecules are broken in the permanentworking regime of the reactor. This rate takes into accountthe recombination processes that give back some SiH4. Atypical feature, which was not understood, is the rapid varia-tion of the SiH 4 concentration at the beginning of the dis-charge. This feature was found to be a systematic one andcould be correlated to the transitory peak in the SiH radicalconcentration. This peak is observed. also when the plasmais established and corresponds to about three times the con-centration in the stable regime. The total pressure in thereactor is higher by about 50% when the plasma is estab-lished. This is an obvious indication of the increase in thetotal number of molecules.

The infrared detection of SiH in a SiH 4 plasma was firstmade at the Laboratoire d'Infrarouge.34 The vibration-rotation bands 1-0 and 2-1 in the electronic fundamentallevel 2II were measured with a moderate signal-to-noise ratioaround 5 um. Line positions were then available for thetests on this species in the present experiment with thediode-laser beam. The P(6.5) line of the 1-0 transition at1872.35 cm-' was initially chosen for the detection of SiH.Effectively, it is among the strongest transitions. The dou-bling that is due to the electronic interactions that separatethe lines into two components just vanishes at this J = 6.5

LASERUNLOCKI

LASE I \ P1tO9mt,rr

LOCKED 9

|L \'P=151mtorr/

PLASMA!

OFF 0 OFF TIMEOs 1s

Fig. 3. Lower trace: variation of the concentration of SiH4 ob-served with the diode-laser beam frequency-locked to the SiH 4transition in a particularly strong rf parasitic environment. Thetime axis is not located at the zero-intensity level. P is the totalpressure in the reactor. Upper trace: the same but with the laserunlocked.

Chollet et al.


' (7,5)

10

t +0.026cm-'

5_ 1

1855.600-cm7'

ON J '/Vv \ A A (Y 4

052 cm

ON

OFF SIH ~~

PLASMA

1838.400 c:m'

Fig. 4. Detection with the diode laser of SiH in its electronic funda-mental level. The plasma-absorption path L and plasma pressure Pare different for each series of spectra made at different times. Forthe P1(7.5) spectra, L = 44 m and P = 70 mTorr; for the P1(8.5) L =66 m and P = 40 mTorr. The destruction of silane is of the order of96 and 94%, respectively.

The sensitivity of the detection of SiH under the condi-tions of the experiment, which is reported on the left-handside of Fig. 4, is (6 + 2) X 10+8 molecules cm- 3 . The SiHconcentration, calculated by assuming for the transitionprobabilities A,,,t and for the oscillator strengths fv'," thevalues given in Ref. 35, is indeed (1.7 + 0.5) X1011 moleculescm- 3, and the P1(7.5) signal-to-noise ratio is 1:300. In otherwords, the smallest absorption coefficient detectable undersuch conditions is equal to approximately (1.2 J 0.4) X 10-8cm-1.24

The high-information Fourier-transform spectra were re-corded without the need to monitor the source with thediode laser. In fact, the reactor worked under stable condi-tions in the permanent regime. A low-resolution spectrumof the emission of the silane plasma is given in Fig. 7. Themain feature is the P3 band of SiH 4 with the Q branch locatedat 2189 cm-1 . SiH also clearly appears on the low-wave-number side of the silane transition, particularly the P

2

value, which also approximately corresponds to the maxi-mum of the rotational intensity distribution. Unfortunate-ly, a SiH4 line was found at exactly the same place. Althoughsome confidence could have been provided in the diagnosismade with P1(6.5), which actually served for the first diodedetection of SiH in our silane plasma, the lines P1 (7.5) andP1 (8.5) were used instead to optimize all the parameters ofthe plasma for SiH production.

Figure 4 shows diode-laser beam spectra around 1855.6and 1838.4 cm-1 . The lower traces correspond to silanelines only. When the plasma is switched on (intermediatetrace), the lines decrease considerably and new lines appearthat are better seen on the upper trace, as it is identical tothe intermediate one but vertically multiplied by a factor of10. These new features correspond to the expected SiHlines. They are easily recognized by their doublet structuresand by the spacing between each component, which is twiceas large for J = 8.5 than for J = 7.5. The recordings thatcorrespond to the P1(8.5) line are noisier than those for theP1(7.5) line made later with improved working conditions.Intensities of the SiH lines are certainly strong enough tofrequency lock the diode laser. Variations in the pressure,flow rate, gas mixture (SiH 4 alone, SiH 4 + H2, SiH 4 + N2,SiH 4 + Ar) and the rf power were then made possible bynoting the efficiency of the SiH production on the maximumof the second-derivative signal of the P1(7.5) line. Figure 5shows an example in which the rf power is varied. Thisparameter was found to be the most sensitive. At the timethe test was done, 600 W was the maximum available rfpower, and obviously the saturation in SiH production wasnot obtained. This is shown in Fig. 6, which presents theresults of Fig. 5 differently by plotting the SiH concentrationversus the rf power. Under the particular conditions of thisexperiment (silane only, silane pressure before plasma: 0.1Torr, flow rate 50 S.C.C.M.), the rf power threshold for theappearance of SiH in the reactor is of the order of 230 W.Since the excited volume is about 15 liters, this correspondsto a power density of 1.4 X 10-2 W cm- 3, or 4 X 10-18 W perSiH4 molecule.

