R. S. Ram, J. Lievin and P. F. Bernath- Emission Spectroscopy and Ab Initio Calculations on IrN

8/3/2019 R. S. Ram, J. Lievin and P. F. Bernath- Emission Spectroscopy and Ab Initio Calculations on IrN

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Emission Spectroscopy and Ab Initio Calculations on IrN

R. S. Ram,* J. Lievin, and P. F. Bernath*,

*Department of Chemistry, University of Arizona, Tucson, Arizona 85721; Universite Libre de Bruxelles, Laboratoire de Chimie Physique Moleculaire,CP 160/09, Av. F. D. Roosevelt 50, Brussels, Belgium; and Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

Received November 9, 1998; in revised form June 10, 1999

The emission spectrum of IrN was recorded in the near infrared using a Fourier transform spectrometer. The IrN moleculeswere excited in an Ir hollow cathode lamp operated with a mixture of Ne and a trace of N 2. Numerous IrN bands observedin the 75009200 cm1 region were assigned to a new a3X1 electronic transition with the 0 0 bands of the a30X

1

and a31X1 subbands near 9175 and 8841 cm1, respectively. A rotational analysis of several bands of the 00 and 01

sequences was obtained and molecular constants were extracted. The effective Hunds case (a) constants for the new a3 stateare: T00 8840.31747(88) cm

1, A0 340.53329(93) cm1, G(1/2) 984.3629(23) cm1, Be 0.4699116(27) cm

1,e 0.0030058(50) cm

1, and re 1.6576432(47) . The spectroscopic properties of the ground state and several low-lying

electronic states of IrN were also predicted by ab initio calculations. These calculations are consistent with our assignment ofthe a3X1 transition and also support our previous assignments of the A 1 and A1 electronic states [R. S. Ram and P. F.Bernath, J. Mol. Spectrosc. 193, 363 (1999)]. The excited a3 state of IrN has an 122214311421 electron configurationand the configurations of the other low-lying electronic states are also discussed. 1999 Academic Press

INTRODUCTION

The interaction of transition metals with nitrogen has im-ortant applications in catalysis and in surface science as wells for comparison with ab initio calculations. Diatomic transi-on metal nitrides also serve as simple models for the study of

metalnitrogen bonding in inorganic chemistry. Like severaliatomic transition metal oxides (14) and hydrides (5 8),ransition metal nitrides are also of potential astrophysicalmportance. For the 5d transition metal nitride family, high-esolution data are available for HfN (9), WN (10), ReN (11,2), OsN (13), IrN (14, 15), and PtN (16, 17). In spite of thisxperimental interest, no theoretical calculations are availableor the 5d transition metal nitrides, except for PtN (18).

The first spectroscopic observation of IrN was made in 1996y Marr et al. (14) who observed a 11 transition near5 190 cm1 by laser excitation spectroscopy. This transition

was labeled as the A1X1 electronic transition by compar-son with the isoelectronic PtC molecule (1922). For PtC two

dditional low-lying electronic states labeled as A 1 and A were observed below the A1 state (21). To locate thenalogous A 1 and A 1 states of IrN, we have recentlynvestigated the spectrum of IrN in the 10 00020 000 cm 1

egion (15). In this work we identified a new A 1X1

ransition of IrN near 13 135 cm1 and have observed numer-us additional bands with vibrational levels up to v, v 4 forhe A1X1 transition. We have also noted interactionsetween the v 0 and 1 vibrational levels ofA 1 state andhe v 2 and 3 vibrational levels ofA1 state, causing globalerturbations.

A comparison of the observed states of IrN and PtC indi-cated that an electronic state analogous to the A 1 state ofPtC remained to be observed for IrN. To search for this statewe have recently recorded the spectrum of IrN below 10 000cm1. Our spectra revealed the presence of two new transitionsin the near infrared, one of the 1X1 type (0 9181.8072 cm1) and the other of the 1X1 type (0

8841.2561 cm1

). The1

X1

transition near 9182 cm1

ismost probably analogous to the A 1X1 transition of PtC.The observation of a nearby 1X1 transition suggests thatthe two new transitions are the subbands of a 3X1 tran-sition. We have, therefore, assigned the two new infraredtransitions as the 30

1 and 311 subbands of the a3

X1 transition of IrN. The analysis of this transition is re-

ported in this paper.To better understand the electronic structure of IrN and to

confirm the assignments, ab initio calculations were performedand the spectroscopic properties of several singlet and tripletstates were calculated. Our assignments of the X1, a3, A1 and A1 states and their observed spectroscopic parametersare in excellent agreement with the predictions of the ab initiocalculations.

