Download Chapter 7 Components of Optical Instruments

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Chapter 7 Components of Optical Instruments
Problems: 1, 2, 3, 6, 8, 11, 12, 13, 15, 21, 23
UV, and IR instruments have enough in common with Visible instruments that we can
look at the basics of all three in this one chapter. Typically call these optical
instruments, even though you can’t see UV and IR light with your eye.
7A General Design of Optical Instruments
Six different phenomena can be measured in a ‘optical’ instrument
absorption
fluorescence
Phosphorescence
scattering
emission
chemiluminescence
While individual machines are configured differently lots of similar components
Typically 5 major components
Source of radiant E
Transparent container for sample
a device for selecting a specific portion (frequency or wavelength) of the radiant
E
A transducer to convert radiant E into an electrical signal
Signal processor to turn the electrical signal into something you can use
Figure 7-1 how these components assembled in these 6 different instruments
In absorption, detector is in line with source vs in fluorescence, scattering and
phosphorescence detector is literally at 90o to source.
In some instruments sample and wavelength selector may be switched
In emission and chemiluminescence no need for source
Figures 7-2 and 7-3 summarize optical characteristics of various components as a
function of radiation being used
7B Sources
Need to make enough light to be easily detected
Power of light needs to be constant
For most sources need a constant voltage supply to give that constant output
For double beam instruments can sacrifice constant power supply since make
measurements of sample and reference almost simultaneously
Figure 7-3a summarizes sources. Note 2 types, Continuum and Line
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7B-1 Continuum Sources
Intensity of output varies smoothly over a wide range of wavelengths
UV region
Most common Deuterium arc lamp
If need high intensity use high pressure Xe, Hg, or Ar arc lamp
Vis
Most common tungsten filament
IR
Heat various inert solids to about 1500-2000K, peak intensity
1.5-1.9:m.
More details in subsequent chapters
7B-2 Line Sources
Output intensity restricted to a few discrete line
Used in Atomic Absorption, Atomic & molecular fluorescence, Raman,
polarimetry and refractometry
Most common Hg or Na vapor lamps -give a few sharp lines in UV and Vis
(Also used for street lamps!)
Also ‘hollow cathode lamps’ used in atomic absorption and fluorescence
7B-3 Lasers
Technically another line source
High intensity, narrow range of wavelengths(-.01 nm), also coherent (all
wave are in phase)
First invented 1960
Light amplification by stimulated emission of radiation (laser)
Early lasers had limited choice of wavelengths (only red in the Vis)
Now Dye lasers have more and different wavelengths
Components of a laser
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Figure 7-4
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Main component - lasing medium, can be ruby, a semiconductor,
solution of an organic dye, a gas(Ar. or Kr)
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Activate lasing medium by pumping. Start with a few photons of
proper wavelength (pumping source)
Not used in gas lasers - use electrodes connected to gas chamber
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Laser worked as an oscillator, Using mirrors at both ends, the Em
radiation travels back and forth inside lasing medium. Each time it
traverses, releases more photons and builds up more intensity.
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This also makes light parallel, all non-parallel light leaves
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tube and is not amplified
Have mirror at one end weaker than other, so as builds up intensity
over minimum value, starts shooting out the end
Mechanism of Laser action
See Figure 7-5
First excite a few molecules to excited state
Lots of excited state but drop to lowest excited state almost
instantly
From here you know the drill
(nonradiative transfer, fluorescence or phosphorescence)
Fluorescence would be in all directions in at all phases due
to nothing keeping chromophores together
But one I haven’t mentioned, Stimulated emission
7-5c
As light passes through molecule and make molecule emits its
radiation
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Parallel to stimulating radiation
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In phase with stimulating radiation
Note molecules in lasing medium can still absorb this E, but then it is
poised to be restimulated so no net loss! 7-5d
Population inversion
In normal population most molecules are in ground state, just a few in
excited state
As you pump the lasing material most molecules are in excited state few
in ground - call this population inversion
Under these conditions lees chance for absorption, more chance for
stimulated emission, so light beam gets stronger yet!
Three- and Four-level laser
Have described a 3 level laser: ground, highly excited, and lowest excited
state
4 level laser uses these, but stimulated emission is to a 4th, vibrationally
excited/electronically non excited non-ground state
Advantage is easier to get population inversion, so takes less energy to
pump
For us I don’t see any reason to worry about this feature
Examples if useful lasers
Read for your own interest, I don’t think I’ll cover in class Lets skip to 7C
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7C Wavelength Selectors
For most analysis would like to analyze each wavelength independently of all others
Unless you are using a laser, this won’t happen.
