1. BY Dr. Bhawna Lecturer in Chemistry Govt College, Mohali
PHOTOCHEMISTRY BY
Dr. Bhawna
Lecturer in Chemistry
Govt College, Mohali

2. INTRODUCTION
Photochemistry is defined as that branch of chemistry which deals with the study of interaction of radiation with matter resulting into a physical change or into a chemical reaction (called photochemical reaction) whose rates and mechanisms are studied after initiation by the radiant energy. Though the term radiation includes all types of electromagnetic waves from very low frequency microwaves to medium frequency infrared, visible and ultraviolet radiation to high frequency X-rays and γ-rays, the radiation of photochemical importance are those which lie in the visible and ultraviolet region from Å ( nm).
Thus the two main processes studied under photochemistry are :
(i) Photophysical processes
(ii) Photochemical processes or photochemical reactions.

3. Photophysical processes
Photophysical processes are those processes which take place in the presence of light but do not result into any chemical reaction. These processes arise on account of the absorption of light by the substances followed by the emission of the absorbed light. If the absorbed light is emitted instanteniously, the process is called fluorescence. If the absorbed light is emitted after some time lag, the process is called phosphorescence. Further, if the energy of the light absorbed is sufficiently high the electrons may not only jump to the outer levels but may leave the atoms completely ; the process is called photoelectric effect. Thus photophysical processes include phenomena like fluorescence , phosphresence and photoelectric effect.

4. Photochemical reactions are those reactions which take place by absorption of light by the reacting substances. These reactions are generally brought about by the absorption of light radiations of the visible and ultraviolet region which lie between Å ( nm) as already mentioned above. In these cases, the light energy absorbed is stored within the substance and then used for bringing about the reaction. A large number of different types of reactions can be brought about by exposure to suitable light e.g. synthesis, decomposition, oxidation, reduction, polymerisation, isomeric change etc.

5. Two well known examples of photochemical reactions are given below :
(i) Combination of hydrogen and chlorine to form hydrogen chloride
H2(g) + cl2(g) Light→ 2HCl(g)
(ii) Photosynthesis of carbohydrates in plants taking place in presence of chlorophyll. the green colouring matter present in the leaves
6C02 + 6H20 → C6H
From air Glucose

6. Thermochemical Reactions Photochemical reactions
DIFFERENCE BETWEEN PHOTOCHEMICAL REACTIONS AND THERMOCHEMICAL REACTIONS
Thermochemical Reactions
Photochemical reactions
(i)
These reactions involve absorption of heat.
These reactions involve absorption of light.
(ii)
They can take place even in the dark.
The presence of light is the primary requisite for the
reaction to take place.
(iii)
Temperature has significant effect on the rate
Temperature has very little effect on the rate of a
of a thermochemical reaction.
photochemical reaction. Instead, the intensity of light has
a marked effect on the rate of a photochemcal reaction

7. The free energy change (ΔG) of a thermochemical
iv)
The free energy change (ΔG) of a thermochemical
The free energy change (Δ G) of a photochemical reaction
reaction is always negative.
may not be negative. A few examples of photochemical
reactions for which ΔG is positive and still they are spontaneous
(i)Synthesis of carbohydrates in plants.
(ii)Ozonization of oxygen.
(iii)Decomposition of HCl into H2 and Cl2*
In these reactions, activation energy arises
In these reactions, activation energy is acquired by absorption
from the intermolecular collisions or
of photons of suitable energy
it is supplied in the form of heat.

8. When a reaction mixture is exposed to heat
(vi)
When a reaction mixture is exposed to heat
When a reaction mixture is exposed to light radiation, the
radiation, all the reactant molecules absorb
molecules of one of the reactants may get excited
these radiations and get excited almost to
independent of the presence of other species
the same extent i.e. there is no selectivity
(eg .only Br2 molecules in a mixture of H2 and Br2)

9. LAWS GOVERNING ABSORPTION OF LIGHT
When a monochromatic light (light consisting of a single wavelength) is passed through a medium some light is absorbed by the medium and hence the intensity of the incident light decreases. The decrease of intensity on passing through a pure homogeneous medium (i.e. a pure liquid) is governed by Lambert's law and the decrease of intensity on passing through a solution is governed by Beer's law.