MATERIALS AND METHODS

The experimental conditions for the observation of the nearinfrared bands of IrN were the same as described in ourprevious paper (15). In summary, the IrN bands were producedin a hollow-cathode lamp by discharging a mixture of 2.5 Torr

ournal of Molecular Spectroscopy 197, 133146 (1999)Article ID jmsp.1999.7911, available online at http://www.idealibrary.com on

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f Ne and about 10 mTorr of N2 with 600 V and 310 mAurrent.

The spectra in the 300013 000 cm1 region were recordedwith the 1-m Fourier transform spectrometer associated withhe McMathPierce solar telescope of the National Solar Ob-ervatory at Kitt Peak. The spectrometer was equipped with a

CaF2 beam splitter, InSb detectors, and red pass (Schott 780)

filters. In total 30 scans were co-added in about 3.8 h ofintegration. In addition to the IrN bands, the observed spectraalso contained Ir and Ne atomic lines as well as strong N2molecular lines. Although there is some overlapping from theN2 bands, particularly in the 70008900 cm

1 region, the IrNbands were clearly distinguished by their smaller line spacing.We have calibrated our IrN spectra using the measurements ofNe atomic lines made by Palmer and Engleman (23). The

spectra were measured using a program called PC-DECOMPdeveloped by J. Brault. The peak positions were determined byfitting a Voigt lineshape function to each spectral feature. Theabsolute accuracy of the wavenumber scale is expected to beon the order of 0.002 cm1. The strong lines of IrN appearwith a typical signal-to-noise ratio of 10:1 and the precision ofmeasurements of strong and unblended IrN lines is expected tobe better than 0.003 cm1.

AB INITIO CALCULATIONS

Ab initio calculations were performed on the low-lying ,, , and states of singlet and triplet spin multiplicities ofIrN. The same computational approach as in our recent workon RuN (24) was adopted. It consists of a complete activespace self-consistent field (CASSCF) calculation (25) followedby an internally contracted multireference configuration inter-action (CMRCI) (26) calculation. All valence orbitals, i.e., thefour , two , and one orbitals arising from the 5d and 6sorbitals of iridium and the 2s and 2p orbitals of nitrogen, wereoptimized at the CASSCF level. A state averaging procedurewas used to optimize a common molecular orbital basis set for

describing the states of interest in a given spin multiplicity.These orbitals were then used in a second step for a CMRCIcalculation in which all valence electrons were correlated. TheCMRCI energies were corrected for Davidsons contribution(27) for unlinked four-particle clusters. The quasi-relativisticpeudo-potentials and the corresponding basis sets, developedin Stuttgart, were used to represent the 60 core electrons of theiridium atom (28). This core does not include the 5s and 5p

FIG. 1. A schematic energy level diagram of the known electronic statesf IrN and PtC.

FIG. 2. A compressed portion of the 00 and 11 bands of the a3

0X1

system of IrN.

34 RAM, LIEVIN, AND BERNATH



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rbitals. For the sake of uniformity, a similar pseudo-potentialnd the corresponding basis set was used for the nitrogen atom29). The iridium atom basis set was augmented by a single f

Gaussian orbital, with an exponent of 0.8, and a single dolarization function, with an exponent of 0.8, was added onitrogen. The size of the CASSCF (CMRCI) wavefunctionsanged between 1200 and 1300 (170 000 and 310 000) con-guration state functions (C2v symmetry), depending on thepace and spin symmetries. We refer to our RuN work (24) for

more details on the computational procedure that we justummarize here.All calculations were performed with the MOLPRO pro-

ram package (30), running on the Cray J916 computer at theULB/VUB computer center.