1

0

, SiH concentration- (x 10

1mol cm

3)

600W

510W

420W

- 330W

_ 0 5 10 15 20

Fig. 5. Variation of the SiH concentration versus the rf power inthe permanent plasma regime. This recording, made in 20 sec,clearly demonstrates the convenience of the diode laser for diagnos-tics on plasma.

SH concentration- (x10"1Mol crn 3)

1.4 x10, 2Wcrn

3

TIME (sec)

2

I

0 14

r f power(watt)

I I I I

0 6 10 0 200 300 400 500 600

Fig. 6. A different presentation of results from Fig. 6. The cre-ation of SiH under the particular conditions of the experimentbegins for a total rf power of 230 W in the plasma. The correspond-ing power density is given in the figure.

28Si H 1-o 01.6%

A1

>

S - - r

Chollet et al.

plI I .

i


7 Q

P

SiH4

)3 band

R

H

I -~ ~ I I '- ' '_-_1956cm- 2256cm1

Fig. 7. The silane-plasma emission recorded with the Fourier interferometer and partly shown at low resolution. Boxed insert: upper trace,the same doublet from the diode laser as in Fig. 4. Lower trace, very small portion with full resolution of the Fourier-transform spectrum.

branch of the 1-0 band. Its vibrational and rotational tem-peratures are, respectively, of the order of 3200 and 700 K.A narrow window on the spectrum with full resolution isopened on the P(7.5) transition. The lower trace repre-sents the Fourier-transform spectrum of the doublet with anapodized instrumental resolution of about 5.4 X 10-3 Cm-.The wave-number scale may be appreciated by the A dou-bling equal to 26 X 10-3 cm-'. This axis is expanded byapproximately a factor of 2500 when compared with the low-resolution wave-number scale. The upper part of the high-resolution window gives the diode-laser spectrum alreadyshown in Fig. 4. The diode and Fourier-transform record-ings are rather similar. The signal-to-noise ratio and theresolution are better in the diode spectrum.

In the infrared, SiH was recently observed by Brown andRobinson using the laser magnetic resonance (L.M.R.) tech-nique.3 6 Eight lines were measured, and the Q2(1.5) linerecording was published as a typical spectrum. It is repro-duced in the upper trace of Fig. 8, which gives below, on thesame wave-number scale, the same line extracted from theFourier-transform spectrum shown in Fig. 7.

The silane plasma spectra described here were recordedmainly for the study of SiH and to allow for the interpreta-tion of nearly 200 observed lines connecting four vibrationallevels in the 2II electronic ground state.2 7 In addition, theycontain additional information not yet fully exploited.Spectra of other unstable species may be present in theforest of silane lines. For instance, SiH2 could be seenaround 2031 cm-', where some transitions do not seem tobelong to SiH4. Such additional information is relevant forstudies not only of unstable molecules but of stable ones aswell. As we said in our preliminary remarks, transitionsbetween highly excited levels of stable species are also possi-ble to observe. This is true for SiH4 , which must be further

analyzed. Some remarkable broad lines, which we haveobserved throughout the spectra, provide a pertinent illus-tration of these comments.

Figure 9 shows some of these lines. The strongest one,located at about 2468 cm-', is the Ba atomic-hydrogen tran-sition, the first of the Brackett series characterized by theprincipal quantum n = 4. It is asymmetric, and no convinc-ing explanation has been found for this asymmetry. In fact,the fine structure should distort the B profile on the high-wave-number side, as at first had been proposed as a possible

Fig. 8. L.M.R. (see Ref. 36) and Fourier-transform spectra(F.T.S.). The A doubling of the Q2(1.5) line of the 1-0 band of SiHis of the order of 0.016 cm-'.

Si H, 1 - O. P1 (7.5)

DIODELASER

F.T.S.

L.M.R.

02(1.5) V

F.T.S.

