Transcript What disperses radiation into component wavelengths?

What disperses radiation into
component wavelengths?
• Prisms
Uses refractive index of
material to bend
different wavelengths
of light different
amounts
• Gratings
How does a grating disperse light?
Tiny parallel grooves
are etched on
reflective surface
Grooves are spaced on
order of magnitude of
wavelength of light
from one another
Reflection produces
constructive and
destructive
interference
• Grating Movie
• An echellete grating with 1200 blazes/mm
at an incident angle of 30o to normal.
What wavelength would appear at 45o
reflection?
• Find the distance between the grooves.
• Echellete Grating: Has relatively broad faces
where reflection occurs and narrow unused
faces.
• Concave grating: Gratings formed on concave
surfaces do not require auxillary collimating and
focusing mirrors or lenses.
– Concave surface both disperses and focuses the
radiation on the exit slit. Advantage in terms of cost
and in decreasing the amount of optical surfaces
therefore increasing energy throughput
• Holographic gratings: Blazes etched with light
instead of mechanically, cheaper and better
reproductions
• Echelle monochrometer: Higher dispersion than
echellete uses higher angle of incidence. Short
side of blaze is used instead of long. Grating is
relatively coarse (<300 groves/mm). Gives lots
more orders of light, must use prism to get rid of
higher orders. Resolution order of magnitude
greater than others
• Prisms are much
cheaper than grating
monochrometers yet
gratings are still the
most frequently used
type of
monochrometers.
Why?
Why are grating MCs used more
frequently than prism MCs?
Criteria monochrometers are evaluated on:
a.) Spectral purity:
b.) dispersion of the monochrometer
c.) resolution
d.) light gathering power
Bottom line: choose
large slit width if
quantitative analysis
is what you need
Larger slit
more light
will be
coming in
Slit Width
Intensity
Choose small slit
width if qualitative
analysis is your need
Smaller
Resolution slits allow
for better
resolution
More light coming in will illuminate more of the
monochrometer, more constructive interference
of light & thus more intense but if slit width is too
wide it is difficult to resolve adjacent spectral
lines
Detectors
• Ideally detectors should give signal directly
proportional to radiant power
S = kP
s = electrical response
k = calibration sensitivity
P = power
Reality:
s = kP + kd
kd = dark current
Types of Radiation Detectors
1.
Photoelectric detectors
a. signal results from absorption of single photons
b. UV, VIS, IR
c. do not have constant response with wavelength
2. Thermal detectors
a. signal results from average power of incident
radiation
b. IR
c. does have constant response with l but much lower
than photoelectric detectors
Photoelectric Detectors
1. Photovoltaic cells
a. used primarily in
visible region
b. produces a current
that is proportional to
radiant power striking it
c. no external electrical
energy required, cheap
d. amplification of signal
is difficult
e. can easily measure
response at high levels
but difficult at low levels
Vacuum Phototubes
• Tube operated at a high
voltage so that current is
proportional to radiant power
• Photon beam strikes surface
material & it emits electrons
which go to the anode
• Different surfaces can be used
for sensitivity in different
spectral regions
Spectral Response for different
photoemissive surfaces
•
Curve 1 is the response of a
bialkali type of cathode with a
sapphire window; curve 2 is for a
different bialkali cathode with a
lime glass window; curve 3 is for a
multialkali cathode with a lime
glass window; and curve 4 is for a
GaAs cathode with a 9741 glass
window. The curves labeled 1%
and 10% denote what the
response would be at the indicated
value of quantum efficiency.
Photomultiplier Tubes
Best detectors for
sensitive response
Detectors are judged on
3 criteria:
1. Spectral sensitivity
2. Gain
3. Rise time
Dark Current
• PMT will have current even when no
signal is present.
• Can be minimized by cooling detector or
be offset because magnitude is generally
constant
Rise time:
• Origin is spread of arrival
times of avalanched
electrons caused by
range of velocities and
path lengths that
electrons may have
• Minimized by electrostatic
focusing based on
geometries
• Using high gain dynodes
reduces # stages
• High electric field strength
increases velocities of
ejected electrons
Photoconductivity Detectors
• Most sensitive in IR
region
• Resistance decreases
when irradiated
Photodiodes/ Photodiode Arrays
Animation
• LPDA big advantage: Can
measure entire spectrum
simultaneously
• Allows distinction of
overlapping peaks on
chromatogram
• Measurement of very transient
phenomena
• All l can be aquired &
recorded in milliseconds or
less
• Spectrum is focused on a a
series of diodes
• LPDA has reverse optics, l
selector is after sample
Instrument Designs
Temporal Designs: have a single detector,
successive bands are examined
sequentially in time
1. Non-dispersive: can only measure a few
wavelengths, use filters as wavelength
selectors
2. Dispersive: can measure multiple
wavelengths through wavelength
selectors that disperse light
Spatial Instrument Designs
• Multi-channel
– Non-dispersive: 3 wavelengths, 3 slits, 3
detectors
– Dispersive: LPDA
Suggest Optical Components and
Materials for Construction of
Instruments for:
a. Investigation of the fine structure of Absorption
bands in the region of 450-700nm
b. Portable device for determining iron content in
natural water based upon absorption of
radiation by red Fe(SCN)2+ complex
c. Determining wavelengths of flame emission
lines for metallic elements in region from 200780nm