It was demonstrated in Chapter 6 that impact theory is able to describe qualitatively the main features of the drastic transformations of gas-phase spectra into liquid ones for the case of a linear molecule. The corresponding NMR projection of spectral collapse is also reproduced qualitatively. Does this reflect any pronounced physical mechanism of molecular dynamics? In particular, can molecular rotation in dense media be thought of as free during short time intervals, interrupted by much shorter collisions?

It seems that an affirmative answer is hardly possible on the contemporary level of our general understanding of condensed matter physics. On the other hand, it is necessary to find a reason for numerous successful expansions of impact theory outside its applicability limits.

One possibility for this was demonstrated in Chapter 3. If impact theory is still valid in a moderately dense fluid where non-model stochastic perturbation theory has been already found applicable, then evidently the ‘continuation’ of the theory to liquid densities is justified. This simplest opportunity of unified description of nitrogen isotropic Q-branch from rarefied gas to liquid is validated due to the small enough frequency scale of rotation–vibration interaction. The frequency scales corresponding to IR and anisotropic Raman spectra are much larger. So the common applicability region for perturbation and impact theories hardly exists. The analysis of numerous experimental data proves that in simple (non-associated) systems there are three different scenarios of linear rotator spectral transformation.

As is seen from relations (2.5)–(2.8), isotropic scattering is independent of orientational relaxation. Since the isotropic component of the polarization tensor is invariant to a molecule's reorientation, the corresponding correlation function K0 describes purely vibrational relaxation. This invariance does not mean however that vibrational relaxation is completely insensitive to angular momentum relaxation. Interaction between vibrations and a molecule's rotation determines the rotational structure of the isotropic scattering spectra observed in highly rarefied gases. The heavier the molecule, the smaller is the constant αe of the Q-branch rotational structure. In fact this thin structure is easily resolved only in hydrogen and deuterium. The isotropic Raman spectrum of most other gases is usually unresolved even at rather low pressure and when describing its shape at higher densities one may consider J a classical (continuous) variable.

Within the framework of the impact theory J(t) is a purely discontinuous Markovian process. The same is valid for the corresponding frequency, or ‘rotational component’, which changes its position in the spectrum after each collision. This phenomenon, known as spectral diffusion or rotational frequency exchange, is accompanied by adiabatic dephasing of the vibrational transition caused by these same collisions. Both processes contribute to observed spectral transformation with increasing collision frequency, however they have opposite effects. While frequency exchange leads to collisional narrowing, dephasing results in the spectrum-broadening. If dephasing is weak and the collision frequency is small, the tendency for the spectrum to narrow prevails.

Debye's theory, considered in Chapter 2, applies only to dense media, whereas spectroscopic investigations of orientational relaxation are possible for both gas and liquid. These data provide a clear presentation of the transformation of spectra during condensation of the medium (see Fig. 0.1 and Fig. 0.2). In order to describe this phenomenon, at least qualitatively, one should employ impact theory. The first reason for this is that it is able to describe correctly the shape of static spectra, corresponding to free rotation, and their impact broadening at low pressures. The second (and main) reason is that impact theory can reproduce spectral collapse and subsequent pressure narrowing while proceeding to the Debye limit.

The above capabilities of impact theory are illustrated in preceding chapters by consideration of the isotropic scattering spectrum, which consists of one Q-branch. The peculiarity of the present problem is that in the spectrum of orientational relaxation there are always several branches, and, generally speaking, one cannot consider their transformation independently. The very first attempt to build a quasi-classical impact theory of rotational structure drew one's attention to this fact as being of principal importance. It made the theory similar to the quantum theory of unresolved atomic spectra, whose Stark or Zeeman components interfere with each other during collisions. Interference of the same nature takes place between rotational branches of vibrational spectra, described classically. Increase of collisional frequency causes spectral collapse, but very rarely does an atomic spectrum narrow afterwards.

The quasi-classical theory of spectral shape is justified for sufficiently high pressures, when the rotational structure is not resolved. For isotropic Raman spectra the corresponding criterion is given by inequality (3.2). At lower pressures the well-resolved rotational components are related to the quantum number j of quantized angular momentum. At very low pressure each of the components may be considered separately and its broadening is qualitatively the same as of any other isolated line in molecular or atomic spectroscopy.

At the beginning, line shape theory concentrated on calculation of the width and shift of an isolated line broadened by collisions considered as instantaneous. This approach, known as ‘impact theory’, which originated with the pioneering work of Lorentz and Weisskopf, was initially purely adiabatic. The assumed adiabaticity of collisions excluded in principle any interference between spectral lines in the frame of impact theory. The situation changed with enhanced study of Stark multiplets of atoms in plasmas. The Stark sublevels were so weakly split in a weak electrical field of ions that a condition similar to (1.7) was met (ΔEτc ≪ 1) and a non-adiabatic generalization of impact theory became necessary. Transitions between Stark sublevels as an effective mechanism of their broadening were first taken into account by Kolb. Subsequently nonadiabatic theory was employed to describe overlapping Stark multiplets. It was mentioned that a qualitatively new feature arises when collisions are non-adiabatic: collisionally induced interference between components of the Stark structure causes spectral collapse.