10. Lambert's Law .
This law put forward by Lambert (in 1760) states as under When a monochromatic light is passed through a pure homogeneous medium, the decrease in the intensity of light with thickness of the absorbing medium at any point X is proportional to the intensity of the incident light i.e. the intensity of light at the point X (i.e. just before entering into the dx). Mathematically,
-dl / dx α I or -dl / dx =kI

11. where dl is the small decrease in intensity of the light on passing through a small thickness dx, I is the intensity of the incident light just before entering the thickness dx and the constant of proportionality k is called the absorption coefficient depending upon the nature of the absorbing medium.
The intensity I at any point X (at a distance x from the start of the medium), can be found in terms if the original intensity I0 as follows :
Equation (1) can be written as
dI/ I= -k dx (2
When x = 0,I = I0
x = x, I =I

12. Integrating equation (2) between the limits x = 0 to x and I= I0 to I, we get
This equation expresses how the original intensity I0 is reduced to intensity I after passing through a thickness x of the medium*. The difference between I0 and I obviously gives the intensity of the light absorbed.

13. Thus
Iabs =I0 - I = IO-I0 e –kx
Equation (3) can be written as
2.303 log I/I0 = -kx
log I/I0 = - kx/2.303
log I/I0 = - k ‘x
I/I0 = 10 - k ‘x (6)
I = I0 x 10 - k ‘x
Where k’= k/2.303 is called extinction coefficient of the substance ( i.e. of the absorbing medium). Now it is this quantity which is called absorption coefficient or absorptivity of the substance.

14. The physical significance of the extinction coefficient or absorptivity follows more clearly from equation (6) which can be rewritten as
K’ = -1/x log I/I0 = 1/x log I0 /I
If log I0 /I = 1 , K’= 1/x
But log I0 /I = 1 means I0 /I = 10
i.e I = IO/10
Hence extinction coefficient or absorptivity may be defined as follows :
The extinction coefficient or absorptivity is the reciprocal of that layer thickness (expressed in cm) at which the intensity of the light falls to one-tenth of its original value.

15. Beer's Law
. This is an extension of Lambert' law, put forward by Beer (in 1852). It is applicable to solutions and states as under :
When a monochromic light is passed through a solution, the decrease in the intensity of light with the thickness of the solution is directly proportional not only to the intensity of the incident light but also to the concentration 'c' of the solution. Thus for a solution, mathematically, we have
-dl/dx α I x c
-dl/dx = εIc (ε is pronounced as epsilon) (8)
where ε is a constant of proportionality and is called molar absorption coefficient. Its value depends upon the nature of the absorbing solute and the wavelength of the light used.

16. Equation (8) can be rewritten as
dI/I = - εc dx
As before, integrating this equation between the limits x = 0 to x and I= I0 to I , we get
ln I/IO = - εcx
Or I/I0 =e - εcx
Or I =I0 e - εcx
This equation shows that the intensity of a monochromatic light decreases exponentially from I0 to I with increase in thickness x and concentration c.

17. The intensity of the light absorbed by the solution when it passes through a length x of the solution will be given by
Iabs = I0 - I = I0 - I0 e- εcx = I0(I- e- εcx) ..(12)
2.303 log I/ I0 = - εcx or logI/I0=- εcx/2.303
or logI/I0=- έcx _(13)
or I/I0= 10 - έcx
or I = I0 x έcx
where έ = ε/ was earlier called molar extinction coefficient of the absorbing solution. Now it is this quantity which is called molar absorption coefficient or molar absorptivity of the absorbing solution.

18. The physical significance of molar extinction coefficient or molar absorptivity, follows from equation (13), which can be written as
έ = -1/cx log I/I0 = 1/cx log I0/I
If c=1 M and log I0/I = 1 then έ = 1/x
As before, log I0/I = 1 means I0/I = 10 i.e. I = I0/10
Hence molar extinction coefficient or molar absorptivity may be defined as the reciprocal of that thickness of the solution layer of 1 molar concentration which reduces the intensity of the light passing through it to one-tenth of its original value.
It may be pointed out that gases follow Beer's law and not Lambert's law.

