5.33 Lecture Notes,Introduction to Spectroscopy
What is spectroscopy?
Studying the properties of matter through its interaction with different frequency
components of the electromagnetic spectrum,
Latin:,spectron”—ghost or spirit
Greek:, σκοπειν,—to see
With light,you aren’t looking directly at the molecule—the matter—but its
“ghost.” You observe the light’s interaction with different degrees of freedom of
the molecule,Each type of spectroscopy—different light frequency—gives a
different picture → the spectrum,
Spectroscopy is a general methodology that can be adapted in many ways to
extract the information you need (energies of electronic,vibrational,rotational
states,structure and symmetry of molecules,dynamic information),
I want you to understand how light interacts with matter and how you can use this
to quantitatively understand your sample,
I want you to understand spectroscopy the way you understand other common
tools of common measurement like the watch or the ruler,
You will see that spectroscopy is a set of tools that you can put together in
different ways to understand systems → solve chemical problems,
The immediate questions that we want to address are,
What does light do to sample?
How do you produce a spectrum?
What EXACTLY is a spectrum a measurement of?
What does a spectrum measure?
Interaction of light with a sample can influence the sample and/or the light,
Method involves,(1) excitation and (2) detection,
The basic idea,
2a
Characterize light after
sample,
1
Light (E.M,wave) Sample
characterize change in
sample,(photoacoustic
other excitation source 2b
spec.)
This borders on
photochemistry
In most spectroscopies,we characterize how a sample modifies light
entering it,
1) Absorption,Change in amplitude of incident light
Sample attenuates light → transmission T=I/I
0
I
0
I
sample detector
We measure the absorption of light at different frequency/wavelength
ω/λ/ν (characteristic frequency/wavelength of
light entering sample)
5.33 Lecture Notes,Introduction to Spectroscopy Page 2
2) Emission,(or scattering) Excitation induces emission of light from the
sample (usually of different frequency),
(Emitted in all directions)
λ
in
sample
λ
out
detector
Includes, Fluorescence (emission from excited electronic singlet states)
Phosphorescence (emission from excited electronic triplet states)
Raman Scattering (light scattering involving vibrational transition)
3) Optical Rotation,Change of phase of light incident on sample (rotation of
polarization)
Let’s work on describing absorption,
A
ω/λ/E
Let’s look at a typical absorption spectrum,
5.33 Lecture Notes,Introduction to Spectroscopy Page 3
What are the axes?
X-axis,Characterizes the input light in terms of frequency-wavelength-energy
Wavelength λ (nm,μm,?),
Frequency ν (cycles/sec or s
-1
or Hz) =
ω
=
c
2π λ
ω = 2πν (rad/sec) (angular frequency)
ν = ω/2πc = 1/λ expressed in units of cm
-1
Energy E = hν (usually expressed as cm
-1
using E/hc = ν/c)
Conversions
ν ( cm
-1
) = 10
7
/ λ( nm )
ν ( eV ) = 1240 / λ( nm )
y-axis,
Absorption
ν=? log
I
0
=ε νA ()
I
() c L
(Beer’s Law)
I
0
= light intensity incident on the sample
I = light intensity that got through the sample
ε = molar decadic extinction coefficient
(
M
1
cm
1
)
– the molecular quantity
c = concentration (M)
L = sample length (cm)
How do your measure absorption spectra?
Measure the change of intensity of light at different frequencies as it passes
through a sample,
5.33 Lecture Notes,Introduction to Spectroscopy Page 4
Two types of spectrometers,
1) Dispersive
2) Fourier transform
Dispersive spectrometer,Separate different frequency components
red
blue
sample
detector
slit
prism
I
0
I
A
ω/λ ω/λ
Well talk about Fourier transform spectrometers later,
This is a way of processing all wavelength/frequencies simultaneously →
IR/NMR
5.33 Lecture Notes,Introduction to Spectroscopy Page 5
What is spectroscopy?
Studying the properties of matter through its interaction with different frequency
components of the electromagnetic spectrum,
Latin:,spectron”—ghost or spirit
Greek:, σκοπειν,—to see
With light,you aren’t looking directly at the molecule—the matter—but its
“ghost.” You observe the light’s interaction with different degrees of freedom of
the molecule,Each type of spectroscopy—different light frequency—gives a
different picture → the spectrum,
Spectroscopy is a general methodology that can be adapted in many ways to
extract the information you need (energies of electronic,vibrational,rotational
states,structure and symmetry of molecules,dynamic information),
I want you to understand how light interacts with matter and how you can use this
to quantitatively understand your sample,
I want you to understand spectroscopy the way you understand other common
tools of common measurement like the watch or the ruler,
You will see that spectroscopy is a set of tools that you can put together in
different ways to understand systems → solve chemical problems,
The immediate questions that we want to address are,
What does light do to sample?
How do you produce a spectrum?
What EXACTLY is a spectrum a measurement of?
What does a spectrum measure?
Interaction of light with a sample can influence the sample and/or the light,
Method involves,(1) excitation and (2) detection,
The basic idea,
2a
Characterize light after
sample,
1
Light (E.M,wave) Sample
characterize change in
sample,(photoacoustic
other excitation source 2b
spec.)
This borders on
photochemistry
In most spectroscopies,we characterize how a sample modifies light
entering it,
1) Absorption,Change in amplitude of incident light
Sample attenuates light → transmission T=I/I
0
I
0
I
sample detector
We measure the absorption of light at different frequency/wavelength
ω/λ/ν (characteristic frequency/wavelength of
light entering sample)
5.33 Lecture Notes,Introduction to Spectroscopy Page 2
2) Emission,(or scattering) Excitation induces emission of light from the
sample (usually of different frequency),
(Emitted in all directions)
λ
in
sample
λ
out
detector
Includes, Fluorescence (emission from excited electronic singlet states)
Phosphorescence (emission from excited electronic triplet states)
Raman Scattering (light scattering involving vibrational transition)
3) Optical Rotation,Change of phase of light incident on sample (rotation of
polarization)
Let’s work on describing absorption,
A
ω/λ/E
Let’s look at a typical absorption spectrum,
5.33 Lecture Notes,Introduction to Spectroscopy Page 3
What are the axes?
X-axis,Characterizes the input light in terms of frequency-wavelength-energy
Wavelength λ (nm,μm,?),
Frequency ν (cycles/sec or s
-1
or Hz) =
ω
=
c
2π λ
ω = 2πν (rad/sec) (angular frequency)
ν = ω/2πc = 1/λ expressed in units of cm
-1
Energy E = hν (usually expressed as cm
-1
using E/hc = ν/c)
Conversions
ν ( cm
-1
) = 10
7
/ λ( nm )
ν ( eV ) = 1240 / λ( nm )
y-axis,
Absorption
ν=? log
I
0
=ε νA ()
I
() c L
(Beer’s Law)
I
0
= light intensity incident on the sample
I = light intensity that got through the sample
ε = molar decadic extinction coefficient
(
M
1
cm
1
)
– the molecular quantity
c = concentration (M)
L = sample length (cm)
How do your measure absorption spectra?
Measure the change of intensity of light at different frequencies as it passes
through a sample,
5.33 Lecture Notes,Introduction to Spectroscopy Page 4
Two types of spectrometers,
1) Dispersive
2) Fourier transform
Dispersive spectrometer,Separate different frequency components
red
blue
sample
detector
slit
prism
I
0
I
A
ω/λ ω/λ
Well talk about Fourier transform spectrometers later,
This is a way of processing all wavelength/frequencies simultaneously →
IR/NMR
5.33 Lecture Notes,Introduction to Spectroscopy Page 5
