spctroscopy

• This module will mainly look at spectroscopy from an analytical chemistry point of view, focusing on how we make measurements, and how we interpret the measurements to identify "what is present" and "how much is present". We will also discuss what happens when light interacts with matter.

  
• Lecture 1 will introduce the concepts of spectroscopy and molecular absorption spectroscopy.
• Spectroscopy


  The study of the interaction of light with matter
• Spectroscopy can give us information about

  • What is present in a sample ("qualitative analysis)
  • How much of an analyte is present in a sample ("quantitative analysis")
  • The structure of the material in the sample, at the atomic, molecular, and larger scales
• Many observed colours are due to absorption

  
• Absorption is best explained by considering the particle nature of light (photons)
• Reflections, interference colours and refraction are best explained by considering the wave nature of light

  
• Structural colour is due to regular variations in refractive index of materials
• Refractive index


  A measure of how the path of a beam of light will bend (refract) as it passes into a given material

  
• In a vacuum the speed of light, c, has the value 2.998 x 10^8 m s^-1. In other materials light travels slightly slower.
• Rayleigh scattering


  Short wavelengths scattered most strongly
• Mie and geometric scattering

  All wavelengths scattered
• If matter is excited (given energy) in some way, it may emit light
• Interaction of electromagnetic radiation with matter

  • The speed, wavelength (λ) and wavenumber (ν̃) of e.m.r. change with the medium it travels through because of the refractive index of the medium; However, the frequency (ν) remains unchanged.
  • Types of interactions: Absorption, Reflection, Transmission, Scattering, Refraction
  • Each interaction can provide useful information about a sample
  • By applying e.m.r. of different frequencies, different types information can be obtained

  
• Wavelength (λ)


  The distance between two consecutive peaks or troughs in a wave
• Frequency (ν)


  The number of wave cycles that pass a given point per unit of time
• Photon


  A particle of electromagnetic radiation
• E = hν, where E is the energy of a photon, h is Planck's constant, and ν is the frequency of the photon
• The brilliant red colour seen in fireworks is due to the emission of red light at a wavelength near 650 nm.
• The intensity of light can be considered from both a wave and a particle point of view

  
• The visible region extends from approximately 700 nm (red) to 400 nm (blue)
• The infra-red region is at slightly lower energy (longer wavelength) than the visible region, while the ultraviolet is at slightly higher energy (shorter wavelength)
• Wavenumber (ν̃)


  The number of waves per unit distance, usually expressed in cm^-1
• Molecular Absorption Spectroscopy (MAS) involves the study of electronic transitions in the UV-visible region of the electromagnetic spectrum (200 – 700 nm)

  
• The structure of a molecule determines the wavelengths where it absorbs and therefore its colour

  
• Wavelengths


  Unit: μm
• Wavelengths


  • 18
  • 3/21/24
  • 10

  
• Molecular Absorption Spectroscopy (MAS)


  Also referred to as UV-Visible spectrophotometry. It involves the study of electronic transitions in the UV-visible region of the electromagnetic spectrum (200 – 700 nm)

  
• Molecular structure


  Determines the wavelengths where a molecule absorbs and therefore its colour

  
• pH-dependent forms of anthocyanin pigments in red wine

  • pH 2 (520 nm)
  • pH 7
• Molecular structures of molecules (chromophores)

  • Reflected colour under white light (λ not absorbed)
  • Molecular absorption spectra (λ's absorbed)

  
• Boltzmann distribution


  Predicts the number of molecules or atoms in an excited state (nupper) relative to the number in the ground state (nlower)
• At 25 °C (298.1 K), the occupancy of the S1 state is negligible ("all" molecules are in the ground state)
• Types of Electronic Transitions

  n → π* and π → π* transitions: Observed for compounds with lone pairs and multiple bonds (λmax = 200-600 nm)
• For simple organic molecules such as formaldehyde, the UV-vis molecular absorption spectrum corresponds to transitions from high energy bonding (σ or π) or non-bonding orbitals (n) into low-energy antibonding (π*) orbitals
• What happens in a molecule once it absorbs a photon in the UV-visible region?


  1. Absorption of the photon causes an electron to move to a higher energy orbital (the molecule is now in an excited state)

  2. Vibrational relaxation and internal conversion

  3. Fluorescence

  4. Phosphorescence
• Vibrational relaxation


  Non-radiative process, energy is dissipated to other vibrational modes as kinetic energy, very fast transition (10-14 - 10-10 s)
• Internal conversion


  Energy dissipated through collisions, can occur when the energy difference between different excited states is small, non-radiative process
• Fluorescence


  Radiative process, most often observed between the first excited electronic state (S1) and the ground state (S0), the energy of the emitted photons is always less than that of the exciting (absorbed) photons (Stokes shift), slow process compared to vibrational relaxation (10-9 – 10-6 s)

  
• Phosphorescence


  Radiative process, requires intersystem crossing (ISC) from a singlet state (e.g. S1) to a triplet state (e.g. T1), the transition T1 → S0 is formally forbidden, so T1 has a long lifetime (phosphorescence occurs over a long time: 10-6 – 10 s)