Notizen 737
An Automatic Scan Waveguide
Microwave Fourier Transform Spectrometer
M . K r ü g e r , H . H a r d e r , C . G e r k e , a n d H . Dreizler Abteilung Chemische Physik im Institut für Physikalische Chemie der Universität Kiel, Kiel, FRG
Z. Naturforsch. 48a, 737-738 (1993);
received April 1, 1993
The design and performance of an automatic scan wave- guide microwave Fourier transform spectrometer is described, which is of great help in assigning microwave spectra.
In 1990 we r e p o r t e d o n a w a v e g u i d e m i c r o w a v e F o u r i e r transform ( M W F T ) spectrometer with a single m i c r o w a v e source [1] which p r o v i d e d the s a m e p e r f o r - m a n c e c o n c e r n i n g resolution a n d sensitivity as f o r m e r set u p s [ 2 - 4 ] . T h e a d v a n t a g e of t h e s p e c t r o m e t e r is its simpler c o n s t r u c t i o n a n d o p e r a t i o n .
T h i s scheme is n o w in use in the f r e q u e n c y ranges 8 - 1 8 , 2 - 4 [5], a n d 4 - 8 G H z . In the a p p e n d i x we give the M W p a r t s we use.
We a d d e d a modification t o the spectrometers which allows a n a u t o m a t i c f r e q u e n c y sweep to p r o d u c e sur- vey spectra. T h u s the M W F T s p e c t r o m e t e r s , which w o r k at a selectable fixed f r e q u e n c y a n d c o v e r a fre- q u e n c y b a n d of s o m e ten M H z d e p e n d i n g o n the p o l a r i z a t i o n conditions, a r e c o n v e r t e d t o s c a n n i n g in- s t r u m e n t s .
T h e h a r d w a r e of the s p e c t r o m e t e r s is similar t o t h a t given in Fig. 1 of [1]. An essential m o d i f i c a t i o n is the r e p l a c e m e n t of the p h a s e stabilized M W s o u r c e (parts 1 - 3 , 5 - 9 of Fig. 1 [1]) by a M W synthesizer c o n n e c t e d to the reference frequency 10 M H z of t h e s p e c t r o m e - ter.
We tested a n d used the synthesizers H e w l e t t - P a c k - ard (HP) 8340B, 0.01 - 2 6 . 5 G H z (1 - 4 Hz), H P 8 6 7 3 G , 2 - 2 6 . 5 G H z (1 - 4 kHz) a n d R o h d e & Schwarz S M P D , 0 . 0 0 5 - 2 7 2 0 M H z , with frequency doubling ([5], Fig. 1) (2 Hz). T h e n u m b e r s in p a r e n t h e s e s give t h e m i n i m a l f r e q u e n c y step width. T h e o u t p u t p o w e r of t h e synthe- sizers is f r o m 10 t o 13 d B m . T h e f r e q u e n c y a n d M W p o w e r is set via an I E E E b u s by the c o m p u t e r 44 [1], T h e center f r e q u e n c y of t h e digitally driven Y I G - F i l - ter 18 [1] is also a d j u s t e d by t h e c o m p u t e r .
Reprint requests to Prof. Dr. H. Dreizler, Abteilung Chemi- sche Physik, Institut für Physikalische Chemie, Universität Kiel, Olshausenstr. 40, W-2300 Kiel 1, FRG.
After a selected n u m b e r of e x p e r i m e n t cycles t h e averaged transient decay is s t o r e d in the c o m p u t e r m e m o r y . F o r each polarizing f r e q u e n c y the s u m of t h e m a g n i t u d e s of all signal values of the decay (decay sum), the m a x i m u m a m p l i t u d e of t h e w h o l e s p e c t r u m a n d the sum of the s p e c t r u m a m p l i t u d e s within a small intervall a r o u n d the center f r e q u e n c y a r e calculated a n d printed. This p r o c e d u r e is r e p e a t e d after increas- ing the polarizing frequency in selectable f r e q u e n c y steps. T h e usual step w i d t h s are 0.5 t o 5 M H z . F r o m the d a t a list t r a n s i t i o n s m a y be localized. To give a better visual picture, a s p e c t r u m c a n be calculated by overlaying all spectra in the covered f r e q u e n c y range.
