Definition of Absorption | Emission | Scattering

Content of the Lecture

See the contents of the first part of Light

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AbsorptionIt is the transition from a lower level to a higher level with

transfer of light energy from the radiation field to receiving atoms or molecules.

EmissionIt is the transition from a higher level to a lower level

where light energy is transferred to the radiation field, or where light energy is transferred into a non-radiative decay process (if no radiation is emitted.)

ScatteringRedirection of light due to its interaction with matter is

called scattering, and may (or may not) occur with transfer of energy, i.e., the scattered radiation has a slightly different or the same wavelength.

AbsorptionWhen atoms or molecules absorb light, the incoming energy

excites a quantized structure to a higher energy level. The type of excitation depends on the wavelength of the light.- Electrons are promoted to higher orbital by ultraviolet or

visible light,- Vibration is excited by infrared light, - Rotation is excited by microwave light.

An absorption spectrum is the absorption of light as a function of wavelength.

The spectrum of an atom or molecule depends on its energy level structure. Absorption spectra are useful for identifying compounds .

Measuring the concentration of an absorbing species in a sample is accomplished by applying the Beer-Lambert Law.

The chromophore is the part of the molecule responsible for light absorption.

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Any substance that absorbs visible light will appear colored when white light is transmitted through it or reflected from it.

The color of a solution is the complement of the color of the light that it absorbs - since the substance absorbs certain wavelengths of the white light our eyes detect the wavelengths (colors) that are not absorbed. The unabsorbed color is called the complement of the absorbed color.

If molecules did not absorb visible light, you could simply flick a light on and off, and then sit back while the photons continued to bounce around the room! Likewise, infrared light (= heat = energy!) would not do any good in heating up your home in the winter if it did not get absorbed by matter. Higher energy light photons, like X-rays, tend to want to plow through more matter before they get absorbed. (Hence, their use in medical imaging: they can pass through your "soft" tissue, but are more readily absorbed in your bones, which are denser.)

Absorption and chemical analysisEach range of light corresponds to a range of frequencies (or

wavelengths) of light vibrations. Since every chemical element has its own unique set of allowed energy levels, each element also has its own distinctive pattern of spectral absorption (and emission) lines!

It is the spectral "fingerprint" that astronomers use to identify the presence of the various chemical elements in astronomical objects. Spectral lines are what allow us, from a "spectrum," to derive so much information about the object being observed!

The Physics of PhotosynthesisPigments:

Special molecule that make the mediation of light capture by living things; they absorb wavelengths in the visible region of the spectrum. When a pigment molecule is struck by a photon of light, it can absorb the light energy, and thus it enters into a higher energy level (an excited state). Each pigment absorbs (or reflects)

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its own characteristic wavelengths of light. The following diagram represents the absorption spectrum of pure chlorophyll in solution

- chlorophyll A (green line), and- chlorophyll B (red line)

Chlorophylls and the Accessory PigmentsThe two primary pigments involved in photosynthesis are

chlorophyll A and chlorophyll B. These two molecules efficiently absorb light at the blue and red ends of the spectrum when purified and in solution, and not very efficiently in between (though this may not completely accurately reflect the situation in living cells).

Also, in photosynthesis there is a series of pigments (accessory pigments), covering more of the visible spectrum. The accessory pigments act as antennae to channel the energy they absorb into the reaction center.

A molecule of chlorophyll at the reaction center can then transfer its excited state into bio-synthetically useful energy.

Emission

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Molecules emit light to dissipate the energy of an absorbed photon- By emitting a photon or- By creating heat throughout the medium

Atoms or molecules that are excited to high energy levels can decay to lower levels by emitting radiation (emission or luminescence).

Any emission of light is called luminescenceLight emitted during a chemical reaction = chemi-luminescence

For atoms excited by a high-temperature energy source this light emission is commonly called atomic or optical emission (see atomic-emission spectroscopy),

For atoms excited with visible light it is atomic fluorescence, and its study is the atomic-fluorescence spectroscopy.

(After a photon is absorbed, depending on the energy, a bond may break and photochemistry may occur).

For molecules, chemi-luminescence is called:- fluorescence if the transition is between states of the same

spin (short lived emission = fluorescence)- phosphorescence if the transition occurs between states of

different spin (long-lived emission = phosphorescence)The emission intensity of an emitting substance is linearly

proportional to analyte concentration at low concentrations, and is useful for quantitating emitting species.

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ScatteringWhen electromagnetic radiation passes through matter, most

of the radiation continues in its original direction but a small fraction is scattered in other directions.

Light that is scattered at the same wavelength as the incoming light is called Rayleigh scattering.

Light that is scattered in transparent solids due to vibrations (phonons) is called Brillouin scattering, it is typically shifted by 0.1 to 1 cm -1 from the incident light .

