Engineering Chemistry S Datta

130 ENGINEERING CHEMISTRY
These photons will stimulate other molecules in the state 2 to emit photons of frequency
1 → 2 and drop to state 1 (Fig. 5.5).
3
Photons of frequency 1 → 2 will also induce absorption from
state 1 to state 2 but as the system contains n 2
> n 1
stimulated
emission will predominate over absorption and we will get a
2
net amplification of radiation of frequency ν 1→2
.
The laser system is contained in a cylindrical cavity whose
ends have parallel mirrors. A few photons are spontaneously
emitted when the molecules go from the state 2 to state 1. Those
photons emitted at the cylindrical axes pass out of the system and
have no role in the laser action. Those photons emitted along the
laser axis travel forward and backward between the end mirrors
1
Fig. 5.5. States involved in
laser action.
and stimulate emission for further photons. The presence of end mirrors makes the laser a
standing wave pattern.
Let l be the distance between the mirrors and light only with wavelength = λ, such that
nλ
= l , where n is an integer, will resonate in the cavity. This makes the laser nearly
2
monochromatic, the laser output is highly monochromatic and highly directional. These
properties make laser useful in spectroscopy and kinetics. Thousands of different lasers
exist; the material in which the laser action can be employed may be a solid, a liquid or a gas.
The frequency emitted lies in the IR, visible or UV region.
Kinds of lasers
• Solid-state metal ion laser.
• Semiconductor laser or diode laser.
• Gas laser.
• Chemical laser.
• Dye laser.
An example of chemical laser: One can use the reaction:
H + F 2
⎯⎯→ HF + F,
which is an elementary step of the mechanism of reaction:
H 2
+ F 2
⎯⎯→ 2HF (chain reaction)
A laser in which the population inversion can be produced by a chemical reaction is
called a chemical laser. The HF chemical laser can be used as an antimissile.
Laser Technique
A laser can be used to excite a major fraction of a reactant species to a specific vibration
level in a molecular beam. Then we can study the dependence of the reaction probability on
the vibrational quantum states of these reactants.
This sort of idea can be utilised to a selective excitement of a particular normal vibrational
mode that involves mainly vibration of a particular bond and to break preferentially that
bond. Thus, laser light can be employed to control the outcome of the reaction. For example,
we can take the following two reactions:
(i) H + HOD ⎯⎯→ OD + H 2
(ii)
H + HOD ⎯⎯→ HO + HD.