Spectroscopy

Measuring electromagnetic radiation abasorbed / emitted due to an energy state transition

Infrared

• vibrations are quantised
• IR can cause transitions between vibrational energy states 
• need oscillating dipole moment 
• Coupling of vibrations and E field ocurs

• Antisymmetric and bending are IR active
• Use harmonic theory and mechanical models to describe the energy needed for bond deformation

Increases wavenumber

• Multiple bonds, smaller mass, stretching compared to benfing, assymetry compared to symmetry, polar bonds 

Each bond corresponds to a wavenumber. Wavenumber is not affected by the presence of other atoms/ bonds

Regions

• 4000-1000 cm^-1 --> Functional group
• 1300 - 1000 cm^-1 --> Polymer fingerprint
• 1000-650 cm^-1--> Ring region

Can use Infrared to make qunatitative measurements about concentrations

• using Beer Lambert law: It is the relation of absorabcnec to molar concentration
• Subtract absorbance of the sample with the background, by measuring the Absorbacne of the solvent
• Can use law to get relative concentrations [keep path length the same]
• The law is only valid for low concentrations

• Polychromatic light reflected by mirror hits sample, sent through monochromator 
• use diffraction grating at specidic position to satisfy Bragg's equation to choose 1 wavelength to be detected
• See how that wavelengdth has been absorbed through sample
• Cheap, slow
• Labour intensive --> need to move grateing
• Can't identify compounds

• Use superposition to turn polychromatic light into monochromatic light
• Use half-transparent mirror to get transmitted and reflected beam
• Have fixed mirror and movable mirror
  • Which gnerates a path difference between the beams 
• Only 1 wavelength causes constructive interference at a ceratin position of the mirror
• Resolution --> inverse of the maximum displacement of the mirror
• The position of the mirror is known using a laser (Ne laser)
  • fast , high signal to noise
• To read data quickly must use a higher noise detector ( pyroelectric blometer)

Sample preparation

• Support, container and solvent should be transparent to IR e.g. NaCl and KBr
• Liquids : use NaCl plates, aqueous: use H₂O insoluble material
• Solids: powder into mull (using oil, which may interfere), then compress in disc, dissolve in voltaile solvent

Effects of Hydrogen bonding

• Change wavenumber that certain bonds appear ( C =O and N-H)
• once bonds are broken ( at high temperature), the wavenumber changes back
• Can use Synchrotron to produce finely focused IR imaging

• Complementary to IR as weak absorption in IR is strong in Ramen ( and vice versa)
• light scattering occurs

Mechanism

• E-feld creates dipole between electron and nuclei
  • source of EM, emits scattered light
• Intensity dependent on the polarisability of a bond
• Symmetric bonds give strong signal has they have high polarisability
• Rayleigh scattering: electrons excited to virtual energy then drops back
• Stokes Ramen: excited to virtual state then back down to vibrational state
• Anti-stoke: from thermal state (not ground) to virtual state and then back to ground
• The energy transition in ramen and IR for a particular bond type is the same
• Can get fluoresene interferring with the signal

Fluorescence

• Exitation to real energy sate, then to lower E state
• Photon emission from this can overtake small Raman signals

Laser: need highly monochromatic light to isolate wavenumbers

Design

• Laser shine on sample , use monochromator (mirrors)  perpendicular to the sample to detect one wavelength

Sample preparation

• Liquids and aqeous solutions: focus laser on small spot, use capillaries, quartz cells, light piping
• Solids: collect back scattered light; signal from top surface