Followership

Molecules containing 𝜋-electrons or non-bonding electrons (n-electrons) can absorb energy in the form of ultraviolet or visible light to excite these electrons to higher anti-bonding molecular orbitals (Alias et al., 2013). The more easily excited the electrons (i.e. lower energy gap between the HOMO and the LUMO), the longer the wavelength of light it can absorb. There are four light possible types of transitions (𝜋 - 𝜋 *, n - 𝜋 *, 𝜍-𝜍*, and n-𝜍*), and they can be ordered as follows: 𝜍 - 𝜍 * > n - 𝜍 * >𝜋 - 𝜋 * >n – 𝜋 *.

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Organic compounds especially those with a high degree of conjugation, also absorb light in the UV or visible regions of the electromagnetic spectrum. The solvents for these determinations are often water for water-soluble compounds, or ethanol for organic-soluble compounds. (Organic solvents may have significant UV absorption; not all solvents are suitable for use in UV spectroscopy. Ethanol absorbs very weakly at most wavelengths). Solvent polarity and pH can affect the absorption spectrum of an organic compound. While charge transfer complexes also give rise to colours, the colours are often two intense to be used for quantitative measurement.

The Beer-Lambert law states that the absorbance of a solution is directly proportional to the concentration of the absorbing species in the solution and the path length, (Alias et al., 2013).

The nature of the solvent, the pH of the solution, temperature, high electrolyte concentrations and the presence of interfering substances can influence the absorption spectrum.

Uv-Vis Spectroscopy is also used in the semiconductor industry to measure the thickness and optical properties of thin films on a wafer. It is used to measure the reflectance of light and can be analyzed via the Forouhi-Bloomer dispersion equations to determine the index of Refraction (n) and the extinction coefficient (k) of a given film across the measured spectral range. For convenience of reference, definitions of the various spectral regions have been set by the Joint Committee on Nomenclature in Applied Spectroscopy as shown in Table 2.2

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Table 2.2 UV-Vis Regions and Equivalent wavelengths

Region Wavelength (nm)

Far ultraviolet 10 – 200

Near ultraviolet 200 – 380

Visible 380 – 780

The human eye is only sensitive to a tiny proportion of the total electromagnetic spectrum between approximately 380 and 780 nm and within this area we perceive the colours of the rainbow from violet through to red. Fig 2.6 shows the EM spectrum.

Fig. 2.6 The electromagnetic spectrum (Young and Freedman, 2008)

Besides the sun, the most conveniently available source of visible radiation which is familiar is the tungsten lamp. If the current in the circuit supplying such a lamp is gradually increased from zero, the lamp filament at first can be felt to be emitting warmth, then glows dull red and the gradually brightens until it is emitting an intense white light and a considerable amount of heat.

Table 2.3 shows the relationship between light absorption and colour.

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Table 2.3 Wavelengths for colours in the visible spectrum (Young and Freedman, 2008) Wavelength (nm) Colour

400 - 440 Violet 440 - 480 Blue

480-560 Green

560-590 Yellow

590-630 Orange

630-700 Red

A close relationship exists between the colour of a substance and its electronic structure. A molecule or ion will exhibit absorption in the visible or ultraviolet region when radiation causes an electronic transition within its structure. Thus, the absorption of light by a sample in the ultraviolet or visible region is accompanied by a change in the electronic state of the molecules in the sample. The energy supplied by the light will promote electrons from their ground state orbitals to higher energy, excited state orbitals or anti bonding orbitals.

The instrument used in ultraviolent-visible Spectroscopy is called a Uv-Vis Spectrophotometer which measures the intensity of light (IT) passing through a sample (intensity of the transmitted radiation) and compares to the intensity of light (I₀) before it passes through the sample (intensity of the incident radiation). The ratio IT/I0 is called the transmittance (T) and is usually expressed as a percentage (%T). The absorbance, A is based on the transmittance A= -log(%T/100), the percentage transmission is %T=IT/I0 x 100.

The Uv–Visible spectrophotometer can also be configured to measure reflectance. In this case, the spectrophotometer measures the intensity of light reflected from a sample (I_R), and compares

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it to the intensity of light reflected from reference material (I_o) such as a white tile. The ratio I_R/I_o is called the reflectance, and is usually expressed as a percentage (%R).

Absorption spectroscopy is a technique where the absorption of electromagnetic wave is measured as a function of the frequency or wavelength. The absorption process induces an interaction between electromagnetism and the sample, which can be interpreted through variations in the absorption spectra (Alias et al., 2013). An absorption spectrum is a fingerprint of a molecule or polymer material. Uv–Vis absorption is a commonly used analytical tool for studying the interaction between electrons and radiation. There are four basic components, namely the light source, monochromator, sample holder and optical detector. In general optical absorption spectrophotometer uses either a single or double beam (Tkacheno, 2006).

Fig. 2.7 (a) shows the simplest single beam which is easy to utilize and is compatible. A double beam, as shown in Fig. 2.7 (b), consists of two beams from the same source divided into two optical paths.

38 (a)

(b)

Fig. 2.7 (a) Single and 2.7 (b) Double-channel UV-vis spectrophotometers.

(Alias et al., 2013)

The first beam passes through the reference and second beam passes through the sample. The main advantage of the double beam channel is the reference material that makes measurement more stable, precise and accurate compared to the single beam channel (Porro and Terhaar,

Light Source

Monochromator

grating

Mirror 2 Mirror 3

Detector Sample

Mirror 4

Mirror 1

Light Source

Monochromator

grating

Mirror 2 Mirror 4

Detector 1 Reference

Mirror 5

Mirror 1

Mirror 3

Mirror 6

Detector 2 Sample

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1976). However, one of drawbacks of the double beam channel is the low ratio signal - to – noise which affected the sensitivity of the system.

In absorption spectroscopy, normally two types of interference can occur namely; spectra interference and interference from the absorption species (Skoog et al., 2007). Spectra interference appears from the overlapping line in the monochromator. Interference filter are needed to limit the number of wavelength. Interference from the absorption species involves thick or opaque thin film. In thick thin film, the absorbance depends on the optical length path in the sample. To avoid this circumstance, diffuse reflectance measurement with integrated sphere can be used (Bosch and Sanchez, 2004).

In the absorption process, the intensity of light decreases when propagates in the medium as shown in Fig 2.8(a) and Fig 2.8(b) shows the light absorption process in the film.

Fig. 2.8 (a) The light absorption process in thin film 2.8 (b) The normalized light intensity absorbed in the thin film as the function of sample thickness x with 𝜶 = 0.5 m ^-1

(Alias et al., 2013)

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where di = small change in the intensity and I = absorbed intensity.

The ratio intensity loss (di /I) is proportional with small changes in the sample thickness dx. This relation can be given by equation 2.10 (Alias et al., 2013)

𝑑𝐼

𝐼 = −𝛼𝑑𝑥 2.10

If I_o is the initial or incident light intensity before entering the medium, I is the transmitted light and x is the sample thickness. The intensity absorbed by the sample can be calculated by integrating equation 2.10 of both side as

𝑑𝐼

𝐼 = −𝛼 𝑑𝑥₀^𝑥

𝐼

𝐼𝑂 2.11

𝐼 = 𝐼₀ 𝑒𝑥𝑝 (−∝ 𝑥) 2.12

This equation is known as Lambert-Beer Laws. According to this law, the light intensity exponentially decays with the sample thickness. The coefficient of proportionality in equations 2.11 and 2.12 is known as absorbance coefficient and it measures how strong the sample absorbs light at a specific wavelength (Alias et al, 2013).