Notizen 837
Pure Rotation Spectrum of NNO in the Far Infrared Region
Koichi M . T. Y a m a d a
I. Physikalisches Institut Universität zu Köln D-5000 Köln 41, West Germany
Z. Naturforsch. 45 a, 837-838 (1990);
received April 4, 1990
The pure rotational spectrum of N N O has been observed as an impurity in the N O spectrum which has been recorded with a high resolution Fourier transform spectrometer. The observed high-J transitions in the ground vibrational state were analyzed by a least-squares fit together with the avail- able millimeter and submillimeter wave data. It has been proved that the highly precise data of Maki et al. [3] can be used as a wavenumber standard for the far infrared.
Recently M a k i a n d c o w o r k e r s have revised the spectroscopic p a r a m e t e r s of t h e N N O molecule in- tending to supply a g o o d w a v e n u m b e r s t a n d a r d in the infrared region [ 1 - 3 ] . T h e y h a v e m e a s u r e d the vibra- t i o n - r o t a t i o n t r a n s i t i o n s of this molecule in the infra- red region with high precision by using h e t e r o d y n e technique. T h e y have p r o v i d e d also a very a c c u r a t e set of the c o n s t a n t s for the g r o u n d state [3]. H o w e v e r , t o the best of o u r knowledge, the p u r e r o t a t i o n a l transi- tions in the far i n f r a r e d (FIR) h a v e n o t been m e a s u r e d yet. In the course of o u r m e a u r e m e n t s of N O [4], we have accidentally f o u n d the s p e c t r u m of N N O in t h e region f r o m 20 to 50 cm ~~1 as a n i m p u r i t y . By analyz- ing these d a t a t o g e t h e r with t h e available millimeter (mmw) a n d submillimeter ( s u b - m m w ) w a v e d a t a [5, 6]
u p to 552 G H z , we have c o n f i r m e d the g r o u n d state c o n s t a n t s of [3].
T h e m e a s u r e m e n t h a s been carried o u t with a Bruker I F S 120 H R v a c u u m M i c h e l s o n s p e c t r o m e t e r at the m o l e c u l a r s p e c t r o s c o p y l a b o r a t o r y , P h y s i k a - lisch-Chemisches Institut, U n i v e r s i t ä t Giessen. T h e sample of N O was s u b l i m a t e d at a low t e m p e r a t u r e , in o r d e r to prevent c o n t a m i n a t i o n by water, a n d w a s transfered at a total pressure of a b o u t 150 P a i n t o t h e 1.8 m long cell sealed with polyethylene w i n d o w s . T h e total pressure was a r o u n d 156 P a . T h e s p e c t r u m w a s recorded in the region f r o m 20 t o 90 c m- 1, using a myler beamsplitter a n d I n S b d e t e c t o r at liq. H e t e m - Reprint request to Dr. Koichi M. T. Yamada, I. Physikali- sches Institut, Universität zu Köln, D-5000 Köln 41, West Germany.
p e r a t u r e . Small a m o u n t s of water, which p r e s u m a b l y c a m e o u t of the cell wall, have been detected. In addi- tion to the N O s p e c t r u m very regularly spaced lines were observed, w h o s e s e p a r a t i o n indicated N N O t o b e t h e carrier caused by a small i m p u r i t y in t h e N O gas.
Table 1. Measured transitions of N N O in the ground state.
J'- -J" Observed Obs-Calc Unit
24 -23 20.10268 -0.00012 cm - 1 a
25--24 20.93954 0.00000 c m- l a
26--25 21.77625 0.00006 cm - 1 a
27--26 22.61283 0.00010 c m- l a
28 -27 23.44908 -0.00007 c m "1 a
29 -28 24.28544 -0.00002 c m "1 a
30 -29 25.12173 0.00008 cm - 1 a
31--30 25.95782 0.00012 c m ~l a
32--31 26.79374 0.00012 c m- l a
33--32 27.62954 0.00013 c m "l a
34 -33 28.46510 0.00003 c m ~l a
35--34 29.30064 0.00007 c m_ l a
36--35 30.13594 0.00001 c m_ l a
37--36 30.97119 0.00005 c m ~l a
38 -37 31.80625 0.00006 c m_ l a
39 -38 32.64120 0.00013 c m "1 a
40--39 33.47586 0.00006 c m ~l a
41--40 34.31028 -0.00008 c m- l a
42--41 35.14466 -0.00007 c m ~l a
43--42 35.97887 -0.00008 c m- 1 a
44 -43 36.81301 0.00005 c m_ l a
45--44 37.64684 0.00003 c m- 1 a
46--45 38.48049 0.00004 c m ~l a
47--46 39.31394 0.00004 c m ~l a
48 -47 40.14717 0.00002 c m_ l a
49 -48 40.98036 0.00016 c m_ l a
50--49 41.81312 0.00007 c m "1 a
51--50 42.64565 -0.00003 c m ~l a
52--51 43.47817 0.00007 c m ~l a
53--52 44.31021 -0.00008 cm - 1 a
55--54 45.97391 -0.00010 cm~1 a
57--56 47.63681 0.00002 c m_ l a
5-- 4 125613.69600 -0.00111 M H zb
6 - 5 150735.04600 0.00308 M H zb
7-- 6 175855.62300 -0.00558 M H zb
8-- 7 200975.25600 -0.07138 M H zc
10-- 9 251211.55700 -0.00068 M H zb
12--11 301442.70000 -0.02022 M H zb
15--14 376777.75300 0.00822 MHzd
16 -15 401885.80200 0.01432 MHzd
17--16 426991.80800 0.00456 MHzd
18--17 452095.67000 0.00467 M H zd
19 -18 477197.24700 0.00033 MHzd
20 -19 502296.42300 0.00228 MHzd
21--20 527393.05100 -0.00977 MHzd
22--21 552487.03600 -0.00411 MHzd
a Present work: estimated uncertainty of 3 MHz.
b From [5]: estimated uncertainty of 10 kHz.
c From [5]: estimated uncertainty of 100 kHz.
d From [6]: estimated uncertainty of 10 kHz.
