Chapter – 8 Fluorescence Raditionless Transitions Radiation Transitions Fluorescence

Fluorescence Sir George Gabriel Stokes, 1st Baronet Mathematician and Physicist. Worked at Cambridge. In 1852, in his famous paper on the change of Wavelength of light, he described the phenomenon of Fluorescence, as exhibited by Fluorspar and Uranium Glass. Stokes Shift Low-intensity lighting and haze in a concert hall allows laser effects to be visible Fluorescent minerals

Raditionless Transitions

How to measure fluorescence lifetime ? A streak camera (ultra fast high speed detector) is an instrument for measuring the variation in a pulse of light's intensity with time. Molecules are excited by a very short pulse (close to a d-pulse) at t = 0 and the decay of florescence intensity is measured. Usually by Time Correlated Single Photon Counting (TCSPC).

Excited-State Lifetimes of Atoms The excited state of an atom will have an intrinsic lifetime due to radiative decay given by: Nj(t) = Nj(0) exp (-t/)  is the radiative lifetime defined as 1/ Aji. The above expressions give the radiative lifetime Spontaneous or induced emission usually occurs shortly after the system is promoted to the excited state, returning the system to a state with lower energy. This return to a lower energy level is often loosely described as decay and is the inverse of excitation.

Life Time of Excited State

We have used the following expressions nj(t) = nj(t=0) exp(- Aji t) for fluorescence emission Nt = N0 exp ( -  t) for radioactive decay

Host-Guest System

Atomic-Absorption Spectroscopy Light source – Lasers, Xe-Lamp, Halogen lamp Test Sample Detector - Monochromators, Photomultiplier tubes Recorder

Applications Optical Properties of Semiconductor Optical Measurements in semiconductors are useful: To study the band structures of semiconductors To determine the energy bandgap Free carrier Used to study lattice vibrations.

Absorption spectrum of a film of propyl gallate 40 nm thick on a silica substrate Band gap = 1238/ (nm) = 1238/ 340 = 3.64 eV 340 nm Absorption edge

C.B. = Conduction Band

Fluorescence Spectrometer

Light Source : Spectral output over wide range Tungsten halogen lamp, Mercury Lamp, Xenon Lamp Wavelength Selection: Interference Filter , Monochromators Detectors: Photomultiplier Tube Read out Devices: Digital Display Sample Holder: (i) Normal Incidence. Fluorescence given off in directions. Normally collected from the front surface of the cell at Right angle to the incident beam. (ii) in-line with the incident beam