The 020 level in the ground state of NH2

JOURNAL

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MOLECULAR

SPECTROSCOPY

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493-495 (1981)

NOTE The 020 Level in the Ground State of NH, Accurate molecular constants have been determined for the 008 and 010 levels in the ground state of NH%, but only approximate constants are available for a few of the higher levels. For the 000 level the rotational constants have been determined primarily from a combination of optical (1-3) and laser magnetic resonance (LMR) studies (4) while the spin-rotational constants have been derived mainly from microwave-optical double resonance (MODR) experiments (5-6). For the 010 level no MODR data are available and the molecular constants have been obtained using optical (2, 7) and LMR data (8). Approximate values of Y,,,A, B, and C have been derived for the 020, 100, 110, and 040 levels on the basis of laser-excited fluorescence studies (9). In the present work the emission spectrum of NH, was photographed in the region 4000-8900 A using a 7.3-m vacuum Ebert spectrograph. The source was a radiofrequency discharge through flowing ammonia. Approximately 27 000 lines were measured with an estimated precision of 0.003 cm-*, though larger systematic errors (-0.02 cm-‘) were frequently encountered between different sets of measurements. The assignment of the lines was greatly assisted by the extensive laser-excitation studies which have been carried out (10, II). Although the precision of these resonance fluorescence measurements was only 0.2 A, the combination of these results with the more precise emission measurements, using the method of combination differences, frequently led to unambiguous assignments for the lines. To date, approximately 7000 assignments have been made; -3500 involve the 000 level, -2000 the 010 level, -1000 the 020 level, and -500 the higher vibrational levels of the ground state. A similar study has been reported earlier but with lower resolution (II). The Hamiltonian used in fitting the data has been given earlier (3). It consists of the complete sextic A-form reduced asymmetric rotor Hamiltonian of Watson (I2) plus the leading eighth-power term, together with the quartic A-form reduced spin-rotation Hamiltonian of Brown and Sears (13). Lower state term values were obtained by subtracting the frequency of each assigned line from the excited state term values determined earlier (3), care being taken to minimize any effects of systematic error between the emission and absorption data. For the 020 level most of the rotational term values were determined up to N = 8 and K, = 5. For the 010 and 000 levels, higher rotational term values were also observed, e.g., up to N = 16 and K, = 16 for the 000 level. As pointed out earlier, the number of terms required for the rotational Hamiltonian depends on the range of energy levels being fitted and the precision desired. Since the present work is directed mainly towards the determination of the molecular constants for the 020 level, and a comparison with the constants for the 010 and 000 levels, the range of levels was restricted to N = 8 and K, = 8 for all the data. Under these circumstances the corresponding levels for the ground state of H,O (14) are fitted by the above Hamiltonian with a standard deviation of kO.002 cm-‘. The molecular constants for the three levels are given in Table I, together with the standard deviations of the fits of the optical data. For the 000 level the fitting was accomplished using the 3 1 MODR frequencies and 24 LMR frequencies discussed earlier (3) together with the optical data both in emission and absorption. The constants are considered to be marginally superior to those presented earlier and are included mainly for comparison purposes. The constants for the 010 level were obtained by fitting 55 zero-field frequencies calculated by Dr. J. M. Brown from the LMR data in (B), 6 frequencies with AK, = -3 from (Z5), 32 diode laser frequencies kindly provided by Dr. E. Hirota and Mr. K. Kawaguchi, and 2001 combination differences derived from our emission data. The constants are considerably more accurate than those given in (2,7) and give a better overall fit to the term values than the constants given in (8). The latter give a good fit to the terms directly involved in the measured frequencies, but give a less satisfactory representation of other terms in the same range of N and K,. Further refinement of the constants can be expected as further diode laser measurements become

493

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NOTE TABLE I Molecular Constants for the (020). (010) and (000) Levels in the .@B, Ground State of NHta constant

(020)

(010)

778443.,,(18)

(000) 710238.,,(6)

A

863441.,,(69)

B

397452.62(32)

39-iO68.53(14) 388292.6*(4)

236719.28(32)

240849.,e(lZ)

C

AK

1792.2,0(79)

1059.7*,(18)

'NK

-195.4,,(34)

-161.6,,(9)

AN *K 6N HK HKN

35.61,9(31)

40.3228(96) 129.9,5(22) 16.471+~(15)

31.632rC12) 30.1,,(6)

14.70,3(21)

12.752~(6)

7.79~3(27)

4.34,,(6)

1.88,,(3)

-0.64,,(17)

-0.60,,(3)

