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Wednesday, April 11, 2018




The reactions which occur with the absorption of radiations (photons) are called photochemical reactions. Photon supplies the necessary energy to the reactants, enabling them to react to yield products. It may be emphasized that a photochemical reaction requires the absorption of a definite energy by the reactants to proceed to products.

Although the term ‘radiation’ includes a variety of rays, the radiations important from the standpoint of photochemistry lie, almost exclusively, in the visible and ultraviolet (UV) regions, i.e., the wavelength ranges from 2000 to 8000 Å (200–800 nm). Photochemistry is, therefore, mainly concerned with rates and mechanism of reactions resulting from the exposure of a system to radiant energy which lies in the visible and UV regions, i.e., the wave length ranges between 2000 and 8000 Å (200–800 nm).

Photochemical reactions are usually cleaner and more efficient than other type of reactions. Important applications of photochemistry like photo polymerization, photohalogenation, nitrozation, pharmaceutical synthesis (e.g. synthesis of vitamin D) and photolethography (photo-resistant technology) for micro-electronics industries etc. have led to the development in the area of Photochemistry. A few examples of such photochemical reactions are the conversion of oxygen into ozone, the decomposition of hydrochloric acid and ammonia and the photosynthesis of carbohydrates in plants. In all these reactions, a part of the radiant energy supplied to the system is converted into free energy of the products. Moreover, these reactions are important because these reactions are several times faster than thermal reactions. Reactions as fast as 10-9S and the associated processes as fast as 10-15S are often observed.


Thermochemical reactions take place either by the absorption or by the evolution of heat energy and are referred to as endothermic and exothermic reactions. Some examples of these reactions are discussed in the following sections

Endothermic reactions: The reactions take place with the absorption of heat energy, i.e., enthalpy change (H) in their case is positive. For example, the following chemical reactions proceed with the absorption of heat energy.

1. H2 (g) + I2 (g)→ 2 HI (g), ∆H = 49.7 kJ
2. N2 (g) + O2 (g) → 2 NO (g), ∆H = 180.5kJ
3. C (s) + 2 S (s)→CS2 (l), ∆H = 92.00 kJ

Exothermic reactions: These reactions take place with the evolution of heat energy, i.e., enthalpy change (∆H) in their case is negative. For example, combustion reactions proceed with the evolution of heat energy.

1. C (s) + O2 (g) → CO2 (g), ∆H = –393.5 kJ
2. H2 (g) + ½ O2 (g)→ H2O (I), ∆H = –285.9 kJ
3. CH4 (g) + 2 O2 (g)→CO2 (g), + 2 H2O (I), ∆H = –890.3 kJ
Ordinary chemical reactions are referred to as thermal or dark reaction in order to distinguish them from photochemical reactions.Photochemical reactions: These take place only after the absorption of light radiations from the visible and UV regions of the spectrum. Some typical photochemical reactions include:

(a) Dissociation, e.g., 2 HBr→ H2 + Br2
(b) Rearrangement, e.g., Fumaric acid→ Maleic acid
(c) Addition reactions, e.g., Br2 + (C6H5)2C = C (C6H5)2→ (C6H5)2 C Br – C Br (C6H5)2
(d) Polymerization, e.g., nC2H2→ (C2H2)n (polyacetylene)
(e) Photocatalytic reactions, e.g., CO2 + H2O[chlorophyll](CH2On)+O2+[chlorophyll]
(f) Combination, e.g., H2 + Cl2→ 2 HCl
(g) Decomposition, e.g., 2 O3→ 3 O2
(h) Substitution, e.g., C6H12 + Br2→ C6H11Br + HBr; etc.

Light has to be absorbed by the reactants.
If coloured light is used, the reactants may not be initiated by all colours. A photon of red light is less e1nergetic than a photon of blue light. Usually reactions that are initiated by blue light cannot be initiated by red light. Reactions that are initiated by red light can often be initiated by green, yellow, blue and violet lights.
The rate of reaction depends on the intensity of light.
bIn some cases, the molecule that absorbs light may transfer its extra energy to another molecule which may undergo a reaction. This process is called photosensitization.
The main points of differences between these two types of reactions are given in Table( Differences between photochemical and thermochemical reactions.)


