What is photoluminescence?

Photoluminescence is the emission of light from a material under optical excitation. Absorption of visible or UV radiation raises molecule to an excited state. Electron absorbs quantum of energy and jumps to a higher energy orbital. When electron drops back to the ground state, excitation energy can be liberated by...


What is Fluorescence?

Fluorescence is a luminescence, i.e. optical phenomenon in cold bodies, in which a molecule absorbs a high-energy photon, and re-emits it as a lower-energy (longer-wavelength) photon. The energy difference between the absorbed and emitted photons ends up as molecular vibrations (heat). Usually the absorbed photon is in the ultraviolet, and the emitted light (luminescence) is in the visible range, but this depends on the absorbance curve and Stokes shift of the particular fluorophore. Fluorescence is named after the mineral flurite (calcium fluoride), which exhibits this phenomenon.

Phosphorescence is the result of a radiative (light emitting) transition involving a change in teh spin multiplicity of (in most cases) a molecule from excited state singlet to excited state triplet. This transition is quantum mechanically forbidden as is the transition that leads to light emission. These forbidden transitions are kinetically slow, whcih introduces a delay between photo-excitation (exposure to light of one wavelength) and emission (release of light of a different wavelength). So-called "glow in the dark" materials are phosphorescent materials with a very long (seconds, minutes, even hours) delay between excitation and emission. Most phosphorescent compounds have triplet lifetimes on the order of milliseconds.

Fluorescence vs Phosphorescence 

Usually re-emission of radiation, if it occurs, is within nanoseconds (10^-9 sec)of the excitation= FLUORESCENCE. PHOSPHORESCENCE is a much slower (and rarer) process. After source of excitation radiation is switched off, phosphorescence emission continues for periods that vary from milliseconds to weeks. Phosphorescence is caused by electron becoming transferred into a triplet state. (Electrons of the same spin in the one orbital). Triplet states have long lifetime so phosphoresence persists. Mostly requires low temperatures - at higher T, triplet state is de-activated by quenching.

The Morse curves (see above) display energy transitions for an electron, in a covalent bond A-B, excited by hn, then re-emitting the energy as photoluminescence

• Before excitation, molecules and bonds are mostly in the lowest vibrational level of the ground electronic state. Hence excitation (absorption of hn) begins from lowest level.
  But - for quantum mechanical reasons - excited electron enters a higher vibrational level in the excited state.
• In the excited state, the excess vibrational energy is rapidly lost.. So de-excitation (with emission of hn) takes place from the lowest vibrational level of excited state.
  But electron dropping back to the ground state re-enters it first at a higher vibrational level before losing the excess vibrational energy.

1. How intensity of light source varies with wavelength
2. How response of photomultiplier detector varies with wavelength

What can we say about Radioactivity?

Emission of energy from an atom's nucleus in the course of its natural or artificial disintegration. Energy is emitted in the form of alpha, beta, and gamma particles (X rays).

What is Excitation wavelength?

What is of phosphor? 

A phosphor is a substance that exhibits the phenomenon of phosphorescence (sustained glowing without further stimulus). The chemical element phosphorus (Greek. Phosphoros, meaning "light bearer") was discovered by German alchemist Hennig Brand in 1669 through a preparation from urine. Working in Hamburg, Brand attempted to distill salts by evaporating urine, and in the process produced a white material that glowed in teh dark. Since that time, the term phosphorescence has been used to describe substances that shine in the dark without burning. Phosphorus itself is NOT a phosphor; it is highly reactive and gives-off a faint glow upon uniting with oxygen. The glow observed by Brand was actually caused by the very slow burning of the phosphorus, but as he saw no flame nor felt any heat he did not recognize it as burning. 

Phosphors are transition metal compounds or rare earth compounds of various types. The most common uses of phosphors are in CRT displays and fluorescent lights. CRT phosphors were standardized beginning around World War II and designated by the letter "P" followed by a number.