Microwave Spectroscopy

BY J. N. MACDONALD AND J. SHERIDAN

1 Introduction

The literature of microwave spectroscopy has been somewhat less in the period than for several years. There is no general evidence, however, of a reduction in activity. Trends mentioned in previous volumes have continued, particularly those towards wider use of combined techniques such as microwave-optical double resonance. Continued success in studies of relatively unstable species, formed by a variety of reactions, remains a noticeable feature.

A volume has appeared which contains several reviews in this field, including coverage of millimetre and sub-millimetre wave spectroscopy, sub-millimetre wave spectroscopy by methods employing acoustic detectors and forbidden rotational transitions. The least squares fitting of data to molecular properties is also reviewed.

A theoretical discussion has been given of how optical rotation may arise in rotational spectroscopy, predicting the dependence of such effects on the molecular parameters.

2 Techniques

Use of essentially established techniques for the identification and estimation of substances remains active. An automated solid state spectrometer operating in the range 18 to 26 GHz for analysis and process control has been described by workers at the Philips Laboratories at Hamburg. A balanced system of two cells, with their stark-modulations in anti-phase, is used to reduce noise. Tests of aspects of the analytical use of microwave (MW) spectroscopy which have been pointed out in the past are reported, with results which are not without promise. Rinehart has given an up-to-date review of analytical microwave spectroscopy, including references to current developments, not all of which have been fully published. Details of the use of half sine-wave modulation for simplifying spectra for analytical purposes have been given.

Microwave spectroscopy continues to be used with advantage to identify products in complex reaction systems. Bak and co-workers have recently identfied N-methylmethyleneimine in normal and fully deuteriated forms from pyrolysis of dimethylamine and [2H7]dimethylamine at low pressures in a quartz tube. Similar treatment of other substances is reported. Among products being sought is cyanoform, HC(CN)3 [see Section 4A (vi)], the MW spectrum of which has meanwhile been characterized for samples more specifically synthesized." Similar means were used to study MW spectra of cis- and trans-forms of dithioformic acid, HCSSH, formed by pyrolysis of HC(SH)3 (see Section 4C). Several known substances have been identified by MW techniques in the products of the reaction of ozone with vinyl chloride.

Intensity studies have been applied analytically to monitor concentrations of HCN in several vibrational states in the plasma used in the well-known HCN-laser. The total HCN concentration reaches a maximum at very low laser current. As this current increases, the populations of the 10°0 and 00°1 states increase in comparable fashion, the data indicating that the laser action is less critically dependent on metastability in the 10°0 state than has been proposed. The pressure dependences of the populations of these two excited states differ in ways consistent with de-excitation factors which have been thought to limit the operating pressure of this laser.

Isotopic analytical work with deuteriated propenes has been extended by a study of reactions of 1,1-dideuterio propene on Ω-zeolite. The MW analysis of the products gives clear support for an associative mechanism of double-bond shift. Double-bond migration was not detected in 2-cyclopenten-l-one, by such methods, when this substance was treated with deuterium on a nickel catalyst; this is understandable in view of the conjugation in the starting material which such migration would destroy. The isotopic stereochemistry of the resulting deuteriated cyclopentanones was followed by MW spectra and it was found that the predominant dideuterio species has deuterium atoms on the same side of the quasi-plane of the ring at the α- and β-adjacent positions. This predominance is interpreted, perhaps not convincingly, as indicating associative adsorption of the double bond grouping in eclipsed conformation; inversion at one of the carbons is perhaps more strongly excluded than associative adsorption with staggered conformation. In contrast, reduction of 2-cyclopenten-l-one by magnesium in deuteriated acetic acid gave equal deuteriation on both sides of the quasi-plane of the ring.

Beam maser spectroscopy has been extended into the sub-millimetre wave region. Details are given of an instrument in which advantage is taken of the rotational cooling in nozzle beams, sixty-three of which are used in an array with state selectors, directed into a Fabry–Perot cavity. Deuterium hyperfine splittings in D20 and ND3 were measured with linewidths, at half peak intensity, of about 20 kHz.

Several papers deal with the practice and basic theory of double resonance techniques. For MW–MW double resonance, a spectrometer is described in which the pump and signal radiations operate in individual crossed open resonators, extra isolation being achieved by making the electric vectors of the two radiations mutually perpendicular. Up to three watts of pump power in the range 8.2 to 12.4 GHz are applied via a travelling wave amplifier. High effective pump power levels are obtainable and double resonances can be observed with sample pressures as high as 0.5 Torr, while flexibility and stability over large frequency ranges can be maintained. Tests on lines of allyl alcohol indicate that the sensitivity can be raised above that for a conventional Stark modulated system. Such methods point to ways in which double resonance modulation may facilitate the most effective use of cavity cells.

For laser-MW double resonance work, H. Jones has described two types of cell which allow ready detection of resonances and two-photon transitions when optical detection is used in conjunction with wide scanning of the microwave radiation in such cells within the laser cavity. Scan rates of 1 GHz min-1 were possible, with MW pumping power densities down to a few mW cm-2. The use of such an arrangement to measure 30 transitions in the v2 band of ammonia with high accuracy, through two-photon transitions, is also described.

Further theoretical work on optical-MW double resonance deals with the case where the optical frequency is off-resonance. Line shapes for fixed optical and variable MW frequencies are calculated for a variety of saturation conditions and for both MW and optical detection. This unified treatment of two-photon transitions and the optical Stark effect relates directly to several areas of...