Early events and some later developments in microwave spectroscopy

Journal of Molecular Structure, 97 (1983) 17-32 Elsevier Scientific Publishing Company, Amsterdam -Printed

in The Netherlands

EARLY EVENTS AND SOME LATER DEVELOPMENTS IN MCROWAVE SPECE?OSCOPY

WALTER

GORDY Physics Dept., Duke University, Durham, N.C.,27706,U.S.R

ABSTRACT

The early history of microwave spectroscopy is reviewed. New direction& in the field are indicated. These include: further extensions of coherent submillimeter wave spectroscopy, microwave spectroscopy of molecules in interstellar space, microwaveinfrared laser double resonance, spectroscom of ionized molecules and transient molecular radicals, studies of hydrogen-bonded molecular complexes and atom-molecule complexes, observations of "forbidden"rotational transitions in symmetric-top and sphericaltop molecules, and new developments in high-temperature spectroscopy.

IN’IROWCTION

as well as a pleasure, for me to partiIt is a great honor, cipate in this symposium dedicated to the memory of my dear friend and respected fellow scientist, Prof. Dr. Werner Zeil. I first met Professor Zeil at a spectroscopy conference in Paris, in 1953. We sat on a bench outside the conference hall and speculated about future possibilities for microwave spectroscopy. Both of us were extremely optimistic, but in retrospect I must say that neither of us overestimated the potentzialitiesof this new, developing field of spectroscopy. During the years that followed, he and I were to meet and discuss these developments at many conferences in Europe and in America. Among the last that we attended together was the Fourth European Microwave Spectroscopy Conference here in Tbbingen in1977, at which he was host. The last was one held at my base, In between meetings we had many Duke University, in i979. exchanges of papers and letters. Our laboratories were linked romantically in 1965, when one of my favorite students, Brenda Pruden, married one of his favorite students, Manfred Winnewisser. Manfred and Brenda, now known by you and by most of the spectroscopistsof the world, are here with us to honor Professor Zeil. His contributionsto microwave spectfoscopywere many and important; the students he trained and inspired continue to contribute to the progress of spectroscopy.

18

When 1 first visited Professor Zeil, in1965, he was at the university in Xarlsruhe. On the campus, near his laboratory, he proudly showed me a statue of Beinrich Bertz. Inside a building directly behind the statue, Bertz had, in 1888, detected the first microwave radiation. Because the first applications of this new electromagnetic radiation that Nertz discovered were in telegraphic communications and radio at kilocycle wavelengths and because no applications of microwaves came until some fifty years later, it was widely believed that Wertz made his discovery at long-wave radio frquencies. Bowever, what he first produced and detected at Karlsruhe were centimeter microwaves. He produced the new radiation with a spark-gap generator and measured the wave lengths by detecting nodes of standing waves produced by reflections from the walls of the room in which he worked. Because Nertz discovered microwaves, it is appropriate that microwave spectroscopists now designate their frequency units as "hertz" rather than as "cycles per second." I am happy about this chanye even though it is causing many tedious changes of "PIc/sec" to “MHz” in our book, Microwave&lecular Soectra& which Robert Cook and I are in process of revising. EARLY MISMRY The oscillator tubes and associated tunable circuits employed in early radio could not be extended to the high-frequencymicrowaves that Hertz discovered. For a coherent, tunable microwave source, spectroscopy had to wait about fifty years until the Ovarian brothers working with Professor W.W. Bansen at Stanford University developed the versatile klystron source that they called the "rumbertron,"a name which did not stick. 'iheklystron source was the heart of all early microwave spectrometers. It is the device that led to the birth of high resolution microwave spectroscopy. Physicists working with klystrons in the early microwave radar sets at such places as M.I.T.or Oxford, in Bell or Westinghouse Laboratories, were the natural leaders in developments of microwave spectroscopyimmediately after World War Il.

