R. S. Ram and P. F. Bernath- Fourier Transform Emission Spectroscopy of the Low-Lying Electronic...

8/2/2019 R. S. Ram and P. F. Bernath- Fourier Transform Emission Spectroscopy of the Low-Lying Electronic States of NbN

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Fourier Transform Emission Spectroscopy of the Low-LyingElectronic States of NbN

R. S. Ram and P. F. Bernath1 Department of Chemistry, University of Arizona, Tucson, Arizona 85721

Received December 8, 1999; in revised form February 24, 2000

The high-resolution spectrum of NbN has been investigated in emission in the 300015 000 cm1 region using a Fouriertransform spectrometer. The bands were excited in a microwave discharge through a mixture of NbCl5 vapor, 5 mTorr of N2, and 3 Torr of He. Numerous bands observed in the near-infrared region have been classied into the following transi f 1 c 1 , e 1 a 1 , C 3 0 A3 1 , C 3 0 A3 1 , C 3 1a 1 , C 3 1 A3 0 , d 1 A3 0 , and d 1 b 1 . Theseobservations are consistent with the energy level diagram provided by laser excitation and emission spectroscopy [Y. AzG. Huang, M. P. J. Lyne, A. J. Merer, and V. I. Srdanov,J. Chem. Phys. 100, 41384155 (1993)]. The missingd 1 statehas been observed for the rst time and its spectroscopic parameters are consistent with the theoretical predictions of

Langhoff and W. Bauschlicher, Jr. [ J. Mol. Spectrosc. 143, 169179 (1990)]. Rotational analysis of a number of bands hasbeen obtained and improved spectroscopic parameters have been extracted for the low-lying electronic states. The observof several vibrational bands withv 1 has enabled us to determine the vibrational intervals and equilibrium bond lengths fothe A3 0 , a 1 , b 1 , d 1 , andC 3 1 states. 2000 Academic Press

INTRODUCTION

Over the last two decades considerable interest has devel-oped in the spectroscopy of transition metal-containing mole-ules, partly due to the availability of many improved experi-

mental and theoretical techniques. The inherent complexity of he electronic spectra of many transition metal oxides, nitrides,arbides, and halides has now been unraveled and successfullynterpreted. The availability of high-qualityab initio calcula-ions has proven to be very helpful in the analysis of theseomplex spectra. In part, the recent interest in the transition-

metal-containing molecules can also be attributed to their im-portance in, for example, catalysis (1, 2) and astrophysics (3).Transition-metal atoms have relatively high abundances inmany stars (3) and several transition-metal hydrides and oxideshave also been detected (3 8 ). There is a possibility thatransition-metal nitrides may also be found. So far nitride

molecules have not been observed in stellar atmospheres, inpart due to the lack of precise spectroscopic data required for meaningful search in complex stellar spectra. In recent years,he electronic spectra of several transition-metal nitrides have

been analyzed at high resolution and considerable progress hasbeen made in theoretical studies (912 ), although much morework is needed. Only limited and fragmentary spectroscopicdata are available for a number of transition-metal nitrides. Forxample, in the V B transition metal family, some high-reso-ution data are available for VN (13, 14 ) and NbN (1518 ), but

TaN (19) remains poorly characterized.

The electronic spectra of NbN were rst observed by and Rao in 1969 (15). The transition they saw was initiaassigned asA3 X 3 and the rotational analysis of the3 33

2 subband was reported. The rotational analysis of thetwo subbands was difcult to achieve because of the preof large nuclear hyperne splittings at lowerJ values. Thehyperne structure of this transition was later analyz

Femeniaset al. (16 ) and more recently by Azumaet al. (17 ),and molecular constants including the hyperne paramhave been determined. This transition was renamed asB 3 X 3 by Azumaet al. (17 ), who also recorded the laser extation spectra of theC 3 X 3 , e 1 X 3 2 and f 1 a 1transitions and measured the spinorbit intervals by obtion of weak spin satellite branches (17, 18 ). The3 23 1 and3

33 2 intervals of 400.5 0.1 and 490.5 0.1 cm 1 weredetermined for theX 3 state of NbN. In addition, Azumaet al.(18) have observed a number of transitions between thelying singlet and triplet electronic states by recordin

wavelength-resolved uorescence following the selectiveexcitation of different states. The term energy positions spin components of theA3 , B 3 , andC 3 triplet states aswell as the positions of a 1 , b 1 , c 1 , e 1 , andf 1 singletstates were also determined in this work (18).