OBSERVATIONS

In our recent study of RuN (24) we have noted that thelectronic structure of RuN is very similar to that of thesoelectronic RhC (31, 32) and we expected a similar corre-pondence between IrN and PtC. Because no previous obser-

ations of IrN were reported below 1 m and no theoreticalalculations were available to guide us in the search for IrNransitions in the near infrared, we used the isoelectronic PtC1922) molecule as a guide. A comparison of the knownlectronic states of IrN and PtC indicates that the A 1 and1 states are observed for both IrN and PtC. For PtC, andditional low-lying transition labeled as A 1X1 waseported near 1 m (21), suggesting that a similar transitionould be found for IrN. We have observed several new IrNands in the 75009200 cm1 region, which were classified

nto two transitions, one with 0 and the other with

1. A rotational analysis indicates that these two transitions havea common lower state which is the ground state of IrN. Anupdated schematic energy level diagram of the observed elec-tronic transitions of IrN is presented in Fig. 1, where theobserved electronic states of the isoelectronic PtC were alsoprovided for comparison. The two new transitions were as-signed as the 30X

1 and 31X1 subbands of the a3

X1 transition and this assignment is supported by our ab

initio calculations. The branches in different bands were picked

out using an interactive color LoomisWood program runningon a PC computer, which was particularly helpful in finding theweaker lines of the minor isotopomer, 191IrN.

(A) The a30X

1 Transition

The 00 band of the 30X1 transition has an R head at

9194.3 cm1. This band is the strongest in intensity, while the11 band with an R head at 9146.8 cm1 has about 40% of theintensity of the 00 band. The 22 and other higher vibrationalbands of this sequence are too weak for identification andanalysis. The bands in the 01 sequence were also identified

with the help of the known ground state molecular parametersfrom the previous study (15). The bands with R heads at8083.1, 8048.2, 8015.4, 7984.4, and 7954.8 cm 1 were identi-fied as the 01, 12, 23, 34, and 45 bands of the a30X

1

transition. A search for the 10 sequence bands was unsuc-cessful partly due to the poor response of the InSb detector near10 000 cm1 where these bands are expected. The 02 or 2 0sequence bands were also not observed because of their veryweak intensity. A rotational analysis of the 00, 11, 01, 12,23, and 34 bands was obtained. The rotational structure ofeach band of this transition consists of only two branches, one

FIG. 3. An expanded portion of the 00 band of the a 3 1X1 system of IrN.

135EMISSION SPECTROSCOPY AND AB INITIO CALCULATIONS ON IrN



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TABLE 1

Observed Line Positions (in cm1) for the a3X

1 System of 193IrN




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TABLE 1Continued




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R and one P, consistent with a 0 transition. A portion ofhe spectrum of the a30X

1 subband of IrN is presented inig. 2, where the R heads of the 00 and 11 bands were

marked. The lines belonging to the minor isotopomer 191IrNwere also identified in our spectra but the molecular constantswere determined only for the most abundant 193IrN isotopomer.

TABLE 1Continued




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B) The a31X

1 Transition

On the lower wavenumber side of the a30X1 0 0 band,

nother band was observed with a 0 0 R head at 8848.9 cm1.This band involves a 1 transition with the ground state

0 vibrational level as its lower state. We have assigned thisand as the 00 band of the a31X

1 subband. The ex-ected position of the 11 band is overlapped by a very strongtomic line with extensive ringing, and therefore, the rotationaltructure could not be measured. The 22 or other higher

ibrational bands of this sequence were also not identifiedecause of their very weak intensity. The 01 and 12 bands ofhis subband were identified using the ground state vibrational

intervals obtained from the analysis of the A1X1 andA 1X1 transitions (15). A rotational analysis of the 00,01, and 12 bands of this subband were carried out. Eachband of this subband has three branches, one P, one Q, and one

R with the Q branch being the most intense. The R and Pbranches appear with similar intensity. A portion of the 00band of this transition is presented in Fig. 3 with the P, Q and

R branches marked.

