So far, the energy necessary for a change to take place had to be brought in by the reactants. In this chapter the required energy is provided by light. Often, light allows us to promote the system to an electronically excited state where the potential and hence the dynamics are different from those of the ground electronic state. But light-induced chemistry or “photochemistry” is not only a new way of driving chemical reactions. Photochemistry offers a degree of control – a selectivity – that is unique in its potential and variety. Our ability to tailor light toward specific applications is part of the secret. This control ranges from a simple requirement such as wavelength or polarization selection that allows us to access a particular excited state to more complex tasks such as manipulations that produce short light pulses that can freeze the motion and to pulse shaping in frequency and time for the purpose of guiding nuclear motions. Another important aspect is the ingenuity of scientists in matching up light and matter. In particular we have learned to take advantage of the changing character of the intramolecular dynamics of molecules as the energy increases. At low levels of excitation polyatomic molecules offer us a great individuality through their spectroscopic fingerprints, whereas at higher levels of excitation they have a quasi-continuum of vibrational states and can be made to take up lots of energy like a good heat sink. This saga is nowhere near to a closed chapter.

Modern spectroscopic techniques such as laser-induced fluorescence, resonance-enhanced multiphoton ionization (REMPI), cavity ringdown, and ZEKE are important tools in the physical and chemical sciences. These, and other techniques in electronic and photoelectron spectroscopy, can provide extraordinarily detailed information on the properties of molecules in the gas phase and see widespread use in laboratories across the world. Applications extend beyond spectroscopy into important areas such as chemical dynamics, kinetics, and analysis of complicated chemical systems such as plasmas and the Earth's atmosphere. This book aims to provide the reader with a firm grounding in the basic principles and experimental techniques employed in modern electronic and photoelectron spectroscopy. It is aimed particularly at advanced undergraduate and graduate level students studying courses in spectroscopy. However, we hope it will also be more broadly useful for the many graduate students in physical chemistry, theoretical chemistry, and chemical physics who encounter electronic and/or photoelectron spectroscopy at some point during their research and who wish to find out more.

There are already many books available describing the principles, experimental techniques, and applications of spectroscopy. However, our aim has been to produce a book that tackles the subject in a rather different way from predecessors. Students at the advanced undergraduate and early graduate levels should be in a position to develop their knowledge and understanding of spectroscopy through contact with the research literature.

A severe restriction of conventional photoelectron spectroscopy is its low resolution. The main limitation is instrumental resolution, particularly that caused by the electron energy analyser, as was discussed in Chapter 12. Resolving rotational structure is not a realistic prospect for conventional photoelectron spectroscopy but even vibrational structure may be difficult to resolve. In addition to the instrumental resolution must be added other factors such as rotational and Doppler broadening which, if they could be dramatically reduced, might make a sufficient difference to improve many photoelectron spectra. A potential solution is to combine conventional photoelectron spectroscopy with supersonic molecular beams. Supersonic expansions can produce dramatic cooling of rotational degrees of freedom and, if part of the expansion is skimmed into a second vacuum chamber, can be converted to a beam with a very narrow range of velocities. This is precisely the approach adopted by Wang et al. [1], the molecular beam being crossed at right angles by HeI VUV radiation (58.4 nm) to produce a near Doppler-free photoelectron spectrum. The resolution achieved is in the region of 12 meV (100 cm-1).

The ultraviolet photoelectron spectra of CO2, OCS, and CS2 in molecular beams are discussed here. These illustrate some of the important concepts involved in the interpretation of the photoelectron spectra of polyatomic molecules.

The presence of a solvent interacting with a system throughout its evolution from reactants to products brings about qualitative changes from the corresponding gas-phase reaction. There are changes in both the reaction rate and the dynamics. The energetic effects due to the solvent reflect the electronic reorganization that takes place as the system transverses the reaction path.The SN2 ion–molecule reaction, shown in Figure 11.1 for the generic X- + CH3X exchange reaction, provides an example in which the charge delocalization in the transition-state region causes qualitative changes in the energy profile along the reaction coordinate when the reaction is in the presence of a polar solvent. As becomes clear in this chapter, even the reaction coordinate itself is not exactly the same in solution as it is in the gas phase, because the solvent adaptation to the changing system must also be considered.

A solute molecule at room temperature undergoes of the order of 1013 collisions per second with solvent molecules. The solvent can therefore hinder the large-amplitude motions that often accompany chemical transformations (e.g., as in a twist isomerization, Figure 11.14). The cage effect, where the solvent hinders the separation of the products, Figure 1.8, or the approach of the reactants, was one of the first examples of the role of the solvent. The cage effect remains a major difference between gas-phase and solution dynamics. There can also be dynamical effects.