There have been several theoretical calculations that pthe dipole moments, dissociation energies, ionization ptials, and spectroscopic properties of the low-lying elecstate of NbN (1922 ). The ionization potential of 7.175 eV calculated by Berceset al. (20) and the binding energy of 4.eV was calculated by Sellers (21) for theX 3 state of NbN.

The permanent electric dipole moment of NbN was exper

1 Department of Chemistry, University of Waterloo, Waterloo, Ontario,

Canada N2L 3G1.

ournal of Molecular Spectroscopy201, 267279 (2000)oi:10.1006/jmsp.2000.8099, available online at http://www.idealibrary.com on

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ally determined by Fletcheret al. (22) as 3.26(6) and 4.49(9)D for theX 3 1 andB 3 2 states, which compares well with theb initio value of 3.65D (22) obtained for theX 3 state. Theirb initio values of r e 1.695 and e 1010 cm1 alsoompare well with the experimental values of 1.663 and

1002.5 cm1, respectively. A very high-qualityab initio cal-ulation has been performed by Langhoff and Bauschlicher23) on the spectroscopic properties of the low-lying singletnd triplet electronic states of NbN. The ordering of states as

well as the energy positions as determined experimentally byAzumaet al. (18) were found to be in excellent agreement withhe predictions of Langhoff and Bauschlicher (23). The onlyxception was thed 1 state, which was not observed directly

by Azumaet al. (18) but was predicted by Langhoff andBauschlicher (23) to be near 13 000 cm1.

In the present investigation we have recorded the emissionpectrum of the red and near-infrared regions of NbN using a

Fourier transform spectrometer. Numerous electronic transi-ions have been observed between the low-lying singlet andriplet states. The analysis of new bands is consistent with thenergy level diagram provided for NbN by Azumaet al. (18).

The missingd 1 state has now been located from the obser-vation of thed 1 A3 0 and d 1 b 1 transitions. Aotational analysis has been performed for the numerous elec-ronic transitions and improved spectroscopic constants have

been evaluated.

EXPERIMENTAL DETAILS

The NbN emission bands were excited in an electrodeless

microwave discharge through a owing mixture of 3 Torr of He, about 5 mTorr of N2, and a trace of NbCl5 vapor. Thedischarge tube was made of quartz and had an outer diameterof 12 mm. Anhydrous NbCl5 powder was placed in a smallbulb which was continuously heated by a heating tape duringhe experiment. The partial pressure of NbCl5 vapor in the

discharge tube was adjusted to maintain a constant bluishwhite color in the discharge. This experimental arrangementwas chosen to simultaneously study the electronic spectra of NbCl and NbN in two experiments.

In the rst experiment the discharge was made with 3 Torrof He and a trace of NbCl5 vapor. This experiment wasntended for the observation of NbCl spectra. Several NbCl

bands, particularly those near 6704, 6799, and 6862 cm1, wereobserved with very weak intensity. In addition to NbCl, anumber of NbO bands, with the most intense ones at 9306,12 500, 12 571, 12 815, and 13 034 cm1, were also observed.The NbCl and NbO bands were distinguished by their lineeparation, with the branches in the NbCl bands having amaller line spacing compared to those in NbO. The NbO

bands were very recently observed by Launilaet al. (24) andhave been assigned to several doubletdoublet transitions.

In the second experiment5 mTorr of N2 was added to the

ow, keeping the other experimental parameters the same. In

this case it was noticed that the NbO and NbCl bands appeared along with new bands in the 800013 700 1region. These bands were later attributed to NbN usinresults recently published by Azumaet al. (18). The NbNbands appeared strongly when the discharge had an inbluewhite color. The emission from the discharge tubsent directly into the 8-mm entrance aperture of theFourier transform spectrometer of the National Solar Obtory at Kitt Peak. The spectra in the 180015 000 cm1 intervalwere recorded using liquid-nitrogen-cooled InSb detectCaF2 beam splitter, and RG695 lters. A total of six scansco-added in about 63 min of integration at a resolution ocm 1.

