Microwave Spectroscopy

BY J. N. MACDONALD AND J. SHERIDAN

1 Introduction

We have retained a general continuity with Volume 3, and regarded work in Chemical Titles before NO. 7 of 1974 as normally covered earlier. For timeliness, however, we have included as many as possible of the papers appearing up to the manuscript stage. Inevitably, some recent studies will be considered in the next Report. Subject to limitations through inconsistencies in titling and keyword listing, a d through the widening of fields which are closely related to microwave (MW) spectroscopy, we have covered finalized publications to September 1975.

While much work remains in what may now be called classical MW spectroscopy, an increasing proportion concerns newer methods and areas, notably double resonance (DR) involving i.r. and u.v.-visible radiation, and studies of time-dependent effects. After considering techniques, in which we include chemical analytical procedures and some purely analytical applications, we deal with some generalities of the derivation of molecular information from spectra and proceed to the discussion of predominantly structural studies through a convenient empirical classification of the molecules concerned, chosen to group together structures with their present MW interest in common. We treat separately the MW spectroscopy of molecules in space, and work primarily concerned with time-dependence and collisional effects.

The flow of new studies towards formal publication is, as always, valuably indicated in the proceedings of conferences, notably the Third European MW Spectroscopy Conference at Venice in 1974, the Fourth Colloquium on High Resolution Molecular Spectroscopy at Tours in 1975, and the 29th and 30th Symposia on Molecular Structure and Spectroscopy at Columbus, Ohio, in 1974 and 1975.

Only in a few randomly chosen examples have we been able to illustrate the strong interdependence of MW spectroscopy and fields such as paramagnetic resonance spectroscopy, laser spectroscopy and molecular beam spectroscopy. The e.p.r. spectroscopy of gaseous free radicals, and related MW absorption studies, have been reviewed. The interrelation of MW, laser and e.p.r. spectroscopies is well shown by the analysis of the laser magnetic resonance spectrum of HO2 in terms of rotational constants. Molecular beam spectroscopy at MW frequencies has yielded striking information on loosely bound molecular complexes; a review has appeared, and specific cases are considered in Section 4A(ii).

Trends have continued towards more consistent success in the study of unstable species and of labile chemical systems and reaction mechanisms, and towards richer information on excited states of molecules. If any one achievement is to be singled out, it is the study of a charged species, CO+, for the first time in a direct MW absorption experiment.

Several valuable surveys of the interpretation of MW data and related information have been published as the proceedings of a conference held at Dartmouth College, Hanover, New Hampshire, in 1973.

2 Techniques

A. Single-radiation Methods. — A spectrometer employing source-modulation, usually at 50 kHz, has been described. Phase-sensitive detection at twice the modulation frequency gives the second derivative of the line-shape, and frequencies up to 80GHz can be used. A critical comparison with Stark-effect modulation systems is made, and the convenience of source modulation for high-resolution work on dense spectra is illustrated for the case of CSCl2. Another complete system, in this instance with computer control, maximizes sensitivity with special regard to work on transient species; the data can also be transferred directly to a large computer for processing. This system was used in the work on CO+ already mentioned. Details of a high-resolution cavity spectrometer for Zeeman studies (see Vol. 3, p. 4) have been given. A superheterodyne system is used with a superconducting magnet. While flexibility must be less than when a broad-band cell is placed in the field, this cavity instrument allows field-strengths as great as 65 kG, with consequent advantages for the study of the second-order Zeeman effect. It is shown that the nominal inhomogeneity of the field influences the precision less strongly than might at first appear, and that magnetic susceptibility anisotropies and molecular quadrupole moments can be derived with accuracies comparable with those from beam studies. A pulsed K-band emission spectrometer with a cell inside an interferometer allows high sensitivity with a flexible cellgeometry for work on timedependent effects. Spectrometers using acoustic detectors (Vol. 3, p. 99) in the range 200 — 870 GHz have been further described by Krupnov and his group;lL the use of high source-powers to increase Sensitivity is discussed, as is the theory of the molecular dynamics of acoustic detection with a view to maximizing performance.

Modulation devices have been evolved for work on unstable species. A Stark-effect modulator which reverses the field direction in alternate 'field-on' half cycles of square wave avoids charge accumulation on the Stark electrode in the study of discharges. Zeeman effect modulator, either square wave at 1 kHz or sinusoidal at 25 kHz, give high sensitivities for detection of SO and OH, and six weak transitions of the latter are reported for the first time.

In a procedure related to the saturation modulation method (Vol. 3, p. 100) spectra are detected through the difference in cavity resonance curves in the presence and absence of power saturation; this has promise for high frequencies and for cavity studies in which mode-contours require suppression.

The first level-crossing resonances in the MW range have been observed when the levels of the J = 1, K = 1 ->J = 2, K = 1 transition of CH3F are tuned by the Stark effect of a field perpendicular to the electric vector of the radiation. Further theoretical study has been made of the influence of absorption sidebands due to RF Stark fields in level-crossing experiments.

Other specialized technical developments are mentioned in Section 6 in connexion with time-dependent effects. Many relevant background developments are treated in the Proceedings of the 1st International Conference on Sub-millimetre waves and their Applications, at Atlanta in 1974.

B. Multiple-radiation Methods. — The MW–MW double resonance technique is now widely applied. A spectrometer with square-wave amplitude modulation of the X-band pump-power in a PIN diode has been briefly described. The pump radiation is then amplified to 2 watts in a...