Dong-Sun Lee / cat - lab / SWU
2010-Fall version
Chapter 25
Instruments for Optical
Spectrometry
Components of Various Types of Instrument for Optical Spectroscopy
1) Absorption measurement
2) Fluorescence measurements
3) Emission spectroscopy
Instrumentation for spectrometry
1. Light sources
continuum source
line spectrum
Spectral source types. The spectrum of a continuum source (a)
is much broader than that of line source (b).
Black Body Radiation
Any object surface can
radiate heat to and
receive heat from
outside, if an object can
absorb all the incident
radiation, regardless of
the frequencies and
directions, this object is
called Black Body. A
ball cavity with a small
hole can be regarded as
a black body, since any
radiation entering the
ball cavity can only
reflect inside it, thus
totally absorbed.
Spectral distribution of blackbody radiation.
Low pressure mercury arc lamp : 253.7 nm Hg line
Hollow cathode lamps : line sources / AA spectrometry
Laser source
H2 + Ee  H2*
H2*  H’ + H’’ + h
Ee = EH2*
= EH’ + EH’’ + h
A deuterium lamp
A tungsten lamp
Intensity of a tungsten
filament at 3200K and a
deuterium arc lamp.
Light sources.
Laser
Laser is the acronym of Light Amplification by Stimulated Emission of
Radiation. A device which produces light with a narrow spectral width. Laser
is light of special properties, light is electromagnetic (EM) wave in visible
range. Lasers, broadly speaking, are devices that generate or amplify light,
just as transistors generate and amplify electronic signals at audio, radio or
microwave frequencies. Here light must be understand broadly, since lasers
have covered radiation at wavelengths ranging from infrared range to
ultraviolet and even soft x-ray range.
A laser is a cavity that has mirrors at the ends and is filled with lasable
material such as crystal, glass, liquid, gas, or dye. These materials must have
atoms, ions, or molecules capable of being excited to a metastable state by
light, electric discharge, or other stimulus. The transition from this metastable
state back to the normal ground state is accompanied by the emission of
photons which form a coherent beam.
Laser construction
A laser system generally consists of three important parts:
- An energy source (usually referred to as the pump or pump source);
- A gain medium or laser medium;
- A mirror, or system of mirrors, forming an optical resonator.
Laser cavity. The electromagnetic wave travels back and forth between the mirrors,
and the wave is amplified with each pass. The output mirror is partially transparent
to allow only a fraction of the beam to pass out of the cavity.
(a) Energy-level diagram illustrating the principle of operation of a laser.
(b) Basic components of a laser. The population inversion is created in the
lasing medium. Pump energy might be derived from intense lamps or an
electric discharge.
Amplification of light
All lasers contain an energized substance that can increase the intensity of light
passing through it. This substance is called the amplifying medium or, sometimes, the
gain medium, and it can be a solid, a liquid or a gas. Whatever its physical form, the
amplifying medium must contain atoms, molecules or ions, a high proportion of which
can store energy that is subsequently released as light.
In a neodymium YAG (Nd:YAG) laser, the amplifying medium is a rod of yttrium
aluminium garnate (YAG) containing ions of the lanthanide metal neodymium (Nd). In
a dye laser, it is a solution of a fluorescent dye in a solvent such as methanol. In a
helium-neon laser, it is a mixture of the gases helium and neon. In a laser diode, it is a
thin layer of semiconductor material sandwiched between other semiconductor layers.
The factor by which the intensity of the light is increased by the amplifying medium is
known as the gain. The gain is not a constant for a particular type of medium. It's
magnitude depends upon the wavelength of the incoming light, the intensity of the
incoming light, the length of the amplifying medium and also upon the extent to which
the amplifying medium has been energized.
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Schematic of a Nd:YAG laser.
Energizing the amplifying medium
Increasing the intensity of a light beam that passes through an amplifying
medium amounts to putting additional energy into the beam. This energy comes
from the amplifying medium which must in turn have energy fed into it in some
way. In laser terminology, the process of energizing the amplifying medium is
known as "pumping".