RESULTS

While obtaining the vibrational assignment of the near-infrared bands of IrN, we noted that the excited state vibra-

TABLE 1Continued

TABLE 2

Molecular Constants (in cm1

) for the X1

, a30, and a

31 States of

193IrN from an Empirical Fit




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onal intervals of the a30 and a31 spin components are

onsiderably different, indicating an interaction of the a3tate with the other nearby states. The rotational analysis ofoth subbands was initially obtained by treating the two ex-ited spin components as separate states. Since both subbandsnvolve the ground state as their common lower state, theotational assignments in different bands were accomplishedy comparing the lower state combination differences to thealues obtained in our previous study of the A1X1 and 1X1 transitions (15). The rotational constants wereetermined by fitting the observed line positions of the twoubbands to the customary empirical energy level expressions:

FvJ T

v B

vJJ 1 D

vJJ 1 2

HvJJ 1 3 [1]

FvJ T

v B

vJJ 1 D

vJJ 1 2

HvJJ 1 3 1/ 2qJJ 1

qDJJ 12. [2]

n the final fit the blended lines were given reduced weights.The lines overlapped by N2 molecular lines or strong atomic

nes were excluded. In the final fit, the lines of both subbandswere combined with the lines of the A1X1 and A 1X

1 transitions (15) and a global fit was obtained to deter-mine a single set of molecular constants for each vibrationalevel. The observed line positions of the a30X

1 and a31X

1 subbands are provided in Table 1 and the molecular

constants obtained for the X1, a30, and a31 states of

193IrN are provided in Table 2.At the final stage of the analysis, we tried to combine the two

subbands in order to obtain a Hunds case (a) fit for the exciteda

3 state. Initially the lines of only the 00, 11, 01, and12 bands were included since the v 0 and 1 vibrationallevels were observed in both a30X

1 and a31X1 tran-

sitions. The observed transition wavenumbers were fitted withthe usual N2 Hamiltonian for the 3 state as given by Brownet al. (33). An explicit listing of the 3 state matrix elementsis provided by Brazier et al. (34). In this fit the molecularconstants T

v, A

v, ADv, Bv, Dv, qv, qDv, and pv were determined

with the standard deviation of fit of 0.95. The spectroscopicconstants such as A

vare only effective values because we are

missing the 30 and32 spin components. The inclusion of

the 23 and 34 bands, which were observed only for thea

30X1 subband, caused a deterioration in the quality of

the fit so that we decided not to include these bands. Theconstants obtained from the case (a) fit of the 00, 11, 01,

and 12 bands are provided in Table 3. The constants of Table3 are very much affected by global interaction with other statesand this results in, for example, very strong vibrational depen-dence of constant A.

AB INITIO RESULTS

The potential energy curves calculated at the CMRCI levelof theory are shown in Figs. 4 and 5 for the singlet and tripletspin manifolds, respectively. The energy scale used in thesefigures is relative to the minimum energy of the ground X1

TABLE 3

Molecular Constants (in cm1) for the X1 and a

3 States of 193IrN

from the Case (a) Fit of the a3 State




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lectronic state. The low-lying excited states in the singlet

ystem are, in order of increasing term energy: 11, A 1,1, 11, 21, and 11. The energy ordering within the

riplet system is the following: a3, b3, c3, 13, 13,3, and 23. The relative energy scale between both spinystems is better illustrated in Fig. 6.

The correlation diagram drawn in Fig. 7 allows an interepre-ation of the electronic state ordering predicted by our CMRCIalculations. This figure correlates the molecular orbitals ofrN to those of the iridium and nitrogen atoms in their groundtates. This diagram was obtained from size-consistent full-alence CASSCF calculations performed on the different spe-

ies: IrN, Ir(4

F), and N(4

S). The IrN orbitals were obtainedrom a state-averaged CASSCF calculation performed, at annternuclear distance of 1.6 , on the singlet states correlatingo the ground dissociation channel Ir(4F) N(4S), i.e., the

X1, 11, A 1, and 11 states. One can verify in Fig. 4 that

hese states do converge to the same dissociation limit at largenternuclear distances. Figure 7 provides a helpful one-electronicture for discussing the electronic structure of IrN. Note that

we have chosen to label only the valence orbitals. The dottednes connecting the orbitals of IrN to those of the atomicissociation products give qualitative information on the

LCAO content of the molecular orbitals, as provided by theanalysis of the CASSCF wavefunctions. One finds that

(i) The 2and 3MOs are, respectively, constructed frombonding and antibonding combinations of 5d(Ir) and 2p(N).

(ii) 3has, in addition, some of the 6s(Ir)2p(N) bondingcharacter.

