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The main vibrational spectroscopic techniques or equipment used throughout this thesis are described briefly.

Attenuated Total Reflection – Fourier Transform Infrared (ATR-FTIR)

ATR-FTIR spectrometers are one of the most versatile infrared spectrometers, which are present in many active academic and industrial research labs. Their widespread use may be attributed to their increasing affordability, low maintenance, ease of use and ability to rapidly identify and classify any sample without prior sample preparation, so long as the sample can make physical contact with the ATR sample crystal.

ATR-FTIR is a total internal reflection technique that measures the infrared absorption of the sample at the surface of the internal reflecting element (further referred to as ATR crystal, Figure 1.14).[75] Total internal reflection is obtained when the angle of infrared light reaching the ATR crystal surface is equal or larger than the critical angle (Equation 1.2) if the refractive index of the ATR crystal (𝑛2) is higher than the sample (𝑛1) (Snell’s law). The electric field of the

infrared beam under total internal reflection extends beyond the interface of reflection as it is polarised in the direction of the sample where infrared absorption can occur, which is known as the evanescent wave. The depth of penetration (dp) of this evanescent wave is related to the wavelength (λ) and

incident angle (θi) of the infrared beam, and the refractive index of the ATR

Figure 1.14. Schematic representation of ATR at θi ≥ θc. Notations used: θi / r = angle of incidence/reflection, θc = critical angle, n1 = refractive index of the sample, n1 = refractive index of the ATR crystal, dp = penetration depth of resulting evanescent wave into sample, IR = infrared beam.

Equation 1.2 sin𝜃_! =𝑛! 𝑛_! Equation 1.3 𝑑! = 𝜆 2𝜋 sin!_𝜃−(𝑛 !÷𝑛!)!

Two different ATR crystal materials are used throughout this thesis, diamond (𝑛 = 2.4) and silicon (𝑛 = 3.42). Both of the ATR crystals are mounted into ATR accessories with the angle of incidence of the IR beam fixed to 45°. Assuming a typical refractive index of 1.5, the critical angle would be ~39° (diamond) or ~26° (silicon), meeting the requirements for ATR. The penetration depth can be calculated for the typical infrared wavenumber ranges as shown in Figure 1.15.

Figure 1.15. Penetration depth of the evanescent IR wave into the sample at different wavenumbers including selected peak labels with a diamond or silicon ATR crystal, IR beam incidence of 45° and a refractive index of the sample of 1.5.

Relatively low penetration depth is typically obtained in the mid-infrared region

(~4000 – 400 cm-1) between ~0.5 – 6 µm, whereas the penetration depth starts

to vary more significantly in the far-infrared region (~650 – 10 cm-1). The difference in penetration depth throughout the spectral region is why ATR spectra are not directly comparable to transmission spectra with fixed path lengths. The effective path length of ATR spectra can be calculated (approximately) for quantitative analysis as described by Griffiths et al., if desired.[75a] Nevertheless, ATR-FTIR allows for measurement of strongly absorbing samples, such as water, without having to work with short path length transmission cells.

As outlined above, ATR-FTIR spectra are dependent on the refractive index of the sample. There are samples, such as solid and crystalline materials, that exhibit anomalous dispersion, which results in rapid changes to the refractive index of the sample whilst scanning through an absorption peak.[76] This results in asymmetric line shapes and shifts in absorption maxima to smaller wavenumber values and has to be accounted for. Such effects were observed throughout this thesis and corrections were carried out accordingly, which are outlined in Chapter 2.2.

Raman Microspectroscopy

Raman microspectroscopy is used to complement the infrared spectroscopic measurements. Raman microscopes are in general not limited to a lower

wavenumber limit of ~400 cm-1 such as mid-infrared spectrometers due to the

material of optics used, weak infrared sources and/or detector limits.

This lower wavenumber region (~650 cm-1 _{– 10 cm}-1_{) is of particular interest to}

inorganic materials as it is where metal to ligand vibrations occur.[77] Raman microspectroscopy allows one to collect spectra between ~4000 cm-1 – 50 cm-1, whereas far-infrared spectrometers have to be typically used in order to obtain spectra in the lower wavenumber region. Far-infrared spectrometers have their own limitations and restrictions, which are discussed further in Chapter 4.1. Raman scattering, as described above, is a weak process and the monochromatic lasers of different wavelengths and microscope lenses employed into the Raman microscopes allow one to minimise the unwanted scattering effects. Fluorescence can be a particular problem for many inorganic metal complexes.[77a]

Synchrotron Radiation as Infrared Sources

Charged particle accelerators, i.e. synchrotrons, were originally used in the field of particle physics to allow the study of subatomic particle collisions. However, second and third generation synchrotrons were designed to store accelerated electrons created with strong magnetic and electric fields into electron storage rings. The electron storage ring utilises electromagnets to focus, steer and maintain the high-energy electrons in a vacuum, whereupon they emit extremely intense radiation, referred to as synchrotron radiation (SR). SR emits a wide range of electromagnetic radiation, including infrared radiation, which can be tuned by the electron energy.

The brilliance of infrared SR of a 3 GeV SR source (such as the Diamond Light Source) shows at least two orders of magnitude advantage over a typical Globar infrared source (Figure 1.16).[78] Furthermore, this advantage increases more significantly in the far-infrared region. Apart from the high brilliance, SR is highly collimated, allowing for an effectively parallel source and its energy is intrinsically stable (directly proportional to the electron current), whereas thermal sources such as the Globar do not exhibit those properties.[79]

This has resulted in the installation of infrared beamlines at synchrotrons, with at least 20 active infrared beamlines in 2016, continuously advancing their equipment and throughput to utilise SR.[80] _{Further discussion on the particular}

advantages of SR and application towards the research carried out in this thesis in the far- and mid-infrared regions can be found in Chapter 4.

Figure 1.16. Calculated SR brilliance of a 3 GeV SR source, a typical Globar infrared source and an ideal bending magnet SR diffraction limited source. Adapted from Marcelli et al.[ 7 8 ] Reprinted with permission from Elsevier.

Wavenumber (cm-1₎ 1000 100 10 10000 1011 1013 1015 1017 1019 Bri lli an ce (p ho to n s -1 mm -2 mra d -2 0.1% bw )

3 GeV 50x30 mrad2 _{IR front end}

Diffraction limited SR source Globar source ~ 1500K