Raman scattering

Raman effect

When radiation hits molecules of a substance, the radiation may be

1. scattered
2. completely absorbed
3. partially absorbed (Raman effect)

Raman scatter

• The partial transfer of energy from the radiation to the molecule (Raman effect) becomes stored as the vibrational energy in the bonds between atoms in the molecule.
• Because energy of radiation is proportional to frequency of the radiation
• Thus,
  --> when energy is released back as radiation, it can only be released at a lower frequency.
• cf: Rayleigh scattering is where energy is completely absorbed and released at the same frequency.
• Cannot occur with gases with single atoms
  * e.g. He, Xe, Ar

Principle

Measurement of the change in light frequency will identify the gas and the partial pressure.

Raman effect is comparatively weak, thus an intense laser is needed to produce enough Raman scatter to be measurable at a detector.

Setup

• Laser of high intensity travels through a gas chamber
• Detectors are setup to be perpendicular to the axis of the laser
• Each detector has a fliter that allows radiation of a specific wavelength to pass
• One particular type of machine (?Rascal II) has 8 detectors: O2, N2, N2O, CO2, C-H bond and 3 volatile anaesthetic agents
  * C-H bond is used to measure volatile anaesthetic agents in general
  * the 3 volatile anaesthetic agents need to be specified at the time of purchase so that the manufacturer can install the appropriate filters

Advantage

• Rapid
• More accurate than mass spectrometry
• Not affected by water vapour

Disadvantage

• May degrade volatile gases

Other notes

Raman effect

(Additional information from Britannica)

Raman effect is the change in the wavelength of light that occurs when a light beam is deflected by molecules. The phenomenon is named for Sir Chandrasekhara Venkata Raman, who discovered it in 1928. When a beam of light traverses a dust-free, transparent sample of a chemical compound, a small fraction of the light emerges in directions other than that of the incident (incoming) beam. Most of this scattered light is of unchanged wavelength. A small part, however, has wavelengths different from that of the incident light; its presence is a result of the Raman effect.

Raman scattering is perhaps most easily understandable if the incident light is considered as consisting of particles, or photons (with energy proportional to frequency), that strike the molecules of the sample. Most of the encounters are elastic, and the photons are scattered with unchanged energy and frequency. On some occasions, however, the molecule takes up energy from or gives up energy to the photons, which are thereby scattered with diminished or increased energy, hence with lower or higher frequency. The frequency shifts are thus measures of the amounts of energy involved in the transition between initial and final states of the scattering molecule.

The Raman effect is feeble; for a liquid compound the intensity of the affected light may be only 1/100,000 of that incident beam. The pattern of the Raman lines is characteristic of the particular molecular species, and its intensity is proportional to the number of scattering molecules in the path of the light. Thus, Raman spectra are used in qualitative and quantitative analysis.

The energies corresponding to the Raman frequency shifts are found to be the energies associated with transitions between different rotational and vibrational states of the scattering molecule. Pure rotational shifts are small and difficult to observe, except for those of simple gaseous molecules. In liquids, rotational motions are hindered, and discrete rotational Raman lines are not found. Most Raman work is concerned with vibrational transitions, which give larger shifts observable for gases, liquids, and solids. Gases have low molecular concentration at ordinary pressures and therefore produce very faint Raman effects; thus liquids and solids are more frequently studied.