Instead get a ‘band’ if wavelengths Figure 7-11 the narrower band the cleaner the
analysis
Terms from figure to remember
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Nominal wavelength - the one at the center of the distribution
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Effective band width - he range of wavelengths at ½ height
How do we take light from a continuous source and select it down to a band at a
selected, nominal wavelength?
Two selectors, filters and monochrometers
7C-1 Filters
Two types, interference and absorption
Interference
uses optical interference to remove certain wavelengths
Typical construction two metal films separated by a transparent dielectric
(Dielectric - an insulator with no charged particles- generally transparent)
The film-dia-film then sandwiched between glass plates for mechanical
support
Thickness of dielectric film controls wavelengths filtered
As light passes through mirror-like metal films and dielectric, certain
wavelength removed by destructive interference. If light has right match
for dielectric and thickness, then it passes though (up to 80%T)
Equations given but don’t memorize, look them up if you ever need them
Filters like this used from UV down to 14:m in IR
Typical bandwidth about 1.5% os peak wavelength, can be as little at
.15%
Interference Wedges
Same design as interference filter, but use a wedge of dielectric instead of
uniform thin film
Can change wavelength that you transmit by moving filter
Can use like a prism or grating!
Overall bandwidth at any point is at least 5x larger than regular filter
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Absorption Filters
Less expensive than absorption filters
Simply a glass or gelatin with dye that absorbs light of different colors
Bandwidths from 30 ro 250 nm in visible range
Ones with narrow bandwidth don’t let much light through (10% T)
Cut-off filters Have 100% T in one region , then 0% T above or below
some cut-off value
In general not nearly as selective as Interference Filters, but still have
many uses
7C-2 Monochrometers
Used to vary wavelength of transmitted light over a wide range of wavelengths
Similar design in UV, Vis and IR, but materials differ
Components of Monochrometers
Figure 7-16
1.
Entrance slit
2.
Collimating lense or mirror to make light parallel
3.
Prism or grating to disperse light into component wavelengths
4.
Focusing element to get light refocused
5.
Exit slit to pass only correct band of light
Most monochrometer are isolated from environment with entrance and
exit windows
Dispersing element - reflection grating or prism
Prism
Older instruments, used to be cheaper
Non-linear, shorter wavelengths bent more than longer
wavelengths
Reflection grating
Cheaper to make now so almost universally used
Linear, all wavelengths bent in similar manner
Prism Monochrometers
Built same manner for UV, vis, or IR
But need to change materials of prisms and lenses to
transmit the appropriate wavelengths
Figure 7-18 Would use either a full prism, or a mirrored ½ prism
Grating Monochrometers
Can use either a transmission or reflection grating
Are cheap to make because are usually replica gratings
Make a single master, shaped like figure 7-19 by cutting
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onto a hard, flat, polished surface with a diamond, usually
about 3-10 cm long
UV 300-2000 grooves/mm, 1200-1400 most common
Infrared 20-200 grooves/mm, 100 most suitable
After master is made, make plastic mold over the top
Coat plastic with metal (Al, Au, Pt) to make reflective
Echellette Grating
(Type shown in Fig 7-19)
Broad flat edges for maximum reflection
Each face acts as a point source of radiation, so get interference
as waves recombine in reflected beam
Let’s not worry about individual geometry, but go the equation
n8=d(sin(i)+sin®))
I is angle between incident light and prism normal
R is angle between reflected light and prism normal
D is distance between grooves
N is order of reflection
Note this says that at a particular angle you won’t get just 1 light,
say 900nm, but also higher orders 900-1st order, 450, 2nd order,
300-3rd order etc
By proper design can get 90% intensity in 1st order
Usually use filters to get rid of other orders
Concave gratings
Make grating in a concave surface
Surface serves to focus light as well as make it monochromatic
Don’t need lenses for focusing
Instrument is cheaper to make
Less components to have light bounce off, so get more light
through the instrument
Holographic gratings
Made by two lasers hitting a photoresist surface
Then dissolve portion of the surface hit by the lasers
Can get nearly perfect gratings up to 6000 lines/mm, up to 50 cm
long cheaply
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Again can make replicas cheaply
Performance Characteristics of Grating Monochrometers
Several characteristics you have to look at
Spectral Purity
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light that get out of monochrometer can be contaminated
with other wavelengths
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called scattered or stray radiation
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can be scattered off imperfections in grating
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can be scattered off dust
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Reduced by putting in baffles to block light from other
sources
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Paint all interior parts flat black to absorb scattered light
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Seal off interior with windows to prevent dust and fumes
from getting in
Dispersion of Grating
The ability of grating to separate light of different wavelengths
Lots of equations here I think the only one I really want is:
D-1 = d8/dr =
D-1 is called reciprocal linear dispersion nm/mm or Å/mm
For most integuments D-1 = d/nF, ie is linear as a function of
distance
Resolving Power of Monochrometers
How well it can separate slightly different wavelengths
Resolving power (R)=8/)8
Can be shown that R=n/V
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n is the order (so better resolving at higher orders)
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V is number of scratches in grating that are illuminated by
source (so either wide illumination or blazes close together)
Light Gathering Power
For most spectrometers you want as much light hitting the detector
as possible. The f/ number or speed provided a measure of this .
