Spectroscopic Investigation of the Electronic Structure of YN: The X1Σ+ Ground State and the New B1, C1 and D1 Excited Electronic States

Journal of Molecular Spectroscopy 211, 86–92 (2002) doi:10.1006/jmsp.2001.8483, available online at http://www.idealibrary.com on

Spectroscopic Investigation of the Electronic Structure of YN: The X 1 Σ+ Ground State and the New B1, C1, and D1 Excited Electronic States Zygmunt J. Jakubek,1 S. G. Nakhate,2 and Benoit Simard Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Ontario, Canada K1A 0R6 Received July 2, 2001; in revised form October 20, 2001

YN molecules were produced in a free jet molecular beam apparatus by a laser vaporizing yttrium metal in the presence of He doped with NH3 . Laser excitation spectra were observed in the range 18 250–19 850 cm−1 . The ground state was confirmed to have 1 +  symmetry. The fundamental vibration in the ground state was measured to be 650.6(1) cm−1 . Three new electronic states, B1, C1, and D1, were observed at 18 974.7(1), 19 023.3(1), and 19 824.0(1) cm−1 , respectively. The fundamental vibrations and equilibrium internuclear distances were found to be 718.3(1) cm−1 and 1.939(8) for the B1 state and 723.5(1) cm−1 and 1.9194(3) for the C1 state. Two additional electronic states were identified with the help of a deperturbation procedure, one of which is either the 1  + or the 3 0− state. The newly observed electronic states cannot be accounted for based on the existing ab initio results. We expect that these states correlate with the excited asymptote Y(4d 1 5s 2 2 D) + N(2 D).

metal nitride, which is isovalent with YN. However, as far as experimental gas-phase spectroscopic results are concerned, only a single study of ScN has been reported (6). The relatively little attention devoted to the spectroscopy of transition-metal nitrides sharply contrasts with the expected importance of this class of molecules in catalysis (7), surface science, astrochemistry of certain types of stars (8), and other areas of science. Following our earlier high-resolution spectroscopic studies of the YNH (9, 10) and YH (11, 12) molecules, we continue systematic spectroscopic investigation of an yttrium–ammonia reaction and its products with this study of the electronic structure of YN. The first spectroscopic investigation of gas-phase YN was reported in 1994 by Ram and Bernath (3). They studied a sequence of v = 0 bands of the A1  + –X 1  + transition by Fourier transform infrared emission spectroscopy. Some of the bands were strongly perturbed, but the perturbers were not identified. Rotational analysis resulted in accurate rotational constants for the six lowest vibrational levels of the A1  + and X 1  + states. However, vibrational energies could not be determined, as no v = 0 bands were observed. The lowest electronic state observed by Ram and Bernath was identified as the X 1  + ground state, in agreement with the ab initio prediction of Shim and Gingerich (4). However, the assignment was rather indirect, based on similarity of the observed spectrum with those of ScN (6) and CaO, which is isoelectronic with ScN, and through comparison with the ab initio calculations of Kunze and Harrison (13) on ScN. The support of the assignment of the A1  + and X 1  + states attributed to the ab initio results was questionable, as no other than the ground state low-energy excited 1  + states were predicted for either YN or ScN at the time of experiment. Only recently were Daoudi et al. (5) able to calculate a low-energy

I. INTRODUCTION

Transition-metal-containing molecules present a serious challenge from both experimental and theoretical points of view. Their systematic experimental study received a strong boost about two decades ago with the development of laser ablation molecular beam sources (1). Recent progress in theoretical methods, as well as high-performance computing technology, made systematic and reliable theoretical investigation of this class of molecules feasible. Transition-metal-containing molecules are computationally more demanding than molecules containing main group elements, because of the extent of electron correlation required for even qualitatively correct results. For this reason, experimental results on excited electronic states of transition-metal-containing molecules are of great importance for theorists, as they provide a critical test of adequacy of selected methods and levels of theoretical treatment. The current status of electronic structure calculation on first-row transition-metal- and main-group-element-containing diatomics has recently been reviewed by Harrison (2). Second-row transition-metal-containing diatomics, in particular YN and other nitrides, have received much less attention from either theoreticians and experimentalists then their first-row counterparts. In fact, only one paper (3) on gas-phase spectroscopy of YN has been published so far. Also, only a single ab initio calculation of the electronic structure of YN has been reported (4). More abundant are ab initio results on the electronic structure of ScN (2, 5), a first-row transition 1

