Modern Spectroscopy

380 9 LASERS AND LASER SPECTROSCOPY
Figure 9.34 A LIDAR device using a Cassagrain telescope
Measurements of ozone (O3) concentrations in the atmosphere are of particular
importance. Ozone absorbs strongly in the ultraviolet region and it is this absorption which
protects us from a dangerously high dose of ultraviolet radiation from the sun. The vitally
important ozone layer lies in the stratosphere and is typically about 10 km thick with a
maximum concentration about 25 km above the surface of the earth. Extreme depletion of
ozone in a localised part of the atmosphere creates what is known as an ozone hole.
Ozone shows two absorption systems, from the X˜ 1 A1 ground state, in the near ultraviolet
region. The C˜ –X˜ , Huggins, system is in the region ca 310–374 nm and the D˜ –X˜ , Hartley,
system in the region ca 220–310 nm, these limits being imprecise because the systems
overlap. The Hartley system is very much more intense than the Huggins system and is
responsible for the ozone layer being opaque to radiation from the sun in this region.
Determination of the concentration, and its height distribution, of ozone or any molecular
atmospheric pollutant is carried out using the DIAL (differential absorption LIDAR)
technique. For a molecule such as carbon dioxide which shows sharp, discrete absorption,
the gas concentration at a particular height (using a particular time delay for the returned,
backscattered laser radiation) is obtained by comparing the intensity of the backscattered
laser beam with the laser wavelength tuned onto and then off an absorption feature of the
molecule being investigated. Any background absorption and backscatter by, for example,
an aerosol which may be present in addition to the gas of interest is taken account of by