1970.6crmf1

v

t w - w - K

1,

Chollet et al.

SiH


H2 _.b] [*.- o.10oCM-

H

0.070 cm-

...*t 0.040 cm

AA v, -i A+ A

RYDBERG TRANSITION5g-4f

1

'7'- 1 .-w * . . . , ~4 . .2_4 6 9

A46 12468 2469

Fig. 9. Atomic- and molecular-hydrogen transitions from the plasma emission. See text for comments on the asymmetry of the Brackett-aatomic line. The molecular-hydrogen lines are connecting states with high I for the Rydberg electron. Unknown features are present at about2469 cm-1.

interpretation. However, the observed shape is not satisfac-torily reproduced within the assumption of a Boltzmannianpopulation of the levels. A possible explanation would bethat these j levels have a non-Boltzmannian distributionthat is due either to the effect of cascades in the atom or topreferential dissociation processes of the parent hydrogenmolecule. 49 It should not be overlooked that this asymme-try could result from an overlapping with a molecular-hy-drogen line. Much narrower (0.040 instead of 0.070 cm-1)lines of Fig. 9 around 2459.5 cm'1 belong to molecular hydro-gen and connect high-i states of the Rydberg electron. Theyhave been identified as 5g-4f transitions by Herzberg andJungen.3 7 The vibration and rotation quanta of the freeionic core H2+ are good quantum numbers because of thealmost complete uncoupling of the orbital angular momen-tum of the electron from the molecular axis. This removespractically any regularity from the observed spectrum. TheRydberg molecular-hydrogen transitions identified in Ref.37 were observed by Herzberg et al.38 in a hollow cathode,with Fourier-transform spectrometers, and much better sig-nal-to-noise ratios than those of Fig. 9. The observationhere, however, gives narrower profiles for both atomic andmolecular hydrogen. This could be due to the lower tem-perature in the reactor. The lines at 2459.5 cm-' were notresolved in Ref. 41. They were predicted as R1(3) v = 0 andR1(3) v = 1, with a separation40 of 0.12 cm-1, which is actual-ly observed as shown in Fig. 9.

These Rydberg spectra of molecular hydrogen were ob-tained as by-products during experiments in which the hy-drogen resulting from dissociation of SiH4 was considered tobe merely troublesome. They were present in all the plasmaemissions including the N2 + H2 plasma described below. Itshould be underlined here that, as stated in Ref. 37, transi-tions between high-i energy levels of the Rydberg electronare only rarely observed in diatomic (or polyatomic) mole-cules, whereas they are fairly common in atoms.

Nitrogen + Hydrogen PlasmaThe nitrogen + hydrogen plasma was generated mainly toinvestigate the NH radical. In addition to the fundamentaland practical interest expressed above, the study of infraredvibration-rotation of NH in its 32; electronic ground stateserved also as a good test for at least two reasons. First, theattempt to generate species other than SiH was a good checkof the versatility of the plasma reactor. Such versatility isan important characteristic that was required of this newsource of unstable molecules. Although it was demonstrat-ed to be an efficient source of the difficult-to-produce SiH,whether it could work properly for another species was notknown. The second reason was that several infrared spectraof NH have already been made by laser39 or Fourier-trans-form techniques.5'40 The high quality of these earlier resultsprovided ready evaluation of the quality of ours.

The results on NH given here were obtained rapidly. Be-cause an appropriate diode was not available, the diode-laserspectrometer was not used. Only mixtures of nitrogen andhydrogen were tried because NH3 was not readily available.A computer on line with the Fourier interferometer checkedin real time that NH was effectively in the reactor. In a fewminutes the calculated spectrum of the plasma emission wasresolved enough to permit a comparison with the spectrumof Sakai et al. in Fig. 9 of Ref. 5 and to show evidence of NH.Variations of pressure and ratio mixing to optimize the NHproduction were also made with the help of the real-timecomputer spectra. As expected, the concentration of NHwas optimum for equal pressures of nitrogen and hydrogen.It was also verified that the total pressure in the reactor wasthe same for the plasma and the nonplasma regimes, unlikethe case of the silane plasma. This indicates that the totalnumber of molecules in the reactor is independent of theworking regimes. The NH vibrational and rotational tem-peratures are found to be of the order of 5000 and 650 K,respectively.