E-mail: [email protected]. On leave from Spectroscopy Division, Bhabha Atomic Research Centre, Mumbai 400 085, India. 2

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excited 1  + electronic state in ScN in good agreement with the observation of Ram and Bernath (6). Their conclusions regarding the electronic character of the ground state and some of the low-energy excited electronic states strongly disagreed with the earlier results of Harrison (2). Indirect and unconvincing assignment of the ground state of YN (and ScN) motivated us to conduct this spectroscopic investigation of YN in a jet-cooled molecular beam in order to unequivocally establish the symmetry of the ground state. We also expected that our study would shed some light on the electronic structure of YN and test the ab initio predictions of Shim and Gingerich (4).

spectra, the monochromator was set at a fixed wavelength corresponding to fluorescence either to the ground state or to some excited state while the pulsed dye laser was scanned. The excitation spectra were obtained with a typical resolution of 0.04 cm−1 . The transition wavenumbers were calibrated using a Fizeau wavemeter (New Focus). The manufacturer-specified absolute precision of the calibration was ∼0.35 cm−1 in the range of our spectra. However, by checking the wavemeter readings against some yttrium atomic lines positions we verified that the actual absolute precision was significantly better, possibly ∼0.1 cm−1 . The internal precision of a single scan was ∼0.03–0.05 cm−1 .

II. EXPERIMENTAL

III. DATA ANALYSIS AND RESULTS

The YN molecules were produced in a free-jet molecular beam apparatus. The experimental conditions were similar to those used in our investigation of YNH (9). A rotating yttrium rod (Goodfellow, 99.9%) was ablated with the third harmonic beam (354.7 nm, 10–20 mJ/pulse) of a Nd:YAG laser (Lumonics, YM200) focused to an ∼1 mm2 spot in the presence of He carrier gas doped with ∼1–2% NH3 (14 NH3 , Matheson; 15 NH3 , MSD Isotopes, 95% 15 N).The He gas containing the YN molecules and other products of the yttrium plasma reaction with ammonia expanded into vacuum, cooling the internal degrees of freedom of the molecules. The molecules were excited at right angles to the supersonic expansion about 4 cm downstream from the nozzle by an excimer-pumped (Lumonics HyperEx-400) pulsed dye laser (Lumonics, Hyperdye 300). The resulting fluorescence was viewed through a 1-m Spex monochromator equipped with a cryocooled photomultiplier tube (R943-02, Hamamatsu). In order to record excitation

III.1. Appearance of the Spectra The excitation spectra were observed in the range 18 250– 19 850 cm−1 for the Y14 N and Y15 N isotopomers. Observation of the spectra for two isotopomers was intended as an aid in assignment of the vibrational quantum numbers. Seven bands clustered in three groups were observed and rotationally analyzed. The bands were red-degraded and each of them had P, Q, and R branches. The middle group of the excitation spectrum consisted of two bands with band origins at 18 975 and 19 023 cm−1 in Y14 N (see Fig. 1). The branches of the 18 975-cm−1 band ran up to J = 28. In the 19 023-cm−1 band the Q branch stretched up to J = 36, while the P and R branches were significantly shorter (J ≤ 25). The bands showed upon 14 N → 15 N isotopic substitution red shifts of approximately 8 and 12 cm−1 , respectively. The bands’ intensities were comparable in the Y15 N isotopomer. However, in Y14 N the 19 023-cm−1 band was weaker than the 18 975 cm−1 one by approximately a

Y14N

7.88 cm

-1

12 cm

-1

15

Y N

18920 18930 18940 18950 18960 18970 18980 18990 19000 19010 19020 19030 -1

Wavenumber (cm ) FIG. 1.

