Transcript Document

Components of Optical
Instruments
Lecture 9
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4. Silicon Diode Transducers
A semiconductor material like silicon can be
doped by an element of group V (like arsenic
and antimony) would have more electrons as
a group V atom (5e) replaces a silicon atom
(4e). The thus doped semiconductor is called
an n-type semiconductor. In contrast, when a
group III element (3e) is doped in a silicon
matrix, replacement of a silicon atom (4e)
with a group III atom (like indium or gallium,
3e) results in the formation of a less
electrons semiconductor or a p-type.
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A diode is a device that has a greater
conductance in one direction than the
other. A diode is manufactured by
forming adjacent n-type and p-type
regions within the same silicon or
germanium single crystal. The term pn
junction refers to the interface between
these two regions.
A diode can be connected to a power
supply (a battery) in one of two modes:
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A silicon diode transducer consists of a
reverse-biased pn junction formed on a
silicon crystal. The application of a reverse
bias creates a depletion layer that will
ultimately result in zero current. When a
beam of radiation hits silicon diode, holes
and electrons will be formed in the depletion
layer thus producing a current proportional
to the intensity of incident radiation.
Silicon diodes are more sensitive than
phototubes but far less sensitive than
photomultiplier tubes. They can be used in
both UV and visible regions
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5. Multichannel Photon
Transducers
The simplest multichannel transducer ever
made is the photographic film where the full
image can be captured in one shot. However,
the time required for handling and
developing the film makes it difficult to
practically use it in conventional
instruments, although it is still in use in
some techniques like x-ray diffraction
spectroscopy. There are two other major
classes of multichannel photon transducers,
which find important applications and use in
spectroscopic instruments.
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a. Photodiode arrays (PDA)
These are simply linear arrays of silicon diodes
described above. The number of linear diodes used
in each photodiode array usually 64 to 4096 with
1024 silicon diodes as the most common. One can
imagine the complexity of the electronic circuitry
used in such an array as well as the data handling
and manipulation requirements. The entrance slit is
usually fixed at a size enough to fill the surface area
of one silicon diode. The entire spectrum can thus
be instantaneously recorded. The arrays are also
called diode array detectors (DAD).
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b. Charge Transfer and Charge
Coupled Transducers
The photosensitive elements are, in contrary to
PDAs, arranged in two dimensions in both
charge injection devices (CID) and chargecoupled devices (CCD). Therefore, these are
very similar to photographic films. For
example, a commercially available
transducer is formed from 244 rows with
each row containing 388 detector elements.
This will add up to a two-dimensional array
holding 16672 detector elements (pixels) on
silicon chip that is 6.5 mm by 8.7 mm.
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Introduction to Atomic
Spectroscopy
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Technique – Flame Test
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An Introduction to Optical
Atomic Spectroscopy
In optical atomic spectrometry, compounds are first
converted to gaseous molecules followed by
conversion to gaseous atoms. This process is
called atomization and is a prerequisite for
performing atomic spectroscopy. Gaseous
atoms then absorb energy from a beam of
radiation or simply heat. Absorbance can be
measured or emission from excited atoms is
measured and is related to concentration of
analyte.
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Atomic Energy Level Diagrams
As a start, we should be aware that only valence
electrons are responsible for atomic spectra
observed in a process of absorption or
emission of radiation in the UV-Vis region.
Valence electrons in their ground states are
assumed to have an energy equal to zero eV.
As an electron is excited to a higher energy
level, it will absorb energy exactly equal to the
energy difference between the two states. Let
us look at a portion of the sodium energy level
diagram where sodium got one electron in the
3s orbital:
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The dark lines represent most probable
transitions and in an atomic spectrum they
would appear more intense than others. It
should also be indicated that two transitions,
of very comparable energies (589.0 and 589.6
nm), from the 3s ground state to 3p excited
state do take place. This suggests splitting of
the p orbital into two levels that slightly differ
in energy. Explanation of this splitting may
be presented as a result of electron spin
where the electron spin is either in the
direction of the orbital motion or opposed to
it.
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Both spin and orbital motion create magnetic
fields that may interact in an attractive
manner (if motion is in opposite direction,
lower energy), or in a repulsive manner when
both spin and orbital motion are in the same
direction (higher energy). The same occurs
for both d and f orbitals but the energy
difference is so small to be observed. A Mg+
ion would show very similar atomic spectrum
as Na since both have one electron in the 3s
orbital.
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In cases where atoms of large numbers of
electrons are studied, atomic spectra
become too complicated and difficult to
interpret. This is mainly due to presence of a
large numbers of closely spaced energy
levels
It should also be indicated that transition from
ground state to excited state is not arbitrary
and unlimited. Transitions follow certain
selection rules that make a specific transition
allowed or forbidden.
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Each type of atoms would have certain
preferred or most probable transitions
(sodium has the 589.0 and the 589.6 nm).
Relaxation would result in very intense lines
for these preferred transitions where these
lines are called resonance lines.
Absorption of energy is most probable for the
resonance lines of each element. Thus
intense absorption lines for sodium will be
observed at 589.0 and 589.6 nm.
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