Light that is scattered due to vibrations in molecules or optical photons in solids is called Raman scattering. Raman scattered light is shifted by as much as 4000 cm -1 from the incident light.

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Beer-Lambert Law (This is a First-Order Kinetic Equation)

Introduction

Beer's law is the linear relationship between absorbance S

and concentration C of an absorbing species.

The general Beer law is usually written as:

S = () * b * cWhereS absorbance (dimensionless) as measured,

() absorptivity coefficient (it is wavelength-dependent),b length of the pathway,c concentration of the analyte.When working in concentration units of molarity, the Beer

law is written as:

S = * b * CWhere

molar absorptivity coefficient with units of M-1 cm-1.(It is wavelength-dependent)

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InstrumentationExperimental measurements are usually made in terms of

transmittance (T), which is defined as:

T = I /I o

Where

I o initial light intensity.

I light intensity after passing through the sample.

The relation between absorbance (S) and transmittance

(T) is given by Lambert law:

S = - log T = - log (I /I o) log (1/T) = log (I o/I )

Absorption of light by a sample

Modern absorption instruments can usually display the data as either transmittance, %-transmittance, or absorbance.

An unknown concentration of an analyte can be determined by measuring the amount of light that a sample absorbs and applying Beer's law.

If the absorptivity coefficient is not known, the unknown concentration can be determined using a working curve of absorbance versus concentration derived from standards.

Derivation of the Beer-Lambert lawThe Beer-Lambert law can be derived from an approximation

for the absorption coefficient for a molecule by approximating

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the molecule by an opaque disk whose cross-sectional area, ,

represents the effective area seen by a photon of frequency .If the frequency of the light is far from resonance, the

area is approximately zero, and if is close to resonance, the area is a maximum.

Taking an infinitesimal slab, d z, of sample:

I o intensity entering the sample at Z = 0,

I z intensity entering the infinitesimal slab at Z = z,

d I intensity absorbed in the slab,

I intensity of light leaving the sample.

cross-sectional area of one molecule,

N number of molecules per cm3

A total area, cm2

Then, the total opaque area on the slab due to the absorbers is

*N*A*d zThe fraction of photons absorbed will be obtained by

dividing this total opaque area by the total area A

*N*A*d z/A ► * N * d z- = * N * d z- d I/I z = * N * d z

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- (1/I z) d I = * N * d zIntegrating this equation (from Z = zero to Z = b) gives:

- l n (I / I o) = * N * b- [l n I - l n I o] = * N * bl n I o - l n I = * N * b

Now, since concentration (C in mole/liter) could be

calculated using the number of molecules in one liter = N*1000 and Avogadro number 6.023x1023:

C in moles/liter = N in molecule/cm3 * (1000/6.023x1023)

C * 6.023 * 1023 = N*1000 in molecules/liter

C * 6.023 * 1020 = NHence,

l n I o - l n I = * N * bl n I o - l n I = * C * 6.023*1020 * bl n I o - l n I = * 6.023 * 1020 * b * C

As (logX = lnX/2.303) we have to divide both sides by

(2.303) to transform l n into log:

log (I o/I)= S = * (6.023 * 1020 / 2.303) * b * C= * (2.61x1020) * b * C= * b * C

log I o - log I = * b * Clog I o - log I = slope * C

Where = slope / b = * 2.61x1020

Plotting (log Io - log I) versus (C) gives a linear relationship with (positive slope = *b) and (intercept = zero).

Typical values of molecule cross-section and molar absorptivity

(cm2) (M-1 cm-1) 112

Absorption by atoms 10-12 3x108

Absorption by molecules 10-16 3x104

Infrared 10-19 3x10Raman scattering 10-29 3x10-9

Limitations of the Beer-Lambert law in Chemical AnalysisThe linearity of the Beer-Lambert law is limited by chemical

and instrumental factors.Beer's law shows us that absorbance (A) is proportional to

the concentration (c) of the absorbing species. It works very well for dilute solutions ( 0.01 M) of most substances. As a solution becomes more concentrated, solute molecules begin to influence each other as a result of their close proximity.

In concentrated solutions, the properties of each molecule (including the absorption of light) are likely to change result: a graph of absorbance (A) vs concentration (c) is no longer a straight line

Another factor affecting the linearity of Beer's law:The possibility that a non-absorbing solute in a solution is

interacting with the absorbing species - this can affect the apparent absorptivity

Causes of non-linearity include: • deviations in absorptivity coefficients at high concentrations

(>0.01M) due to electrostatic interactions between molecules in close proximity.

• scattering of light due to suspended particulates in the sample• fluorescence or phosphorescence of the sample.• changes in refractive index at high analyte concentration.• shifts in chemical equilibria as a function of concentration.• non-monochromatic radiation, deviations can be minimized

by using a relatively flat part of the absorption spectrum such as the maximum of an absorption band.

• stray light.

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