Microwave Spectroscopy

BY J. N. MACDONALD AND J. SHERIDAN

1 Introduction

We have continued the system employed in Volume 4 to summarize a continued flow of work on the many facets of this field.

A marked feature has been the continuance of successful attacks on the microwave (MW) spectroscopy of unstable species and energetically unfavourable conformations of molecules. The charged species HCO+ and HN+2, previously regarded as sources of microwave emissions from space, have now been studied in the laboratory, and the isomer of hydrogen cyanide (HNC), also present in space, has been investigated in great detail. A second conformer has been shown to be present in as familiar a substance as formic acid, and details of its structure have been established. The scope, power, and reliability of the techniques are clearly increasing to a point where some long-standing gaps in our knowledge of simple molecules in the gas phase are being rapidly filled. The range of molecules studied is similar to that in former years. The astrophysical aspects of the subject continue very active, and may receive further incentive from the commissioning of the new 300GHz radio-telescope near Amhurst, Massachusetts.

Few reviews have appeared, but Sutter and Flygare have given a comprehensive account of the theory and practice of MW Zeeman spectroscopy. Work with molecular beam maws has been reviewed by Lainé and by Dymanus. The related field of spectroscopy with coherent laser sources has been surveyed by Shimim. The spectroscopy of van der Waals molecules, including the important contributions of MW work, has been reviewed.

2 Techniques

A. Single-radiation Methods. — A notable publication describes details of a pulsed system for Fourier transform MW spectroscopy (Vol. 4, p. 68), with systematic discussion of the factors and procedures which can allow measurements in the time domain to give considerable improvements in signal-to-noise ratio and in resolution due to the absence of power broadening. The system developed, which is tested at rather low frequencies, some 4 — 6 GHz, uses a 4 m length of 'empty' C-band guide as a cell. Data confirming the advantages and promise of the methods are presented for lines of CH2O and CD2O.

In this connection, we may note the use of Fourier-transform methods in the lower resolution study of spectra of water vapour at 168 — 189 GHz, with use of harmonic radiation from IMPATT diode oscillators.

Details of a spectrometer using a cell of the usual Stark effect waveguide type, but tuned to resonance for selected absorptions, have been given. While the sensitivity here claimed is not exceptional, this type of cell is that described by others (Vol. 4, p. 5) as especially suited to analytical work at extreme sensitivities.

B. Multiple-radiation Methods. — Two papers by Stiefvater describe in detail techniques of MW-MW double resonance modulation spectroscopy developed as a standard laboratory method which has already demonstrated its value in the detection and assignment of complex spectra, including many lines due to weakly populated states and rare isotopic species (see Vol. 4, pp. 3 and 45). In the first paper the general principles, including the use of double resonance 'maps', are developed and it is noted that the ready detection of resonances when the pump frequency is some way from its exact resonance value allows 'double search' procedures in which signal and pump frequencies are swept. In a second part, the lineshapes and their integrated intensities are considered: there are two readily distinguished lineshapes which allow deductions to be made about the location of the energy level common to the two transitions and so facilitate spectral assignments. The integrated intensities of lines can be found by procedures which do not require knowledge of the absolute values of the pump power.

In the above work the pump frequency is modulated in and out of resonance at 100kHz through a square-wave voltage applied to the reflector of the pump klystron. In a description of a related instrument the Zurich group have employed also an amplitude modulation of the pump power at 30 kHz with a PIN diode and, in describing the instrument, have made interesting comparisons of the frequency and amplitude modulation options in terms of leakage signals and other electronic difficulties. The two types of lineshape are also considered and compared, in examples, with calculated shapes.

A double resonance spectrometer of essentially conventional Stark effect modulated type, but with the attractive facility to sweep the pump frequency automatically over the whole of the 8–12.4 GHz band, has been described. Double resonance connections between transitions in this band and selected signal transitions may then be readily found.

The Kiel group have reported further on RF–MW double resonance, in which the 'RF' frequency applied as pump radiation to the Stark effect electrode (see Vol. 3, p. 3) can be as high as 4 GHz and so enter the low MW range. The signal frequencies have also been extended to 160 GHz. Experimental details are given and many examples of resonances are quoted, with particular reference to CH3OCl and CD3OCl, which illustrate the value of the methods in allowing high sensitivities in the measurement of rotational transitions at very low frequencies.

Double resonances involving MW with i.r. or higher frequency radiation from lasers continue to be widely reported, and the use of lasers for DR work has been reviewed. Takami has developed the theory of such cases, with both MW detection and optical detection. Lineshapes and peak values of the DR signals are considered, as are the effects of varying optical saturation on such observations; circumstances which may lead to anomalous line-shapes are indicated.

The shifts of frequency of MW lines in the presence of a non-resonant laser field have been used, following earlier theory, to determine i.r. transition frequencies with accuracies, in the case of NH3, similar to those obtainable with Stark laser spectroscopy .

C. Techniques for Chemical Analysis. — A further review of analytical MW spectroscopy has been published by Lovas. Most aspects referred to have been frequently discussed in the past (e.g. Vol. 4, p. 5) but there is timely emphasis on the potential of MW methods for the monitoring of complex reactions, including such important areas as pyrolysis processes, catalytic syntheses, such as that of HCN from methane, ammonia and oxygen, and the methanol synthesis. The plea, implicit even in this paper from the United States, for more response from the chemical processing side in pointing to problems in...