There are several ways of pumping an amplifying medium. When it is a solid, pumping
is usually achieved by irradiating it with intense light. This light is absorbed by atoms
or ions within the medium raising them into higher energy states. Xenon-filled
flashtubes positioned as shown below are used as a simple source of pumping light.
Passing a high voltage electric discharge through the flashtubes causes them to emit an
intense flash of white light, some of which is absorbed by the amplifying medium. The
assembly of flashtubes is enclosed within a polished metal reflector (not shown in the
diagram below) to concentrate as much light as possible on the amplifying medium. A
laser that is pumped in this way will have a pulsed output.
Pumping an amplifying medium by irradiating it with intense light is referred to as
optical pumping. The source of pumping light can be another laser. Some types of laser
that were originally pumped using xenon-filled flashtubes are now pumped by laser
diodes.
Gaseous amplifying media have to be contained in some form of enclosure or tube and are
often pumped by passing an electric discharge through the medium itself. The mechanism by
which this elevates atoms or molecules in the gas to higher energy states depends upon the gas
that is being excited and is often complex. In many gas lasers, the end windows of the laser tube
are inclined at an angle and they are referred to as brewster windows. Brewster windows are
able to transmit a beam that is polarized in the plane of the diagram without losses due to
reflection. Such a laser would have an output beam that is polarized.
The diagram illustrates pumping by passing a discharge longitudinally through the gaseous
amplifying medium but, in some cases, the discharge takes place transversely from one side of the
medium to the other. Many lasers that are pumped by an electric discharge can produce either a
pulsed output or a continuous output depending upon whether the discharge is pulsed or
continuous.
Various other methods of pumping the amplifying medium in a laser are used. For example, laser
diodes are pumped by passing an electric current across the junction where the two types of
semiconductor within the diode come together.
Creating a Population Inversion
Finding substances in which a population inversion can be set up is central to the
development of new kinds of laser. The first material used was synthetic ruby. Ruby is
crystalline alumina (Al2O3) in which a small fraction of the Al3+ ions have been replaced
by chromium ions, Cr3+. It is the chromium ions that give rise to the characteristic pink or
red color of ruby and it is in these ions that a population inversion is set up in a ruby laser.
In a ruby laser, a rod of ruby is irradiated with the intense flash of light from xenonfilled flashtubes. Light in the green and blue regions of the spectrum is absorbed by
chromium ions, raising the energy of electrons of the ions from the ground state level
to the broad F bands of levels. Electrons in the F bands rapidly undergo non-radiative
transitions to the two metastable E levels. A non-radiative transition does not result in
the emission of light; the energy released in the transition is dissipated as heat in the
ruby crystal. The metastable levels are unusual in that they have a relatively long
lifetime of about 4 milliseconds (4 x 10-3 s), the major decay process being a transition
from the lower level to the ground state. This long lifetime allows a high proportion
(more than a half) of the chromium ions to build up in the metastable levels so that a
population inversion is set up between these levels and the ground state level. This
population inversion is the condition required for stimulated emission to overcome
absorption and so give rise to the amplification of light. In an assembly of chromium
ions in which a population inversion has been set up, some will decay spontaneously
to the ground state level emitting red light of wavelength 694.3 nm in the process. This
light can then interact with other chromium ions that are in the metastable levels
causing them to emit light of the same wavelength by stimulated emission. As each
stimulating photon leads to the emission of two photons, the intensity of the light
emitted will build up quickly. This cascade process in which photons emitted from
excited chromium ions cause stimulated emission from other excited ions is indicated
below:
The ruby laser is often referred to as an example of a three-level system. More than
three energy levels are actually involved but they can be put into three categories.These
are; the lower level form which pumping takes place, the F levels into which the
chromium ions are pumped, and the metastable levels from which stimulated emission
occurs. Other types of laser operate on a four level system and , in general, the
mechanism of amplification differs for different lasing materials. However, in all cases,
it is necessary to set up a population inversion so that stimulated emission occurs more
often than absorption.