The (0, 0) B1–X 1  + and the (0, 0) C1–X 1  + bands of the Y14 N (top trace) and Y15 N (bottom trace) isotopomers.

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FIG. 2.

Rotational structure of the (1, 0) D1–X 1  + band of the Y14 N isotopomer.

factor of 5 (see Fig. 1). The group at the highest-wavenumber end of the excitation spectrum showed three bands with band origins at 19 693, 19 747, and 19 824 cm−1 in Y14 N. The 14 N/15 N isotope shifts were approximately equal to 10, 14, and 21 cm−1 , respectively. Intensities of the three bands were approximately equal in the Y15 N isotopomer. In Y14 N the 19 693 cm−1 band was weaker than the two others by approximately a factor of 5. The 19 824-cm−1 band showed a regular structure of very long branches (J ≤ 37) (see Fig. 2). The branches of the other two bands were significantly shorter, extending up to J = 16 and J = 29 for the 19 693 and 19 747 cm−1 bands, respectively. The 19749.5 19749.0

T −0.3615∗J''(J''+1) /cm−1

19748.5 19748.0 19747.5 19747.0 19746.5 19746.0 19745.5 19745.0 0

500

1000

J''(J'' + 1) FIG. 3. Reduced term value plot for the v = 1 C1 state of the Y14 N isotopomer. Open/solid circles indicate e/ f -symmetry levels, respectively.

19 747 cm−1 band was visibly perturbed in all three branches (see Fig. 3). No apparent perturbations were present in other Y14 N or Y15 N bands. The two bands at the lowest-wavenumber end of the spectrum with band origins at 18 324 and 18 373 cm−1 in Y14 N had approximately equal relative intensity, but were several times weaker than the five other bands. Their rotational structures extended up to J = 24. These bands were not studied in the Y15 N isotopomer. III.2. Quantum Number Assignments Rotational quantum number assignment of P and R branches was carried out using the well known combination differences method. The assignment of Q branches could not be established using the combination differences method as no corresponding branches terminating on the same-symmetry levels of the upper states were present in the spectra. Instead, they are based on observation of the first lines (see Fig. 2). Combination differences for the lower states of the bands 18 975, 19 023, 19 693, 19 747, and 19 824 cm−1 coincided within the experimental precision with those of the v = 0 level of the X 1  + state observed by Ram and Bernath (3)(see Fig. 4); the combination differences of the lower state of the bands 18 324 and 18 373 cm−1 coincided with those of the v = 1 level of the X 1  + state. Since our spectra were obtained in a jet-cooled molecular beam, we are entitled to conclude that the X 1  + state, first observed by Ram and Bernath (3), is the ground state of the YN molecule indeed. The upper state combination differences of the 18 975 and 19 023 cm−1 states (e-symmetry levels only) turned out to be the same within the experimental precision as those of the 19 693 and 19 747 cm−1 state, respectively. Vibrational assignments of the upper states based on the isotope effect was inconclusive. The observed isotope shift values upon 14 N → 15 N substitution were intermediate between those expected for v = 0 and v = 1 levels. However, since no bands originating from v

= 0 could be found to the red of the bands

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their more precise (experimental precision 0.002 cm−1 ) infrared data on v = 0 A1  + –X 1  + bands. A test fit with the v = 0 and v = 1 X 1  + ground state molecular constants treated as adjustable parameters showed that their values were statistically equivalent to those of Ram and Bernath, but much more poorly determined (see also Fig. 4). The X 1  + state was described by the well-known Hamiltonian

0.432

B" /cm−1

0.430

0.428

H = Tv + Bv J (J + 1) − Dv [J (J + 1)]2 ,

0.426

0.424

0.422

0.420 0

200

400

600

800

1000 1200

where Tv is a rotationless energy calculated with respect to the J = 0 v = 0 level of the X 1  + ground state, Bv is a rotational constant, and Dv is a centrifugal distortion constant. Molecular constants for the B1, C1, and D1 states were treated as adjustable parameters. Since only the  quantum number for the upper states could be conclusively assigned, an effective Hamiltonian of the form   H ef = Tv + Bv [J (J + 1) − 1] − Dv [J (J + 1) − 1]2 + Hv [J (J + 1) − 1]3

