Condensed molecular solids

The samples studied in connection with work discussed here consist of ultra-thin films of molecular solids, either polymers or condensed molecular solids. It is useful, however, to describe first the molecular photoelectron emission process, at least from a phenomenological view point, and then present the extra issues in dealing with solids. 

The next issue in the overview of PES involves surfaces of polymer thin films and condensed molecular solids. There are several issues to be mentioned (1) inter-molecular polarization (relaxation) effects and (2) surface sensitivity. First, under (1), the concept of intra-molecular was described above. When molecules... 

In the UPS or XPS of solid films, the front of the sample (the polymer or condensed molecular solid film) can be maintained in electrical contact (equilibrium) with the metallic substrate, because the films employed are so thin that electron tunneling from the substrate, or numerous other effects, prevent any positive surface electronic charge from building up during the course of the measurements. In this way, electronic sample charging effects23 40 41 are avoided in works reported herein, and thus will not be covered. 

In chapter 7, all works discussed on model molecular systems for conjugated polymers refer to condensed molecular solid ultra-thin films, generally prepared by condensation of molecules from the effusion of a Knudsen-type cell, in UHV, on to clean metallic substrates held at low temperatures. Clean is defined as atomically clean as determined by core-electron level XPS, such that there is intimate contact between the molecules at the substrate-film interface, without the influence of, for example, a metallic oxide, hydrocarbon... 

In Fig. 7.10, is shown a portion of the C(ls) XPS spectrum for /ramj-polyacetylene, DP7, and condensed (molecular solid) benzene. The main C(ls) peaks are not displayed, but only the relatively weak satellite features that appear on their high binding energy side of the... 

It is perhaps obvious that the nature of the interface between a molecular solid (polymer) and a (clean) metal surface is not necessarily equivalent to the interface formed when a metal is vapor-deposited (essentially atom-by-atom ) on to the (clean) surface of the polymer or molecular solid. Atoms of all metals are active in the form of individual atoms , even gold atoms. In the context of the new polymer LEDs, some of the works discussed in chapter 7 involve the study of the early stages of formation of the interface in the latter configuration (metal-on-polymer interfaces). Very little has been reported on conjugated polymer-on-metal interfaces, however, primarily because of the difficulties in preparing monolayers of polymer materials on well defined metal substrates appropriate for study (via PES or any other surface sensitive spectroscopy). The issues discussed below are based upon information accumulated over two decades of involvement with the surfaces of condensed molecular solids and conjugated polymers in ultra-thin form, represented by the examples in the previous chapter. 

In the first monolayer of conjugated model material, a model molecular solid or a polymer adsorbate, assume that no chemistry (covalent bonding) occurs, since, in the absence of, for example, mechanical rupturing, the bonds at the surface of the molecular film are completely satisfied. This assumption is supported by the fact that, at least for condensed molecular solids, vapor-deposited films may be re-evaporated (removed) from the surface by gentle heating in UHV. 

A word of caution the case of conjugated polymer films is generally identical to the case of condensed molecular solid films, as described above, but with occasional small differences. In general, the Fermi level is found to lie very near the centre of the energy gap, Eg. Small amounts of impurities, defects, or other charge donation (acceptor) species, however, can move the position of the Fermi... 

In the examples of our work on organic molecular and polymeric solids that follow, first some contributions to the UPS line widths in condensed molecular solids are discussed for two prototype systems, anthracene and isopropyl benzene then the UPS of two.aromatic pendant group polymers, polystyrene and poly(2-vinyl pyridine), are discussed and compared with some spectra concerning the simplest linear conjugated polymer, polyacetylene. 

Homogeneous and Inhomogeneous Contributions. Two other contributions to UPS linewidths for molecular solids have been articulated in a study of isopropyl benzene films at low temperatures ( ). The shape and size of the isopropyl benzene molecule prohibited the explicit observation of the surface effect discussed for anthracene. Isopropyl benzene was of interest as a model molecule for polystyrene, however. The measurements were carried out on condensed molecular-solid films in the temperature range 15°K < T < 150°K. 

Study was to characterize the electronic excitations of the polymers in terms of those of the molecular building blocks of the polymers. To this end, the appropriate model monomer molecules were studied in both the gas phase and the condensed molecular solid phases. Some technical details of the photoelectron spectroscopy of the polymer films are given in the Appendix. Some of the general practices discussed there apply to the condensed molecular solids as well. 

In a condensed molecular solid, however, there are also intermolecular relaxation (polarization) effects that occur in addition to the intramolecular effects (1, 11), as discussed above. In fact, in any dielectric medium, the total net positive charge density on the molecular cation induces corresponding electronic, and ultimately atomic, distortions in the surrounding medium. 

The first optical absorption bands of benzene and pyridine are known to occur near 5 eV. The optical data on 1000 A thick, solution cast, PS and PVP as well as on the model molecules ethyl benzene and 2-ethyl pyridine in the gas phase were recorded on a Cary 14 double beam instrument. The data are therefore limited to >1 u <6 eV by the optics of the instrument. Condensed molecular solid spectra were not necessary since the photoemission studies had already shown that the polymer and condensed molecular spectra were equivalent. Therefore a comparison of gas-phase model-molecule spectra with that of the corresponding polymer spectra was sufficient. 

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