The spectral line positions were determined using areduction program called PC-DECOMP developed bBrault. The peak positions were determined by tting a lineshape function to each line. The strong N2 lines observed inour spectra were used for calibration of the molecular liNbN. The strong N2 lines recorded in a separate experimwith an Os hollow-cathode lamp operated with 10 mTorr2and 2.3 Torr of Ne were calibrated using the Ne line posof Palmer and Engleman (25). This calibration was then tranferred to the NbN spectrum using the N2 lines as a transfestandard. The molecular lines of NbN have a typical wi0.05 cm1 and appear with a maximum signal-to-noise rat12:1 so that the best line positions are expected to be acto about 0.002 cm1.

LOW-LYING ELECTRONIC STATES OF NbN

The ground state of NbN is now well established as3

state arising from the conguration, 4d 15s 1 (17, 18, 23 ).Until 1989 the only electronic transitions known for NbNtheB 3 X 3 , C 3 X 3 , andC 3 A3 transitions, wherethe A3 , B 3 , andC 3 states arise from 4d 2, 4d 14d 1,and 4d 14d 1 electron congurations, respectively. A vhigh-qualityab initio study of the spectroscopic propertiesNbN was carried out by Langhoff and Bauschlicher (23), whocalculated the properties of numerous low-lying singletriplet electronic states below 20 000 cm1 as well as thequintet states below 30 000 cm1 using the MRCI anMRCI Q methods. Here MRCI denotes the multireferconguration interaction procedure based on SA-CA(state-averaged complete-active-space self-consistent calculations and Q refers to a Davidson-like correctionthe singlet manifold the ordering of the electronic statepredicted to bea 1 (4d 15s 1), b 1 (5s 2), c 1 (4d 2),d 1 (4d 2), ande 1 (4d 14d 1). In a recent publicationAzumaet al. (18) have reported the observation of a numtransitions between the low-lying singlet and triplet elecstates which are consistent with the ordering of states preby Langhoff and Bauschlicher (23). The new transitions weobserved by wavelength-resolved uorescence with a

monochromator following selective laser excitation of v

268 RAM AND BERNATH

Copyright 2000 by Academic Press


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xcited states. Thed 1 state was not observed by Azumaet l. (18) and a high-lyingf 1 (4d 14d 1) state was observedt 22 312 cm1. A summary of the low-lying electronic states

of NbN has been provided in an energy level diagram in thepaper by Azumaet al. (18). All of these states, except theX 3tate, have been observed in our present investigation of NbN

using high-resolution Fourier transform emission spectros-opy. The missingd 1 state has now been identied near

13 908 cm1. An updated energy level diagram of the low-ying electronic states has been provided in Fig. 1, where the

observedT 00 values of the low-lying states are also providedor reference.

OBSERVATIONS

A number of new NbN bands have been observed in the800014 000 cm1 region. The branches in different bandswere sorted using a color LoomisWood program running on PC computer. These bands have been found to involve

numerous low-lying singlet states and spin components of the

C 3

and A3

states. A schematic energy level diagram of

observed low-lying electronic states is presented in FAlthough most of the states shown in this gure were obpreviously by Azumaet al. (18), the infrared transitionmarked in Fig. 1 have been observed at high resolutiona Fourier transform spectrometer. Also, thed 1 state of NbNhas been observed for the rst time in our work.

(a) The d 1 b 1 , d 1 A3 0 Transitions

Two infrared transitions, which have 00 origins nearand 8796 cm1, have a common upper state and are assignethe d 1 b 1 , d 1 A3 0 transitions. These assignmenlocate thed 1 state at 13 908 cm1. This state was noobserved by Azumaet al. (18) but was predicted by Langhoand Bauschlicher (23) at 14 908 and 12 878 cm1 using theMRCI and MRCI Q calculations, respectively. It was noby Langhoff and Bauschlicher that the MRCIQ calculationswere in better agreement with the experimental valuesrotational structure of both of these transitions is slightgraded toward higher wavenumbers because of the slsmaller bond length of thed 1 state compared with the bonlengths of theb 1 andA3 0 states.

The spectrum of thed 1 b 1 transition near 8045 cm1is overlapped with a strong N2 band near 8056 cm1. In spiteof overlapping, the NbN rotational lines were easily dguished because of relatively small separation betweensecutive rotational lines in a branch. Our color Loomisprogram was also very helpful in identifying the lines overlapped regions. This band consists of oneR and onePbranch and lines have been identied up toR(37) andP (42).The 00 band is followed by relatively weaker 11 anbands with origins near 8069 and 8093 cm1. The rotationalstructure of all three bands has been analyzed.