The new born microwave spectroscopy cut its teeth on the ammonia inversion spectrum. This spectrum, which occurs in the 1.2 cm range, was the first to be measured in the microwave region. The first measurements, made in 1934 by Cleeton and Williams (ref.11,were of low resolution,with a semi-optical spectrometer employing a split anode magnetron source with an echelette grating for measurement of wave length. During World War 11 a rediscovery of the ammonia inversion absorption spectrum

19 was made in a rather unusual manner, A group of scientists in the M.I.T.Padiation Laboratory were testing the new K-band oscillator by observing signals from a radar secretly located in a building in the Back Bay area of Boston, across th'e Charles River from M.I.T. Each day, at about the same time in the morning, the signals mysteriously were blacked out temporarily. This puzzling phenomenon was finally solved when an alert technician noticed that when the signals disappeared there was always a garbage boat going down the river. From the boat came the unmistakable smell of ammonia. Someone in the groupmust have read and remembered that paper by Cleeton and Williams; the puzzle was quickly solved. The inversion spectum.of this molecule is so strong that if ammonia were a normal component of the atmosphere, K-band radar would have been impossible. The effectiveness of the K-band radar was seriously reduced, especially in humid weather, by absorption of the 5_l-6_5 transition of H20. Early infrared spectroscopistshad predicted that the transition would fall in the vicinity of l-cm wave to the length (ref.2),but their predictions were either unknown developers of K-band radar or not taken seriously by them. The absorption intensity (ref.3) and center frequency of the transition (ref.4) were accurately measured by early microwave spectroscopists, but only after K-band radar had been developed. If the microwave measurements had preceded the choice of the radar The band, the unfortunate choice could have been avoided. construction of a K-band spectrometer without K-band radar components may, however, have been as costly and time-consuming as developing the radar. Anyway, the K-band radar did work well on dry days. After having developed K-band radar on top of a water vapor line, the scientists of the Radiation Laboratory at M.I.T.became seriously concerned about spectral absorptions of atmospheric gases. When J.H. Van Vleck calculated that a fine structure oxygen band occurred in the region of a half-centimeter (ref.51, there was great interest in experimental confirmation of his calculations by measurement of the frequency distribution and set out to strength of the absorption. In 1944 Robert Eeringer make these measurements, but millimeter wave oscillatorswere not yet available. Knowing that it would be difficult to develop one to operate in the half-centimeter-wave region, he decided to try using a crystal harmonic generator to double the frequency of the available K-band klystrons, the Meher tubes, which gave a few milliwatts of,power. The crystal doubling circuit was highly tuned'in order to maximize the half-centimeter-wave power. The

20

resulting narrow-banded output made it necessary to observe the spectrum point by point. Themeasurementswere made at15 spot frequencies in the wave length range of 0.62 to 0.48 mm on oxygen gas and oxygen-nitrogenmixtures at atmospheric pressures, and the results were found to be in rough agreement with Van Vleck's theoretical predictions of the intensity and spread of the absorption. The peak absorption, which occurred at a halfcentimeter wavelength, was observed to be strong, 67 db/km, for pure 02. It was evident that anyone who aspire8 to construct a millimeter-wave radar should stay away from the half-centimeter wave region. These measurements, which represent the first millimeter wave spectral measurements with a coherent microwave source, were described in an internal Radiation Laboratory report in 1944 and were published in the Phvsical Review in 1946 (ref.6). The report is now of historical interest only since the millimeter wave spectrum of oxygen has been accurately measured and remeasured with modern high-resolutionsweep spectrometers. Reports of high-resolution, microwave-sweep spectroscopy began in 1946 with the publication in the March issue of Rature of a note, "Ammonia Spectrum in the lcm Wave-length Region," by B. Bleaney and R.P. Penrose, submitted February12 of the same year (ref.71. They resolved for the first time the rotational structure of the inversion vibrational band. The fact that they resolved no hyperfine structure may have been due to their not having expected any: the pressure they used, 1.2 mm, would have smeared it out. As it happens, a number of other, quickly established, laboratories were seeing their first microwave lines at the time the Bleaney-Penrose article appeared. There were good reasons why they all chose to look at ammonia first: it was the strongest known, or suspected, absorption, and it occurred conveniently in the K-band region for which tunable oscillators from radar were immediately available. The next to report was W.E, Good, who on April 20, 1946, submitted a letter to the editor of the PhvsicaL R-U which had the title, "The Inversion Spectrum of Ammonia" (ref.8). In this first note, Good, like Bleaney and Penrose, mentioned only the rotational fine structure, but in his full length paper submitted a month later to the Phvsical Review (ref.91 he reported observation of a definite hyperfine structure at pressures below low2 mm of Hg and showed an oscilloscopic picture of a beautifully symmetric satellite structure on the J = 3, K = 3 fine structure line. This structure was unpredicted,and its origin was at the time unknown. Confirmation of Good's observations was quickly reported by a joint group from M.I.T. and Harvard , who measured the satellite