Light incident upon a system can be transmitted, refracted, scattered or absorbed. The fraction of incident light absorbed by a system depends on the thickness of the medium that is traversed. The intensity of absorption at a particular wavelength can be determined by passing a monochromatic beam of light through a sample of known thickness and measuring the intensity of transmitted light which is found to decrease. The decrease of intensity of incident light at any given wavelength on passing through a transparent absorbing substance is given by Lambert’s law, according to which,

Figure A beam of intensity I passes through medium of thickness dx, the intensity of the beam is reduced to I – dI The rate at which the intensity decreases with thickness of the medium is proportional to the intensity of the incident light. Mathematically, it may be expressed as

where I is the intensity of light of a particular wavelength (energy per unit area per unit time); dI is the change in light intensity produced by absorption radiation on passing through of a thin layer of thickness dx (Fig. 6.1); α is called absorption coefficient or extinction coefficient. The minus sign is introduced because there is reduction in intensity.

Equation (6.1) can be also written as

Integrating Eq. (6.2) and applying boundary conditions, when x = 0, I = I0 and when x = x, I = I.

Equation (6.3) can also be rewritten as

The equantityα′ = α/2.303 is called absorption coefficient or absortivity of the substance.

(α′ and α both represent the same term i.e. absorption coeffiecient, but the term only differ in their numerical values)

Equation (6.5) can be rewritten as

The factor log I0/I is called optical density or absorbance (A) and I0/I is termed as transmittance (T).

This implies that absortivity is equal to the reciprocal of thickness of that layer in which the intensity of light falls to one tenth of its original value.

Note: If It is the intensity of transmitted light and I0 is the intensity of incident beam of light, then, the intensity of absorbed light, Ia, is given by,

Lambert–Beer’s Law: Later, Beer found that the absorption coefficient (α) was proportional to the concentration (c) of solution and combined it with Lambert’s law, this is known as Lambert–Beer’s law and is stated as

On passing a beam of monochromatic radiation through a solution, the rate of decrease of intensity with the thickness of the solution is proportional to the intensity of incident radiation as well as concentration of the solution.

where e is the absorption or extinction coefficient. Integration of Eq. (6.10) gives

Substituting these boundary conditions, we get

Equation (6.14) is one of the well-known forms of Lambert–Beer’s law, changing natural logarithm in Eq. (6.13) to the logarithm to the base 10, we get

The quantity log I0/I is known as the absorbance (A) and E is called the molar absorption coefficient.

The Eq. (6.17) tells that the absorbance A is directly proportional to the concentration of the solution and the length of the path. The length of the path can be kept constant and hence absorbance measurement can be used for measuring unknown concentrations of the coloured solutions. The molar absorption coefficient depends on temperature, T, at which the solution is kept and the wavelength, λ, of the incident light.

From Eq. (6.17),

If C = 1 and logI0/I=1 then 1 = E x

This gives us the definition of molar absorption coefficient. Thus, molar absorption coefficient is the reciprocal of that thickness of the solution layer of one molar solution which decreases the intensity of light to one-tenth of its original value.

Relation Between Transmittance and Absorbance of Solution

The absorbance of solution is denoted by A and is given as

Limitations of Lambert–Beer’s Law

The law is not obeyed if radiation used is not monochromatic.

The temperature of the solution should not be allowed to increase, as the absorption band in UV–visible spectra will shift towards longer wavelength.

The law is applicable for dilute solutions only; as in concentrated solution, the refractive index of the solution changes and high concentration of solute shows strong interionicinteractions which can disturb the ability of a solute to absorb a given wavelength of the incident radiation.


Grotthus–Drapper Principle of Photochemical Activation: (First Law of Photochemistry)

Grotthus–Drapper principle states that only the light that is absorbed by a substance is effective in producing a photochemical change. This is known as the first principle of photochemistry.

If I0 is the incident light, It is the transmitted light and Ia is the absorbed light, then

Alternatively, according to Grotthus and Draper,

When light falls on a body, only that fraction of incident light which is absorbed can bring about a chemical change. This implies that reflected and transmitted light do not produce any chemical change.

It is important to note that it is not necessary that all the light which is absorbed will bring about chemical change. It is possible that atoms and molecules may absorb light and then they may re-emit in the form of line or band spectrum of same or different frequency. Sometimes, the light absorbed may bring about phenomena such as fluorescence and phosphorescence. The absorbed light can also be converted into thermal energy.

This law is a qualitative one and does not give any correlation between the amount of light absorbed by a system and the number of molecules which have reacted.