Also note this is the same f number you use in setting a camera
lens
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f=F/d
f=f/ number
F is focal length of lens (or monochrometer)
D -s diameter of lens (I don’t know what this corresponds to in a
monochrometer
f/2 lens gathers 4x mor light than an f/4 lens
(Lens on our good Nikon is 1.8)
Typical f/ numbers are between 1 and 10 (lower is better)
Echelle Monochrometers
Somewhat different diffraction grating, used in specialized
instruments, let’s skip
7C-3 Monochrometer slits
two metal pieces, make a set of jaws, two faces of jaw must be parallel. Can be
fixed or adjustable
Entrance slit serves to define source of radiation
its image is focused on exit slit
Entrance and exit slits usually the same size
Effect of slit width on resolution
Figure 7-22
As narrow slit, get smaller bandwidth
Bandwidth-span of wavelengths for exit slits at a given setting
effective bandwidth, ½ of bandwidth
Figure 7-23&7-24
as slitwidth get narrower, gt better resolution so if need fine details this is
better
Tradeoff. As slit width is narrower, less light get through so signal is
weaker. If need sensitivity need slitwidth wider!
7D Sample Containers
Need something to hold sample
must be transparent to the wavelength of light you are using
must be os appropriate size for sample (both total volume and pathlength)
Returning for figure 7-2
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quartz or fused silica for UV will go as far as 3:m into the IR
Silicate glasses 350-2000 nm
NaCl common in IR
7E Radiation Transducers
What shall we use for a detector
7E-1 Introduction
Ideal
1.
High sensitivity
2.
High signal to noise
3.
Constant response for all wavelengths
4.
fast response time
5.
0 response for no light
6.
Signal directly proportional to radiant power of light P
S=kP
Almost all transducers fail in #5 have some ‘dark current’
Current in absence of signal
usually constant so can make circuit to get rid of this offset
Types of Radiation Transducers
Ones that respond to Photons
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Called photon transducers of quantum detectors
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Have an active surface to absorb radiation
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Absorbed photons cause emission of electrons develop a
photocurrent
•
Absorbed photons moves electrons into conduction band, this
enhances conduction in a semiconductor
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Used in UV, Vis, and Near IR
If used wavelength >3:m (3000A) must be cooled with liquid
N2 or dry ice to prevent noise
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Detect individual photon events
Ones that respond to heat
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called heat transducers
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used for IR
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respond to average power of radiation
Summarized figure 7-25
Heat transducers (H,I)response doesn’t vary with wavelength, but very low
Photon transducers give lots mor signal, but wavelength response highly
variable
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7E-2 Photon Transducers
Several types, (1) Photovoltaic cells, (2) phototubes, (3) photomultiplier tubes,
(4) photoconductivity transducers, (5) silicon photodiodes (6) charge transfer
devices
Photovoltaic of Barrier Layer cells
Cheap
Rugged
Not very sensitive, need lots of light
Hard to amplify signal
Response primarily in visible
Used in simple, portable instruments where want rugged and low
cost
Vacuum Phototubes
Figure 7-27
Cathode and wire sealed in a vacuum
Photons hit photoemissive cathode, electrons released, attracted to
anode
If potential between cathode and anode is >90V all electrons
emitted hit the cathode, and current directly proportional to
light intensity
Current about 10X SMALLER than photovoltaic cell, but easily
amplified
Change metal surface of cathode to change wavelength response
High sensitivity
Red sensitive
UV sensitive
Flat response
Also pretty popular and cheap to make, this is what is used in spec
20's
Photomultiplier Tubes
Figure 7-29
Sort of similar to phototube but redesigned for extreme sensitivity ,
so good at low light levels
Starts like phototube, with photoemissive surface
Instead of electrons going directly to cathode, goes toward a
‘dynode’
As electrons hit dynode they release additional electrons
Each dynode about 90 V more positive than previous, and each
amplifies the signal several fold
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Extremely sensitive especially in UV
Extremely fast
Have some dark current but this can be eliminated with cooling
