PHOTOLUMINESCENCE
SPECTROSCOPY
AWNISH KUMAR TRIPATHI
What is luminescence?
 Luminescence is a general term for the emission of light from a cool
object. (In contrast to, e.g., incandescence – the emission of light from a
hot piece of metals such as the filament in a light bulb.) There are many
examples of naturally occurring luminescence. Most of these fall into one
of 3 categories:
 Phosphorescence: Absorption and slow re-emission of light. Most
commonly observed in minerals.
 Fluorescence: Absorption and fast re-emission of light. Naturally seen
in deep sea organisms and some insects and plants.
 Chemiluminescence: Emission of light driven by a chemical reaction.
The most common form of natural luminescence (often called
“bioluminescence”). Seen in aquatic organisms, insects and plants.
Phosphorescence
• Certain minerals glow in the dark when exposed to ultraviolet (UV)
light. Some of them continue to glow even after the ultraviolet light
is turned off. This “slow” re-emission of light is known as
phosphorescence.
• The first phosphorescent mineral was reported in the early 1600s.
These minerals, or “inorganic phosphors” have many interesting
uses – for example, they are used in making the cathode ray tubes
still used in older color televisions.
Fluorescence
• Like phosphorescence, fluorescence involves absorbing and reemitting light. However, fluorescence is very fast, and disappears as
soon as the source of light to removed.
• An interesting example of natural fluorescence is the exoskeleton of
scorpions – scorpions glow under UV light! For people who like to
collect scorpions, it makes them easy to find at night with a portable
UV lamp. Also the SIGNEAGE glow
Chemi-luminescence
• Chemiluminescence is the generation of light by the
chemical reactions. This is the most common form of
luminescence in living organisms such as fireflies etc,
deep sea fish like the Anglerfish use it to lure prey close
to their mouth and microorganisms use it to signal
distress.
About Photoluminescence
The phenomenon of temporary light absorption
and subsequent light emission is called
Photoluminescence.
The Jablonski Diagram
Different time scales of
phenomena
•
Absorption (10-15 s)
•
Internal conversion (10-12 s)
•
Intersystem crossing (heavy-atom
enhanced)
•
Quenching by external quenchers
(diffusion-controlled)
•
Fluorescence (10-9 s)
•
Phosphorescence/Inorganic
luminescence (10-5 - 101 s).
Kasha’s Rule: Photoluminescence always from S1 (fluorescence)
or T1 (phosphorescence)
MOLECULES: electronic, vibrational and rotational transitions
are possible (STOKE’s Shift)
S1
A
Phosphorence
T1
A
A*
B
A*
B
B
Emission
Absorption
S0
B


STOKE’s Shift: wavelength (energy) shift of emission peak w.r.t. absorption peak.
Building blocks of carbon-based conjugated Organic Materials
Three types of structures;
– Aliphatic Conjugated segments: acetylenes (PAs)
– Aromatic Conjugated segments: para-phenylenes, para-phenylene-vinylenes
etc.
– Heterocyclic Conjugated segments: pyrroles, thiophenes etc.
N
S
1. trans-PA exhibits identical
ground state energy in A and Bphase.
2. trans-PA is degenerate ground
state conjugated polymer.
E
A-phase PA
LUMO
soliton state
HOMO
A-phase
B-phase
B-phase PA
A-phase + B-phase
Positive soliton
Generation methods
1. Chemical doping
2. Photo-generation
3. Charge injection
emergence of soliton
state in the Band-gap
Dimerization of PA
Neutral soliton
Negative soliton
• Most conjugated
polymers have nondegenerate ground
states.
• EAromativc < EQuinoid .
E
Aromatic
Quinoid
Aromatic
polaron states
Quinoid
Positive polaron
Positive bipolaron
Spin
configuration
Optical
transitions
Negative polaron
Spin
configuration
Optical
transitions
Spin
configuration
Optical
transitions
Negative bipolaron
Spin
configuration
Optical
transitions
Exciton
Singlet
exciton
Triplet
exciton
Important Characteristics of Photoluminescence (PL)
• Each substance (solid, fluid and plasma) has its own
excitation and emission spectra.
• The PL intensity is proportional to sample absorption, the
intensity of excitation source and the lifetime of the
recombining species.
• The Photoluminescence emission of a substance is always
at a higher wavelength than the light used to excite the
substance (Stokes-shift phenomenon).
• The Photoluminescence of a substance is sensitive to
many factors and may change or even disappear under
different conditions.
Advantage of Using Photoluminescence Technique
as Research Tool
• It is very sensitive, fast, safe and cost effective.
• This requires very small amount of substance ,of the
given sample, for the measurement.
• It allows to measure very small changes.
• It allows to track conformational changes of the
molecular systems.
• It is widely used in almost every research fields.
What Can be Measured of a Photoluminescence
Substance
•
Photoluminescence intensity
•
Excitation spectra
•
Emission spectra
•
Photoluminescence lifetime
•
Anisotropy & Polarization
•
Time-based Photoluminescence intensity
•
Time-based Photoluminescence lifetime
•
PL Quantum efficiency.
Components for building the PHOTOLUMINESCENCE (PL) setup
Computer
Components
• Light Sources:

Broad band
source


CW Laser
Hg or Xe arc lamp (rich UV
radiation).
Tungsten-halogen lamp (for
visible
and
infrared
excitation).
CW Laser (as an intense
excitation monochromatic
source).
Optical chopper
• Optical chopper: for chopping the incoming
light beam. Chopping frequency should not
be multiple of power-line frequency.
• Integrating
sphere: to
obtain integrated no. of
photons generated during
the processes.
Integrating
sphere
Integrating sphere
Integrating sphere is a versatile tool
used in many optical
measurements.
Its function is to angularly and
spatially integrate the incoming
radiation.
In pratice, it acts as a diffuser and
an attenuator
It consists of input and output ports
and reflective cavity coating
(magnesium oxide. Barium sulfate,
Polytetrafluoroethylene (PTFE)
etc.).
•
http://www.labsphere.com/uploads/technical-guides/a-guide-to-integrating-sphere-theory-andapplications.pdf
•
http://www.light-measurement.com/ideal-integrating-sphere/
•
http://www.newport.com/Flange-Mount-Integrating-Spheres/378467/1033/info.aspx
Integrating sphere (2)
• The operational principle is simple;
• highly reflective and diffusive coating causes multiple reflections inside
the sphere and eventually some part of it end up at the active area of the
photodiode.
Integrating sphere (3)
Used in power measurements to attenuate (and
integrate) signal.
→ cheaper detectors can be ussed when power is lower!
Comes in many sizes.
Application determines the size.
• Neutral Density (ND) filters: to
reduce the intensity of the
excitation source (whenever
required).
Optical filters
Monochromator
•
Monochromators / Filters: needed to select both wavelength of
the excitation and emission. Monochromators are generally used
when dealing with narrow absorption or emission peaks while
filters may be used when peaks are not as narrow. When filters
are used, one is limited to wavelength range that passes through
the particular filter used.
– Instruments using filters are called fluorometers.
– Instrument
using
monochromators
are
called
spectrofluorimeter.
PMT
• Detectors: Photo-detectors
or photomultiplier Tubes
(small signals).
Preamplifier
• Pre-amplifier: to amplify the PMT
signal and to convert the PMT
current into voltage.
Lock-in amplifier
• Lock-in Amplifier: to obtain
the signal with high signal to
noise ratio.
A Spectro-fluorometer
Schematic diagram of PL measurement setup
LASER Source
Chopper
Integrating sphere
Filter
(to reject laser beam)
Beam
Expander
PMT
PreAmp
Monochromator
ΔP
1
Reference
input
Ainput
Lock-in Amp
ΔT (), 2
2
Computer
PL Quantum Efficiency Measurement
A
Area under the curve= LA
B
Area under the curve = LB
Area under the PL curve = PB
C
Area under the curve = LC
Area under the PL curve = PC