19. Units of molar absorptivity or absorption coefficient
έ = - 1 /cx log I/I0
log I/I0 is a dimensionless quantity.
In SI units, x is in metres (m) and c is in mol m -3. Hence, we have
έ=(m3 mol -1)(m -1)=m 2 mol -1

20. LAWS GOVERNING PHOTOCHEMICAL REACTIONS
There are two important laws of photochemistry as explained below :
1. Grotthus-Draper Law. This law is also called First law of photochemistry. It was put forward by Grotthus on theoretical grounds (in 1818) and confirmed by Draper by experiment (in 1839). It states as under:
When light falls on a body, a part of it is reflected, a part of it is transmitted and the rest of it is absorbed. It is only the absorbed light which is effective in bringing about a chemical reaction.

21. The above law, however, does not imply that the absorbed light must always result into chemical reaction. The absorbed light may simply bring about phenomena such as fluorescence, phosphorescence etc., Similarly, the absorbed light energy may be simply converted into thermal energy e.g. in case of potassium permanganate solution, the light energy is absorbed strongly but no chemical effect is produced. Further in some cases, it is observed that light energy may not be absorbed by the reacting substance directly but may be absorbed by some other substance present alongwith the reacting substance. The energy thus absorbed is then passed on to the reacting substance which then starts reacting. This process is called 'photo-sensitization' . A well known example is that of photosynthesis of carbohydrates in plants where chlorophyll acts as a photosensitizer.

22. 2. Stark-Einstein's Law of Photochemical Equivalence.
This law is also called second law of photochemistry. It was put forward by Einstein (1905) and Stark (1908). It states at under:
Every atom or molecule that takes part in a photochemical reaction absorbs one quantum of the radiation to which the substance is exposed.
If v is the frequency of the absorbed radiation, then the energy absorbed by each reacting atom or molecule is one quantum i.e. hv where h is Planck's constant. The energy absorbed bv one mole of the reacting molecules will then be given by
E = Nhv (1)
where N is Avogadro's number (i.e. the number of molecules present in one mole of the substance)
Putting v =c/λ We can write E = Nh c/λ (2)
where c is the velocity of light and λ is the wavelength of the absorbed radiation

23. The energy possessed by one mole of photons (or the energy absorbed by one mole of the reacting molecules) is called one einstein.
In CGS units
E = 2.86 x 105 / λ (Å) kcal per mole
In SI units
E= x / λ(m) kJ/mol
Thus the energy per Einstein is inversely proportional to the wavelength of the radiation. Shorter the wavelength of a radiation , greater is the energy per Einstein. For example the order of wavelength of different radiations is
X rays < Ultraviolet < Visible < Infrared
Hence the energies per Einstein of these radiations are in the order
X rays > Ultraviolet >Visible > Infrared

24. INTERPRETATION OF EINSTEIN'S LAW -'QUANTUM EFFICIENCY'
If Einstein's law is correct, every reacting molecule will absorb one quantum of radiation. Hence the number of reacting molecules should be equal to the number of quanta absorbed. However, it is found that in a number of cases, a small amount of the light absorbed can bring about a large amount of reaction, whereas in some other cases, large amount of the light absorbed can bring about only a small amount of reaction. This was explained on the basis that all the molecules present do not absorb the radiation, only a few molecules absorb the radiation. These molecules are said to have been 'activated' These activated molecules then react with the other molecules and thus bring about a large amount of chemical reaction. On the other hand, in certain cases, the activated molecules get deactivated and no reaction takes place. The first stage (involving the absorption of light) is called the primary process. The reactions involved in the second stage are called secondary processes.

25. To relate the number of quanta absorbed with the number of reacting molecules, a term called quantum efficiency or quantum yield (φ) has been introduced. It is expressed as
φ = Number of molecules reacting in a given time
Number of quanta of light absorbed in the same time
= Number of moles reacting in a given time
Number of einsteins of light absorbed in the same time .
Hence quantum efficiency may be defined as the number of moles reacting per einstein. light absorbed.
If Stark-Einstein's law is strictly obeyed in the form already stated, φ should be equal to unity. However, studies have shown that the law is applicable only to the primary processes. Hence it is sometimes preferred to state it as follows :
Each molecule that gets activated to initiate the reaction absorbs one quantum of light.

26. φ = Number of moles reacting per sec
EXPERIMENTAL DETERMINATION OF QUANTUM YIELD OF A PHOTOCHEMICAL REACTION
Theory of the process. For experimental purpose, the expression for the quantum yield may be written as
φ = Number of moles reacting per sec
Number of quanta absorbed per sec.
= Rate of chemical reaction
Quanta absorbed per sec
Thus to determine the quantum yield of any photochemical reaction, we have to measure (i) the rate of chemical reaction in terms of molecules per sec.
(ii) the quanta absorbed per sec.