The 00 band of thed 1 A3 0 transition has its originear 8796 cm1. This region of the spectrum is relatively from N2 overlapping. The band also consists of singleR andPbranches, as expected. A part of the spectrum of this tranis provided in Fig. 2, whereP -branch lines of the 00 banhave been marked up to the bandhead. We have identi

FIG. 2. A portion of thed 1

b1

00 band of NbN near theP head.

FIG. 1. A schematic energy level diagram of the observed electronicransitions of NbN. The positions of theA3 0 , a 1 , b 1 , c 1 , d 1 , C 3 1,nd f 1 states have been drawn with theX 3 2 X 3 1 interval xed to 400.5m 1 as determined by Azumaet al. (18) (see text for details). The positionsf X 3 3, B 3 2, B 3 3, B 3 4, andC 3 2 states, marked by the broken lines,ave been taken from the paper of Azumaet al. (18). The transitions marked

with broken lines have been taken from the thesis of Huang (26 ).

269THE LOW-LYING ELECTRONIC STATES OF NbN



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ines up toR(53) andP (54) in this band. The 11 and 22bands of this transition have also been observed near 8821 and8846 cm1 with relatively weak intensity and a rotationalnalysis of 00, 11, and 22 bands has been obtained.

b) The C 3 1 a1 and C 3 1 A

30 Transitions

Two bands observed near 12 261 and 12 346 cm1 have anxcited state in common. These bands were assigned as the

00 bands of theC 3 1a 1 and C 3 1 A3 0 transitions.Both of these transitions consist of P , Q , andR branches withhe Q branch being the most intense.

The C 3 1a 1 transition at 12 261 cm1 has a relativelyopen structure and the rotational lines in the three branches arelearly resolved starting from close to the origin. The higherJ ines withJ 35 slowly broaden due to the onset of small-doubling in theC 3 1 state but the split lines could not be

measured because of their very weak intensity. Lines up toR(34),P (34), andQ (39) were observed in the 00 band. Apart of the spectrum of this transition near theQ head ispresented in Fig. 3, where someP and Q lines have beenmarked. The 11 band of this transition has anR head at

12 195.7 cm1 and all the three branches were identied inband. The rotational analysis of both 00 and 11 bandobtained.

In the 00 band of theC 3 1 A3 0 transition near 12 34cm 1, the lines of theQ branch are not resolved and most ofQ lines pile up in a 2 cm1 interval to the lower wavenumbside of the origin. This is a result of the near equality rotational constants in the upper and lower states. In thithe R andP branches have a open structure. The 11 banthis transition appears with aQ head near 12 309.5 cm1 andthe Q branch is partly resolved at higherJ values. A com-pressed portion of the spectrum of NbN in theC 3 A3region is provided in Fig. 4, where theQ heads of the 00 an11 bands have been marked. The 00Q heads of theC 3 0 A3 1 and f 1 c 1 transitions, presented in the followsections, have also been marked in this gure. TheR and Plines of the 00 and 11 bands were picked out with thof our LoomisWood program and were rotationally anaThe line positions of the 00 and 11 bands of theC 3 1 X

32 transition (26 ) with the same excited state were aincluded in the nal t. To constrain theG (12) vibrational

interval of theX 3 2 spin component, the difference betwthe Q -branch lines of the 10 and 11 bands of theB 3 3 X 3 2 transition (26 ) were also included in the t. This wnecessary to determine the vibrational intervals of the estates.

(c) The C 3 0 A3

1 and C 3

0 A3

1 Transition

Two bands withQ heads near 12 297.7 and 12 304.3 cm1,which appear with moderate intensity, have been found ttheir lower states in common. These two bands haveidentied as the 00 bands of theC 3 0 A3 1 andC 3 0 A3 1 transitions. TheQ branch of theC 3 0 A3 1 00band is shaded toward the higher wavenumbers and is resolved while theQ branch of theC 3 0 A3 1 00 band isunresolved with slight shading toward lower wavenum

FIG. 3. A portion of theC 3 1a 1 00 band of NbN near theQ head.

FIG. 4. A compressed portion of the spectrum of NbN showing theQ heads of theC 3 0 A3 1 , C 3 0 A3 1 , C 3 1 A3 0 , andf 1 c 1 electronic

ransitions of NbN.

