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J. Phys. Chem. 1980, 84, 1758-1765
Millimeter Wave Spectrum of Barium Sulfide in a Low-Pressure Flame.+ Current Millimeter Wave Measurements of High-Temperature Species David A. Helms,$ Manfred Wlnnewlsser, * and Gisbert Winnewisser5 Physikalisch-Chemisches Institut, Justus-Liebig-Universitat Giessen, Heinrich-Buff-Ring58, 0-6300Glessen, West Germany, and Max Pianck Institut fur Radioastronomie, Auf dem Hugel 69, 0-5300Bonn, West Germany (Received September 14, 1979)
The millimeter wave spectra of six isotopic species of BaS have been obtained in a chemiluminescent flame by the reaction Ba + OCS = BaS + CO entrained in argon gas. The six isotopic species 134Ba32S, 135Ba32S, 136Ba32S, 137Ba32S, 138Ba32S, and 138Ba34S were measured and analyzed in the vibrational and electronic XIZ ground state in the 70-350-GHz region. Data on the first excited vibrational states of the 135Ba32S, 136Ba32S, 13'Ba3%, and Yll, Y12, and 138Ba32S species were also obtained. From these measurements the Dunham constants Yol,YO2, Yzl have been determined. The internuclear distance of BaS has been reevaluated: r,(BaS) = 2.5073184(15) A. The increased sensitivity and the extended spectral coverage of the spectrometer well into the submillimeter wave region are the result of three essential improvements of our system: (1)increase in the interaction time of the chemical reaction through the placement of the chemiluminescent flame in a large-diameter (35 cm) reaction sphere; (2) introduction of a dynamic stabilization scheme for the coherent radiation source (reflex klystron); (3) introduction of a bench-built helium-cooled (1.7 K) photoconducting InSb detector.
Introduction In recent years, there has been renewed spectroscopic and astrophysical interest in the diatomic group II/VI compounds, especially the refractory oxides and sulfides of the alkaline earth metals Ba, Sr, Ca, and Mg. In recent papers on CaO, SrO, and BaO Winnewisser and co-workers1,2presented a comprehensive introduction into the rotational spectroscopy of these species. The authors adopted a modified version of the production method given by Field and Broida and their respective co-worker~.~,~ In these methods diatomic metal oxides are produced by the gas-phase oxidation of metal vapor entrained in a carrier gas. In the present work this scheme has been further modified to achieve efficient production of metal oxides and sulfides in low-pressure flames. To obtain the millimeter wave spectra of Bas in a chemiluminescent flame with high signal-to-noise ratio, a considerable improvement in spectrometer design and sensitivity was necessary. This paper reports the spectroscopic data for BaS and gives an account of the experimental work involved in obtaining these spectra in the millimeter and submillimeter wave region. The present technique is believed to be applicable to many molecular species of spectroscopic and astrophysical interest, in particular to metal hydrides and metal halides. Previous microwave work (below 70 GHz) on BaS was carried out by Tiemann, Ryzlewics, and Torring in 1976j who vaporized 85% pure Bas at a temperature of 2000 K. For the detection of the lines of the main isotopic species, 138Ba32S,saturation modulation6 was employed giving a signal-to-noise ratio of approximately 15:l with a time constant of 3 s after the lock-in amplifier. However, the sensitivity reached was not sufficient to detect other isotopic species of Bas. Melendres et al.7 determined by molecular beam electric resonance spectroscopy the elec*Address correspondence to this author at the PhysikalischChemisches Institut. 'Presented in part at the Conference on Microwave Spectroscopy and Coherent Radiation in honor of Professor Walter Gordy, Duke University, Durham, NC, June 18-20, 1979 as paper D17, and at the Thirtyfourth Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, Ohio, June 11-15, 1979, as paper TC'5. I. Physikalisches Institut, Universitat zu Koln, D-5000 Koln, West Germany. Physics Department, Rice University, Houston, TX 77091. 0022-3654/80/2084-1758$01 .OO/O
trical dipole moment in the rotational state J = 1with the aid of the rotational constant Bo which was at that time only known from the rotational analysis of the optical spectra reported by Barrow et a1.8
