Cepillo Dental

2.2.1 Introduction

A syno p sis o f th e R a m a n e x p e rim e n ta l p ro ced u re fo llo w ed in th e a c c u m u la tio n o f all th e s p e c tra in th is th e s is is g iv e n below . T h is is to c la rify th e u ses an d lim ita tio n s o f th e tec h n iq u e in th e s tu d y o f H M M C s . A s c h e m a tic d ia g ra m show ing th e a rra n g e m e n t o f lasers and s p e c tro m e te rs is show n in F ig u re 2 .2 .1 .

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L a s e r, with ion type. S p e c tro m e te r C ryo stat

Spex

1401

Figure 2.2.1 Schematic plan view of the Raman experimental set-up. The dotted lines represent the table boundaries, the arrowed lines the optical paths. The Pellin-Broca prisms are mounted in the central chamber.

There are two key points to note in the diagram: each laser can be directed to any of the spectrometers, and the equipment is mounted on a single table base to minimise vibrations. The Nicolet Fourier Transform (FT) Raman spectrometer is not shown. It does not have a usable cryostat, the 1064 nm Nd:YAG laser with which it is fitted is too powerful to focus on HMMCs at room temperature and small crystal samples cannot be aligned accurately with the laser. While the FT Raman technique may prove to be of some value to linear-chain study in the future, no results collected with this machine are reported here. The Dilor Raman microscope has a very short spectral response range, and it is not specially adapted for work at low temperature. However, crystals or particular surfaces of a crystal can be examined easily using the microscope, and the accumulation time of the machine is very fast. It offers the best means for checking the uniformity of large samples of crystals (see section 4.4), because crystals can simply be scattered onto a glass slide and analysed very quickly.

All the spectra reported in this thesis were recorded on one of the two scanning spectrometers. The Spex 14018/R6 (usually abbreviated as R6) double monochromator, with Jobin-Yvon holographic gratings (1800 line mm'^), was used with 406.7 nm excitation since it is the more sensitive of the two instruments in the blue part of the visible spectrum. The majority of the spectra were recorded on the Spex 1401 double monochromator, with Bausch and Lomb gratings (1200 line mm'^). The Spex 1401 has good response over a large range, and furthermore it is equipped with a Charged Coupled Device (CCD) camera, which makes alignment of small crystals much easier than it is on the R6.

2.2.2 Optical path

The exciting radiation necessary to promote Raman scattering comes from one of four different lasers. In theory this should have made a large number of possible wavelengths available, but the age and unreliability of the apparatus meant that not all were possible (the large argon ion laser fell into disrepair). The lasers, and the excitation lines from each that were used, are listed in Table 2.2.1.

Table 2.2.1 Lasers and the wavelengths that are used in this work

Krypton ion Argon ion

Colour CR-3000K CR-52 CR-18 1-70 blue 406.7 468.0 476.2 turquoise 488.0 green 530.9 497.0 488.0 497.0 514.5 yellow 568.2 red 647.1 676.4 752.5 647.1 676.4

OR = Coherent Radiation. I = Innova.

The radiation emitted by lasers is coherent and virtually monochromatic, and polarised in the same vertical plane. The radiation from any laser can be directed into the central chamber (see Figure 2.2.1) where it is be aligned with a Pellin-Broca prism; the prism diverts plasma lines from the optical path. The beam is reflected by a series of mirrors, which are adjusted so that it is exactly vertical as it enters the cryostat. Crystals or pressed discs are mounted on a sample block made of copper, which has its face set at 30 ° to the vertical. The block is in thermal contact with a reservoir containing liquid nitrogen. The cryostat is kept under vacuum to prevent water from condensing on the windows through which the beam passes. The laser beam is filtered to provide the requisite intensity and is focused onto the sample with an adjustable lens. Any light emitted by the sample passes through a vertical window to be collected by a second lens, and focused by it onto the slits of the spectrometer. The optical paths within the instrument are designed so that this should result in near maximum intensity at the detector. The set-up of the cryostat and lens is shown in Figure 2.2.2. The detectors are G a As photomultiplier (PM) tubes (type RCA31034), which are thermoelect rically cooled. They give a signal that is relayed to a DPC2 digital photometer. Each scanning or data collection operation is driven by computer programme (written by Dr. S. P. Best) run on a personal

computer (PC). The spectral data collected in this way are calibrated against the Rayleigh emission line. The signal is usually optimised for each sample by finding the maximum intensity for a known peak, by adjusting the various lens foci and mirror orientations. The whole process of obtaining a good signal is simplified by the CCD camera mounted inside the spectrometer (this only applies to the Spex 1401). CCD alignment involves diverting the light entering the slits to the camera along a path length identical to the slit-detector distance. The image that is seen reproduces exactly the signal that arrives at the detector. It is also possible to see the contours of the sample, which is particularly useful for single crystal studies.

Under vacuum Liquid Nitrogen Reservoir Focusing lens Sample block- Radiation is focused onto the entrance slits

of the spectrometer

Incident radiation:

filtered and focused _{single crystal}

Figure 2 .2 .2 A representation o f the cryostat, with sample block, and the path taken by the las er beam . A single H M M C crystal is shown. It is m ounted so that the electric part o f the electromagnetic w ave oscillates along the chain axis.