J(J+1) FIG. 4. Graphical determination of the rotational (B) and centrifugal distortion (D) constants of the ground state from combination differences. The solid straight line connects points calculated using the B0 and D0 values taken from Ref. (3). The broken lines indicate limits around the solid line corresponding to experimental precission of 0.1 cm−1 . Different symbols represent points obtained from different bands: diamond—(1, 0) D1–X 1  + , circle— (0, 0) B1–X 1  + , star—(0, 0) C1–X 1  + , triangle— (1, 0) B1–X 1  + , cross— (1, 0) C1–X1  + .

18 975 and 19 023 cm−1 , these bands were assigned the v = 0 vibrational quantum number in the upper electronic states. Similarly, the bands 18 324 and 18 373 cm−1 were also assigned the v = 0 quantum number based on the similarity of their upper state combination differences with those of the bands 18 975 and 19 023 cm−1 , respectively. The remaining three bands, 18 975, 18 324, and 19 693 cm−1 , were assigned the v = 1 quantum number. Finally, the presence of strong Q branches in all seven bands of our excitation spectra indicated that they were  = ±1 transitions; thus, the upper states were  = 1 states. The procedure described above led to the following labeling scheme for the excitation bands: the bands 18 324, 18 975, and 19 693 cm−1 are the (0, 1), (0, 0), and (1, 0) bands of the B1–X 1  + electronic transition, respectively; the bands 18 373, 19 023, and 19 747 cm−1 are the (0, 1), (0, 0), and (1, 0) bands of the C1–X 1  + electronic transition, respectively; and the 19 824 cm−1 band is the (1, 0) band of the D1–X 1  + electronic transition. III.3. Least Squares Fit The set of transition wavenumbers of the excitation bands was reduced to molecular constants by a nonlinear least squares fitting procedure. The molecular constants for the v = 0 and v = 1 levels of the X 1  + ground state were constrained in the fit at the values obtained by Ram and Bernath (3) from the analysis of

± 0.5[qv + q H v J (J + 1)2 ]J (J + 1) TABLE 1 Molecular Constants (in cm−1 ) of the X 1 Σ+ Ground State and the B1, C1, D1 Excited States and Their Perturbers

TX BX D X × 107 TB1 B B1 D B1 × 107 H B1 × 108 q B1 × 104 TC1 BC1 DC1 × 106 qC1 × 104 q H,C1 × 109 TD1 B D1 D D1 × 106 q D1 × 104 T p1 B p1 q p1 × 102 α1, p1 α2, p2 × 102 T p2 B p2 a1, p2

v=0

v=1

0.0 [0.42666973]a [7.0923]a 18974.660 0.38833 7.4 −1.336 4.78 19023.285 0.390173 −3.071 4.11

650.642 [0.4233284]a [5.9670]a 19692.979 0.35209 116.

(18)b (35) (14)

20.7 19746.824 0.365162 2.406 3.17 1.402 19823.989 0.384628 −2.516 1.82 19755.59 0.2684 −1.882 1.289 −6.2 19758.02 0.3255 0.404

(12) (11)b (64) (69) (43) (69) (9)b (38) (33) (20) (18)b (20) (67) (11) (11) (58)b (20) (20)

(8)b (12) (38) (34) (25) (7)b (36) (38) (26)

(5)b

Note. Numbers in parentheses represent one standard deviation in units of the last quoted digit. a Parameters (from Ref. 3) constrained in the fit. b This value represents a statistical error. In addition, there is a systematic error equal to ∼0.1 cm−1 .