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Fig. 4). Although theR and P branches of these bands areweaker in intensity and are not distinguishable at rst sight, ourLoomisWood program was very helpful in identifying theines. Also the known combination differences for theC 3 0tate based on the previous work of Azumaet al. (18) were

very useful in making denite assignments. The 11 bands of hese two transitions could not be identied due to their very

weak intensity. Rotational analysis of both 00 bands has beenarried out. The line positions of theC 3 0 X 3 1 and

C 3 0 X 3 1 00 transitions (26 ) were also included in thenal t.

d) The e 1 a 1 and e 1 b 1 Transitions

Two bands observed withQ heads near 13 660 and 12 994.5m 1 have been identied as the 00 bands of thee 1 a 1nd e 1 b 1 transitions. The 11 bands of these two tran-itions were not identied. Thee 1 a 1 00 band at 13 660m 1 consists of three branchesP , Q , andR, with theQ branch

being the most intense. A part of the 00 band of this transitions presented in Fig. 5, where some lines in theQ branch have

been marked. The highJ lines of this band slowly broaden andplit in two components nearJ 35 because of the small-doubling in thee 1 state. The highJ lines after the -dou-

bling is resolved were very weak and could not be reliablymeasured. We have identied lines up toR(37), P (42), andQ (46) in this band.

Thee 1 b 1 00 band at 12 994.6 cm1 is much weaker

n intensity than thee1

a1

band and is overlapped with N2nd NbO lines present in the same region. TheQ lines of thisband could be readily identied from the predictions based ononstants of thee 1 andb 1 states. TheR andP lines of this

band are too weak to be measured. This band was not includedn the nal t.

e) The f 1 c 1 Transition

A band observed with aQ head at 12 400 cm1 has beendentied as the 00 band of f 1 c1 transition. This bandonsists of P, Q, andR branches with theQ branch being the most

ntense. The rotational structure of this band is partly overlapped

with the lines of theC 3 1 A3 0 00 band on the lower wavnumber side and a NbO band on the higher wavenumber scould be positively identied with the help of our Loomisprogram. The previously reported data for thef 1 a 1 transition(26 ) were also helpful in the assignment of this band. A parQ branch of this band near the band origin is presented in No -doubling has been observed as expected for a1 1 tran-sition and we have identied the lines up toR(48),P(44), andQ(55). The lines of the 00f 1 a 1 transition (26 ) were alsoincluded in the nal t.

ANALYSIS

The observed line positions of bands of the different eletransitions are provided in Table 1. The molecular constandifferent states were determined by tting the observed lisitions to the following customary energy level expression

(for 1 , 3 0 , 1 , 1 , and1 states)

F v J T v Bv J J 1 D v J J 1 2, [1]

(for 1 , 3 1 , and 3 1 states)

F v J T v Bv J J 1 D v J J 1 2

1/ 2 qJ J 1 q D J J 1 2 . [2]

Two ts were obtained for the determination of nal spscopic constants. In the rst t the lines of theC 3 0 A3 1and C 3 0 A3 1 transitions were combined with the lpositions of theC 3 0 X 3 1, C 3 0 X 3 1 transitions pre-viously observed by laser excitation spectroscopy (18, 26 ). Inthe second t lines from thed 1 A3 0 , d 1 b 1 ,C 3 1 A3 0 , C 3 1a 1 , e 1 a 1 , andf 1 c 1 transitionswere t simultaneously. The rotational lines of theC 3 1 X 3 2, e 1 X 3 2, andf 1 a 1 transitions from the previolaser excitation work (26 ) were also included in this t so tall of the new bands are connected together and referenv 0 of theX 3 2 spin component. The badly blended li

FIG. 6. A portion of thef 1

c1

00 band of NbN near theQ head.

FIG. 5. A portion of thee 1 a 1 00 band of NbN near theQ head.




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TABLE 1Observed Line Positions (in cm 1) of the Near-Infrared Transitions of NbN

Note. O-C are observed minus calculated line positions in units of 103

cm1

.

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TABLE 1 Continued




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TABLE 1 Continued

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TABLE 1 Continued




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TABLE 1 Continued

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were given reduced weights and overlapped lines were ex-

luded in order to improve the standard deviation of the t. Therst t is connected tov 0 of the lowest energyX 3 1 spinomponent while the second t is connected tov 0 of the

X 3 2 spin component. Because we have no direct connectionbetween the energy levels in our two separate ts, we havehosen to x thev 0 X 3 2 X 3 1 interval to exactly 400.5m 1. The molecular constants obtained for the different elec-ronic states are provided in Table 2.

