Optical Spectroscopy with dispersive Spectrometers
Basics - Building Blocks - Systems - Applications

This page summarizes chapter 4 of the book
"Applications of Dispersive Optical Spectroscopy Systems",
ISBN 9781628413724, SPIE monographs, Bellingham, WA, USA

Application – L2
Luminescence, dynamic
Lifetime Measurement of
(Photo)Luminescence - Fluorescence – Phosphorescence

This page presents the directory, the signs and symbols, conversions, and equations of the book, while the details are an exclusive part of  the book.

L2.0 Introduction
Photoluminescence phenomena are pump (excitation) and relaxation (emission) processes. All of the excited states will return to the original, unexcited, state, quickly or slowly. So called static luminescence is the equilibration between the two sceneries. While the excitation light constantly pumps some electrons up to a higher state of energy, some others return to the ground state, and emit light. The time span between excitation and the end of the emission has a spread, which is defined by the chemical structure, the chemical environment, and thermal situation of the sample. The emission starts quickly after excitation - but the delay never is zero. While new electrons are pumped up, others already relax and emit light. Not all electrons return from the excited state at the same time, they follow an exponential relaxation function (e-function). The time between any point of the decay curve (like the end of the excitation), and an emission level of 1/e (or 36.8 %) of the reference point is called the LIFETIME. It may reach from the upper pico second range to hours or even days.
the definition of lifetime
Graph L21 demonstrates the normalized decay of every kind of luminescence.
The combination of all effects leads to an overlay of emission curves. A constructed example follows:
Graph L22 multiple overlyed emission curves
Graph L22 displays a selection of several emission curves, and the measured output.
Example how convolution worksDerivative of Decay Curves
Graph L23 displays a constructed example of the output signal

L2.01 A look on the instrumentation available.
There are two basic methods available and offered on the market: Pulsed and modulated systems. Both methods can combine with polarization analysis, and with parallel wavelength detection.
L2.0.2 Analysis of the change in the state of Polarization
L2.1 Pulsed methods
L2.1.1 The Synchrotron

L2.1.3 LED´s
L2.1.1 Synchronized Integration, also called Boxcar Integration, or Pulse/Sample Analysis
time scheme of a  Boxcar integrator
Graph L24 illustrates the signal timing in Boxcar mode.

How does a Boxcar system work?

Block diagram of a Boxcar system
Graph L25 shows a typical Boxcar setup.
L2.1.2 Single Photon Counting

Block diagram of a TCSPC system
Graph L26: schematic of a TCSPC system.

L2.2 Continuous Methods
L2.2.1 Phase Modulation Analysis

 
graph L27: the parametrs of phase/modulation
Graph L27 illustrates the parameters to recalculate the Lifetime from a modulated excitation/emission scheme

The general equations for lifetime spectroscopy by phase/modulation are:
For the calculation of the phase angle, equation F41
F41:  tan
F = w  * tp

For the calculation of the modulation factor, equation F42
F42:  m = [ 1 +
 w2  * t2m ]-1/2 
whereas
F is the resulting phase angle,
w is the circular frequency of modulation,
t is the lifetime,
m is the resulting modulation factor.

The behaviour of the two curves is next shown for a lifetime of 11 ns:
graph L28: resposne curves for 4  different frequencies
Graph L28 demonstrates both the change in phase shift F, and the modulation factor m, as function of frequency.

L2.2.1.1 How does a phase/modulation system work?
graph L29 System schematic of a phase-modulation instrument
Graph L29: the setup of a phase-modulation system.

Graph L30: typical scheme of phase and modulation
Graph L30 is the reproduction of a Phase-Modulation data representation, at a lifetime of 11 ns.
L2.2.2 Multi Harmonic Fourier Transform Systems (MHF)

graph L30  the sum curve of an MHF puls
Graph L31 illustrates a single 4 MHz singe wave (yellow) and the summed curve of the 7 harmonics in binary order (1st to 64th), which are used to excite the sample within 250 ns, not shown is the LF.

L2.3 Methods using parallel Wavelength Detection
L2.3.1 Synchronized CCD Gating

Graph L32: Block diagram of a gated MCP-CCD system
Graph L32 presents a gated, synchronized, MCP/CCD lifetime system.
L2.3.2 Modulated MCP/CCD Analysis
 

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Status April 2012