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was used for all three states, where Tv is a rotationless energy calculated with respect to the J = 0 v = 0 level of the X 1  + ground state, Bv is a rotational constant, Dv and Hv are higher order corrections to Bv , qv is a -doubling constant, and q H v is a higher order correction to qv . A Hamiltonian of a similar form was used for two perturbers of the v = 1 C1 state. In addition, perturbation matrix elements of the form e

H

f

= α 1 ± α2

were used, where α1 and α2 are effective perturbation parameters. The final results of the least squares fit are shown in Table 1. IV. DISCUSSION

Based on the experimental evidence presented above the three excited electronic states, B1, C1, and D1, were all assigned the  = 1 quantum number. In order to better understand the nature of these states we attempted to correlate them with states calculated by Shim and Gingerich (4). They predicted four bound states, 3  − , 5 , 5  − , and 1 , with potential minima in the range 19 700–33 000 cm−1 above the ground state, which all have equilibrium internuclear distances ∼1 a.u. longer than the ground state. The first three of these states, which have relatively shallow (De ≈ 4000–9000 cm−1 ) and strongly asymmetric potentials, adiabatically correlate with the lowest atomic asymptote, Y(4d 1 5s 2 2 D) + N(4 S); the 1  state adiabatically correlates with an excited-term atomic asymptote. In addition, there are three other molecular states originating from the Y(4d 1 5s 2 2 D) + N(4 S) atomic asymptote, 3 , 3 , and 5 . The 3  and 5  states were not calculated by Shim and Gingerich (4). The 3  state adiabatic potential curve was predicted to have the minimum around 10 000 cm−1 above the ground state. Two other low-energy excited states, 1  and 3 +  , and the 1  + ground state adiabatically correlate with the excited-term atomic asymptotes. The four lowest-energy electronic states appear to have equilibrium internuclear distances significantly shorter, by ∼0.5–1 a.u., than the next four higher excited states. The rotational constants of the B1 and C1 states are about 10% (in v = 0 level) smaller than that of the v = 0 X 1  + ground state (see Table 1). However, since they are still much smaller in the v = 1 level, the equilibrium internuclear distances in the B1 and C1 states, which are estimated to be equal to 1.939(8) ˚ respectively, are only slightly longer than in the and 1.9194(3) A, ˚ (3)). The large differences between ground state (1.804 05(50) A the B0 and B1 values (∼0.036 cm−1 for B1 and ∼0.025 cm−1 for C1) suggest that these states have strongly asymmetric potentials. For this reason, one could expect that they correspond to the above mentioned 3  − , 5 , or 5  − states. However, it is difficult to imagine that the 5 1 –1  + and 5 1− –1  + transitions would have sufficiently large intensity to be observable in our experiment. Also, it would not be easy to explain why only the

1− –1  + component was observed and the 3 0− −1  + one was absent from our spectra. In addition, the fundamental vibrations’ wavenumbers of 718.3 and 723.5 cm−1 for B1 and C1, respectively, both larger than the fundamental vibration wavenumber of the ground state, equal to 650.6 cm−1 , would indicate stronger bonding in the B1 and C1 states as compared with the ground state. Since the 3  − , 5 , and 5  − states were predicted by Shim and Gingerich (4) to have wide-open and shallow potentials, as compared with the ground state, this makes them unlikely candidates for the theoretical counterparts of the B1 and C1 states. The D1 state with the rotationless term value of 19 823.989(9) cm−1 and rotational constant equal to 0.384 628(38) cm−1 was only observed in a single vibrational level. Although the rotational constant was closer to those of the v = 0 rather than the v = 1 levels of the B1 and C1 states, we assigned the v = 1 vibrational quantum number to this state based on the size of isotope effect. However, such an assignment should only be considered as a tentative one. Despite our best effort we were unable to observe transitions to neither of the adjacent vibrational levels of the D1 state. This fact posed a serious concern for us. The D1–X 1  + band was a rather strong and well developed band; thus we could dismiss a possibility that D1 was a dark state which gained some intensity due to a local perturbation with the v = 1 B1 and v = 1 C1 states. An alternative assignment of the v = 0 quantum number to the D1 state should also be considered. In that case, one of the reasons for lack of observation of the v = 1 level could be predissociation. However, this hypothesis seems unlikely, as it would require the dissociation energy to be ∼2.5 eV, thus less than half of the value predicted by Shim and Gingerich (4). Another cause of the absence of the v = 1 level could be a small Franck–Condon overlap factor. This option would require the D1 state potential curve to closely resemble that of the X 1  + ground state, which would restrict observable transitions to the v = 0 sequence of bands. If this postulation were true one would also expect to see the next member of the sequence originating from the v = 1 level of the X 1  + ground state, which was absent from our spectra. It is clear that the existing ab initio results cannot explain the electronic structure of YN properly. Until a more extensive ab initio study of YN is carried out, the available results on ScN, which is isovalent with YN, should help to rationalize our observations. The electronic structures of Sc and Y are very similar; they include the same low-lying electronic configurations (with the principal quantum number larger by one in yttrium) and the atomic term ordering and energetics (14, 15) are only slightly different. The only noticeable difference, the presence of the low-lying 5s 2 (1 S)5 p 2 P o term in yttrium, should not affect the picture, as this configuration is not expected to participate in YN bonding, at least at short ranges comparable to the equilibrium internuclear distance of the ground state. In their recent paper, Daoudi et al. (5) showed that the A1  + 3