_1

Chollet et al.

a


N 20.1 14torr

50m I I. N

N 2

.IIi

IL

.I?93 I26-9

CON 2 H 2

O.085+O.O85torr 50m EU,~ 11

N 2 " H 20.085+0.085torr

50m

H

Id

CNI HCN

HCN

NH

al ll 1 f Sl!III II - 1,1 llll lIl I]11,1 JI 11

1700

1 701~~1_ F - trial 1,117'W

2300cm1 3300cm-1Fig. 10. Three different plasma emissions seen at identical resolutions (2 cm-') on the same spectral range. The three spectra are recorded se-quentially. Each trace is normalized to the most intense line. Addition of hydrogen totally suppresses the N and N2 features. Carbon and ox-ygen are due to the contamination of the reactor. NH appears clearly on the three traces.

Figure 10 presents the same portion of three differentspectra (numbered 1698, 1700, 1701) drawn with a resolutionof about 2 cm-'. The entire free-spectral range extendsfrom 0 to 4200 cm-'. The actual low-wave-number limit isdue to the blindness of SbIn detectors below 1700 cm-'.The maximum resolutions of the three spectra are 0.5,0.020,and 0.009 cm-', respectively, and the respective recordingtimes are 1.3, 2, and 4.5 h. The three recordings were madesequentially, one immediately after the other. The nitrogenplasma spectrum shows N2 electronic transitions locatedaround 2700 and 3000 cm-', which were assigned tow'A-all and w3A-B 3llg, respectively. 41 42 The atomicspectrum of N is also present around 2400 cm-'. Surpris-ingly, features belonging to CO, NH, and HCN are present inthis spectrum, which should be concerned only with N. TheAv = 1 sequence bands in the ground state of CO (Ref. 43)are located around 2100 cm-'. The same type of transitionof NH is widely and regularly spread around 3000 cm-', andfaint transitions belonging to the 1)3 mode of HCN are gath-ered around 3300 cm-'. In fact, H atoms were not unexpect-ed in the reactor, which had been used before over a longperiod for the silane plasma. Consequently, hydrogenatedamorphous silicon and other aggregates were deposited onthe walls, which had not been cleaned. That carbon andoxygen existed in the plasma source (which was not leaking),was later realized. The main contamination, apart from theimpurities in the gas samples themselves, was a smallamount of grease spoiling the antenna.

With the mixture of nitrogen and hydrogen, the physiog-nomy of the spectrum is radically changed, as is in the middletrace of Fig. 10. Molecular and atomic nitrogen spectrahave completely disappeared. The previous HCN P3 band ismuch more intense and is accompanied by the correspond-ing hot bands. New HCN transitions4 4

P3-P2 and corre-sponding hot bands are clearly seen around 2600 cm-. NHseems fainter, but this is only because each drawing of thefigure is normalized to its most intense line. The lower tracespectrum shows enhanced NH and hydrogen. This is dueonly to the decrease of the HCN normalizing feature. Theplasma cleans the reactor.

This variation of concentration from one spectrum to an-other should not have occurred under clean experimentalconditions. Nevertheless, Fig. 10 illustrates the ability ofour device to see electronic transitions between highly excit-ed levels. The species created were, in fact, not troublesomein the NH study. This is shown in Fig. 11, where, as anexample, small portions of the spectra 1700 and 1701 aredrawn with full resolution around 2111,2685, and 3175 cm-'.It includes one line of CO, one triplet of NH, and one line ofHCN. The radical NH may be distinguished easily for tworeasons. First, the relative intensities have varied betweenthe two spectra. CO and HCN intensities are lowered by afactor of about 3 in comparison with NH. Second, theshapes of the lines are different in spectrum 1700, whichcorresponds to the period of greater variation of the concen-tration of CO and HCN. The lines of the impurities are

L*L 1 6 93

Chollet et al.

I I l1l1

1l.I

I 1

11 I I I I I I I I


CO (7) HCN (9)

1700

O.1cm 1

(3) (1) (3)

1701

2111.5cm ' 2685cm' 3174.6cm

Fig. 11. Evolution in time of CO, NH, and HCN from resolvedlines selected in the two spectra, 1700 and 1701, already shown inFig. 10 at low resolution. The number next to each profile givesits amplitude in arbitrary units. The relative intensities andthe shape of the observed lines permit unambiguous identificationof NH.

NH1-0

R1 (1)

R2( 1

. -A _ Ai

H(

F.T.S.

O.1c 1

w w V - - I

3187

DIFFERENCEFREQUENCY

^,-J LASER

Fig. 12. Difference-frequency laser (seetransform spectra (F.T.S.). The spacingR2(1) lines is of the order of 0.096 cm-1.

