Page 327 - Instant notes

Practical aspects of spectroscopy 313

(ii) The rate of collisions of the molecules with each other and the walls of the container
establishes a collisional linewidth, δE coll≈ћ/τ coll where τ coll is the average time between
the deactivation collisions. For liquids, and gases at moderate pressures, the collisional
linewidth dominates the natural linewidth.

A third line-broadening process, particularly important for gaseous samples, is the
Doppler effect, in which radiation shifts to higher (or lower) frequency when the source
is moving towards (or away) from the observer, respectively. Since molecules in a
sample are moving in all directions with respect to the detector, with a range of velocities
given by the Maxwell distribution (Topic A2), each spectral transition is spread over a
range of Doppler frequency shifts. Doppler linewidth increases with temperature
because the molecules have a greater range of speeds.

Lasers

The word laser is an acronym for light amplification by the stimulated emission of
radiation. Laser action requires: (i) the production of a population inversion, in which
the population in an upper (excited) state exceeds the population in a lower state, and (ii)
the stimulation of a radiative transition between the two states. The excited state is
stimulated to emit a photon by interaction with radiation of the same frequency. The
more photons of that frequency present, the greater the number of photons the excited
states are stimulated to emit. This positive feedback process is known as the gain of the
laser medium.
A greater population is required in the upper state in order to ensure that net emission
rather than net absorption occurs. The population inversion must be prepared deliberately
(a process called pumping) because the Boltzmann distribution law dictates that
population is greater in the lower energy state at thermal equilibrium. One way of
achieving this is to pump an excited level (using an intense flash of light from a discharge
or another laser) which converts non-radiatively into the upper level of the laser transition
(Fig. 1). In the fourlevel system illustrated the laser transition terminates in another
excited level and population inversion is easier to achieve than when the laser transition
terminates in the ground state. Continuous rather than pulsed laser output is possible if
the population inversion can be sustained.
The characteristics of laser radiation are that it is: (i) intense, (ii) monochromatic
(narrow frequency), (iii) collimated (low beam divergence), and (iv) coherent (all waves
in phase).