A n example is given in F i g u r e 1. H e r e a r a n g e of 315 M H z is displayed of the direct 1-type r e s o n a n c e s p e c t r u m [6] of t r i f l u o r o e t h a n e in its v1 2 = l v i b r a - tional state. In a d d i t i o n p a r t of the F o r t r a t d i a g r a m is given.
T h e scan time d e p e n d s o n t h e n u m b e r of a v e r a g i n g cycles a n d the f r e q u e n c y step width. In the e x a m p l e of Fig. 1 the scan time was a b o u t 2 h o u r s for 315 M H z with the given i n s t r u m e n t settings.
T h e scanning version of t h e M W F T s p e c t r o m e t e r s p r o v e d as very useful for assigning s p e c t r a in the last three years. T h i s m o d i f i c a t i o n allows a m o r e effective use of the spectrometers similar t o t h a t r e p o r t e d in [7].
It s h o u l d be n o t e d t h a t each line h a s t o be r e m e a s u r e d individually to gain highest sensitivity, resolution, a n d f r e q u e n c y accuracy.
We t h a n k the m e m b e r s of o u r g r o u p for help a n d discussion, the 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 e F o n d s der Chemie, a n d t h e L a n d Schleswig-Holstein for funds.
Appendix
C o m p a r e Fig. 1 of [1].
Single sideband modulator 13:
M / A - C O M R H G , I R D S 1 - 4 / 1 6 0 , 1 - 4 G H z , m o d u l a - tion frequency ( M F ) ± 1 6 0 M H z ,
M / A - C O M R H G , I R D S 2 - 2 6 / 1 6 0 , 2-26 G H z , M F =
± 1 6 0 M H z ,
W a t k i n s - J o h n s o n (WJ), W J - M 3 4 C - 4 , 8 - 1 8 G H z , M F = ± 8 0 t o ± 1 6 0 M H z , W J - M 3 4 C - 5 , 8 - 1 8 G H z , M F = ± 100 t o ± 200 M H z .
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738 Notizen
K = 0 J = 25
4900 5100 1/ [MHz]
Fig. 1. An automatic frequency scan with a waveguide M W F T spectrometer from 4825 to 5140 MHz. Part of the direct 1-type resonance spectrum of trifluoroethane, C H3C F3, in its vibrational state v12 = 1 is displayed. In addition, part of the F o r t r a t diagram with K = \ K — l\ is given. The instrument settings are: pressure about 1 Pa (8 mTorr), r o o m temperature, 1.6 • 106
experiment cycles for each polarizing frequency, 10 ns sample interval, 1024 d a t a points of the transient decay, 2 M H z frequency step width, polarizing M W power approximately 1 W, scan time 2 hours.
YIG-filter with 12 bit digital driver 18, 3 dB bandwidth approx. 40 MHz:
W J , 5 2 7 2 - 0 0 4 D A , l - 4 G H z ,
F e r r e t e c , F D 1072 C - D D , 2 - 8 G H z , A v a n t e k , A F P D - 3 1 8 2 1 , 8 - 1 8 G H z .
Amplifier 19 for compensation of the conversion loss of 13 and insertion loss of 18:
T r o n t e c h , P 4 2 G A , 1 - 4 G H z , g a i n 4 2 d B , o u t p u t + 30 d B m , O m e g a M i c r o w a v e , 1 - 0 0 2 9 0 8 , 6 - 1 8 G H z , g a i n 48 d B , o u t p u t + 1 8 d B m .
[1] M. Krüger and H. Dreizler, Z. Naturforsch. 45 a, 724 (1990).
[2] G. Bestmann and H. Dreizler, Z. Naturforsch. 37a, 58 (1982).
[3] H. Dreizler, Mol. Phys. 59, 1 (1986).
[4] A. Bauder, Microwave Fourier Transform Spectroscopy, in: Vibrational Spectra and Structure, Vol. 20 (J. R.
Durig, ed.), Elsevier Scientific Publishing Co., Amster- dam 1993, p. 157.
[5] C. Gerke and H. Dreizler, Z. Naturforsch. 47 a, 1058 (1992).
[6] C. Gerke et al., to be published.
[7] U. Andresen, H. Dreizler, J.-U. G r a b o w , and W. Stahl, Rev. Sei. Instrum. 61, 3694 (1990).