21 spacings and showed they could be interpreted as arising from the 14N nuclear quadrupole splitting (ref.18). The group included names now well known in microwave spectroscopy - B.P. Dailey, R.L. I;yhl,M.W.P. Strandberg, J.H. Van Vleck, and E.B. Wilson. These papers marked the beginning of observations of nuclear quadrupole hyperfine structure in microwave molecular spectra. Later observations of the structure by many spectroscopists in hundreds of molecules advanced significantly the knowledge of the chemical bond. Notable among those who used this new method in studies of the chemical bond was Professor Zeil. made the first of his It is of interest that Charles Tonnes many important observations in microwave spectroscopy on the inversion spectrum of ammonia. In this paper, submitted August 2, 1946, he gave detailed measurements of the line shapes and reported the first detection of power-saturation effects (ref.ll.1. As all of you know, the Stark effect in microwave spectroscopy is exceedingly important for measurement of molecular dipole moments, for identification of spectral transitions,and as an aid to spectral line detection in the Stark modulation spectrographs (ref.12). In a letter to the Phvsical Review submitted on October 1, 1946, Dakin, Good and Coles reported the first measurement of the Stark splitting of a microwave spectral line and the first microwave measurement of a molecular dipole on the J = 1 -> 2 moment (ref.13). These measurementsweremade rotational transition of OCS; the second-order doublet Stark splitting was beautifully displayed on an oscilloscope screen. Some who had worked with microwaves in the radar development centers left after the war to start laboratories in microwave spectroscoW elsewhere. These new laboratories generally did not produce results until after 1946. As for me, I left R.I.T. in January of 1946 to initiate work in microwave spectroscopy at Duke University., Our first results, on the methyl halides, were published in mid 1947. In 1948 I was invited to write an article on the new field for the-Reviews of Modern Phvsics. The review, which appeared in October of that year (ref.lQ),included Spectral line measurements of 51 isotopic species of molecules including linear, symmetric-top and asymmetric rotors, molecular structural determinations of 17 molecules, nuclear quadrupole coupling in 24, In addition it reported and electric dipole moments of 7. measurements of nuclear spin moments and quadrupole moments for several nuclear isotopes. Probably the most important microwave spectal measurement reported in this review was that of Lamb and Retherford on atomic hydrogen , which showed that the 22Sl,2 and 22Pl/2 levels are not degenerate (ref.15). This experiment was

22

rewarded by a Nobel Prize to Lamb. As soon as I began work in microwave spectroscopy, I felt a stifling need for more spectral space. It was evident that most molecular transitions fall at higher frequencies than those of centimeter waves, for which coherent microwave sources were available. Thus, from its beginning, our laboratory concentrated on the problem of extending the range of coherent microwave spectroscopy into the millimeter wavelengths. As a start, we duplicated the harmonic generator source used by Beringer for the 5-mm absorption of 03 and found that this type of source offered hope but that it had to be redesigned to improve its power output and to make it broad-banded before it could be used for coherent sweep spectroscopy. In1948 we succeeded in extending coherent sweep spectroscopy to the 3-mm wave region (100,000 MHz) (refs.l4,16);by 1953 we had reached the submillimeter region, to 0.77 mm wavelength (ref.17). Finally,'by further improvement in the harmonic generators and detectors , we were able in1970 to extend these coherent techniques to measure various spectral lines with high precision up to 813 GHz, or 0.37 mm wave length (ref.18). Most of the earliest workers in microwave spectroscopy were physicists who had gained experience with microwave radar. The later studies of molecules were aided by an increasing number of Some laboratories, chemists who were entering the field. including ours at Duke, enjoyed a healthy mixture of physicists and chemists. In 1949 our laboratory was lifted by the coming of the chemists John Sheridan for a year's visit. Chemists were leadersin establishing some of the early centers for microwave spectroscopy. Among these leaders was Professor Werner Zeil. TWO others, Professors Bak and Sheridan, are speakers at this commemorative session. In the States, E. Bright Wilson, at Harvard, and William Gwinn, of Eerkeley, were among the first chemists to initiate programs in microwave spectroscopy. Many of you in this audience received your training at one of these centers or were students of one of these early leaders. NEM DIRECTIONS After this brief review of the early history of microwave spectroscopy, which some of you never knew and others of you have forgotten, I wish to mention some of the results which the leaders of the second generation of microwave spectroscopists are getting and to indicate the directions in which they appear to be leading. Further extension of coherent submillimeter wave spectroscopy A second-generationmicrowave spectroscopist, RF. Krupnov,

