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SPECTROSCOPIC, DIFFRACTION AND MICROSCOPIC TECHNIQUES, How XRD pattern is…
SPECTROSCOPIC, DIFFRACTION AND MICROSCOPIC TECHNIQUES
(i) Fundamental concepts in spectroscopic and microscopic techniques
Spectroscopy
branch of science that studies the interaction between electromagnetic (EM) radiation and matter.
used as a tool for studying the structures of atoms and molecules.
basic principle - to shine a beam of EM radiation onto a sample, and observe how it responds to such a stimulus. respose is recorded as a function of radiation wavelength.
light is an EM wave and transverse in nature. natural/ordinary light: unpolarized - vibrations take place symmetrically in all directions in the plane perpendicular to the direction of propagation of light.
types of EM radiation interaction with matter:
radiation >> absorbed/transmitted/scattered/reflected/undergo photoluminescence.
photoluminescence >> fluorescence, bioluminescence, Raman scattering.
complement of light absorbed gets transmitted.
colour of an object we see is due to wavelengths transmitted or reflected. other wavelengths are absorbed. more absorbed, darker the colour (more concentrated solution).
interaction of EM radiation with matter is a quantum phenomenon and is dependent on both the properties of radiation and appropriate structural parts of the samples involved.
origin of EM radiation is due to energy changes within matter itself.
in spectrochemical methods, absorbed radiation is measured.
(ii) Principle and applications of UV-Visible spectroscopy technique
PRINCIPLE
different molecules absorb radiation of different wavelengths depending on their structure.
an absorption spectrum will show a number of absorption bands corresponding to structural (functional) groups within the molecule.
in UV-Vis spectroscopy, energy is absorbed by a molecule in UV region (1-400nm) or visible region (400-750nm) resulting in electronic transition of valence electrons.
Beer-Lambert Law
Auxochrome
group of atoms attached to a chromophore which modifies the ability of that chromophore to absorb light.
eg. -COOH, -OH, -SO3H, -NH2, -NH-R, -N-R2
Chromophore
any isolated covalently bonded group that shows a characteristic absorption in the UV-Vis region.
the only molecular moieties likely to absorb light in the 200 to 800nm region are pi-electron functions and hetero atoms having non-bonding electron pairs.
when light passes through a molecular material, absorption can occur. the absorption of light, as it passes through a medium, varies linearly with the distance the light travels and with concentration of the absorbing medium.
extent of absorption is given by Beer-Lambert Law, as expressed by
A = εcl
, where
A >> absorption
ε >> absorptivity coefficient,
l >> path length, and
c >> concentration of the specific analyte. absorptivity characterizes the amount of light absorbed by a specific molecule at a specific wavelength.
Electronic excitations in UV-Visible spectroscopy
sigma to sigma' transitions: energy required is large.
n to sigma' transitions: saturated copounds with atoms containing lone pairs.
lesser energy than sigma to sigma'.
can be initiated by light whose wavelength is in the range 150-250nm.
n to pi' and pi to pi' transitions: need an unsaturated group in the molecule to provide the pi electrons.
spectral region >> 200-700nm.
bases absorption spectroscopy of organic compounds.
based on the functional groups present and attached to chromophores:
Bathochromic shift: absorption maximum shifted to longer wavelength (blue to red) [red shift].
Hypsochromic shift: absorption maximum shifted to shorter wavelength (red to blue) [blue shift].
Hyperchromism: increase in molar absorptivity.
Hypochromism: decrease in molar absorptivity.
(iii) Principle and applications of X-Ray Diffraction (XRD) technique
PRINCIPLE
XRD is a technique used to determine the crystallographic structure of a material. XRD works by irradiating a material with incident X-rays and then measuring the intensities and scattering angles of the X-rays that leave the material.
versatile, non-destrutive characterization technique widely used in materials science and engineering for identifying unknown crystalline materials.
used to study the structure and function of many biological molecules, including vitamins, drugs, proteins and nucleic acids such as DNA.
used to determine strudtural properties (lattice parameters, strain, grain size, epitaxy, phase composition, orientation, atomic arrangement) and to measure film thickness.
yields information on how the actual structure deviates from the ideal one, owing to internal stresses and defects.
diffraction
What is diffraction?
diffraction refers to a phenomena when a wave encounters an obstacle.
in classical physics - the apparent bending of waves around small obstacles and the spreading out of waves past small openings.
Interference between diffracted waves
Interference >> interaction between diffracted waves.
Constructive Interference: waves are in-phase when each of their crests and troughs occur exactly at the same time. those types of waves stack together to produce a resultant wave that has a higher amplitude. for constructive interference, path difference should be multiples of n*λ.
Destructive Interference: if the waves are out of phase by multiples of (n/2)*λ, then destructive interference occurs and the amplitude of the resultant wave will be reduced.
XRD instrument
Components of an XRD instrument:
X-ray tube: source of X-rays
incident-beam optics: condition the X-ray beam before it hits the sample.
goniometer: platform that holds and moves the sample, optics, detector and/or tube.
sample holder
recieving-side optics: condition the X-ray beam after it has encountered the sample.
detector: count the number of X-rays scattered by the sample.
incident angle (w) is defined between the X-ray source and sample.
diffracted angle (2θ) is defined between the incident beam and the detector angle.
incident angle (w) is always 1/2 of the detector angle 2θ i.e. θ.
in a typical XRD instrument, the X-ray tube is fixed, the sample rotates at θ degree/min and detector rotates at 2θ degree/min.
How XRD pattern is produced? Bragg model of diffraction.
crystals are regular arrays of atoms, whilst X-rays are waves of EM radiation. crystal atoms scatter incident X-rays, primarily through interaction with the atom's electrons. >> elastic scattering.
electron >> scatterer.
a regular array of scatterers produces a regular array of spherical waves. in majority of directions, these waves cancel each other out through destructive interference, however, they add constructively in a few specific directions, as determined by Bragg's law:
nλ = 2dsinθ
, where
n >> an integer
λ >> beam wavelength
d >> spacing between diffracting planes
θ >> incident angle.
X-rays scattering from adjacent crystalline planes will combine constructively (constructive interference) when angle θ between plane and X-ray results in path-length difference that is integer multiple "n" of X-ray wavelength "λ".