DISCUSSION

The energy level diagram of the low-lying electronic states

obtained by Azumaet al. (18) was very helpful in assigning the

new transitions observed in the present work. The position

new transitions, except those involving thed 1

state, werereadily calculated by taking the difference between thevalues of the different electronic states. Thed 1 state was notobserved by Azumaet al. (18) and was not marked in their enelevel diagram. The observation of two transitions,d 1 A3 0andd 1 b1 , with a common excited state provided a straforward assignment of the rotational lines. Thed 1 state hasbeen located at 13 908 cm1 at the energy scale adopted bAzumaet al. (18). This compares with the calculated positio12 878 cm1 from MRCI Q calculation of Langhoff anBauschlicher (23).A revised energy level diagram of the obser

low-lying singlet and triplet electronic states of NbN is pr

TABLE 2Spectroscopic Constants (in cm 1) for Low-Lying Electronic States of NbN

Note. All term values marked with a were tted with respect to thev 0 vibrational level of theX 3 2 spincomponent which was held xed to the value of 400.5 cm1 obtained by Azumaet al. (18), while b refers to theundetermined term value for thev 2 vibrational level of theA3 0 state.




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n Fig. 1. The termenergies of different electronic states are on theame scale as given by Azumaet al. (18).The electronic transitions observed in our work involve

ingletsinglet or singlettriplet transitions and hyperne split-ing or broadening has not been detected. As has been noted

previously, theB 3 X 3 and C 3 X 3 transitions showtrong hyperne effects which result mainly from the large Nb

hyperne splitting in theX 3

state (17, 18 ). The molecularonstants obtained from the nal t are presented in Table 2.nspection of this table indicates that the3 0 and 3 1 spinomponents of theA3 state are about 492 cm1 apart, whichs not normal for a Hunds case (b)3 state. This indicateshat theA3 state is better described by Hunds case (c)oupling. This case (c) behavior is a result of the interaction of he A3 0 spin component with the higher lyingb 1 and1 states. As discussed in detail by Azumaet al. (18), the1 (5s 2) state is mixed with thed 1 (4d 2) state bylectrostatic perturbation. Because of this interaction theb 1

tate acquires considerabled 1

character and thus perturbshe A3 0 spin component more strongly because it lies muchloser to theA3 0 state.During the rotational and hyperne analysis of theC 3

X 3 transition, Azumaet al. (17, 18 ) found evidence of strongecond-order spinorbit interaction between theC 3 ande 1tates, both arising from the same valence electron congura-ion, 4d 14d 1. Because of this interaction, theC 3 1 spinomponent is pushed down 650 cm1 from the expected posi-ion by thee 1 state. Thee 1 X 3 2 intercombination tran-ition was observed strongly because of these interactions. We

have also observed theC 3 1a 1 intercombination transitionn addition to theC 3 1 A3 0 andC 3 0 A3 0 transitions.

The molecular constants of theA3 0 , a 1 , b 1 , d 1 ,nd C 3 1 states (Table 2) have been used to evaluate thequilibrium rotational constants for these states, which are

provided in Table 3. Even though the 22 bands were observedn the d 1 b 1 and d 1 A3 0 transitions, thev 2

vibrational levels of theb 1 andd 1 states remain oatingbecause there is no connection with the known vibrationalevels of theC 3 1 state. Equilibrium vibrational constants forhe A3 0 , b 1 , and d 1 states could, therefore, not be

determined in spite of the observation of v 0, 1, and 2

vibrational levels. The equilibrium rotational constants provide

the equilibrium bond lengths of 1.669320(22), 1.649061.663115(20), 1.655800(19), and 1.669781(10) fo A3 0 , a 1 , b 1 , d 1 , andC 3 1 states, respectively.

CONCLUSION

The emission spectrum of NbN has been investigated a

resolution in the 300015 000 cm1

region using a Fourietransform spectrometer. The bands observed in the 814 000 cm1 region have been assigned to a number of tsitions involving both singlet and triplet electronic statetational analysis of these transitions has been carried oimproved spectroscopic constants have been obtained foof the low-lying states. In this work we have locatemissingd 1 state, which was predicted by Langhoff Bauschlicher (23), but was not observed experimentallyprevious investigations (18, 26 ). The observation of the 0and 11 bands in a number of transitions, in conjunctiothe previously reported wavenumbers for several transihave enabled us to determine theG (12) vibrational intervals awell as equilibrium bond lengths forA3 0 , a 1 , b 1 , d 1 ,and C 3 1 states of NbN. Our observations are in exceagreement with theab initio predictions of Langhoff anBauschlicher (23) and the previous work of Azumaet al. (18).