Stark–Einstein’s Law of Photochemical Equivalence — The Second Law of Photochemistry

This law states that each molecule which takes part in the photochemical reaction absorbs one quantum of light energy which induces the reaction. Thus, one molecule absorbs the one quantum to excite one molecule, e.g. A + hv → A*.

The asterisk (*) here means that the molecule A* is in the excited or activited state after absorption of one quantum of energy by the molecule A in the normal state.

The energy acquired by a single molecule (or atom) in absorbing one quantum is dependent on the wavelength (or frequency) of irradiating light. A mole of photons is frequently referred to as an einstein.

The energy (E) absorbed by one mole of the reacting substance is thus given by

where NA = Avogadro number and h = Plank’s constant and v is the frequency of the radiation absorbed.


where C is velocity of light and λ is the wavelength.

Therefore, the energy possessed by one mole of photons or the energy absorbed by one mole of the reacting species is given by,

This is also known as an ‘einstein’ (one einstein (Nhv) is the energy absorbed by one mole of the reacting substance).

However, when the wavelength is expressed in nanometre,

However, in terms of C.G.S. units,

where λ is expressed in Å

It is clear from Eq. (6.27) that the energy per einstein is inversely proportional to the wavelength. Therefore, the energies per einstein of X-rays radiations will be larger than that of UV or visible radiations, since the wavelengths associated with UV or visible radiation are larger than that of X-rays. It may be noted that the shorter the wavelength is, the greater is the energy per einstein. For example, the order of wavelength of some radiations is

X-rays < UV < Visible < Infrared

Therefore, the order of energies per einstein of these radiations would be:

X-rays > UV > Visible > Infrared

The energy, E, can also be expressed in electron volt (eV) using the relation,

1 eV = 1.6022 × 10–19 J

The photochemical equivalence law applies only to the absorption or primary photochemical process. When as a result of primary absorption only one molecule decomposes and the products enter no further reaction, the number of molecules reacting will be equal to the number of energy quanta absorbed. More frequently, however, a molecule activated photochemically initiates a sequence of thermal reactions called secondary reactions, as a result of which several or many reactant molecules will undergo chemical change. However, in certain processes, less than one molecule may react per quantum. The deviations from the law are due to secondary reaction, which may lead to an increase or decrease in the quantum yield from unity.


Quantum efficiency or quantum yield may be defined as the number of moles reacting per einstein of the light absorbed. It is denoted by and is expressed as ‘quantum efficiency or quantum yield may be defined as the number of moles reacting per einstein of the light absorbed’. It is denoted by ø and is expressed as:

If we measure the rate of formation of substance A in mole per second, dNA/dt, the quantum yield øA is given by

The law of photochemical equivalence is applicable only to primary photochemical process. It should be borne in mind that primary processes which are a direct result of the absorption of light, in many cases, are marked by secondary reactions. Some of the secondary reactions occur in long chains and deviations observed with respect to the applicability of the law are many.

The quantity of radiation absorbed by a system can be determined experimentally. It is usually done by measuring the intensity of radiation before and after passing through the reacting system. If the rate of chemical change also is determined by suitable method, it is possible to calculate the amount of chemical change produced by the absorption of a definite amount of radiation. In other words, the number of molecules reacted by the absorption of one quantum of light, i.e., quantum efficiency, can be calculated.

The quantum yields of some well-studied photochemical reactions are given in Table 6.2.

Explanation of the Unexpected Behaviour

In many cases, the quantum yield is higher and in some cases, the quantum yield is lower than what may be expected from the law. The higher yield may be explained by the supposition that the quantum of energy absorbed in the primary process may be more than required to bring about the primary reaction and the excess may be passed on to many other reactants.

The lower quantum yield, on the other hand, is supposed to be due to the following reasons:

The recombination of some of the molecules of the reactants.

The deactivation of the excited molecules before they react.

The insufficiency of the absorbed quantum to activate some of the reacting molecules.

The loss of excitation energy of excited molecules in collision with non-excited molecules. The law of photochemical equivalence is strictly true only in case of a few reactions.

Table 6.2 Effective frequencies of radiations for a few common photochemical reactions and their quantum yields.

Classification of Photochemical Reactions (Based on Their Quantum Efficiencies)

According to quantum yield, photochemical reactions may be classified into the following three categories:

Those reactions in which ø (quantum yield) is a simple integer such as 1, 2, 3 etc. For example, the dissociation of HI and HBr, the ozonization of oxygen and the combination of SO2 and Cl2.