(iii) 1 and 2, respectively, come from bonding and anti-

bonding combinations of 5d(Ir) and 2p(N).(iv) 4 is the antibonding analog of 3.

This orbital analysis is quite similar to the one obtained forRuN using the same computational approach (24), and bothnitrides have similar correlation diagrams (see Fig. 7 of Ref.(24)). The major difference to be stressed is the smaller 2 4energy gap in IrN (0.11 hartrees) than in RuN (0.22 hartrees)coming from a more antibonding 2orbital in IrN. It follows,as discussed below, that single electron promotions to the 2and 4orbitals need to be considered in IrN.

The electron orbital filling represented on Fig. 7, with the 14

valence electrons distributed in the low-lying orbitals, confirmsthe ground configuration as (15):

1222141 432 X1 .

FIG. 5. The low-lying triplet potential energy curves of IrN, from CMRCIcalculations.

FIG. 4. The low-lying singlet potential energy curves of IrN, from CMRCIalculations.




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This prediction is in agreement with the analysis of the corre-ponding CMRCI wavefunctions provided in Table 4 for theinglet states. This table lists the main configurations contrib-ting to the CI expansions, together with their weights in the

wavefunctions. The configuration weights are given by thequare of the corresponding CI coefficients. One sees thatonfiguration 1222141432 contributes 76% to the groundtate wavefunction. All singlet states are characterized by aingle dominant configuration. The weight of these configura-ons, however, only ranges between 70 and 80%. The weightf any single secondary configuration is quite small and theemaining contributions to the wavefunction are distributedver a large number of configurations. These features demon-trate the existence of strong electron correlation effects, al-eady observed in RuN (24). Table 5 provides similar infor-

mation for the triplet manifolds. Configuration mixing is moreronounced in the triplet system, as further discussed below.The electronic structure of the 14 states calculated in this

work can be explained in terms of eight leading configurationseported in Table 6 together with the corresponding electronromotions with respect to the ground configuration (see Fig.). These configurations will be referred to as configurationsA) to (H), Table 4. One finds that:

(i) The singlet and triplet and states below 20 000 cm1

rise from configurations (B) and (C). As detailed in Table 5,strong mixing between these two configurations is observed

for the 3 states, although the corresponding 1 states aremore pure.

(ii) Configuration (D) is the main component of the 11,11, 13, 23, 13 states.

(iii) The 21 and 13 states do not arise from configuration(D), but rather from configurations (E) and (F), respectively,both implying an electron promotion to the 4orbital.

(iv) Competition between three configurations (D, G, and

H), characterized by two electron promotions to the 2orbital,is observed for the 13 state.

The spectroscopic properties calculated from the CMRCIpotential energy curves are given in Table 7: the equilibriuminternuclear distances Re, the harmonic frequencies at equilib-rium e, and the term energies T0, corrected for the zero pointenergy contribution calculated within the harmonic approxi-mation.

DISCUSSION

From our recent studies of some transition metal containingdiatomic nitride molecules we have noted a remarkable resem-blance in the electronic structure of nitrides and isoelectroniccarbide molecules. For example, the electronic structure ofRuN (24) is found to be very similar to that of RhC (31, 32)and the electronic structure of OsN (13) is very similar to that

FIG. 6. Relative energies within the singlet and triplet systems of IrN,om CMRCI calculations.

FIG. 7. The molecular orbital correlation diagram for IrN, from CASSCFcalculations.




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f IrC (14, 35, 36). Likewise the electronic structure of IrN isxpected to be very similar to the electronic structure of iso-lectronic PtC (1922) (see Fig. 1). Interestingly, a similarorrespondence between the oxides and nitrides does not existor the group VIII transition metal derivatives, contrary toxpectations.

So far the electronic spectra of IrN were investigated only inhe 300020 000 cm1 region. As seen in Fig. 1, there is aemarkable similarity between the low-lying A 1 and A1tates of IrN and PtC. The two 1 and 0 states of IrNbserved in the near-infrared region of IrN were assigned as

he 31 and 30 spin components of the a3 state. Thisssignment is also supported by our ab initio calculations. This

work has implications for the A 1 state of PtC. In the lightf our present observations for IrN, we propose that the A 1

tate of PtC should be reassigned as the 30 spin component ofhe analogous a3 state and that the 31 spin component liesnobserved to lower wavenumbers.

