Introduction

The study of order in structures involving biomolecules divides naturally into two parts. On the one hand, one can consider ordered structures in vivo, and on the other hand in man-made systems. The obvious example of thin organic films in the former category is the cell plasma membrane (the term for the exterior membrane of a cell). In 1925 Gorter and Grendel suggested that the cell membrane consisted of a bilayer of lipid molecules with the hydrophilic ends facing outward and the hydrophobic ends facing one another in the interior of the membrane. (The structures of some common lipids are shown in Figure 8.1.) It was a long time before this postulate was definitely confirmed but it is now generally accepted that the plasma membrane is roughly of this general form. The main modifications of this picture are as follows.

1. (a) Many membrane-bound enzymes penetrate the plasma membrane and are stabilised by the fact that the surface of the enzyme consists of two hydrophilic end regions and an intermediate hydrophobic region. These enzymes take part in the transport of particular substances across the membrane and in various important processes in which energy is stored or interchanged by the medium of ion transport. This topic is returned to below.

2. (b) A number of glycoproteins are incorporated into the membrane and are involved in cell recognition processes.

3. […]

Introduction

The name self-assembly is an unfortunate one as it implies the achievement of something approaching the synthesis of artificial life. However, this term has now been generally accepted and so will be used here. It has two distinct but related meanings. The majority of papers bearing this phrase in their titles concern monolayers of organic molecules adsorbed on solid inorganic surfaces. These rather simple systems can be studied and characterised at a level of detail and rigour which it is difficult to achieve with the other systems discussed in this book. They can also be formed using very simple apparatus. For both these reasons they have made a strong appeal to surface chemists. There exists, however, a more limited group of papers in which treatment of the initial organic layer by a succession of reagents has made it possible to build up ordered multilayers. In principle, this latter technique should make it possible to form the analogues of Langmuir–Blodgett Z-type multilayers and thus use relatively simple chemical methods to construct non-centrosymmetric systems of use in technology, as discussed, for example, in Chapter 5. So far such applications of this technique have not proved practicable and the difficulties involved will be discussed later in this chapter.

Monolayers formed from carboxylic acids

In recent years study of the absorption of small molecules on well characterised single crystal surfaces has attracted many research workers.

Introduction

In this chapter we turn to the study of LB films formed from polymers and LB films consisting of alternate layers of two different amphiphiles. In principle, of course, it would be possible to superimpose successive layers of three or more distinct amphiphiles but little has been done in this direction. However, a few examples of more complex alternating structures will be considered.

Polymer LB films naturally divide into the following categories.

1. Systems in which a multilayer structure is formed from molecules containing one or more double bonds and in which polymerisation is subsequently initiated by irradiation by γ-rays, ultraviolet light or an electron beam.

2. Systems similar to the above but in which the constituent monomers contain the diacetylene group.

3. Multilayers formed from polymers bearing both hydrophilic and hydrophobic side groups which are spread as polymers at the air/water interface and are subsequently deposited on a substrate by the LB technique.

4. Rigid rod polymers which have both hydrophilic and hydrophobic characteristics and which are capable of residing with the rod axis horizontal at the air/water interface and which can be deposited on a solid substrate by the LB technique.

Post-formed polymers made from monomers containing one or more double bonds

Studies of polymerisation at the air/water interface have been made repeatedly over the years and an account of early work in this field is given by Gaines. Here the discussion will be largely confined to polymerisation carried out after deposition.

Definitions of order

In the last chapter we have used the word ‘order’ without giving it any precise meaning. Most definitions of order involve thermodynamic concepts. Thus, for example, one might say that the most ordered state of a system is the one to which the system tends as the temperature tends to absolute zero. This definition would, however, be of little service in the present context. Most of the systems which we will discuss are remote from thermodynamic equilibrium. This is true both of the films during their preparation and also of the ‘final’ prepared films. However, these prepared films are in states of metastable equilibrium which are likely to survive for periods long compared with the time taken to carry out experiments on them and, very often, for periods so long as to be, from a human point of view, infinite.