27. Apparatus. The apparatus used for measuring the above two quantities (and hence for the determination of the quantum yield) It consists of the following parts from left to right:
(i) A source of radiation emitting radiation of suitable intensity in the desired special range. The commonly used sources are filament lamps, carbon metal arcs and various gas discharge Tubes.
(ii) Lens of suitable focal length.
(iii) Monochromator or filter which cuts off all radiations except the radiations of the desired wavelength.
(iv) Slit to get a fine beam.

28. (v) A cell placed in thermostat and containing the reaction mixture
(v) A cell placed in thermostat and containing the reaction mixture. The cell is made of glass or quartz and has optically plane windows for the entrance and exit of light. Glass is used only if the wavelength of the light used lies in the visible range. For radiations with wavelength below 3500 Å, the cell completely made of quartz is used.
(vi) Recorder. It is usually a 'thermopile' or an actinometer which is used to measure intensity of the radiation. The principle of a thermopile and that of an actinometer are briefly explained as follows :

29. A thermopile is a set of thermocouples. Rods of two different metals (e.g. Ag and Bi) are joined alternately as shown in Fig. (If only two rods are connected, it constitutes a thermocouple). One set of junctions is soldered to metal strip blackened with platinum or lamp black and the other set of junctions is protected completely from radiation by placing the system in a box as shown in Fig.. The radiations falling on the blackened metal strip are almost completely absorbed by it. As a result, this set of junctions becomes hot. The temperature difference of the hot end and the cold end produces a current which can be measured by connecting a milliammeter to the thermopile. By calibrating the thermopile with radiations of known intensity, the intensity of the desired radiations can be measured.

30. METAL STRIP BLAKENED WITH PT. BLACK
A THERMOPILE

31. A chemical actinometer is a device in which gas mixture or solutions sensitive to light are used The working of this device is based upon the fact that a definite amount of the radiation absorbed brings about a definite amount of chemical reaction. The most commonly employed is the uranyl oxalate actinometer which consists of 0.05 molar oxalic acid and 0.01 molar uranyl sulphate (U02S04) solution in water. When exposed to light, the reaction takes place converting oxalic acid into CO2 and H2O :
Uranyl sulphate acts as photosensitizer (i.e. absorbs the light and then passes it on to oxalic acid). The extent of reaction is measured by titrating the oxalic acid solution with potassium permanganate solution. The actinometer is first calibrated against a thermopile. This actinometer can be used for measuring only the intensity of the radiation lying in the range Å.

32. Procedure
(i)Measurement of the intensity of light absorbed. First, the empty cell or the cell filled with solvent alone (in case of solutions), is placed in the thermostat. The monochromatic radiation is allowed to pass through the cell for a definite time. The reading is taken on the recorder. From this we get the energy of the incident radiation. Now the reactants are taken in the cell and the radiations are allowed to pass through it for definite time and the reading is taken again. The difference between the two readings gives the total energy absorbed by the reactants in a given time. From this the intensity of the radiation absorbed Iabs is calculated as follows :
Iabs = Total energy absorbed
Volume of the reaction mixture x Time in sec

33. (ii) Measurement of the rate of reaction
(ii) Measurement of the rate of reaction. The rate of the reaction can be studied as usual. For this purpose, either the change in some physical property is followed or samples of the reaction mixture are removed periodically from the cell and analysed.
Knowing the rate of reaction and the intensity of light absorbed, the quantum yield of the reaction can be calculated.

34. PHOTOCHEMICAL REACTIONS
A photochemical reaction takes place in two steps
1. Primary process : This involves activation of some molecules by absorption of light.
AB + hv → AB*
Or AB + hv → A + B*
2. Secondary process : This involves reaction of activated molecules of primary process with other molecules resulting into activation or deactivation

35. On the basis of above processes photochemical reactions are of three types
1. Quantum yield is a small integer.
H2 + Br2 → 2 HBr φ=1
2. Quantum yield is very large.
H2 + Cl2 → 2HCl φ=10 6
3. Quantum yield is very small.
NH3 + hv → N2 + H2 φ=0.25

