Question

Define with your words a process of excitation of a material by means of radiation, also explain step by step the transition from the initial state to the excited state. What main characteristic can be observed of this excitation process?

Answer 1

Luminescence is the process of emission of electromagnetic radiation from a material upon excitation. Based upon the means of excitation there are various types of luminescence like (a) Photoluminescence produced by absorption of
light or photons, (b) radioluminescence produced by the impact of higher energy particles from cosmic rays or radioactive matter, high energy particles from accelerators etc., (c) electroluminescence produced by the application of
an electric field, (d) mechano or triboluminescence produced by the application of mechanical force, (e) cathodoluminescence produced by cathode rays or electrons, (t) chemiluminescence produced by chemical process, (g) sonoluminescence produced by high frequency sound waves or phonons and bioluminescence produced by biological processes(''. Thermoluminescence, which should actually be called thermally stimulated luminescence, however is different in mechanism. It refers to emission during the warming up of a material previously excited in some fashion. In optically stimulated luminescence stimulation is by electromagnetic radiation of wavelength higher
than that of exciting radiation. In all these cases luminescent material may be considered as a transformer of energy. Even though luminescence is observed in all forms of matter, the main applications involved are in solid luminescent
materials.

A luminescent material is often called a phosphor, which means 'light bearer'. These are materials, which absorb radiant energy of a certain wavelength range and reradiate at longer wavelengths. These phosphors are mainly inorganic materials, which are prepared by proper heat treatment i.e. by high temperature solid-state reactions. Comparatively few pure solids like silicates, sulphides selenides and oxides of Ca, Zn, Ba. Sr and Hg, luminesce efficiently at room temperature.

Singlet and Triplet Excited State

Understanding the difference between fluorescence and phosphorescence requires the knowledge of electron spin and the differences between singlet and triplet states. The Pauli Exclusion principle states that two electrons in an atom cannot have the same four quantum numbers (nn, ll, mlml, msms) and only two electrons can occupy each orbital where they must have opposite spin states. These opposite spin states are called spin pairing. Because of this spin pairing, most molecules do not exhibit a magnetic field and are diamagnetic. In diamagnetic molecules, electrons are not attracted or repelled by the static electric field. Free radicals are paramagnetic because they contain unpaired electrons have magnetic moments that are attracted to the magnetic field.

Singlet state is defined when all the electron spins are paired in the molecular electronic state and the electronic energy levels do not split when the molecule is exposed into a magnetic field. A doublet state occurs when there is an unpaired electron that gives two possible orientations when exposed in a magnetic field and imparts different energy to the system. A singlet or a triplet can form when one electron is excited to a higher energy level. In an excited singlet state, the electron is promoted in the same spin orientation as it was in the ground state (paired). In a triplet excited stated, the electron that is promoted has the same spin orientation (parallel) to the other unpaired electron. The difference between the spins of ground singlet, excited singlet, and excited triplet is shown in Figure 11. Singlet, doublet and triplet is derived using the equation for multiplicity, 2S+1, where S is the total spin angular momentum (sum of all the electron spins). Individual spins are denoted as spin up (s = +1/2) or spin down (s = -1/2). If we were to calculated the S for the excited singlet state, the equation would be 2(+1/2 + -1/2)+1 = 2(0)+1 = 1, therefore making the center orbital in the figure a singlet state. If the spin multiplicity for the excited triplet state was calculated, we obtain 2(+1/2 + +1/2)+1 = 2(1)+1 =3, which gives a triplet state as expected.

The difference between a molecule in the ground and excited state is that the electrons is diamagnetic in the ground state and paramagnetic in the triplet state.This difference in spin state makes the transition from singlet to triplet (or triplet to singlet) more improbable than the singlet-to-singlet transitions. This singlet to triplet (or reverse) transition involves a change in electronic state. For this reason, the lifetime of the triplet state is longer the singlet state by approximately 104 seconds fold difference.The radiation that induced the transition from ground to excited triplet state has a low probability of occurring, thus their absorption bands are less intense than singlet-singlet state absorption. The excited triplet state can be populated from the excited singlet state of certain molecules which results in phosphorescence.