application of

coherent anti-Stokes Raman
spectroscopy

The intensity of Raman transitions may be enhanced by coherent anti-Stokes Raman
spectroscopy. The technique relies on the fact that, if two laser
beams of frequencies ν1 andν2 pass through a sample, then they may mix together and
give rise to coherent radiation of several different frequencies, one of which is

ν ′ = 2ν1 − ν2

Suppose that ν2 is varied until it matches any Stokes line from the sample, such as the
one with frequency ν1 − Δν ; then the coherent emission will have frequency

ν ′ = 2ν1 − (ν1 − Δν) = ν1 + Δν

which is the frequency of the corresponding anti-Stokes line. This coherent radiation
forms a narrow beam of high intensity.

An advantage of CARS is that it can be used to study Raman transitions in the presence
of competing incoherent background radiation, and so can be used to observe
the Raman spectra of species in flames. One example is the vibration–rotation CARS
spectrum of N2 gas in a methane–air flame .