36. The reaction may be represented as H2 + Br2 → 2 HBr
Photosynthesis of HBr i.e. Photochemical combination of Hydrogen and Bromine to form HBr.
The reaction may be represented as
H2 + Br2 → 2 HBr
The quantum efficiency of this reaction is very low i.e. about 0.01 at ordinary temperatures. This is explained by proposing the following mechanism for the above reaction :
(a)Primary process ;
Br2 + hv → 2 Br
(b)Secondary Process :
(i) Br + H2 endo→ HBr + H
(ii) H + Br2 → HBr + Br
(iii)H + HBr → H2 + Br
(iv) Br + Br → Br2

37. Photosynthesis of HCl from H2 and Cl2
This is an example of a reaction whose quantum yeild is very high i.e to This reaction may be represented as
H2 + Cl2 → 2HCl
The high quantum yield of this reaction is explained by the chain mechanism (proposed by Nernst in 1918). The different steps involved are as follows :
(a) Primary process :
Cl2 + hv → 2Cl Chain initiating step
i.e. a chlorine molecule absorbs one quantum of light and dissociates to give CI atoms.
(b) Secondary process :
(i) CI + H2 exo--→ HCl + H | Chain propagating steps
(ii) H + Cl2 → HCl+ CI I
(iii) CI + CI → Cl2 } Chain terminating step

38. It is interesting to note that the mechanism of the above reaction is similar to that of the photosynthesis of HBr, yet the quantum yield of this reaction is very high whereas that of the photosnthesis of HBr is very low. The difference is explained on the basis of the reaction (i) of the secondary processes. In the present reaction, the reaction (i) of the secondary processes which immediately follows the primary process is exothermic and therefore, takes place very easily and consequently the chain reaction is set up very easily. In the photosynthesis of HBr, the reaction (i) of the secondary process is endothermic and thus has very little tendency to take place .

39. The quantum yield of the above reaction decreases if the reaction vessel contains small traces of oxygen. This is explained on the basis that oxygen leads to the chain termination by the following reactions
H + 02 → H02
CI HCl → H02 + Cl2
The chain mechanism for the photosynthesis of HCl, as discussed above, is further supported by the following facts :
(i) The reaction can be brought-about without exposing the reaction mixture (H2 + Cl2) to light but by simply adding some CI or H atoms into the reaction mixture.
(ii) The quantum yield of the reaction decreases considerably if the reaction is carried out in capillary tubes. This confirms that the chains are terminated on the walls of the reaction vessel.

40. Photolysis of Ammonia.
This is an example of a photochemical reaction whose quantum yield is very low i.e. about 0.25 at ordinary temperatures and pressures. The probable mechanism is supposed to be as follows :
(a) Primary process:
NH3 + hv → NH2 + H
The presence of H-atoms has been proved by suitable methods.
(b) Secondary processes. The ultimate products of the photolysis of amonia have been found to be nitrogen, hydrogen and hydrazine. Hence the following probable secondary reactions have been proposed ;

41. (ii) NH2 + NH2 → N2H4 (iii) H + H → H2 (iv) NH2 + H → NH3
Since the quantum yield of the reaction is very low, the reaction (i) to (iii) take place only to a very small extent. The important secondary reaction is the reaction (iv) only which involves the reversal of the primary process.

42. PHOTO- PHYSICAL PROCESSES
Fluorescence. There are certain substances which when exposed to light or certain other radiations absorb the energy and then immediately or instantaneously start re-emitting the energy. Such substances are called fluorescent substances and the phenomenon is called fluorescence. Obviously, the absorption of the energy results into the excitation of the electrons (present in the atoms or the molecules of the substance) followed immediately by the jumping back of the excited electrons to the lower levels .As a result, the absorbed energy is emitted back..
The word 'instantaneous' used above implies that the time between the absorption of this energy and the re-emission of the absorbed energy is not more sec. Thus fluorescence starts as soon as the substance is exposed to light and the fluorescence stops as soon as the light is cut off.

43. A few examples of the substances showing the phenomenon of fluorescence are :
(i) fluorite, CaF2 (from which the name of the phenomenon is derived)
( ii) certain organic dyes such as eosin, fluorescein etc
(iii) vapour of sodium, mercury, iodine etc.
(iv) certain inorganic compounds such as uranyl sulphate,
It is interesting to observe that in the phenomenon of fluorescence, the wavelength of she emitted light is usually greater than that of the absorbed light. This is explained on the basis that the energy absorbed raises the electron to a sufficiently higher level but the return of the excited electron to the original level takes place in steps, through the intermediate levels.