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Properties of laser light
Monochromatic : one wavelength
Extremely bright : high power at one wavelength
Collimated : parallel rays
Polarized : electric field of waves oscillates in one plane
Coherent : all waves in phase
Coherence can be devided into spatial and temporal coherence. For any em wave,
if at time t=0 and t0 the phase diference between two points in space remains the
same, we say the em wave has spatial coherence; If at a point P, the em wave at t
and t+dt has same phase difference if dt is the same, temporal coherence exists.
Disadvantages of a laser
High maintenance
Limited wavelengths
Common light sources, such as the electric light bulb emit photons in all directions,
usually over a wide spectrum of wavelengths. Most light sources are also incoherent, i.e.,
there is no fixed phase relationship between the photons emitted by the light source.
By contrast, a laser generally emits photons in a narrow, well-defined beam of light. The
light is often near-monochromatic, consisting of a single wavelength or color, is highly
coherent and is often polarised. Some types of laser, such as dye lasers and vibronic solidstate lasers can produce light over a broad range of wavelengths; this property makes
them suitable for the generation of extremely short pulses of light, on the order of a
femtosecond (10-15 seconds).
Laser light can be highly intense — able to cut steel and other metals. The beam emitted
by a laser often has a very small divergence (i.e. it is highly collimated). A perfectly
collimated beam cannot be created, due to the effect of diffraction, but a laser beam will
spread much less than a beam of light generated by other means. A beam generated by a
small laboratory laser such as a helium-neon (HeNe) laser spreads to approximately 1
mile (1.6 kilometres) in diameter if shone from the Earth's surface to the Moon. Some
lasers, especially semiconductor lasers due to their small size, produce very divergent
beams. However, such a divergent beam can be transformed into a collimated beam by
means of a lens. In contrast, the light from non-laser light sources cannot be collimated.
A laser can also function as an optical amplifier when seeded with light from another
source. The amplified signal can be very similar to the input signal in terms wavelength,
phase and polarisation; this is particularly important in optical communications.
The output of a laser may be a continuous, constant-amplitude output (known as c.w. or
continuous wave), or pulsed, by using the techniques of Q-switching, modelocking or Gainswitching.
The basic physics of lasers centres around the idea of producing a population inversion in a
laser medium. The medium may then amplify light by the process of stimulated emission,
which if the light is fed back through the medium by means of a cavity resonator, will
continue to be amplified into a high-intensity beam. A great deal of quantum mechanics and
thermodynamics theory can be applied to laser action, though in fact many laser types were
discovered by trial and error.
Population inversion is also the concept behind the maser, which is similar in principle to a
laser but works with microwaves. The first maser was built by Charles H. Townes in 1953.
Townes later worked with Arthur L. Schawlow to describe the theory of the laser, or optical
maser as it was then known. The word laser was coined in 1957 by Gordon Gould, who was
also credited with lucrative patent rights in the 1970s, following a protracted legal battle.
The first maser, developed by Townes, was incapable of continuous output. Nikolai Basov
and Alexander Prokhorov of the USSR worked independently on the quantum oscillator and
solved the problem of continuous output systems by using more than two energy levels.
These systems could release stimulated emission without falling to the ground state, thus
maintaining a population inversion. In 1964, Charles Townes, Nikolai Basov and Alexandr
Prokhorov shared a Nobel Prize in Physics "for fundamental work in the field of quantum
electronics, which has led to the construction of oscillators and amplifiers based on the
maser-laser principle."
The first working laser was made by
Theodore H. Maiman in 1960 at Hughes
Research Laboratories in Malibu, California,
beating several research teams including
those of Townes at Columbia University, and
Schawlow at Bell laboratories. Maiman used
a solid-state flashlamp-pumped ruby crystal
to produce red laser light at 694 nanometeres
wavelength.