The spectroscopic constants of Table 2 were used to evaluatehe equilibrium constants in the a3 state. Since the a30 and

31 spin components have quite different spectroscopic con-tants, we decided to evaluate the equilibrium constants for

both the spin components separately. The equilibrium con-stants for the X1, a30 and a

31 states are provided in Table8. As seen in this table, the a30 and a

31 states have verydifferent vibrational constants.

There is a very good agreement between the experimentaland ab initio results (see Table 7). The available experimentalvalues agree fairly well with the corresponding theoreticalvalues, with discrepancies of about 1% for Re, between 3 and8% for e, and less than 1400 cm

1 for T0 values. For example,the a3 state is predicted at 7991 cm1, compared to theobserved value of 8840 cm1. The present ab initio results

confirm the previous assignment for the A 1 and A1 states(15) and the new a3X1 transition. No definitive informa-tion about the regular or inverted character of the a3 state canbe deduced from the analysis of the wavefunction, given theobserved configuration mixing.

CONCLUSION

The near infrared region of IrN was investigated using aFourier transform spectrometer. The new bands observed in the75009000 cm1 region were classified into two transitions

TABLE 4

Weights of the Main Configurations in the Low-Lying Singlet Electronic States

of IrN from an Analysis of the CMRCI Wavefunctions

1 States are listed in order of energy.2 Molecular orbital occupancies are given in this column.3 Weights (in percent) are obtained from the square of the corresponding configuration interac-

tion coefficients; configurations having weights greater than 1% are listed.




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TABLE 5

Weights of the Main Configurations in the Low-Lying Triplet Electronic States

of IrN from an Analysis of the CMRCI Wavefunctions

1 States are listed in order of energy.2 Molecular orbital occupancies are given in this column.3 Weights (in percent) are obtained from the square of the corresponding configuration interaction

coefficients; configurations having weights greater than 1% are listed.

TABLE 6

Leading Configurations for the Low-Lying Electronic States of IrN




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TABLE 7

Spectroscopic Properties of the Low-Lying Electronic States of IrN

from CMRCI Ab Initio Calculations

Note. Experimental values are given in parentheses.a Ref. (15).b This work (see Table 3).c r0 value.d G(1/2) value.

TABLE 8

Equilibrium Constants (in cm1) for the X

1

, a30, and a

31 States of

193IrN

a Numbers in parentheses are one standard deviation uncertainty in the last digits.b Equilibrium constants from the case (a) constants of the a 3 state (Table III) are: G(1/2) 984.3629(23) cm1,

B e 0.4699116(27) cm1, e 0.0030058(50) cm

1, and re 1.6576432(47) .c G(1/2) value.




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with 00 R heads at 9194.3 and 8848.9 cm1. These tworansitions were assigned as a30X

1 and a31X1 sub-

ands of the a3X1 transition. A rotational analysis ofeveral bands of both subbands of the most abundant 193IrN

was obtained and the molecular constants were determined.Our observations indicate that the a3 state is globally affected

y other low-lying electronic states, resulting in quite differentibrational intervals in the two spin components of the excited3 state. The a3 state arises from the 1222143114 21lectronic configuration. The spectroscopic properties of theow-lying electronic states of IrN were predicted by ab initioalculations and our experimental assignments are supportedy these calculations.

ACKNOWLEDGMENTS

We thank M. Dulick of the National Solar Observatory for assistance inbtaining the spectra. The National Solar Observatory is operated by the

Association of Universities for Research in Astronomy, Inc., under contractith the National Science Foundation. The research described here was sup-

orted by funding from the NASA Laboratory Astrophysics Program. Supportas also provided by the Petroleum Research Fund administered by theAmerican Chemical Society and the Natural Sciences and Engineering Re-earch Council of Canada. J. Lievin thanks the Fonds National de la Recher-he Scientifique de Belgique (Contract FRFC 2-4551.92) for financial support.

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R. S. Ram, J. Lievin and P. F. Bernath- Emission Spectroscopy and Ab Initio Calculations on IrN

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Transcript of R. S. Ram, J. Lievin and P. F. Bernath- Emission Spectroscopy and Ab Initio Calculations on IrN