23 has had notable success in extending coherent-sweep submillimeter wave spectroscopy beyond the frequency of 813 GHz, which we at Duke reached in 1970. V?ith a strong primary source, a new BWO tube, and an acoustic detector, he has constructed a sweepspectrometer that operates with high sensitivity to frequencies slightly above 1000 GHz. At the Fifth European Conference on Microwave Spectroscopy, held here in Tiibingen in 1977, he described his spectrometer and showed spectral lines observed with it in the lOOO-GHz range. Since that time he and his group at Gorky have observed lines at still higher frequencies. For example, they have measured rotational lines of PH3 in the region of 300 to 1070 GHz. Included in these measurements are "forbidden" Q-branch transitions, AK=+2, AJ=O (ref.19). This submillimeter wave spectrometer, designated PAD, is described and illustrations of its performance are given in a 1979 article by Krupnov (ref.20). I regret that Dr. Krupnov is not here to report his latest successes. It is unfortunate that the tunable primary oscillator tubes that he uses are not, to our knowledge, available outside the U.S.S.R. Those who are restricted to harmonic generation need not despair.

Two second generation Duke microwave spectroscopists,

Frank C. DeLucia and Paul Helminger, have recorded CO lines at frequencies above 1000 GHz, using a modified Duke harmonic multiplier driven by a klystron oscillator.

They displayed lines

on an oscilloscope at frequencies well above 900 GHz. Molecules in interstellar space Strictly speaking, the microwave spectroscopy of molecules in outer space began with the detection of X-doublet transitions of OH by an M.I.T.group in 1963 (ref.21). However, nothing more was discovered until 1968, when lines of NH3 were found (ref.22). In 1969 H20 (ref.23) and H2C0 (ref.241 were observed. After 1968 dozens of molecules--I have lost count of them--have been detected in interstellar space with microwave spectroscopy. These species include free radicals such as CCH and ionized species such as (NNH)+, as well as numerous stable molecules. The sharp spectral lines observed are giving significant information about the composition and dynamics of clouds of matter in space. Also, the measurement of the ratios of certain isotopic species, particularly those of H and D, is giving information about the forms of matter shortly after the big-bang origin of the universe and after later transformations of the matter in stars. A recent description of the astrophysical significance of the observed molecules in space is given by Winnewisser, Churchwell, and

24

Walmsley (ref.25). The first group of molecules observed in outer space had been measured earlier in microwave laboratories. To find molecules in space, one simply searched for frequencies corresponding to those already measured in the laboratory, taking into account possible Doppler shifts in the lines. When the lines corresponding to those observed in the laboratory were observed in outer space, the species could be identified by comparison of the structure of the two spectra. 1Qny molecules - including OH, NH3, H20, CO, B2C0, HCN, and HCCCN - were observed and identified in this manner. Later, more random searches led to the discovery of lines not recorded in the microwave spectral tables. The first of these was an unknown line at 89,190 MNz, detected from several interstellar sources by Euhl and Snyder in 1970 (ref.26). Because they could not assign it to any known species, they tentatively assigned it to an unknown species, which they designated as X-ogen. Shortly afterward Kiemperer (ref.271proposed BCO+ as the unknown species, from theoretical considerations, and predicted that its J=O->l transition should fall at or near the unassigned frequency. Tne suggested identificationwas verified five years later by Woods et al. (ref.28)from the first laboratory measurements of microwave lines of HCO+. In a somewhat similar manner, several other molecular species have been found firstin outer space. Among them are NNH+, COB+, KS+, C2H, C4H, HNC, HC5N, and BC7N. The longest interstellar molecule to be detected, HCCCCCCCCCN, (ref.291 has not yet been successfully prepared on earth. Its identificationis, nevertheless, considered fairly certain from comparison with a theoretical spectrum predicted from a structural projection of the structures of the lower members of its molecular family, HC,N. It appears that the microwave molecular astronomers have begun to lead the laboratory scientists in the discovery of new molecular species. Microwave molecular astronomy is indeed becoming an exciting field for the chemists. The mechanism of formation in interstellar space of these species that are unstable in the laboratory presents a challenging chemical problem. Microwave-infraredlaser double resonance The development of high-powered, tunable infrared lasers has made possible IR-F!W double resonance, which has created a new dimension for both IR and MW spectral measurements. The microwave component in the resonance yields values for the rotational and hyperfine constants comparable in accuracy to those from normal microwave spectroscopy. The technique is exceptionally useful for the assignment of transitions in complex spectra.

