chem 251

• Spectroscopy


  The study of the interaction of light with matter
• 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

  
• The refractive index varies with wavelength
• In a vacuum the speed of light, c, has the value 2.998 x 10^8 m/s. 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
• The speed, wavelength (λ) and wavenumber (ν̃) of electromagnetic radiation change with the medium it travels through because of the refractive index of the medium; However, the frequency (ν) remains unchanged.

  
• Types of interactions between electromagnetic radiation and matter

  • Absorption
  • Reflection
  • Transmission
  • Scattering
  • Refraction
• By applying electromagnetic radiation 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
• Energy (E)

  The energy of a photon, given by E = hν where h is Planck's constant
• The brilliant red colour seen in fireworks is due to the emission of red light at a wavelength near 650 nm.
• Intensity of light


  Can be considered from both a wave and a particle point of view. From the wave point of view, it is related to the amplitude of the electromagnetic wave. From the particle point of view, it is related to the number of photons in the radiation.
• The visible light region extends from approximately 700 nm (red) to 400 nm (blue).
• The infrared region is at slightly lower energy (longer wavelength) than the visible region, while the ultraviolet is at slightly higher energy (shorter wavelength).
• Different units are used across the electromagnetic spectrum, usually focusing on the wavelength or the frequency.
• Molecular absorption spectroscopy (also known as UV-Vis spectroscopy) involves the study of electronic transitions in molecules that occur in the UV-Visible region (λ = 200-700 nm).

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

  
• 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

  
• 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 are observed for compounds with lone pairs and multiple bonds (λmax = 200-600 nm)
• 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 (10-14 - 10-10 s)
• Internal conversion


  Energy dissipated through collisions, can occur when the energy difference between different excited states is small
• 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)
• Processes occurring after absorption of light

  1. Absorption: S0 → S1 or S0 → S2 (10-15 s)

  2. Vibrational relaxation/internal conversion: 10-12 s

  3. Fluorescence: S1 → S0 (10-9 s)

  4. Intersystem crossing: S1 → T1

  5. Phosphorescence: T1 → S0 (10-3 – 104 s)

  
• Lecture 2: Will introduce measurements using molecular absorption spectroscopy
• Pre-reading: Introduction to how light absorption is measured, and relationships between amount of light absorbed and concentration ("Beer's Law")
• Learning objectives associated with this lecture

  • Discuss how UV-VIS radiation interacts with molecules, including the sources of specific transitions
  • Discuss the atomic and molecular factors controlling vibrational and electronic spectroscopy
  • Size (intensity) and shape of spectra
  • Utilise the Beer-Lambert law to relate absorbance and concentration
  • External calibration
  • Standard addition
  • Multicomponent analyses
• Spectrophotometric measurement


  1. Light source

  2. Monochromator

  3. Sample

  4. Detector