Ref. 39) and Fourier-between the R1(1) and

made approximately from the addition of two profiles withdifferent resolutions. As explained in Ref. 45, this denotes astrong temporal variation of the emission of the line duringthe recording of the interferogram. The ratio of the corre-sponding half-widths is of the order of 5. This means thatthe contaminating grease was burned by the plasma in es-sentially one fifth of the total interferogram recording time,that is, in about 30 mn. As shown by the normal shape of alllines in spectrum 1701, there is practically no variation of COand HCN concentration in the spectrum plasma thereafter.This indicates that the only way to remove the parasiticspecies at this stage is to clean the reactor and to use gassamples of higher purity.

The different temporal behaviors of the transitions ofdifferent species were quite helpful in identifying the lines ofNH. Their specific shape, their triplet structure, and the 30

positions of the 1-0 band measured with a PINE-type differ-ence-frequency laser4 6 by Bernath and Amano 39 were alsouseful. The R(1) of the 1-0 taken from Ref. 39 is shown inFig. 12. Also shown in Fig. 12 on the same wave-numberscale is the same line extracted from spectrum 1701.

As stated previously, the spectroscopic results corre-sponding to about 300 lines that connect six vibrationallevels in the 3Z electronic ground state will be published.2 8

ACKNOWLEDGMENTSThis research was supported in part by the Direction desRecherches, Etudes et Techniques. The help of D. Boud-jaadar, R. Farrenq, J. Perrin, and G. de Rosny is acknowl-edged. Thanks are due to J. Collet and A. Ubelmann fortheir technical assistance.

REFERENCES AND NOTES

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'M'A0V -S. i' ',' . X __, w- .

_

Chollet et al.

1-0 N


19. T. Ishiwata, I. Tanaka, K. Kawagushi, and E. Hirota, "Infrareddiode laser spectroscopy of the NO3 v3 band," J. Chem. Phys. 82,2196-2205 (1985).

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21. This was replaced recently by Roots and primary pumps (1000m3/h) with a new extracting-tube cross section of 150 cm2.

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23. From Spectra-Physics Company. The detector is mounted on acold head of the same type as the one used to cool the diodes.

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25. P. Chollet, "Realisation d'un ensemble experimental destin6 Ala production et A l'6tude spectroscopique A large bande d'e-speces transitoires. Analyse infrarouge par laser A diode duradical SiH dans un plasma de silane," Thbse 3me cycle (Uni-versit6 de Paris-Sud, Paris, 1984).

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30. Presented at the VII International Symposium on PlasmaChemistry, Eindhoven, July 1985.

31. Locking the emission of the diode laser to any given observabletransition is easy. The wave number of the laser emission ismodulated with a frequency f (of the order of 1 kHz). Synchro-nous detection at the same frequency as that of the laser beamgives the first derivative of the spectral profile of the observabletransition when the laser is tuned over it. This signal is pointsymmetric, with a mean value equal to zero. Ideally, one shouldservo the wave number of the diode emission exactly at thecenter of this absorption profile. If synchronous detection atfrequency 2f is simultaneously performed with an additionallock-in amplifier, the second derivative of the absorption profileis obtainable. With the servo control of the laser beam, thesecond signal is then directly proportional to the concentration.This sytem is quite convenient, as the transition of interest isitself used for the servo, and it is commercially available (Spec-tra-Physics Company).

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33. The level of parasitic rf emission was in fact much smallerduring the course of all experiments. If it had not been, itwould have been impossible to record any sequential spectra.However, we demonstrate here that even in a severe environ-ment the diode emission may be satisfactorily controlled. Con-sequently, with an appropriate linear tuning of the servo signal,one could record spectra on a linear wave-number scale.47

,48

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41. D. Cerny, F. Roux, C. Effantin, J. D'Incan, and J. Verges, "Highresolution Fourier spectrometry of 14N2 and 5N2. Infraredemission spectrum: extensive analysis of the W 3 -B 3 gsystem," J. Mol. Spectrosc. 81, 216 (1980).

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- X 2+ de CN, W 3 ,-B 3 1g et B '32u - B 3 1g de N2 ,"ThAse d'Etat (Universit6 Claude Bernard, Lyon, 1979).

43. G. Guelachvili, D. de Villeneuve, R. Farrenq, W. Urban, and J.Verges, "Dunham coefficients for seven isotopic species of CO,"J. Mol. Spectrosc. 98, 64-79 (1983).

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48. A. Valentin, L. Henry, Ch. Nicolas, A. W. Mantz, "High preci-sion intensity and broadening parameter measurements by astep by step F. T. controlled diode laser," presented at the 40thMolecular Spectroscopy Symposium, Columbus, Ohio, June1985.

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Chollet et al.

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