25 The subject is too involved for elaboration here. A thorough exposition of infrared-microwave double resonance is given by Harold Jones (ref.30); one of microwave-microwave double resonance, by John Baker (ref.31).An early, pioneering experiment in IR-MW resonance was performed by Shimizu and Oka lref.32). soectroscoov of ionized molecules The microwave spectroscopy of ionized molecules began with the observation of CO+ and of HCO+ by Claude Woods and his associates in 1975 (refs.28,33). As already mentioned, the HCO+ ion had been detected earlier in astronomical sources. Several molecular ions have now been studied. Because Professor Woods, the originator of the laboratory observations, is speaking at this conference on the subject, I need not - and shall not - attempt to describe the techniques used or the results obtained. However, I would like to report the recent observations of NO+ ion by secondgeneration specroscopists in the Duke microwave laboratory (ref.34). Table 1 summarizes the observed lines and the spectral constants obtained. TABLE1 Observed rotational transitions and derived spectral constants of NO'(a)

Transition J”+

J’

F”

Observed Frequency

3

F’

0'1 2+2 I

i

l-+2 l+l 2+3

3 + 4 2+3 I 1'2 I

(MHz

Spectral constant (MHz)

1

238381.20

BO = 59597.132(16)

238383.20

DO

238386.43

eQq(14N) = -6.76(10)

= 0.171(l)

357564.32

(a) Data from Bowman, Herb&,

and De Lucia (ref.34).

Many years ago (19531, we studied the un-ionized NO in the Duke laboratory. I am impressed by the greater simplicity of the ionized species, but no one should conclude that this is true for all ionized species. If a normal, stable molecule, lZ state, is ionized to an odd molecule, CO to CO+ for example, the spectrum is greatly increased in complexity. Ask Professor Wood.

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Transient molecular radicals Sensitive millimeter wave spectrometers with "free-space" absorption cells, usually enclosed in glass bulbs or cylinders, have made possible the generation and detection of short-lived neutral radicals as well as charged molecules. The resonance spectra of gaseous free radicals which have been observed are the subject of an excellent monograph by Carrington (ref.351,and they need not be discussed here. More recently, zero-field rotational spectra of some of these radicals have been measured. As for ionized molecules, a new impetus to search for rotational lines of neutral free radicals has come from prior observation of certain ones in interstellar space. Rotational spectroscopy of transient molecular species on earth is developing rapidly. At the Ninth Austin Symposium on Molecular Structure, l-3 Flarch1982, Prof. Eizi Birota reported that scientists in his Institute of Molecular Science in Okazaki, Japan, had measured rotational spectra of the diatomic free radicals, CF, SF, Ccl, SiF and Sic1 and the triatomic species, Do2, FSO, ClSO, and WSO. I make no attempt to report all such radicals observed lately. I illustrate with one, HCC, recently observed in the Duke laboratory (ref.361and first observed in interstellar space by Tucker, Kutner, and Thaddeus in 1974 (ref.37). In the laboratory the C2B radicals were produced by a glow discharge of a mixture of He, CO, and CD4 at liquid air temperature and 25 mTorr pressure. Measurements were made on the l->2, 2->3, and 3->4 rotational transitions in a frequency range The following values for the spectral of 174 to 350 GHz. constants were obtained: SD=43674.515(6)MNz,D0=105.3(4) HIHz,and yo=-62.23(35)Ms,Bz.The related radical C4N has now been detected in interstellar space (ref.38),but I do not know whether any of you have observed it in your laboratories. Studies of hydrogen-bondedmolecular complexes I was particularly impressed by a paper on microwave spectral studies of hydrogen-bondeddimers in the gaseous state that was given by D.J. Millen at the 1977 conference here at Tbbingen. Perhaps my low-resolution infrared studies of hydrogen bonding in the to my liquid state, undertaken during the thirties, contributed interest in Millers's paper. As all chemists know, hydrogen bonding is of great significance in determining the properties of many liquids and solids. This bonding is of crucial importance in determining biological functions of proteins and nucleic acids, including the preservation and duplication of genetic information. Because of their importance, hydrogen bonds have been studied in