Molecular symmetry is of great importance in the discussion of spectroscopy. It helps to simplify the explanation of complex phenomena, such as molecular vibrations, and is an important aid in the derivation of electronic states and transition selection rules. It also simplifies the application of molecular orbital theory, which is often applied to assign or predict electronic spectra. In many cases, it provides strikingly simple answers to complicated questions.

In its original form, group theory is a rigorous mathematical subject. No attempt will be made here to be rigorous – the aim is simply to summarize the basics as they apply to symmetry, in light of which the spectroscopic applications of the theory can become clearer. Although the concepts introduced here might be valid for any object with symmetry elements, we will apply these only to molecules. This appendix is not intended to be a comprehensive account of point group symmetry and group theory. Instead the intention is to review some of the key principles required for applications in electronic spectroscopy. A newcomer to the subject of symmetry and group theory is first advised to consult an appropriate textbook on this topic, such as one of those listed in the Further Reading at the end of this appendix.

Symmetry elements and operations

We begin with two fundamental concepts, symmetry operations and symmetry elements. Symmetry operations are transformations that move the molecule such that it is indistinguishable from its initial position and orientation.

As described in Chapter 11, laser-induced fluorescence (LIF) spectroscopy is one of the simplest and yet most powerful tools for obtaining high resolution spectra. Its high sensitivity is particularly convenient for the investigation of extremely reactive molecules, such as free radicals and ions. In this Case Study we illustrate how LIF spectroscopy can be used to obtain important information on a small carbon cluster, the C3 molecule. The spectra presented were originally obtained by Rohlfing [1], who produced C3 by pulsed laser ablation of graphite. This is a violent method for vaporizing a solid and the plasma formed above the graphite surface will undoubtedly contain carbon atoms, clusters such as C2, C3, and various cations and anions. To reduce spectral congestion, the laser ablation source was combined with a supersonic nozzle to produce a cooled sample for spectroscopic probing.

The LIF spectrum was obtained by crossing the supersonic jet with a tunable pulsed laser beam and measuring the intensity of fluorescence as a function of laser wavelength. As discussed in Section 11.2, an LIF excitation spectrum is similar to an absorption spectrum but the signal intensity depends not only on the absorption probability, but also the fluorescence quantum yield of the upper state.

In this chapter we recognize that reactants (and hence products) have internal structure. We need to describe the potential energy that gives rise to the forces that act during the collision and to determine the dynamics. With this background we examine what features of the potential energy play a special role in the dynamics and how the internal states of the reactants participate. The rate of chemical reactions for polyatomic reactants is discussed in Sections 6.1 and 6.2. The derivation therein makes essential use of two features of the potential that we examine in this chapter. These are the potential energy barrier that separates reactants and products and the not unusual possibility that there are two (or more) barriers, between which there must be a hollow. To fully appreciate the approximations that are made in deriving expressions for the reaction rate that are based only on structural information, it is necessary to examine the dynamics, as we do in this chapter. We begin by reminding you about the Born–Oppenheimer separation of electronic and nuclear motions that underlies our work in this chapter, and the possibility of its failure.

The Born–Oppenheimer separation: a caveat

This chapter is based on generalizing the concept of an interatomic potential to the polyatomic case. Specifically, we are interested in many-atom systems that have enough energy to undergo a chemical change. It is therefore important to point out that the concept of a unique potential function governing the motion of the atoms is an approximation.

The photoelectron spectroscopy of anions is, in many respects, directly analogous to the photoelectron spectroscopy of neutral molecules. However, an important difference is that an electron in the valence shell of an anion is much more weakly bound than in a neutral molecule. In fact there are some molecules, such as N2, that are unable to bind an additional electron at all. The binding energy of an electron in an anion, which is known as the electron affinity (EA), is the energy difference between the neutral molecule and the anion. The electron affinity is defined as a positive quantity if the anion possesses a lower energy than the neutral molecule, i.e. the electron is bound to the molecule and energy must be added to remove it.

The photoelectron spectrum of an anion, also known as the photodetachment spectrum, can provide information on both the anion and the neutral molecule. A good example of this is the photoelectron spectrum of, which was first recorded by Ervin, Ho, and Lineberger [1].

The experiment

The most common method for generating anions in the gas phase is an electrical discharge. Ervin et al. produced by a microwave (ac) discharge through a helium/air mixture. A variety of neutral and charged species would be expected under such conditions, including several possible anions and cations.