ACKNOWLEDGMENTS

We thank M. Dulick of the National Solar Observatory for assistaobtaining the spectra. The National Solar Observatory is operated by theation of Universities for Research in Astronomy, Inc. under contract wNational Science Foundation. The research described here was suppofunding from the NASA laboratory astrophysics program. Support wprovided by the Petroleum Research Fund administered by the Americaical Society and the Natural Sciences and Engineering Research CouCanada. We thank A. Merer for providing a copy of the Ph.D. thesis of G

REFERENCES

1. M. Grunzein The Chemical Physics of Solid Surfaces and Heteneous Catalysis (D. A. King and D. P. Woodruff, Eds.), Vol. 4, pElsevier, New York, 1982.

2. F. A. Cotton, G. Wilkinson, C. A. Murillo, and M. Bochman, AdInorganic Chemistry, 6th ed., Wiley, New York, 1999.

3. C. Jascheck and M. Jascheck, The Behavior of Chemical Elem

Stars, Cambridge University Press, Cambridge, UK, 1995.

TABLE 3Equilibrium Constants (in cm 1) A 3 0 , a

1 , b 1 , d 1 , and C 3 1 States of NbN

278 RAM AND BERNATH



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4. D. L. Lambert and R. E. S. Clegg,Mon. Not. R. Astron. Soc. 191, 367389(1980).

5. R. Yerle,Astron. Astrophys. 73, 346351 (1979).6. B. Lindgren and G. Olofsson,Astron. Astrophys. 84, 300303 (1980).7. O. Engvold, H. Wohl, and J. W. Brault,Astron. Astrophys. Suppl. Ser. 42,

209213 (1980).8. T. Tsuji,Ann. Rev. Astron. Astrophys. 24, 89125 (1986).9. R. S. Ram, J. Lievin, and P. F. Bernath,J. Chem. Phys. 109, 63296337

(1998).

0. R. S. Ram, J. Lievin, and P. F. Bernath,J. Mol. Spectrosc. 197, 133146(1999).1. R. S. Ram, J. Lievin, and P. F. Bernath,J. Chem. Phys. 111, 34493465

(1999).2. I. Shim, K. Mandix, and K. A. Gingerich,J. Mol. Struct. (Theochem) 393,

127139 (1997).3. B. Simard, C. Masoni, and P. A. Hackett,J. Mol. Spectrosc. 136, 4455

(1989).4. W. J. Balfour, A. J. Merer, H. Niki, B. Simard, and P. A. Hackett,J. Chem.

Phys. 99, 32883303 (1993).5. T. M. Dunn and K. M. Rao,Nature 222, 266 (1969).

16. J.-L. Femenias, C. Athenour, and T. M. Dunn,J. Chem. Phys. 63, 28612868 (1975).

17. Y. Azuma, J. A. Barry, M. P. J. Lyne, A. J. Merer, J. O. Schroder, anFemenias,J. Chem. Phys. 91, 112 (1989).

18. Y. Azuma, G. Huang, M. P. J. Lyne, A. J. Merer, and V. I. Srd J. Chem. Phys. 100, 41334155 (1994).

19. T. M. Dunn, private communication.20. A. Berces, S. A. Mitchell, and M. Z. Zgierski,J. Phys. Chem. 102,

63406347 (1998).

21. H. Sellers,J. Phys. Chem. 94, 13381343 (1990).22. D. A. Fletcher, D. Dai, T. C. Steimle, and K. Balasubramanian,J. Chem.Phys. 99, 93249325 (1993).

23. S. R. Langhoff and W. Bauschlicher, Jr.,J. Mol. Spectrosc. 143, 169179(1990).

24. O. Launila, B. Schimmelpfennig, H. Fagerli, O. Gropen, A. G. TakU. Wahlgren,J. Mol. Spectrosc. 186, 131143.

25. B. A. Palmer and R. Engleman, Atlas of the Thorium SpectrumAlamos National Laboratory, Los Alamos, 1983.

26. G. Huang, Ph.D. thesis, University of British Columbia, C1993.



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