We thus need a different definition of order. Here it is suggested that we view the most ordered state as the one which corresponds most closely to some preconceived structure which we wish to bring about. From a practical point of view the extent to which we can tolerate disorder may vary widely depending on the context. Thus, for example, in a system analogous to a chain of DNA encoding the structure of a particular enzyme, a single defect may render the system useless.

The scope of this book

The major scientific advances which have taken place since the Second World War have, in some cases, had little influence on everyday life while, in other cases, they have had the most profound effect on it. Thus, for example, particle physics and astronomy have revolutionised our basic concepts of the structure of matter but have had only minor effects on the life of the non-specialist. On the other hand there are two particular fields of study whose influence has affected the inhabitants of all civilised societies. One of those is solid state electronics and the computer revolution which it has given rise to. The other is the advance in biochemistry and organic chemistry which has provided the physician with a large range of effective drugs and has transformed medicine from an art to an applied science. It is thus not surprising that the idea has arisen that a synthesis of these two fields might bring about remarkable new advances. The name molecular electronics has been proposed for this concept though different people have rather different ideas as to what is meant by this expression.

To give effect to this concept it is necessary to design and construct molecules which have certain desired physical properties and then to learn how they can be assembled in particular well ordered ways. The first problem is the province of the chemist and here considerable progress has already been made.

Spectroscopy is concerned with the interaction of light with matter. This monograph deals with collision-induced absorption of radiation in gases, especially in the infrared region of the spectrum. Contrary to the more familiar molecular spectroscopy which has been treated in a number of well-known volumes, this monograph focuses on the supermolecular spectra observable in dense gases; it is the first monograph on the subject.

For the present purpose, it is useful to distinguish molecular from supermolecular spectra. In ordinary spectroscopy, the dipole moments responsible for absorption and emission are those of individual atoms and molecules. Ordinary (or allowed) spectra are caused by intra-atomic and intra molecular dynamics. Collisions may shift and broaden the observable lines, but in ordinary spectroscopy collisional interactions are generally not thought of as a source of spectral intensity. In other words, the integrated intensities of ordinary spectral lines are basically given by the square of the dipole transition matrix elements of individual molecules, regardless of intermolecular interactions that might or might not take place. Supermolecular spectra, on the other hand, arise from interaction-induced dipole moments, that is dipole moments which do not exist in the individual (i.e., non-interacting) molecules. Interaction-induced dipole moments may arise, for example, by polarization of the collisional partner in the electric multipole field surrounding a molecule, or by intermolecular exchange and dispersion forces, which cause a temporary rearrangement of electronic charge for the duration of the interaction.

In Chapter 5 the absorption spectra of complexes of interacting atoms were considered. If some or all of the interacting members of a complex are molecular, additional degrees of freedom exist and may be excited in the presence of radiation. As a result, besides the translational profiles discussed in Chapter 5, new spectral bands appear at the rotovibrational transition frequencies of the molecules involved, and at sums and differences of such frequencies – even if the non-interacting molecules are infrared inactive. The theory of absorption by small complexes involving molecules is considered in the present Chapter.

We will be concerned with the spectral bands in the microwave and infrared regions. The translational and the purely rotational bands appear both at low frequencies and form in general one composite band, especially at the higher temperatures where individual lines tend to overlap (‘rototranslational band’). Moreover, various rotovibrational bands in the near infrared will be considered, such as the fundamental and the overtone bands. Even high overtone bands in the visible are of interest, e.g., of H2. We have seen in Chapter 3 that induced spectra of the kind are readily discernible in gases whose (non-interacting) molecules are infrared inactive, but evidence exists that suggests the presence of induced absorption in the allowed molecular bands as well. Induced absorption involving electronic transitions will be briefly considered in Chapter 7.

The existing bibliographies on collision-induced absorption (CIA) list more than 800 original papers published in the 45 years of history of the field. Furthermore, a number of review articles focusing on one aspect of CIA or another are listed, along with compilations of lectures given at summer schools, advanced research seminars or scientific conferences. A monograph which attempts to review the experimental and theoretical foundations of CIA, however, cannot be found in these carefully compiled listings.