If get too much light (even just room light) can be destroyed
Equipment has to be designed so you can’t expose to room light
You have to be careful as well
Silicon Diode Transducers
Essentially a transistor that passes more current when exposed to
light
More sensitive than phototubes, less than photomultipliers
Can be used between 190 and 1100nm
Can be built into arrays (next section)
7E-3 Multichannel Photo Transducers
All above devices can only read one point source of radiation
Would like to make in linear array so can read entire spectrum at once
or 2D array so cam read en entire picture at once (digital cameras, video
recorders)
Three major devices. Photodiode arrays (PDA), Charge Injection devices (CID’s),
Charge Coupled devices (CCD’s)
Photodiode Arrays (PDA)
Same as photodiode above, but build one after another into the face of a
silicon chip
Build with integrated circuitry to read off values
End up with a single chip that can read 1000's of 8’s in a fraction of a
second
Basis of phododiode array spectrometer (The ocean optics machine)
Charge-Transfer Devices
Photodiode not as sensitive, and does not have as good signal/noise or
dynamic range at PMT so for critical work need something better
Charge transfer devices almost as good as PMT’s for above properties, +
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can be made into linear or 2D arrays on chips
Light E hits an n-doped silicon substrate
Each photon makes a + charge in substrate
Accumulate this charge over a period of time
read off charge in 1 or 2D array using
Measure voltage change in region with a Charge-injection device
Measure charge with a charge sensing amplifier (Charge Coupled
Device)
Let’s not worry about details of these two devices just one or two
advantages
Charge injection devices - can read even while integrating
Charge Coupled device - can’t read until integration is over
But is more sensitive to low light levels
7E-4 Photoconductivity Transducers
Transistors that conduct more when hit with near infrared light (.75 to 3 :m, 7503000nm)
Made from sulfides, selenides, and stibnides of Pb, Cd, Ga, In
Range extended further into IR by cooling to suppress heat noise in transistor
itself
7E-5 Thermal Transducers
IR radiation so low in E, that none of the above methods work well in IR
In general the IR radiation hits thermal transducer, will raise the temperature
slightly (a few thousandths of a K) and this change in temp can be measured by
some change in the materials properties
Transducer needs to be small so there is less to heat up
Need to focus as much of the IR light on the transducer as possible
Thermal heat from the surroundings is going to act as noise on the signal
need to shield transducer
best to put transducer in vacuum
need to use chopper configuration on transducer to help filter out noise
Thermocouples
simplest, a pair of junctions between 2 different metals (figure 3-11)
A potential difference exists between junction reflecting difference in T
In IR use bismuth and antimony
IR light is shown on one junction, other is nearby but shielded from heat
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sources
used in well designed instrument can detect T differences of .000001K
good for much IR work
Bolometers or thermisters
high changes in resistance for small temp changes
generally not as good in the near IR so not as common
A germanium bolometer at 1.5K (very cold) is almost ideal in 5-400cm-1
(2000-25:m) range
Pyroelectric Transducers
Extremely fast response
use in FT IR where need response in m to : seconds
7F Signal Processors and readouts Let’s not bother
7G Fiber optics fibers of glass or plastic that can transmit light for hundreds of feet,
and are flexible so can bend as needed. Useful in many kinds of instruments but,
again, let’s not worry about the details
7H Types of Optical Instruments
Spectroscope about the simplest instrument. Light source, monochrometer, use
your eye to detect peak transmittance
Colorimeter use your eye to judge relative absorbance between sample and
standards
Photometer one step up, use electronics to measure absorbance, usually uses
filters instead of a true monochrometer. Fluorometer same but used for fluorescence
Spectrograph entire spectrum displayed across detector, originally detector a
photographic plate, now similar to a photodiode array spectrometer
Spectrometer can measure intensity as a function of wavelength
Spectrophotometer additional electronics so can measure absorbance as a
function of wavelength
Instrument scan be divided into ones that can only work on one wavelength of
information at a time, and one that gather information from several wavelengths
at one tine, the latter are called multiplex instruments.