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state observed by Ram and Bernath in ScN (6) at ∼5800 cm−1 correlates with the excited asymptote Sc(3d 1 4s 2 2 D) + N(2 D). The corresponding A1  + state was also observed by Ram and Bernath in YN (3) at ∼3900 cm−1 . Including the A1  + state, the Sc(3d 1 4s 2 2 D) + N(2 D)(and Y(4d 1 5s 2 2 D) + N(2 D) as well) asymptote gives rise to 30 electronic states (5): 1,3  + (2), 1,3 −  (3), 1,3 (4), 1,3 (3), 1,3 (2), and 1,3 . Although no other potential curve than that of the A1  + state was calculated by Daoudi et al. (5), we may expect that some of the other states will also be sufficiently strongly bound to have the potential minima at about 19 000 cm−1 above the ground state, and thus to have potential depths comparable with that of the X 1  + ground state. If that is true the B1, C1, and D1 states could correspond to three of the four 1  states or, possibly, the four 3 1 states. Several perturbations were detected for the Y14 N isotopomer. Only two of them were treated explicitly in the fit. Effects of others, because of insufficient experimental data, were included in the effective values of the fit parameters. The first perturber of the v = 1 C1 state with rotationless term value of 19 755.59(18) cm−1 (see Fig. 3) affects both the e- (near J = 8) and f -symmetry (near J = 9) levels. Interaction strength is approximately equal in both symmetry components. The small rotational constant of the perturber equal to 0.2684(20) cm−1 indicates that it could be a high vibrational level of an unidentified electronic state. We can estimate that if the rotational constant of the perturber changes with v as fast as that of the C1 state (B1 − B0 = 0.025011 cm−1 ), five vibrational quanta would be required to bring down the rotational constant value from ∼0.39–0.40 cm−1 at v = 0 to ∼0.268 cm−1 , the value obtained for the perturber. The small value of the rotational constant could also be associated with a longer bond in the perturbing state. Average internuclear distance in the perturbing state would be about 17% or ∼0.5–0.6 a.u. longer than in the v = 0 C1 state. The second perturber (Fig. 3) with the rotationless term value of 19 758.02(58) cm−1 affects only e-symmetry levels (near J = 16); thus it has to be of 1  + or 3 0− symmetry. Possible candidates for this perturber are between the above mentioned states correlating with the Y(4d 1 5s 2 2 D) + N(2 D) asymptote. The second perturbation is weaker than the first one. The largedifference in the values of the rotational constants (B = 0.3255(20) cm−1 vs 0.2684(20) cm−1 ) of the perturbers indicates that, most likely, they are not members of the same spin multiplet. Other perturbations could not be treated explicitly. The third perturber of the v = 1 C1 state (Fig. 3) affects only f -symmetry levels (at J > 21); thus it has to be of 1  − or 3 0+ symmetry. Its effect was accounted for by introducing the q H v [J (J + 1)]3 term in the Hamiltonian. The q H v constant is an effective parameter here and should not be interpreted as a centrifugal correction to qv . The J = 1, . . . , 5 e-symmetry rotational levels of the v = 1 B1 state are pushed to higher energy as compared with the f symmetry levels by up to 0.3 cm−1 . The perturbing state has to be of 1  + or 3 0− symmetry. Since no extra lines were observed