Yet the field is of great significance and continues to attract numerous specialists from various disciplines. CIA is a basic science dealing with the interaction of supermolecular systems with light. It has important applications, for example in the atmospheric sciences. CIA exists in all molecular fluids and mixtures. It is ubiquitous in dense, neutral matter and is especially striking in matter composed of infrared-inactive molecules. As a science, CIA has long since acquired a state of maturity. Not only do we have a wealth of experimental observations and data for virtually all common gases and liquids, but rigorous theory based on first principles exists and explains nearly all experimental results in considerable detail. Ab initio calculations of most aspects of CIA are possible which show a high degree of consistency with observation, especially in the low-density limit.

In this Chapter, we will briefly look at a number of topics related to collision-induced absorption of infrared radiation in gases. Specifically, in Section 7.1, we consider collision-induced spectra involving electronic transitions in one or more of the interacting molecules. In Section 7.2, we focus on collision-induced light scattering, which is related to collision-induced absorption in the same way that Raman and infrared spectra of ordinary molecules are related. The collision-induced Raman process arises from the fact that the polarizability of interacting atoms/molecules differs from the sum of polarizabilities of the non-interacting species. Closely related to the collision-induced Raman and infrared spectroscopies are the second (and higher) virial coefficients of the dielectric properties of gases, which provide independent measurements of the collision-induced dipole moments, Section 7.3. Finally, we look at the astrophysical and other applications of collision-induced absorption in Sections 7.4 and 7.5.

Collision-induced electronic spectra

Collision-induced electronic spectra have many features in common with rovibrotranslational induced absorption. In this Section, we take a look at the electronic spectra. We start with a historical note on the famous forbidden oxygen absorption bands in the infrared, visible and ultraviolet. We proceed with a brief study of the common features, as well as of the differences, of electronic and rovibrotranslational induced absorption.

In this Appendix we attempt to briefly review developments since the early 1990ies in the field of collision-induced absorption in gases. Many of the new contributions were announced, and numerous references to current literature were given in the proceedings of periodic conferences and special workshops. We mention especially the Proceedings of the biennial International Conferences on Spectral Line Shapes and the annual Symposia on Molecular Spectroscopy. New work in collision-induced absorption in gases has been reviewed in the Proceedings of a NATO Advanced Study Institute, a NATO Advanced Research Workshops, and in a recent monograph Molecular Complexes in Earth's, Planetary, Cometary, and Interstellar Atmospheres. A multi-authored volume, a significantly augmented treatment of bremsstrahlung, is also of interest here, for example when electrically charged particles exist in dense, largely neutral and hot environments, e.g., in shock waves, in the atmospheres of “cool” white dwarf stars, in sonoluminescence studies, etc.

Binary Interaction-Induced Dipoles.Ab initio quantum chemical calculations of interaction-induced dipole surfaces are known for some time (Section 4.4, pp. 159 ff.) Such calculations were recently extended for the H2—He and H2—H2 systems, to account more closely for the dependencies of such data on the rotovibrational states of the H2 molecules.

The theory of collision-induced absorption developed by van Kranendonk and coworkers and other authors has emphasized spectral moments (sum formulae) of low order. These are given in closed form by relatively simple expressions which are readily evaluated. Moments can also be obtained from spectroscopic measurements by integrations over the profile so that theory and measurement may be compared. A high degree of understanding of the observations could thus be achieved at a fundamental level. Moments characterize spectral profiles in important ways. The zeroth and first moments, for example, represent in essence total intensity and mean width, the most striking parameters of a spectral profile.

While spectral moments permit significant comparisons between measurements and theory, it is clear that some information is lost if a spectroscopic measurement is reduced to just one or two numbers. Furthermore, for the determination of experimental moments, substantial extrapolations of the measured spectra to low and high frequencies are usually necessary which introduce some uncertainty, even if large parts of the spectra are known accurately. For these reasons, line shape computations are indispensible for detailed analyses of measured spectra, especially where the complete absorption spectra cannot be measured. Moreover, one might expect that the line shape of the induced spectra, with its ‘differential’ features like logarithmic slopes and curvatures and the dimer structures, depend to a greater degree on the details of the intermolecular interactions than the spectral moments.