Experimental Section The measurements were carried out with an improved and modified version of the millimeter wave spectrometer which has been described in detail earlier.gJOThis basic spectrometer system consisted of a harmonic generator radiation source with point contact video detection and a dedicated PDP 8/I computer system for signal averaging and frequency measurement. This design was modified and improved in three different respects: (1) The free space absorption cell and reaction vessel (Figure 1,ref 2) for the low-pressure flame has been replaced by a reaction sphere with a diameter of ca. 35 cm. (2) The coherent radiation source (reflex klystron) has been frequency stabilized while maintaining the fast sweep capability via a newly developed stabilization scheme. (3) The sensitivity of the spectrometer was greatly improved by the use of an InSb photoconducting detector operating at 1.7 K for the detection of the millimeter waves. Figure 1 summarizes the present spectrometer system and includes a logic flow chart of the stabilization and signal acquisition system employed. This block diagram will form the basis for the following discussions. Production of Metal Oxides and Sulfides. The metal oxides and sulfides were produced in the gas-phase reaction of the metal vapor with a particular oxidant. The oven arrangement used to produce the metal vapor by heating the solid metal was contained in the lower inlet port of the reaction sphere (Figure 1). A detailed description of the oven design and its operation has been presented by Hocking et al.,2whose design was based upon that of' West et al.ll Our initial system2 was built with a Pyrex cross piece of 10-cm i.d. which contained the low-pressure flame. In the present modification the cross piece was exchanged for a reaction sphere of ca. 35-cm i.d. The low-pressure flame can spread through the entire volume, allowing the metal vapor to collide with oxidant molecules under conditions minimizing wall collisions. The path of oxidation has been lengthened, which led to considerably higher metal oxide abundances and resulted therefore in a pronounced increase in the signal-to-noise ratio of the ab0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 14, 1980 1759
Millimeter Wav13 Spectrum of BaS
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Flgure 1. Block diagram of the millimeter and submillimeter wave spectrometer with reaction sphere and free space absorption cell as used for the spectroscopy of high-temperature metal oxides and sulfides. The flow chart presents an overview of the hardware and data acquisition system and gives the logic of the free-running klystron sweep-drift stabilization system. KLYSTRON POWER SUPPLY BLOCK DIAGRAMM
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Flgure 2. Comparison of '38Ba1B0rotational transitions recorded with the earlier spectrometer setup (top s p m a y and with the present version of the spectrometer (lower spectra) after computer averaging and smoothing.
sorption lines. This is demonstrated in Figure 2 where typical sample spectra of '%Ba160 in the ground and in the
Figure 3. Electronic circuitry to apply the sawtooth-sweep voltage and error voltage to the klystron reflector electrode for stabilized sweep operation. The sawtooth-sweep voltage is capacitively coupled.
first excited vibrational states are compared as observed with the old and new system. As judged from the signal-to-noise ratios the actual improvement is estimated to
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The Journal of Physical Chemistry, Vol. 84, No. 14, 1980
REFLECTOR 2 CIRCUIT DIAGRAM
Helms et al.
was the OCS oxidant. The flow rates were not determined. The pumping system was always used to full capacity. It consisted of a Leybold OT 1000 oil booster pump with a RUTA 60 forepump with Roots blower. Recently the millimeter wave spectrum of SrS has been observed under similar conditions. These results will be published elsewhere. Frequency Stabilization of Free Running Klystron. Compared to our earlier spectrometer arrangement the advanced frequency drift stabilization system forms the essential improvement of the electronic design of the spectrometer (Figure 1). The heart of the new addition is the klystron sweep stabilizer which produces a correction voltage for each individual sweep so as to keep the klystron frequency stabilized. Klystron controller and modified klystron reflector circuitry ensure proper application of the error voltage to the reflex klystron. These three electronic devices will be discussed in turn. We will omit in this paper a description of the hardware aspects of the signal preparation for data acquisition and the actual acquisition process including the accompanying software since they were discussed in detail in ref 9 and 10. Modification of the Klystron Reflector Circuit. At present one of the most widely used microwave sources for millimeter and submillimeter wave spectroscopy is the reflex klystron. Mechanical deformation of the resonant cavity structure of the tube allows coarse tuning over of N 10% of the klystron's fundamental frequency. Fine frequency adjustment (