Those reactions in which ø (quantum yield) is extremely low (less than 1). For example, the dissociation of acetone vapour, H2O2, NH3 etc.

Those reactions in which ø (quantum yield) is extremely high. For example, the combination of hydrogen and chlorine, carbon monoxide and chlorine, etc.

In order to explain these deviations from the law of photochemical equivalence, it is believed that photochemical reactions involve two distinct processes.

Primary processes are those in which light radiation is absorbed by atoms or molecules of reactants to give excited atoms or molecules or to cause dissociation of molecules yielding atoms (some in the excited state) or free radicals.

It may be pointed out that the law of photochemical equivalence is applicable only to the primary processes in photochemical reaction.

Secondary processesinvolve the excited atoms, molecules or free radicals produced in the primary process. They may take place in dark also and irrespective of the light radiation. The light radiations are essential only for primary processes.


The mechanism and rate law of some photochemical reactions are explained in the following sections.

(A) Kinetics of the photosynthesis of HBr: The quantum efficiency of photosynthesis of HBr from H2 and Br2 is very low, i.e., 0.01. The following chain mechanism has been suggested.

In this photochemical reaction, chain is initiated in the presence of light (Step 1) and this is also a primary process. Steps 2–5 are secondary processes. Step 2 is endothermic, i.e., this reaction takes place to a small extent. The Step 3 depends on the formation of H atoms (Step 2); therefore, it also takes place to a very small extent. In Step 4, HBr is not formed, rather is consumed. Therefore, the quantum yield of this reaction is very low (i.e., 0.01). However, the quantum yield of this reaction increases with the increase in temperature, as Step 2, which is endothermic, takes place more efficiently at higher temperature and as a result Step 3 also occurs to a greater extent and hence result in more formation of HBr and thereby causing high quantum efficiency for this reaction.

The rate of formation of HBr is given by

Applying steady-state condition to H and Br atom (i.e., when the concentration of the short-lived intermediate is very low suggested that the rate at which reactive intermediates are formed is equal to their rate of being used in the reaction).

Adding Eqs (6.31) and (6.32), we get

From Eq. (6.31), we get

Substituting the value of [Br]

Putting the values of [H] and [Br] in Eq. (6.30), we get

T when T = 0 ⇒ [HBr] = 0

On solving the above equation, we get

This expression derived, based on the mechanism proposed, agrees with the experimental expression for the rate of formation of HBr found by Bodenstein and Lutkemeyer.

(B) Kinetics of the photosynthesis of HCl: In this reaction, as many as 106 molecules of HCl are formed when one quanta of light is absorbed. Thus, the quantum yield following chain mechanism has been proposed.

Unlike H2–Br2 reaction, Step 2 in which Cl combines with H2 to form HCl and H is an exothermic step, i.e. even at room temperature, this reaction occurs to a large extent and thus Step 3 is also carried out efficiently because of the large availability of H atom. This explains extremely large quantum yield for this reaction as compared to H2–Br2 reaction. The rate of formation of HCl is given by

Applying steady-state condition to [H] and [Cl] atom, which are short-lived intermediates,

Rate of formation of [H] or [Cl] = Rate of disappearance of [H] or [Cl]

Bodenstein and Unger also experimentally found that the rate of formation of HCl from H2 and Cl2 is also given as k Iabs [H2]. There is a strong support in favour of chain mechanism:

The combination of hydrogen and chlorine can be initiated in the absence of radiations by introducing either chlorine atom or hydrogen atom into the gas.

The quantum yield of this reaction can be decreased to a large extent by working in capillary tubes, since the chains are terminated at the walls. The capillary tubes provide large areas of walls and hence support chain mechanism for H2–Cl2 reaction.

In the presence of even small amounts of oxygen, the quantum efficiency is lowered. O2 has inhibiting effect. The various steps, in the presence of O2, can be depicted as in the following table.

Here, X is any substance which removes oxygen. Steps 4 and 5, which occur in the presence of O2, remove H and Cl and hence decrease the rate of formation of HCl.


As discussed earlier, when a molecule is excited by light, it can lead to a chemical reaction or energy transfer by collision. However, if both fluorescence and phosphorescence do not occur, then the molecule will return to the ground state with the release of energy which is either a short-lived emission, known as fluorescence, or a long-lived emission, known as phosphorescence.