liquids and solids by every means available - infrared, x-ray diffraction, nuclear resonance, etc. Although these studies in condensed matter are significant, they are much less accurate than those of gaseous microwave spectroscopy. The first observation of microwave rotational spectra of hydrogen-bonded structureswas apparently that by Dyke and Muenter on the hydrogen peroxide dimer (Hz0212 in 1974 (ref.39). This was followed by observations on H20 ?? **BF and on HCN"*BF by Millen's group (refs.40-42). In 1972, Dyke, Howard, and Klemparer had determined the structure of the (HF12 dimer from microwave, electric beam resonance measurements (ref.43). Since these first measurements demonstrated the feasibility of observing microwave spectra of hydrogen-bonded complexes in the gaseous state, accurate information has been gained on the energies, lengths, and other properties of the hydrogen bonds which hold together the monomer components of many other molecular complexes. From the Stark effect of the rotational lines, enhancement of the electric dipole moment by hydrogen bonding is being measured (refs.42,44), as illustrated in Table 2. Large dipole enhancements were qualitatively indicated by early infrared observations of vibrational absorptions of bonds directly involved in hydrogen bonding in liquids or solids.

TABLE2 Dipole Moment enhancement due to hydrogen bonding

*.M M ?? 12

U(D)

CVl + V21a

All(D)

HCN ** HF OCO *-* HF SC0 '** HF

5.612b 2.2465' 3.2085'

4.812 1.827 2.542

0.800b 0.420 0.667

??

aSum of moments of separated monomers. (ref.43). bLegon, Millen, and Rogers CBaiocchi, Dixon, Joyner, and Klemperer

(ref.44).

Atom-moleculecomplexes Complexes of rare gas atoms with stable molecules are now being extensively investigated with microwave rotational spectra. These complexes are held together by weak Van der Waals forces, and it is to me surprising that they can be produced in the gaseous state and that they are held together long enough to have been produce observable rotational spectra. These complexes

28 observed mostly by Klemperer and his group at Rarvard (refs.45-47) with techniquesof microwave-molecular beam electric resonance and by Flygare and his group at Illinois University (refs.48-50)with special microwave spectral techniques. The complexes are generally produced by expansion of pressurized mixtures of the constituentsby passage through a restricting supersonic nozzle into the highly evacuated microwave absorption cavity of the spectrometer. Expansion further cools the gases entering an already cooled absorption cell. The rare gas atoms are the predominent component of the pressurized mixture with the molecules contributing only a few per cent. An unusually sensitive microwave spectrometer was developed by Flygare and his group for these observations (ref.50). The supersonic nozzle was pulsed to admit the gas at a frequency synchronized with the phase-lock-in microwave detector. Rotational spectra of molecular complexes with the rare gas atoms - argon, kryton,and xenon - have been measured. The first observation of such complexes appears to be that on ArBCl by Klemperer's group in 1973 (ref.45).The complex was found to have the linear structure Ar*** HCl with the vibrationally averaged Ar**'Cldistance of 4.006 8. The large amplitude of the bending vibrations observed, with the vibrationally averaged bending angle of 450 in ArHCl and 34O in ArDCl, is typical of these weakly bonded complexes. From the observed rotational constants, the Van der Waals bond lengths are calculated. From the centrifugal distortion constants, the bond vibrational frequencies and force constants are obtained. Observations of forbidden rotational transitions of symmetric-top and spherical-topmolecules Early microwave spectroscopists who studied symmetric-tvp molecules were severely restricted by the K=O selection rule that prevented their measurement of the moment of inertia about the symmetry axis. They were prevented from observing a microwave spectrum of a spherical-top molecule such as CR4 because of its lack of a permanent dipole moment. Things have now changed. The second generation has ways of overcoming these restrictionsfor many symmetric-top and spherical-topmolecules. The possibility of detecting forbidden K-transitionsin the ground state of symmetric-top molecules was proposed from theoretical considerations by Hansen in 1967 (ref.51).In the same year Oka considered the possibility of collision-induced K=+3 transitions of NH3 in mixtures with rare gases (ref,52). In 1971 Fox (ref.531 and Watson (ref.541 theoretically predicated the