The verb "to lase" means to give off coherent
light or possibly to cut or otherwise treat with
coherent light, and is a back-formation of the
term laser.
http://www.wordiq.com/definition/Laser
Laser (U.S. Air Force)
Laser types
- Gas Laser
HeNe (543 nm and 633 nm)
Argon(-Ion) (458 nm, 488 nm or 514.5 nm)
Carbon dioxide lasers - used in industry for cutting and welding, up to 100 kW possible
Carbon monoxide lasers - must be cooled, but extremly powerful, up to 500 kW possible
- Excimer(excited dimer or trimer) gas lasers, producing ultraviolet light,
used in semiconductor manufacturing and in LASIK eye surgery;
157 nm (F2)
193 nm (ArF)
222 nm (KrCl)
248 nm (KrF)
308 nm (XeCl)
351 nm (XeF)
- Commonly used laser types for dermatological procedures including
removal of tattoos, birthmarks, and hair:
Ruby (694 nm)
Alexandrite (755 nm)
Pulsed diode array (810 nm)
Nd:YAG (1064 nm)
YAG : yttrium/aluminum garnet
Ho:YAG (2090 nm)
Er:YAG (2940 nm)
- Semiconductor laser diodes,
small: used in laser pointers, laser printers, and CD/DVD players;
bigger: bigger industrial diode laser are available used in the industry for cutting and welding,
up to 10 kW possible
- Dye lasers
- Quantum cascade lasers
- Neodymium-doped YAG lasers (Nd:YAG), a high-power laser operating in the
infrared, used for cutting, welding and marking of metals and other materials;
- Erbium-doped YAG, 1645 nm
- Thulium-doped YAG, 2015 nm
- Holmium-doped YAG, 2090 nm, a high-power laser operating in the infrared, it is
explosively absorbed by water-bearing tissues in sections less than a millimeter thick. It is usually
operated in a pulsed mode, and passed through optic fiber surgical devices to resurface joints,
remove rot from teeth, vaporize cancers, and to pulverize kidney and gall stones.
- Titanium-doped sapphire
- Erbium-doped fiber lasers, a type of laser formed from a specially made optical fiber,
which is used as an amplifier for optical communications.
2. Wavelength selectors (Monochromator)
1) Filter
a. Absorption filter
b. Interference filter
2) Prism
a. Transmission prism = Cornu
b. Reflection prism = Littrow
30
l1 l2
q1
Cornu prism
Dispersion = dq / dl
Approximate transmission limits of prism materials
Flint glass (contains PbO) ; 360nm ~ 2 m m
Quartz(crystalline silica) ; 190nm ~3.3 mm
NaCl or KCl ; 0.3~15 mm
KBr ; 0.3~30 m m
LiF ; 0.2~ 5 m m
CaF2 ; 0.2~12 m m
AgCl ; 0.4~25 m m
CsBr ; 0.3 ~ 50 m m
CsI ; 0.3 ~ 70 m m
KRS-5(TlBr-TlI) ; 1~30 m m
3) Diffraction grating
a. Transmission grating
b. Reflection grating
o
30
l1
q2
l2
o
Aluminized
surface
Littrow prism
Wavelength selectors for spectrometry
Type
Wavelength range
(nm)
Note
Grating
100 ~ 40,000
3000 lines/mm for vacuum UV,
50 lines/mm for far IR
Prism
120 ~ 30,000
Continuously variable
Discontinuous
Interference filter
Absorption filter
200 ~ 14,000
380 ~ 750
Dispersion of radiation along the focal plane AB of a typical prism(a) and
echellette grating (b).
Schematic diagram of diffraction from a grating.
nl = (a – b)
d sin q = a
– d sin  = b
nl = d (sin q + sin  )
Diagram of a Czerny-Turner grating monochromator.
Interference of adjacent waves that are a) 0o , b) 90o and c) 180oout of phase.
Choosing the monochromator bandwidth
Monochromator bandwidth should be as large as possible, but small compared with
the width of peak being measured.
stray light
In every instrument, inadvertent stray light (wavelength outside the bandwidth expected
from the monochromator) reaches the detector. High qulity spectrometers could have
two monochromators in series to reduce stray light.