As the density of a gas increases, free rotation of the molecules is gradually transformed into rotational diffusion of the molecular orientation. After ‘unfreezing’, rotational motion in molecular crystals also transforms into rotational diffusion. Although a phenomenological description of rotational diffusion with the Debye theory is universal, the gas-like and solid-like mechanisms are different in essence. In a dense gas the change of molecular orientation results from a sequence of short free rotations interrupted by collisions. In contrast, reorientation in solids results from jumps between various directions defined by a crystal structure, and in these orientational ‘sites’ libration occurs during intervals between jumps. We consider these mechanisms to be competing models of molecular rotation in liquids. The only way to discriminate between them is to compare the theory with experiment, which is mainly spectroscopic.

Line-shape analysis of the absorption or scattering spectra supplies us with normalized contours Gℓ(ω) which are the spectra of orientational correlation functions Kℓ = 〈Pℓ; [u(t)·u(0)]〉. The full set of averaged Legendre polynomials unambiguously defines the orientational relaxation of a linear or spherical rotator whose molecular axis is directed along the unit vector u(t). Unfortunately, only the lowest few Kℓ are available from spectroscopic investigation. The infrared (IR) rotovibrational spectroscopy of polar molecules gives us G1(ω – ωυ) which is composed of some rotational branches around vibrational frequency ωυ.

The formation of Langmuir-Blodgett films

The technology of the formation of LB films has much in common with the trough technology discussed in Chapter 3. In addition to the equipment described there, one needs a mechanical device to raise and lower the substrate through the air/water interface at a predetermined rate. Various devices have been employed but it is usual to provide the vertical movement by driving a large micrometer screw by an electric motor via a reduction gear train. Practical velocities are such that they are usually measured in millimetres per minute. It is essential to be able to vary the dipping rate as there is an upper effective rate which can be employed for any particular material. This rate is determined by the speed at which water drains from the film as it is withdrawn from the subphase and by the viscosity of the film and hence the rate at which material can approach the substrate. For a material of high viscosity this procedure is difficult to carry out properly and a substantial difference in pressure may occur between the pressure sensor and the region immediately in contact with the substrate. In fully automated troughs the substrate is withdrawn from the subphase and maintained in this position while the film at the air/water interface is respread and compressed to a predetermined dipping pressure. In such systems it is necessary to carry through a cyclic compression and expansion process several times to arrive at a good approximation to an equilibrium situation before the dipping process is reactivated.

In writing any scientific work it is difficult to decide what background knowledge one ought to assume in potential readers. This difficulty is particularly acute when, as in this case, the book is of an interdisciplinary nature and deals with topics which belong properly to physics, chemistry and, to a certain extent, biology. When in doubt I have decided to assume ignorance rather than knowledge. For example, the text is sprinkled with diagrams illustrating the structures of chemical compounds as it is likely that readers with a physics background will be unable to deduce these structures from the names of the compounds. On the other hand, the derivation of formulae, the origins of which are readily available in common text books, has usually been omitted. When derivations are not so easily come by, they have been given. This is true, for example, in the case of the basic expression for the refractive index of a material as experienced by neutrons. I have been unable to find a derivation of this expression in any of the various books on neutron diffraction which I have examined.

The study of thin organic films has expanded enormously in recent years and it has been necessary to be very selective in order to prevent this work degenerating into a bibliography. I have attempted to discuss material in which structure and order are dealt with and to ignore the many papers, interesting from other points of view, in which these matters have not been mentioned or are given only a minor place.

Trough technology

Amphiphilic materials spread at the air/water interface have been the subject of intensive study over a long period of time. The type of apparatus usually used for this purpose has much in common with the apparatus needed to form Langmuir–Blodgett films and, indeed, it is usually possible to adapt the same apparatus for both purposes. In this section the problems which must be overcome if these processes are to be carried out are discussed and the most effective solutions to these problems described.

It has become traditional to use the word ‘trough’ to denote such apparatus and this usage will be adhered to here though the word trough tends to suggest such things as hogwash rather than the ultra-cleanliness needed for effective studies of monolayers. It is indeed this cleanliness which must be our first concern. Many materials which would otherwise be suitable for the fabrication of troughs tend to leach out surface active material into the water based subphase contained in the trough and thus can not be used. Most modern troughs are made in one of the two following ways.

1. Teflon (polytetrafluoroethylene) does not leach out plasticisers and can be purchased in substantial blocks, sheets and rods of various thicknesses. The preferred method is to machine a trough from a solid block of this material. This is a practicable procedure if a good milling machine is available but one is limited to a rather shallow trough. […]

Introduction

As was pointed out in Chapter 1 liquid crystals (or mesophases as they are often called) were first discovered by Reinitzer in 1888 and the first proper classification of liquid crystals was made by Friedel in 1922. Since that time various new categories of liquid crystals have been discovered and named. It would be impossible to give an extensive treatment of this important and wide ranging subject here but, as so many of the systems discussed in this book have a liquid crystalline structure, at least a brief treatment of the topic is essential. Furthermore, several methods of forming ordered thin organic films not treated in other chapters depend on the initial formation of a mesophase. It has been suggested that something like 10% of fine organic materials listed in a typical catalogue of such products are capable of existing in a mesophase within some appropriate temperature range or, in the case of lyotropic liquid crystals, when dissolved at an appropriate concentration in some solvent. It is thus obvious that the subject has immense ramifications and could not be pursued in any great breadth here.

Liquid crystals can initially be divided into thermotropic and lyotropic materials. The first category involves a single molecular species and exists in a temperature range which lies between the melting point of the solid phase and the temperature at which a true liquid is arrived at.