-0.31*2(4)

lo2 HNK

o.oc

o.oc

1.71j2(90)

1.78,,(29)

bK

0.390\(23)

1.00,,(6)

10' hNK

o.oc

O.OC

10' hN

o.oc

102 LK

o.oc

0.87,a(21) -1.26,,(5)

-15914.10(225)

-11975.eoC41)

BS(=“bb)

-1545.05(121)

-1429.,,(40)

CS(=Ecc)

-62.,,(130)

_13.69(36)

s

657.765(7) -124.752(5)

72.1h3(7)

lo2 HN

AS(=caa)

245010.52(5)

AK s 'KN S 'NK s pN S 6K S 6N

-107.9,(10)

GO Bb

0.011

0.010

3.25rc -O.l,gC

-46.9,(4) 2.,8(3) -O.logC

-2.4,,(15) 1.21,,(14) 0.50,,(4) -0.4,,(9) 0.67noC6) -0.42>,(2) -9264.532(10) -1354.6,3(4) 12.169(2) -31.31rC4) 3.2,,(8) -0.1,,(E)

-o.319,3c

-0.437(6)

_0.31963(5)

_0.62,QC

-0.6260c

-0.62,,(6)

-o.15880c

-0.15880c

-0.158,,(3)

2961.210(q)

1497.321(2) 0.008

All canstants are given in MHz except For G0 which is quoted in cm-'. The error limits are 30 and ark right justified to the last digit on the line; sufficient additional digits are quoted below the line to reproduce the original data with full aCC"raCy. Standard deviation of the fit of the optical lines in cm-'. These constants are not determined and were fixed at the values quoted.

available and as the grating measurements from Fourier transform spectroscopy.

for the emission spectrum are replaced by measurements

ACKNOWLEDGMENTS We are indebted to S. C. Ross and C. Zauli for making available to us their latest assignments the absorption spectrum.

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REFERENCES

1. 2. 3. 4.

K. DRESSLER AND D. A. RAMSAY, Phil. Trans. Roy. Sot. London Ser. A 251, 553-602 (1959). J. W. C. JOHNS,D. A. RAMSAY, AND S. C. Ross, Canad. J. Phys. 54, 1804-1814 (1976).

F. W. BIRSS,D. A. RAMSAY, S. C. Ross, AND C. ZAULI, J. Mol. Spectrosc. 78,344-346 (1979). P. B. DAVIES, D. K. RUSSELL,B. A. THRUSH,AND H. E. RADFORD,Proc. Roy. SOC. Ser. A 353, 299-318 (1977). 5. J. M. COOK, G. W. HILLS, AND R. F. CURL, JR., J. Chem. Phys. 67, 1450-1461 (1977).

6. G. W. HILLS,R. S. LOWE, J. M. COOK,ANDR. F. CURL,JR.,J. Chem. Phys. 68,4073-4076(1978). 7. M. VERVLOET,M. F. MERIENNE-LAFORE, AND D. A. RAMSAY,Chem. Phys. Lett. 57, S-7(1978). 8. K. KAWAGUCHI,C. YAMADA, E. HIROTA,J. M. BROWN, J. BUTTENSHAW,C. R. PARENT,AND T. J. SEARS,J. Mol. Spectrosc. 81, 60-72 (1980). 9. M. KROLL,.I. Chem. Phys. 63, 319-325 (1975). 10. M. VERVLOETAND M. F. MERIENNE-LAFORE, .I. Chem. Phys. 69, 1257-1262 (1978). II. M. VERVLOET,Thesis, Reims, 1978. 12. J. K. G. WATSON,in “Vibrational Spectraand Structures” (JamesR. Durig, Ed.), Vol. 6, pp. l-89, Dekker, New York, 1977. 13. J. M. BROWN AND T. J. SEARS,J. Mol. Spectrosc. 75, 111-133 (1979). 14. J. M. FLAUD AND C. CAMY-PEYRET,Mol. Phys. 26, 811-823 (1973). 15. G. W. HILLS AND A. R. W. MCKELLAR,J. Mol. Spectrosc. 74, 224-227 (1979). F. W. BIRSS

Department of Chemistry University of Alberta Edmonton, Alberta T6G 2G2, Canada M.-F. MERIENNE-LAFORE

Laboratoire de Chimie Physique UER Sciences Reims Cedex 51062, France D. A. RAMSAYAND M. .VERVLOET

Herzberg Institute of Astrophysics National Research Council of Canada Ottawa, Ontario KlA OR6, Canada Received September 5, 1980