A photodiode array spectro(graph?) Is an example of a multiplex instrument.
Many multiplex instruments use the Fourier transform (ft) as part of the process
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transform the multiplexed information into one you can understand. Let’s study
the Fourier transform in more detail
7I Principles of Fourier Transform Optical Measurements
First developed by astronomers in early 1950's
Used in far IR instruments in early 1960's
Commercial IR instruments available by late 1960's
Same principles can be used for vis and UV, but not used commercially
7I-1 Inherent advantages of FT spectroscopy
Few optical elements
No slits
lots of power reaches the detector
Because higher power, get higher signal/noise
Extremely high resolving power
All signal reaches detector simultaneously, so can obtain entire spectrum
essentially simultaneously (although it take the mechanism about a second to
actually acquire and interpret the signal)
This last is a very useful property. Using a scanning type machine it might take
10 minutes to acquire a spectrum. If you want to double your signal to noise you
need 4 to average 4 scans together (s/n increases as sqrt(# or scans)) so it
would take you 40 minutes to double your signal to noise. With the FT machine
taking a spectrum a second, in 40 minute you can get 40(60) = 2,400 scans the
sqrt of 2,400 is 48.9, so your signal/noise has increased by a factor of 50 A great
improvement over the scanning instrument
This is called the multiplex advantage
In the IR the chief source of noise is ‘flicker’ in the detector and its circuitry. And
the multiplex advantage does great things to get rid of this noise. In the visible
and the UV, the chief source of noise is in the ‘flicker’ in the source illumination.
Here the signal averaging doesn’t help, in fact this noise tends to gt distributed
throughout the spectrum. This is why FT machines aren’t used in the visible and
UV.
7I-2 Time-Domain Spectroscopy
In a standard UV machine you detect absorbance as a function of frequency (or
wavelength) so we call this frequency domain spectroscopy
In an FT machine you are going to detect your signal as a function of time, hens
this is termed time domain spectroscopy. One the data is acquired in this form
you then need to transform it, using the Fourier transform to turn it into the
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frequency domain data that you are more used to interpreting
Figure 7-40 and your lab exercises help to bring some of these ideas to the fore
(A) time domain data at two different frequencies
(B) when they interfere with each other you get a new pattern
©) the frequency domains of the two different frequencies
(D) the frequency domain of the interference pattern
The Fourier transform is a mathematical operation (actually an integration) of
something in the time domain that transforms it into data in the frequency
domain.
Can do as a math integration, but tedious, or you can have the computer to it for
you, and then it is instantaneous, and you don’t even have to think!
7I-3 Obtaining Time-domain Spectra with a Michelson Interferometer
While each FT instrument has a different way to acquire time domain data, will
focus here on the how this is done with an FTIR
IR wavelength in the :m range, so frequency is in the 3x108/1x10-6 or 3x1014
range. So 300,000,000,000,000 cycles /second. Extremely fast, don’t have a
detector that can respond this fast, so can’t measure the frequencies of the light
directly.
Will instead measure indirectly using Michelson interferometer (figure 7-42) a
device invented in late 1800's!
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Figure 7-42
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Light hits ½ silvered mirror
•
½ goes to fixed mirror
•
½ goes to moving mirror
•
If single wavelength, will get Intensity ups and downs as mover mirror just
enough for cancellation of signal via interference
•
if have lots of frequencies get all sorts of sine waves piled on top of each
other
•
use FT to transform into individual frequencies
•
if have something that absorbs in sample, will filter out that frequency,
when do transform will not see that frequency in the pattern
•
book makes case for moving mirror at a set speed, so not intensity as a
function of distance becomes intensity as a function of time
•
so now can do proper ft of a function of time to a function of frequency
•
key parameter here is distance the mirror moves
)< =1/distance moves
so if want to resolve 1 peaks that are .1 cm-1 apart
.1=1/X
X=10, mirror must move 10 cm in the machine
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More on these instruments in the chapter on IR