R. H. Schwendeman and H. N. Volltrauer
Michigan State University East Lansing, Michigan 48823 V. W. Laurie and E. C. Thomas Princeton University Princeton, New Jersey 08450
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Microwave Spectroscopy in the Undergraduate Laboratory
Since its advent following World War
11, microwave spectroscopy has established itself as a versatile and powerful tool for the determination of a wide variety of molecular parameters (1-4). Although best known for it,s ability to provide precise gas phase values for bond distances and angles, microwave spectroscopy has also been one of the best means of determining such quantities as dipole moments, nuclear quadrupole coupling constants, and barriers to internal rotation. New applications are continually being developed as exemplified by recent studies of ring puckering in four- and five-membered rings ( 5 ) ,polarizahility anisotropies (6),molecular electric quadrupole moments (7), and rotational energy transfer (8). A glance at recent journals and books is convincing proof of the importance of this technique in contributing to our knowledge of molecules. Because it is a primary source of so many of the quantities used every day in chemistry and physics, microwave spectroscopy clearly has a place in the physical chemistry laboratory. Furthermore, microwave spectroscopy can be used to combine in one unifying experiment a number of important experimental techniques, while at the same time providing an illustration of fundamental theoretical principles. For example, the student is introduced to a number of basic electronic techniques and instruments and also gains familiarity with elementary vacuum line manipulations. The experimental verification of simple quantum mechanical calculations to six or seven significant figures provides a very satisfying introduction to a subject which often seems far removed from reality to the undergraduate. Another advantage of a microwave experiment is its open-ended nature. It can be simple or complex as desired, allowing each student latitude for his individual talents. Because of the past lack of commercial instruments and the fact that the instrumentation is somewhat more complex than for many techniques, microwave spectroscopy is not in widespread use at present in undergraduate laboratories. However, with the current emphasis on electronics and quantum mechanics in science we believe that an experiment in microwave spectroscopy satisfies an important need in the chemistry curriculum. Furthermore, with presently available commercial equipment, it is relatively simple to assemble a microwave spectrometer for undergraduate use. I n this paper, we describe the spectrometers currently in use a t Michigan State University and 526 / Journal of Chemical Education
Princeton University and the experiments performed with them. Microwave Spectrometer The MSU and Princeton student microwave spectrometers were assembled by the respective microwave research groups a t the two schools. For the microwave or electron spin resonance spectroscopist, or for anyone with a flair for electronics, constructing his own instrument is the least expensive way to obtain a spectrometer. However, it is possible to assemble a spectrometer completely from commercially available components. There are also several commercial firms which sell or are planning to sell'a student spectrometer. An essential part of the spectrometer is the capacity for accurate frequency measurement. We do not believe that qualitative observation of microwave spectra alone is worth the effort for chemistry or physics students. Accurate frequency measurement adds about $1000 to the cost of the instrument. Detailed descriptions of the spectrometers in use a t Princeton and Michigan State University including electronic circuits and lists of parts and suggested sources have been prepared and will be sent upon request. Only a brief description of the instruments will be given here. A block diagram of the MSU spectrometer is shown in Figure 1; the Princeton spectrometer is
Figure 1.
Block diagram of student microwore spectrometer at Michigan
State Univenity.
similar. The spectrometers may be described under three headings: the vacuum system, the microwave plumbing, and the electronics. Vacuum System The vacuum system required is a very simple one, consisting of a glass manifold fitted with stopcocks and
standard taper joints for the sample bulbs, a thermocouple gauge, a vacuum connection to the sample cell, a trap, and a small mechanical pump. The appropriate pressure of a sample for spectroscopy is easily found by trial and error and is usually about 0.01-0.1 torr. Therefore, for the samples described below, a mechanical vacuum pump capable of producing a limiting low pressure near 0.001 torr is sufficient. If desired, a simple glass diffusion pump using silicone oil can be added. I n addition to increasing the vacuum which can be obtained, a diffusion pump provides faster pumpdown time below pressures of 0.1 tom. Microwave Plumbing
The microwave radiation source is a Varian X-13 reflex klystron oscillator, which is a mechanically-tuned, velocity-modulated electron tube which oscillates from 8.2 to 12.4 GHz and provides more than ample power for spectroscopy. The frequency range of this tube determines the frequency range of the spectrometer. At MSU a klystron power supply was constructed; a t Princeton a commercial klystron power supply was used. The radiation from the oscillator is sent through a cavity frequency meter which can measure the frequency to 0.1% or about 10 MHz. A variable attenuator con'trols the power level of the radiation. Beyond the attenuator the radiation is split into two parts, one part going through the sample cell, the other going to a multiplier-mixer for precise frequency measurement. The sample cell is a length of brass X-band microwave waveguide (0.400 X 0.900 in. i.d.). A thin metal strip parallel to the broad face of the waveguide is centered in the cell and insulated from it by grooved strips of Teflon tape running the length of the cell against the narrow faces (Fig. 2). The metal strip
septvm
Figure 2. Diagram of cross-section of the sample cell for 6 s student spectrometer. Electrical connection to the centrd septum is through a vocuum-tight electric01 connector and a hole in the norrow face of 6 e waveguide.
serves as an electrode and allows the application of an electric field for modulation purposes as well as for Stark effect studies. A vacuum-tight electrical connection to the central metal strip isprovided a t one end of the cell. The ends of the cell contain waveguide flanges grooved to hold rubber O-rings. The cell is made vacuum tight by compressing a sheet of mica (0.002 in. thick) between another flange and each Oring. Alternatively, the flanges on the cell can be flat and the vacuum seal made by attaching the mica windows with a wax such as Apieeon W. After passing through the sample cell the radiation is led to a microwave detector containing a silicon crystal diode. Electronics
The electronics of a typical Stark-modulated microwave spectrometer consist of (1) the microwave system,
which includes the microwave oscillator, its power supply, the microwave plumbing, the crystal detector, and a crystal-current meter; (2) the modulationdetection system, which includes the square-wave generator, the preamplifier, and the amplifier-phasesensitive detector; (3) the frequency measuring system which includes a 5 MHz oscillator, a 450 MHz frequency multiplier, an electronic counter, a crystal-mixer multiplier, and a frequency-selective amplifier; and (4) the frequency sweeping and display system which can be either an oscilloscope with a sawtooth voltage output or a strip-chart recorder and a slow-speed motor drive. The microwave oscillator, its power supply, and the microwave plumbing were described above. The crystal detector is a waveguide-mounted silicon diode which rectifies and filters the microwave signal, providing a dc current which is approximately proportional to the microwave signal power. The current from the crystal detector is monitored by means of a low-resistance (