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838 Notizen
Present work Maki et al. [3]
Combined fit FIR only
B/MHz 12561.63365 (25) 12561.646 (31) 12561.63395 (10)
D/kHz 5.27882 (51) 5.273(17) 5.27915(12)
H / M H z -0.17(40) -1.9(29) -4.921 (138)
Table 2. The rotational and centrifugal distortion constants of N N O in the ground vibrational state3
a Numbers in the parentheses are the standard deviation in units of the last digit quoted.
T h e s p e c t r u m w a s calibrated using the w a t e r lines in t h e r e c o r d e d s p e c t r u m , the positions of which are listed by Guelachvili a n d R a o [7], a n d s o m e s u b - m m w lines of N O which were m e a s u r e d by c o h e r e n t r a d i a - tion sources [8], In t h e r e c o r d e d region we have iden- tified 77 s t a n d a r d lines, a n d the fit of those to an n o n - l i n e a r c a l i b r a t i o n e q u a t i o n ,
v = a ve x p + 6 / ve x p, (1) resulted in a s t a n d a r d deviation of 2.3 M H z , which
represents the a c c u r a c y of the present m e a s u r e m e n t s . T h e n o n - l i n e a r p a r t in (1) represents the diffraction effect at the a p e r t u r e of the r a d i a t i o n source which gives a significant effect at long wavelengths [9],
T h e m e a s u r e d t r a n s i t i o n w a v e n u m b e r s of the N N O lines are s u m m a r i z e d in Table 1 t o g e t h e r with the m m w a n d s u b - m m w d a t a f r o m [5] a n d [6] which were included in the present analysis. T h e c o n s t a n t s ob- tained f r o m the weighted least-squares fit are listed in Table 2. We have used t h e weight reciprocally p r o p o r - t i o n a l t o the s q u a r e of the estimated e x p e r i m e n t a l u n c e r t a i n t i e s : 10 k H z for m o s t of the m m w a n d sub- m m w lines a n d 3 M H z for the F I R d a t a . T h e con- s t a n t s resulting f r o m this fit are listed in Table 2, to- gether with the c o n s t a n t s given by M a k i a n d
c o w o r k e r s [3] a n d the c o n s t a n t s o b t a i n e d f r o m F I R d a t a only.
T h e p r e s e n t c o n s t a n t s agree within o n e s t a n d a r d d e v i a t i o n with t h o s e of [3]. T h e c o n s t a n t s o b t a i n e d f r o m t h e F I R only are also in a g o o d a g r e e m e n t with t h o s e o b t a i n e d f r o m the c o m b i n e d fit. T h e h e t e r o d y n e m e a s u r e m e n t s of M a k i a n d c o w o r k e r s are very pre- cise; t h e u n c e r t a i n t i e s for the three c o n s t a n t s B, D, a n d H are smaller t h a n t h o s e of the present w o r k . F r o m the p r e s e n t result we c o n c l u d e t h a t the t r a n s i t i o n fre- quencies calculated f r o m t h e g r o u n d state c o n s t a n t s r e p o r t e d in [3] s h o u l d be used as a w a v e n u m b e r stan- d a r d in t h e F I R region with an accuracy better t h a n 0.0001 c m "1.
Acknowledgement
A u t h o r wishes t o express his t h a n k s to M r . K. L a t t - ner, U n i v e r s i t ä t Giessen, for his excellent assistance in o p e r a t i n g the F T s p e c t r o m e t e r , a n d to D r . G . Win- newisser for his help in the p r e p a r a t i o n of this m a n u s c r i p t . T h e w o r k in K ö l n was s u p p o r t e d in p a r t by t h e D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t t h r o u g h S F B - 3 0 1 .
[1] J. S. Wells, A. Hinz, and A. G. Maki, J. Mol. Spectrosc.
114, 84 (1985).
[2] M. D. Vanek, M. Schneider, J. S. Wells, and A. G. Maki, J. Mol. Spectrosc. 134, 154 (1989).
[3] A. G. Maki, J. S. Wells, and M. D. Vanek, J. Mol. Spec- trosc. 138, 84 (1989).
[4] A. Saleck, G. Winnewisser, and K. M. T. Yamada, manuscript in preparation.
[5] R. Pearson, T. Sullivan, and L. Frenkel, J. Mol. Spectrosc.
34, 440 (1970).
[6] B. A. Andreev, A. Y. Burenin, E. N. Karyakin, A. F.
Krupnov, and S. Shapin, J. Mol. Spectrosc. 62,125 (1976).
[7] G. Guelachvili and K. Narahari Rao, "Handbook of In- frared Standards", Academic Press, Orlando, Florida, 1986.
[8] H. M. Pickett, private communication.
[9] J. W. C. Johns, private communication.
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