The absorbed photoenergy sends electron from lower energy level to higher energy level and the excited molecule immediately or instantaneously re-radiates or emits a part of the absorbed energy at a greater wavelength, as the electron returns to or reverts back to its normal state within a short time (=10–8s). ‘Florescence’ is instantaneous and starts immediately after the absorption of light and stops the moment when the radiation is cut off.

Examples of fluorescence are solutions of fluorescein and eosin. When their solutions are placed in light, they show fluorescence from green to violet colour; Uranylsulphate, chlorophyll (green pigment of photosynthetic organisms such as plants and algae), fluorspar (CaF2) and petroleum also show this phenomenon. It may be pointed that different substances show the phenomenon of fluorescence in the light of different wavelengths. Thus, fluorspar shows fluorescence with blue light, chlorophyll with red light, uranium glass with green light and so on.

The phenomenon of fluorescence finds wider application such as:

Fluorescence is used for lighting purposes in fluorescent tubes. Mercury arc producing large proportion of UV light is used in a tube coated with fluorescent salts which gives visible light; the intensity of which is not far different from day light.

By mixing fluorescent dyes with coloured paints, the fluorescence of the dye helps the light reflected by the paint to produce extraordinary brightness and lustre. These materials are used as road signs. For example, Brilliant SulphoFlavine FF and Rhodamine 6G dispersed in a special kind of plastic are used for this purpose.

It is used in the industry for testing and identifying materials, e.g., rubber industry.

In analysis, the concentration of riboflavin (vitamin B2) in chloroform has been examined.

In television, the cathode stream of the photoelectric effect is made visible in the cathode ray tube by adding ZnS to which a little Ni is added to cut off phosphorescence which otherwise makes the picture blurred.

The use of fluorescent microscopes and fluoroscope used in X-ray diagnosis help in testing the condition of food stuff, detecting ring wormsetc.A fluorescent dye is used as whitener for washing cloth when mixed with washing powder.


In this process, the absorbed light energy raises the electron to a higher level and the excited electron reverts back to the original state not instantaneously but after some time lag and consequently absorbed light is re-radiated or emitted after some time. Thus, phosphorescence may be regarded as slow fluorescence. In this phenomenon, the emission of light of different wavelengths continues even after the source of light radiation has been cut off.

The familiar examples of phosphorescence are BaS or SrS containing about 2.5% alkali chlorides (NaCl or KCl) and a trace of heavy metal sulphide. Such a mixture is generally used for painting watch dials, electric switches etc.

Many dyestuffs which are fluorescent in aqueous solution become phosphorescent when dissolved in glycerol and cooled.

Photophysical Process—Consequence of Light Absorption(Jablonski Diagram)

In order to understand the mechanism of these photophysical processes, namely fluorescence and phosphorescence, we have to learn certain terms such as spin multiplicity and singlet and triplet states. In the ground state, most of the molecules having an even number of electrons have paired spin. The quantity (2S + 1) is known as spin multiplicity of a state, where 8 is the total electron spin. In the ground state, the two paired electrons (↑↓) have their spin in the opposite direction and the total spin (S) is taken as zero. This can be represented as

Hence, 2S + 1 = 1 and the molecule is said to have a singlet ground state Fig. 6.2(a). The spin multiplicity of the molecule in the ground state is 1.

When a photon absorbs suitable energy (hv), one of the paired electrons goes to higher energy level and the molecule is said to be in the excited state. The spin orientations of the two single electrons may be either parallel or anti-parallel and they are represented in Fig. 6.2(b) and 6.2(c), respectively.

When the spins are parallel, as in Fig. 6.2(b), then

The spin multiplicity of the molecule is 3 and the molecule is said to have the triplet excited state. When the spins are anti-parallel, as in Fig. 6.2(c), then

The spin multiplicity of the molecule is 1 and the molecule is said to have the singlet excited state. Depending upon the energy of the photon absorbed, the electron may jump to any of the higher electronic states 2, 3, 4…. Thus, we may get a series of singlet excited states represented as S1, S2, S3 etc. and triplet excited states represented as T1, T2, T3 etc. According to quantum mechanics, a singlet excited state has higher energy than the corresponding triplet excited state. The energy sequence is as stated in the following equation:

Parallel and anti-parallel spin orientations

When light is absorbed, the electron of the absorbing molecule jumps from S0 to S1, S2 or S3 singlet excited state depending upon the amount of photons absorbed as shown in Jablonski diagram (Fig. 6.3). For each singlet state (S1, S2, S3 etc.), there is a corresponding triplet excited state (T1, T2, T3 etc.). Whatever may be the excited state, singlet or triplet, the molecule of the substance in the ground state (A0) is said to be in the excited state (A). Thus

The activated molecule (A*) returns to the ground state by dissipating its energy through the following types of transitions.