44. The energy thus produced in every jump is smaller and hence the wavelength of the light emitted is large (as E = hv = hc/λ i.e. E α 1/λ) than that of the absorbed light.
In case of mercury vapour, the emitted light has the same wavelength as that of the absorbed light . This phenomenon is called resonance fluorescence. In certain cases, light of shorter wavelength (i.e. of higher energy) is emitted. This is explained by suggesting that the emitted light contains the absorbed energy as well as some energy present in the system before absorption.

45. Phosphorescence. In case of fluorescent materials, the florescence stops as soon as the external light is cut off. However, there are certain substances which continue to glow for some time even after the external light is cut off. Such substances are called phosphors or phosphoresce substances and the phenomenon is called phosphorescence. Thus in a way, phosphorescence is a slow fluorescence. It is found mostly in solids, as might be expected because in solid, the molecules have least freedom of motion. The excited electrons thus keep on jumping back slowly for quite some time.

46. The best examples of the substances exhibiting phosphorescence include zinc sulphide and sulphide of the alkaline earth metals. Usually a trace of heavy metal is added (in the form of its sulphide) which intensifies the light emitted by the phosphorescent substance. Further, it has been found that the fluorescent substances becomes phosphorescent if fixed by suitable methods, e.g. by fusion with other substances etc. For example, many dyes (which show fluorescence) when dissolved in fused boric acid or glycerol and then cooled to get rigid mass become phosphorescent.

47. FLUORESCENCE AND PHOSPHORESCENCE IN TERMS OF EXCITATION OF ELECTRONS (JABLONSKI DIAGRAM)
When energy in the form of light radiations falls on certain molecules. some of the electrons present in them absorb energy and jump to the outer orbitals. The molecules are then said to be in the excited state. Most of the excited species have very short life times, usually less than second They may either undergo a chemical change or return to the ground state.
The excitation of an electron occurs so rapidly (< second) that it is believed that there is not enough time for the orientation of the spin of the electron to change, i.e., the one which was clockwise remains clockwise and the other which is anti-clockwise remains anti-clockwise after the excitation. Thus in terms of electron spin, we can represent an excitation as follows

48. These states are usually expressed in terms of a quantity called spin multiplicity which is equal to 2S + 1 where S is the total electron spin. For a molecule containing all paired electrons S=0and so its spin multiplicity = 1. The molecule is said to be in singlet ground state. After absorbing photon, when the molecule is in excited state with opposite spins, S= ½ -½= 0 so the spin multiplicity = 1 . The molecules is then said to be in singlet excited state. When in the excited state the electrons are present in different orbitals but with spin in the same direction, S= ½ + ½ =1 and spin multiplicity= 2S + 1 =3. The molecule is then said to be in triplet excited state.

49. Depending upon the energy absorbed, electrons may jump to any of the higher excited state and we have a number of singlet excited states called first, second, third singlet excited state represented by S1, S2, S3 etc. The singlet ground state is represented by S0. The corresponding triplet excited states are called first, second, third triplet excited states and are represented by T1, T2, T3 etc. Any singlet excited state is found to have higher energy than the corresponding triplet state i.e. ES1 > ET1, Es2 > ET2 and so on.
The excited species can return to the ground state by losing all of its excess energy by anv one of the paths as shown in Fig., called Jablonski diagram

51. In all these paths, however the first step is the transitions from higher excited singlet state to the lowest excited singlet state S1. This is called internal conversion (IC). The energy is lost only in the form of heat (shown by wavy line in the diagram) due to collision with other molecules.As it does not result into the emission of any radiation, it is therefore, called non-radiative or radiation less transition It is a very fast process and occurs in less than about second. Now from the first excited singlet state, the molecules return to the ground state by any one of the following paths.
Path I. The molecule may lose rest of the energy also in the form of heat so that the complete path is non-radiative.
Path II. The molecule may release the energy in the form of light or ultraviolet radiations in returning to the ground state from the first excited singlet state (shown by straight line in the diagram). Hence it is a radiative transition. This is called fluorescence. It occurs for about 10-8 second after absorption so that a substance fluoresces only in the presence of absorbed radiation.

