This document discusses infrared photochemistry. It begins by defining infrared as invisible electromagnetic radiation with longer wavelengths than visible light. It then discusses how infrared lasers work and their common wavelengths. The document goes on to define photochemistry as chemical reactions induced by light absorption and discusses the laws and mechanisms of light absorption. It explains how infrared radiation is absorbed by molecules and the effects of collisions on energy transfer. The document concludes by discussing how pressure affects infrared-induced reactions and how vibrational excitation by lasers can selectively induce chemical reactions.

1. INFRARED
PHOTOCHEMISTRY
By-
Abhishek Ranjan
BATCH 11,SEM 7
International School Of Photonics,
Cochin University of Science and Technology.

2. INFRARED
 Infrared (IR) is invisible radiant energy, electromagnetic radiation with
longer wavelengths than those of visible light, extending from the
nominal red edge of the visible spectrum at 700 nanometre (frequency
430 THz) to 1 mm (300 GHz) and photons of energy 0.001 to 1.7 ev.
 Most of the thermal radiation emitted by objects near room temperature is
infrared.
 Infrared lasers or IR lasers are propagated at the invisible wavelength of the
light spectrum. IR's frequency is between visible light (near infrared) and
microwave (far infrared).
 Most IR lasers are tuned to 808nm, 980nm, 1064nm respectively. This gives
lasers invisible characteristics and is used extensively in both military and
civilian operations and markets.

3. Photochemistry Meaning
Chemical reactions accompanied with
light.
1. Action of light → chemical change
(light induced reactions)
2. Chemical reaction → light emission
(chemiluminescence)

4. Laws of Photochemistry
1. Only light that is absorbed can produce
photochemical change .
2. A molecule absorbs a single quantum of
light
is becoming excited.

5. Mechanisms of Light Absorption
Excitation:
X2
h *X2
A bonding electron is lifted to a higher energy level
(higher orbital)

6. INTERACTION OF LIGHT AND MATERIALS:
a) X2* → X2 + M* (excess energy transferred to
the surrounding)
b) X2* → X2 + h (fluorescence or phosphorescence)
c) X2* + Y → chemical reaction (excess energy
supplies the activation energy of the
reaction)

7. PHOTOCHEMISTRY
 If we consider an isolated polyatomic molecule in a gas under collision less
conditions , the infrared radiation will produce non equilibrium populations
within the manifold of vibrational and rotational levels of the lowest
electronic level.
 The effect of irradiation with is to transfer population to higher vibrational
states.
 The result of infrared absorption is heating of the tissue since it increases
molecular vibrational activity.

8. EFFECT OF COLLISIONS
 In the absence of collisions absorption will ‘burn a hole’ in the rotational
energy distribution of the lower vibrational level and will enhance the
population of a single rotational state in the excited vibrational state.
 If the presence of collisions, their effect will be to exchange vibrational
energy between excited and unexcited molecules and to exchange
vibrational energy with translational modes.This process leads to gas
heating and is the source of a major complication ;laser driven thermal
reactions.
 Collisions also re-establish equilibrium within the manifold of rotational
energy levels

9. EFFECT OF PRESSURE WITH EXAMPLE
 Thermal effects can b reduced by irradiation at low gas pressure.
 Limestone and isoprene was irradiated with CW CO2 laser with two
pressure regions .
 It was seen that at low pressure a wide variety of reaction products was
observed with the concentration of products maximizing at .05 torr.
 No reaction was observed between 3 to 30 torr.
 Detailed analysis of activation and deactivation showed that at low
pressures sequential absorption of several laser photons could be obtained
and the reaction was vibrationally induced. At high pressure ,normal
thermal chemistry is obtained.

10.  Unlike thermal excitation , the absorption of a limited amount of laser
excites only a single vibrational mode of the absorber.
 If vibrational mode is involved in the route to a particular reaction channel,
then excitation in this mode can increase the probability of the reaction,
the effect of stored energy in such case will be to reduce the activation
energy.
 When the incident laser flux is high,the rapid accumulation of vibrational
energy can lead to unimolecular (one molecule) dissociation .

11. REFERENCES
 Wikipedia
 W W Duley,plenum press