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The Raman line shape of the vi(Ax) mode of the perchlorate anion has been measured over a wide range of concentrations with various group 1 counterions. The line shapes and line parameters are interpreted in terms of equilibria between the free equated perchlorate anion, the solvent-shared ion pair, and the contact ion pair. Association constants of 3.6 X 10~2 (4 M) and 3.7 X 10"1 (4 M) dm3 mol"1 for the first equilibrium are calculated for lithium and sodium perchlorates, respectively. The vibrational relaxation times of the three perchlorate species have been measured, and the variation in the perchlorate anion vibrational relaxation time is described in terms of strength of the anion-water hydrogen bond.

The relatively short wavelength of mm-wave and THz radiation coupled with good transmission through many dielectric materials allows images to be formed of concealed objects. This chapter gives an overview of the detectors, their associated circuitry, and system developments over the past 10 years, focussing on personnel security screening. We will discuss the phenomenology of imaging at these wavelengths, introduce the reader to the basic architectures being used and developed for image forming instruments, show examples of systems, and also discuss the feasibility of spectroscopic THz imaging for security screening applications.

It is well known that millimetre-wave systems can penetrate poor weather, dust and smoke far better than infrared or visible systems. Imaging in this band offers the opportunity to be able to navigate and perform surveillance in these conditions of poor visibility. Furthermore, the ability to penetrate dielectrics such as plastic and cloth has opened up the opportunity of detecting weapons and contraband hidden under people's clothing. The optical properties of materials have a direct impact on the applicability of imaging systems. In the terahertz band solids have absorptions which can be assigned to vibrational modes. Lattice modes occur at the lowest frequencies and polythene, for example, has a lattice mode at 2.4 THz. Solids have no such absorptions in the millimetre bands (30-300 GHz) and image contrast is produced by differences in transmission, reflection and absorption. A novel, real-time, mechanically scanned, passive millimetre-wave imager has been designed. The antenna elements are based on a combination of a Schmidt camera and a conical scanner, both of which have their origins in optical systems. Polarization techniques, which were developed for operation in the centimetric band, are used to fold the optics. Both 35 GHz and 94 GHz versions have been constructed.

Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHz to ~30 THz). Within its frequency boundaries exist a broad variety of scientific disciplines that have presented, and continue to present, technical challenges to researchers. During the past 50 years, for instance, the demands of the scientific community have substantially evolved and with a need for advanced instrumentation to support radio astronomy, Earth observation, weather forecasting, security imaging, telecommunications, non-destructive device testing and much more. Furthermore, applications have required an emergence of technology from the laboratory environment to production-scale supply and in-the-field deployments ranging from harsh ground-based locations to deep space. In addressing these requirements, the research and development community has advanced related technology and bridged the transition between electronics and photonics that high frequency operation demands. The multidisciplinary nature of THz work was our stimulus for creating the 2017 THz Science and Technology Roadmap (S S Dhillon et al 2017 J. Phys. D: Appl. Phys. 50 043001). As one might envisage, though, there remains much to explore both scientifically and technically and the field has continued to develop and expand rapidly. It is timely, therefore, to revise our previous roadmap and in this 2023 version we both provide an update on key developments in established technical areas that have important scientific and public benefit, and highlight new and emerging areas that show particular promise. The developments that we describe thus span from fundamental scientific research, such as THz astronomy and the emergent area of THz quantum optics, to highly applied and commercially and societally impactful subjects that include 6G THz communications, medical imaging, and climate monitoring and prediction.

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