52. Path III. Some energy may first be lost in transition from first excited singlet state to the first excited triplet state i.e. between states of different multiplicity i.e. S1 to T1(or similarlyS2 to T2. S3 to T3 etc., in the form of heat so that it is again a radiationless transition. It is also called intersystem crossing (ISC). Such transitions are normally forbidden but whenever they occur, their rate is very slow. In fur transition from first excited triplet state to the ground state, the rest of the energy may again be lost in the form of heat due to molecular collisions. Thus the complete path is non-radiative.
Further, the energy is lost not only as heat but also in undergoing a chemical reaction .As singlet excited states return quickly to the ground state, there is no chance for a chemical reaction However, if the molecules are in the triplet excited states, they return to the ground state slowly and thus get sufficient time to undergo a chemical reaction. Thus molecules undergoing chemical reaction are those which are first in the excited triplet state.

53. Path IV. After intersystem crossing, the molecule may lose the rest of the energy- in the form of light in going from the excited triplet state to the ground state. This is called phosphorescence. As triplet states have much longer life times than singlet states, some as long as an hour, phosphorescence persists even after the removal of absorbed radiation. This may also be attributed to the fact that the transitions from excited triplet states to ground singlet state are forbidden.
Note that fluorescence involves a radiative transition between two states of the same multiplicity (usually two singlets) whereas phosphorescence arises due to a radiative transition between two states of different multiplicity (usually triplet to singlet). It may be observed that the energy emitted during fluorescence as well as phosphorescence is less than the energy absorbed during excitation, as a part of the energy is dissipated as heat in the internal conversion and intersystem crossing. Hence the frequencies emitted during fluorescence and phosphorescence are smaller or wavelengths are longer than those of the light absorbed for excitation.

54. The paths followed as explained above may be compared with a two-floor building having steps, ramps and windows. First step of all paths is like a ball falling from second floor to the first floor by bouncing on steps going from second floor to first floor. If the ball now falls on the ramp going from first floor to ground floor, it is like path I. If the ball after coming to first floor falls out of the window, it is like path II. If after coming to first floor, the ball moves on some steps and then falls in the ramp going from first floor to ground floor, it is like path III. If the ball after having come to first floor, moves on some steps going from first floor to ground floor and then jumps out of the window, it is like path IV.

55. PHOTOSENSITIZATION
There are many substances which do not react directly when exposed to light. However if another substance is added, the photochemical reaction starts. The substance thus added itself does not undergo any chemical change. It merely absorbs the light energy and then passes it on to one of the reactants. Such a substance which when added to a reaction mixture helps to start the photochemical reaction but itself does not under go any chemical change is called a photosensitizer and the process is called photosensitization. Thus a photosensitizer simply acts as a carrier of energy.
At the first instance it appears that a photosensitizer is similar to a catalyst. However, this is not so as they act by different mechanism.
Examples of Photosensitized Reactions. A few well known examples of the photosensitized reactions are briefly described below

56. 1. Dissociation of hydrogen molecules in presence of mercury vapour
1. Dissociation of hydrogen molecules in presence of mercury vapour. Hydrogen molecules do not dissociate when exposed to ultraviolet light (of 2537 A emitted by the mercury vapour lamp). However, when hydrogen gas is mixed with mercury vapour and then exposed to ultraviolet light, hydrogen molecules dissociate to give hydrogen atoms through the following reactions :
Hg + hv → Hg*
Hg* + H2 → Hg + 2H
where Hg* represents the activated mercury atom. Thus in the above reaction, mercury acts a photosensitizer.
The H-atom being highly reactive can easily reduce metallic oxides, nitrous oxide, carbon monoxide etc.

57. In a similar manner, the dissociation of NH3, PH3 AsH3 and a number of organic compounds is photosensitized by mercury. Certain combination reactions e.g.,
2H > 2H20 ; N2 + 3H NH3; 3O2-----> 2O3 etc. are also mercury photosenstized.
Two well known mercury photosensitized reactions involving dissociation of H2 molecules into H atoms are briefly explained below :
(a) Combination between CO and H2 to form formaldehyde and glyoxal. It is believed to take place through the following steps :
(i) Hg + hv → Hg* (Activation)
(ii) Hg*+H2 → 2H + Hg (Dissociation)
(iii) H + CO → HCO
(iv) HCO + H2 → HCHO + H
(v) 2HCO → HCHO + CO
(vi) 2HCO → OHC—CHO (Glyoxal)

58. THANKS