29

possibility of observing pure rotational spectra of spherical-top molecules in their ground vibrational states. The first detection of the AJ=O, AK=+3 transitions in a symmetric top was achieved in 1974 by Chu and Oka, who obtained K=+l<->+2 transitions in PH3, PD3 and AsH3 (ref.55). In our Duke laboratory, these measurements were extended to several higher-frequency, millimeter-wave transitions (refs.56,57).The first microwave detection of a pure rotational spectrum of a spherical top that was of CH4, accomplished by Bolt, Gerry, and Ozier in 1973 (ref.58). A review of the early work is given by Oka (ref.59). The observations of forbidden transitions are still restricted, but new methods are being developed that make their detection feasible in a wider selection of molecules. The RAD submillimeter wave spectrometer developed by Krupnov has high sensitivity in the shorter submillimeter wave region where all rotational levels tend to be strong. It has been used to record second-order K-transitions in NH3 and PH3 (ref.l9,60). Groundstate, AJ-0, AK=&3 transitions of OPF3 have been measured by Kagann, Ozier, and Gerry (ref.61). A new, effective method for measurement of the separation of symmetric-top K-levels between which normal transitions are forbidden, has been developed by Gzier and Meerts (refs.62,63). Their method, called the avoidedcrossing molecular-beam resonance method, is especially useful for measurement of the moments of inertia and barriers to internal as C!d3C83 (ref.64,). rotation of symmetric groups in such molecules The IR-MW double resonance method is very promising for observations of forbidden spectra; it has been used to measure rotational transitions in CN4,SiB4, and G&J4 (ref.30). New developments in high-temperature spectroscopy The early measurements of microwave spectra at high temperatures were made on substances such as the alkali halides, which could be vaporized in a heated microwave cell. However, many nonvolatile substances dissociate at the temperatures required for their vaporization. In later work, special techniques have been developed for production of such molecules in the vapor state by reactions that occur withinthemicrowaveabsorption cell. For example, copper halide vapors were produced by the passage of the halide gas over heated chips of copper metal within the absorption cell (ref.65). M. Winnewisser and his group at Giessen (refs.6668) have observed the millimeter wave spectra of diatomic metal oxides and sulfides, BaO, EaS, CaO, etc., with samples produced by vaporization of the metal atoms into the microwave absorption cell, where they react with other vaporized chemicals selected to

30 produce the desired oxide or sulfide. Similar methods are employed by Woeft, Tiemann, Tdrring and others at the Free University of Berlin, which has been a leader in high-temperature microwave spectroscopy for many years and has developed many useful techniques for observation of different types of molecules. A review of the techniques and their applications is given by Tiirringand Tiemann (ref.69). The millimeter wave spectra of NaH and MaD have been observed at Duke from hydrides produced by vaporization of metallic sodium into flowing hydrogen ( or deuterium) gases subjected to a glow discharge within the Pyrex "free-space" absorption cell (ref.70). CONCLUSION From these new mines in micro!*~ave spectroscopy and from the rich new veins that I had not time to mention, as well as from yet more of them likely to be opened up, I conclude that there will be plenty of rewarding work left for the third qeneration of microwave spectroscopists. Those of you who wish more thorouqh coverage of these topics than I have given here can find it in a new edition of our book, pli crowave Molecular Spectra, with Robert L. Cook, to be published within a year or two.

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