Absorbance error introduced by different levels of stray light. Stray light is
expressed as a percentage of the irradiance incident on the sample.
Nominal wavelength
Output of an exit slit as monochromator is scanned from l1-Dl to l1+Dl.
Filters
It is frequently necessary to filter (remove) wide bands of radiation from a signal.
Bandwidths for two types of filters(interference filter vs
absorption filter).
(a) Schematic cross section of an interference filter.
(b) Schematic to show the conditions for constructive interference
Transmission spectra of interference filters.
(a) Wide band pass filter has ~90% transmission in the 3- 50 5- mm wavelength range
but <0.01% transmittance outside this range.
(b) Narrow band-pass filter has a transmission width of 0.1 mm centered around 4 mm.
3. Optical Materials and sample containers
Transmittance range for various cell construction materials.
Quartz cell for
UV spectrophotometer
CAT-Lab
/ SWU
4. Detectors for spectrometry
Type
Wavelength range(nm)
Photon detectors
Phototubes
150 ~ 1,000
Photomultiplier tubes
150 ~ 1,000
Silicon diodes
350 ~ 1,100
Photoconductive cells
1,000 ~ 50,000
Thermal detectors
Thermocouples
600 ~ 20,000
Bolometers
600 ~ 20,000
Pneumatic cells
600 ~ 40,000
Pyroelectric cells
1,000 ~ 20,000
A transducer is a type of detector that converts various types of chemical and physical
quantities into electrical signals such as electrical charge, current, or voltage.
Response of several different detectors.
The greater the sensitivity, the greater the output (current or voltage) of the detector
for a given incident power of photons.
Phototube
Schematic diagram of
photomultiplier with nine dynodes.
Comparison of spectra recorded in 5 min
by a photomultiplier tube and a charge
coupled device.
Absorption spectra of hemoglobin with
identical signal levels but different amount
of noise.
Silicon photodiode array
Charge transfer device (CTD)
An operational amplifier current-to-voltage converter used to monitor the current in
a solid state photodiode.
Eout = – IR = – kPR = k’P
P= radient power
G = KP + K’
G = electrical response of the detector in units of current, voltage, or charge.
K’ = dark current
Components and materials of spectroscopic instruments.
Instrumentation of UV-visible spectrophotometer
Types of UV-visible spectrophotometer
1) Single beam spectrophotometer
2) Double beam spectrophotometer
Block diagram for a double-beam in-time scanning spectrophotometer .
3) Diode-array spectrophotometer
Block diagram for a diode array spectrometer.
Power indicator light
Absorbance & Transmittance display
Sample
holder
Wavelength
selector
Power switch
Zero control
Absorbance & Transmittance control
Spectronic 20 spectrophotometer
Procedure
1)
Power on
2)
Select wavelength
3)
0% T adjustment
(Calibration)
Scale of spectronic 20 spectrophotometer
LED
4) Blank (Reference cell) is
inserted into cell holder
5) 100% T adjustment
6) Sample cell is placed in
the cell compartment
7)
Readout absorbance
8)
Power off
Spectronic 20 spectrophotometer
Schematic diagram of optical system of Spectronic 20 single beam UV-visible spectrophotometer
The dual-beam design greatly simplifies this process by simultaneously measuring P and
Po of the sample and reference cells, respectively. Most spectrometers use a mirrored
rotating chopper wheel to alternately direct the light beam through the sample and
reference cells. The detection electronics or software program can then manipulate the P
and Po values as the wavelength scans to produce the spectrum of absorbance or
transmittance as a function of wavelength.
HP8452A diode array UV-visible spectrophotometer
Optical schematic of the Hewlett-Packard HP-8450A diode array UV-visible spectrophotometer.
Q n A
Thanks
Home page
http://www.swu.ac.kr/~dslee
Electronic mail
dslee@swu.ac.kr