Introduction

A large number of studies concerned with the crystallization kinetics of polymer–polymer mixtures have been reported. One reason for this abundance of information is a basic scientific interest coupled with the many ramifications that are inherent to such systems. There is also a pragmatic interest since physical and mechanical properties, as well as environmental features, can be altered and controlled by the appropriate mixing of polymeric components. It is useful to follow the same methodology used in discussing the equilibrium aspects of crystalline polymer–polymer mixtures (see Chapter 4, Volume 1) when analyzing their crystallization kinetics.

The nature of the melt and the pathway followed during the course of the transformation are important factors in governing the crystallization kinetics of binary mixtures. Depending on the molecular weight, the structural regularity of the chain and the value of the interaction parameter χ1, the blends can be either completely miscible over the complete composition range, partially miscible or completely immiscible. Phase separation may occur upon increasing or decreasing the temperature, depending on the change in χ1. Thus a lower or upper critical solution temperature, or both, can be observed. In order for a proper analysis to be made of the crystallization kinetics in blends, it is important that the complete phase diagram be established for partially miscible systems.

Introduction

The discussion of crystallization kinetics, and the mechanisms involved, has so far been limited to quiescent crystallization. In essence, except for polymer structure and composition, the only variable considered was temperature. This has given a limited perspective to the overall subject, since polymers crystallize when subject to applied forces such as hydrostatic pressure, tensile, biaxial and shear deformation among others. Different morphologies and structures result from these types of crystallization. As a result, properties can be drastically altered. The analysis of the crystallization kinetics of polymeric systems when subject to such external forces is the subject of the present chapter. Although this an important area, the literature is not as rich as in some of the other areas that have been discussed. However, the results that have been obtained are interesting and hopefully will stimulate further inquiry.

Effect of hydrostatic pressure

Overall crystallization kinetics

Under hydrostatic pressure the force is applied uniformly in all directions. The discussion will be divided into two parts: the overall crystallization kinetics in this section and spherulite growth rates in the following. The application of hydrostatic pressure increases both the glass and equilibrium melting temperatures. However, since each increases at a different rate the window for crystallization to occur will be altered.

This volume is concerned with crystallization kinetics and mechanisms and serves a crucial role in understanding the actual crystalline state that develops in polymers under real conditions. Attention is focused on the nonequilibrium aspects of the crystalline state that actually forms. It serves as a bridge to Volume 3 and the discussion of morphology, structure and properties. As the reader will find, there are still some important problems in this area that are in need of resolution. In discussing these, the author has tried to be as objective as possible and has presented the diverse viewpoints. Professor R. G. Alamo and Dr. F. C. Stehling read portions of the manuscript and offered constructive criticisms. However, the contents of the volume are the sole responsibility of the author.

It is a pleasure once again to acknowledge a great debt to Mrs. Annette Franklin for her expert typing of the manuscript and preparing it in final format. The author also acknowledges the contribution of Dr. James A. Haigh for the original calculations that appear throughout the book. Most of the illustrations in final form are due to the efforts of Dick Roche and Steve Leukanech. Their contributions are gratefully acknowledged.

The permission granted by American Chemical Society, Anales de Quimica, Chemical Reviews, Colloid and Polymer Science, Deutsche Bunsen-Gesellschaft für Physikalische Chemie, Elsevier, Industrial and Engineering Chemistry, Journal of Applied Physics, Journal of Chemical Physics, Journal of Macromolecular Science, Journal of Material Science, Journal of Physical Chemistry, Journal of Polymer Science, Journal of Thermal Analysis and Calorimetry, Macromolecules, Materials Research Society, Polymer, Polymer Engineering and Science, Polymer Journal, Physical Review Letters, Proceedings of the Royal Society of London, Royal Society of Chemistry, and Springer-Verlag to reproduce material appearing in their publications is gratefully acknowledged.

Introduction

In discussing the crystalline state of polymers the concern up to now has been primarily with equilibrium concepts and expectations. Although the fusion of polymers is well established to be a problem in phase equilibrium, stringent experimental procedures must be employed to approach the necessary conditions. It is concluded from equilibrium theory that for homopolymers of regular structure high levels of crystallinity should be attained. However, this conclusion is contrary to usual experience. Since the crystalline phase only develops at a reasonable rate at temperatures well below the equilibrium melting temperature, the state that is actually observed in a real system is a nonequilibrium one. The constitution and properties of the state that is achieved are the result of the competition between the kinetic factors involved in the transformation and the requirements for thermodynamic equilibrium. In a formal sense one is dealing with a metastable state. This generalization is not unique to polymer crystallization. It also holds for a vast majority of substances that undergo a liquid to crystal transformation. For polymers, however, kinetic control is the dominant factor. For example, the crystallization from the melt of an n-alkane of about one hundred carbon atoms occurs extremely rapidly at a temperature just infinitessimally below its equilibrium melting temperature.