Non-radiative Transition

These transitions include the return of the excited electron within the energy levels of same multiplicity, i.e., either from S3, S2 to the first excited S1 or from T3, T2 to the first excited state T1. These transitions do not involve the emission of any radiations and are thus referred to as non-radiative or radiationless transitions. The energy of the activated molecule is dissipated in the form of heat through molecules collisions. The process is called internal conversion (IC) and takes place in less than about 10–11 s. The activated molecule may also lose energy by another process called intersystem crossing (ISC). This process involves transitions between states (energy levels) of different multiplicity, for instance from S2 to T2 or S1 to T1, which are also non-radiative or radiationless. Such transitions are spectroscopically forbidden, but they do take place at a slower rate comparatively.

Radiative Transitions

These transitions include the return of activated molecule within the singlet state directly from S3, S2, S1 to S0 and also indirectly from singlet excited triplet state (T1) to the ground state (S0). Such transitions are accompanied by the emission of the radiation. These transitions from S1 to S0 are spectroscopically allowed and take place in about 10–8 s. This type of emissions of radiation is called fluorescence.

The transitions from T1 to the ground state S0 are forbidden spectroscopically and take place rather slowly. The lifetime is much longer being of the order of 10–3 s or greater. It is because the transitions involve the inversion of spin which requires time. The emission of radiation from triplet excited state to the ground state, i.e. from T1 to S0, is called phosphorescence.

Both the emitted radiations (fluorescence and phosphorescence) have shorter frequency (greater wavelength) when compared to the absorbed light used for excitation. This is clear because some part of absorbed light energy is dissipated as heat energy due to non-radiative transitions.

Photochemical Reaction

The molecule after activation may lose energy by undergoing chemical reaction with other molecules. The activated molecule in the singlet excited state does not get a chance to react chemically, since they return quickly to the ground state. However, molecules in the triplet excited state return to get a chance to undergo chemical reaction. Therefore, it is necessary for a molecule to be in the activated triplet excited state before it can undergo a chemical reaction.

Mechanism of Fluorescence and Phosphorescence

When a molecule absorbs a photon, it gets excited and after excitation, it can end up in any of vibration level/levels in the first (or second) excited states and these excited states are unstable; the molecule can directly return to ground electronic state with the emission of light which stays for 10–9–10–6s. This phenomenon is referred to as fluorescence. When the excited molecule radiates, the frequency is lower than that of the exciting radiation because the energy gap between the levels is smaller.

Since the vibrationally excited molecules in the ground state can undergo collision with other molecules, they lose energy and return to the zero point level. Fluorescence emission spectrum is the mirror image of the absorption spectrum (Fig. 6.3).

As phosphorescence involves a triplet state, the activated molecules in the singlet excited state must undergo intersystem conversion (ISC), involving loss of energy. Under these conditions, the transition between the states with different multiplicities will take place, although this is forbidden spectroscopically. The triplet state has a long lifetime and hence phosphorescence radiation is emitted slowly. The energy of a triplet state is lower than that of exited singlet state because in the triplet state, the electrons having the same spin tend to avoid each other and as the electrons are at a large distance and hence there is smaller repulsion and thus the energy is lower in the triplet state. That is why the phosphorescence occurs at frequencies lower than fluorescence.

Difference Between Fluorescence and Phosphorescence

Phosphorescence has much longer decay period (10–4 to 100 s) than fluorescence (10–6 to 10–9 s).

Phosphorescence spectrum is not the mirror image of the absorption spectrum whereas fluorescence spectrum is the mirror image of the absorption spectrum.

Phosphorescence is the radiation emitted in a transition between states of different multiplicity whereas fluorescence is the radiation emitted in a transition between states of same multiplicity.

Phosphorescence is not observed in solutions at room temperature whereas fluorescence can be observed in solutions at room temperature.

Phosphorescence is rarely observed in gases. Fluorescence is exhibited by some elements in the vapour state, e.g., sodium, iodine and mercury vapours.



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