this perturbation could not be treated explicitly. The P and R lines terminating on the perturbed upper rotational levels were removed from the fit. However, the effect of the perturbation is still present in the fit in the form of the abnormally large value of the qv parameter (20.7 × 10−4 cm−1 ), which is larger than that in the v = 0 level of the same electronic state by a factor of 5. Both the v = 0 and v = 1 levels of the B1 state experience long-range perturbation from a state at higher energy. This perturbation is manifested by the abnormally large value of the Dv constant in the v = 1 level and the negative sign and the large absolute value of the Hv constant in the v = 0 level. V. CONCLUSIONS

Our jet-cooled molecular beam studies of YN unequivocally confirm that the ground state is of 1  + symmetry, as previously suggested by Ram and Bernath, and it is the same as the lower electronic state of their infrared bands. We observed three new  = 1 electronic states at 18 974.660(8), 19 023.285(7), and 19 823.989(9) cm−1 (v = 1), but their exact identities could not be determined. In addition, by carrying out a deperturbation analysis we located two more electronic states, one of which is either a 1  + state or a 3  − state. The experimental data were reduced to the set of 31 molecular parameters. Our observations cannot be rationalized using the existing ab initio results of Shim and Gingerich (4). The electronic structure of YN turns out to be more complex than it has been suggested by their results. Higher excited atomic asymptotes, in particular Y(4d 1 5s 2 2 D) + N(2 D), can also contribute low energy excited electronic states, as it has been shown by Daoudi et al. (5) for the isovalent molecule ScN. We hope that our observation of the B1, C1, and D1 new excited electronic states will stimulate more extensive ab initio study of YN. Further, more detailed, spectroscopic investigation of the excited electronic states would also be of great value for better understanding of the electronic structure of YN. ACKNOWLEDGMENT SGN is grateful to the Natural Sciences and Engineering Research Council of Canada for financial support.

REFERENCES 1. T. G. Dietz, M. A. Duncan, D. E. Powers, and R. E. Smalley, J. Chem. Phys. 74, 6511–6512 (1981). 2. J. F. Harrison, Chem. Rev. 100, 679–716 (2000). 3. R. Ram and P. Bernath, J. Mol. Spectrosc. 165, 97–106 (1994). 4. I. Shim and K. A. Gingerich, Int. J. Quant. Chem. 46, 145–157 (1993). 5. A. Daoudi, S. Elkhattabi, G. Berthier, and J. P. Flament, Chem. Phys. 230, 31–44 (1998).

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6. R. S. Ram and P. F. Bernath, J. Chem. Phys. 96, 6344–6347 (1992). 7. M. Grunze, in “The Chemical Physics of Solid Surfaces and Heterogenous Catalysis” (D. A. King and D. P. Woodruff, Eds.), Vol. 4, pp. 143–194. Elsevier, New York, 1982. 8. A. J. Sauval, Astron. Astrophys. 62, 295–298 (1978). 9. B. Simard, Z. Jakubek, H. Niki, and W. J. Balfour, J. Chem. Phys. 111, 1483–1493 (1999). 10. Z. J. Jakubek, B. Simard, H. Niki, and W. J. Balfour, J. Chem. Phys. 113, 3591–3601 (2000).

11. Z. J. Jakubek, S. Nakhate, B. Simard, and W. J. Balfour, J. Mol. Spectrosc., to be published. 12. Z. J. Jakubek, B. Simard, and W. J. Balfour, Chem. Phys. Lett., to be published. 13. K. L. Kunze and J. F. Harrison, J. Am. Chem. Soc. 112, 3812–3825 (1990). 14. C. E. Moore, “Atomic Energy Levels,” National Standard Reference Data Series, Vol. I. Natl. Bur. Stand., U.S. GPO, Washington, DC, 1971. 15. C. E. Moore, “Atomic Energy Levels,” National Standard Reference Data Series, Vol. I. Natl. Bur. Stand., U.S. GPO, Washington, DC, 1971.

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