• 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.
  

• 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.
  

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

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.

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.
  

• 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.

• 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".

• 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 >> interaction between diffracted waves.

1. 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.
2. 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.

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.

  
  
  

  

• 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.