3-Light _Matter_s.ppt

7/27/2019 3-Light _Matter_s.ppt

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Light and MatterInteraction


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Electromagnetic Radiation

Properties

Particle-like

Photon

Quantum

Wave-like

Wavelength, Frequency,


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PHOTON

c =

E = h = hc /

Electromagnetic nature of photons are the same Differ in energy and frequency


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Properties of wave


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E = h

= c/

c = 3 x 1010 cm/sec

h= 6.6 x 10-27 erg sec

6.6 x 10-34 Js

Properties of a Photon


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Electromagnetic Spectrum


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Exercises

1. The wavelength of the green light from a trafficsignal is centered at 522 nm. What is the frequency of

this radiation?

2. Calculate the energy (joules) of a photon with awavelength of 5.00 x 104 nm (IR region).

3. What is the wavelength of a photon (nm) emittedduring a transition from ni = 5 state to the nf= 2

state in the hydrogen atom?


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Spectroscopy

Deals with interaction of electromagnetic

radiation (EMR) with a molecule

Transitions between energy levels that

involve the absorption or emission of light

Uses Quantum mechanics to

mathematically describe matter on theatomic scale.


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Interaction of matter and radiation

Absorption lower to a higher level field transition transfer of energy from the radiation field to an absorber.

Emission

higher to a lower level transition transfer of energy from the emitter to the radiation field.

nonradiative decay if no radiation is emitted

Scattering

redirection of light scattering may or may not occur with a transfer of energy,

i.e., the scattered radiation may or may not have a different

wavelength to the incident light.


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Electromagnetic Spectrum

Type of

Radiation

Frequency Range

(Hz)

Wavelength

Range Type of Transition

gamma-rays 1020-1024


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ABSORPTION MECHANISM

Electrons are promoted to higher orbitals by

ultraviolet or visible light

Vibrations are excited by infrared light

Rotations are excited by microwaves


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Jablonski Diagram

Relaxation mechanism for excited state molecules

10-510-8 s 10-4 s

Vibrational relaxation, 10-12 s


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Carbenes /Methylenes

6 electrons in outermost shell, : CH2

Singlet and triplet

cH

H

cH

H

Singlet sp2 < 120o

diamagnetic

triplet sp 180o paramagnetic


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Emission Spectroscopy

Atomic or Optical

Excitation by high-temperature

energy source

Fluoresence or Phosphoresence

Excitation by light

Molecular FlorescenceLaser-induced Fluoresence


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Characteristic of Emission Spectra

continuous all wavelengths of visible light are

present

example; sunlight, red-hot/white-hotiron bar glow

line light emission only at specific

wavelength example; atoms in gas phase


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Line Spectrum of gaseous atom


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Series in Atomic Hydrogen emission spectrum

Series nf ni Spectrum Region

Lyman 1 2, 3, 4 Ultraviolet

Balmer 2 3, 4, 5 Visible andUltraviolet

Paschen 3 4, 5, 6 Infrared

Brackett 4 5, 6, 7 Infrared


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Energy levels of Hydrogen atom and

different emission series


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Spectrometric Techniques

Technique Basis Application

Plasma emission Atomic emission afterexcitation in high Temp

gas plasma

Metals and some non-

metals at trace levels

Flame emission Atomic emission after

flame excitation

Alkali and alkaline earth

metals

Atomic Absorption Atomic absorption afteratomization by flame or

electrothermal means

Trace metals and some

non-metals

Atomic

Fluorescence

Atomic fluorescence

emission after flameexcitation

Mercury and hydrides of

non-metals at tracelevels

X-ray emission Atomic or atomicfluorescence emission

after excitation by

electrons or radiation

Elemental components

of metallurgical and

geological samples


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Spectrometric Techniques


-spectrometry -ray emission afternuclear excitation

Radioactive elements in

environmetal samples

Ultraviolet/Visible Electronic molecularabsorption in solution

Quantitative

determination of

unsaturated organic

compounds

Infrared Electronic molecularabsorption

Identification of organic

compounds

Nuclear Magnetic

Resonance

Nuclear absorption due

to change in spin states

Identification and

structural analysis oforganic compounds

Mass spectrometry Ionization andfragmentation of

molecules

Identification and

structural analysis of

organic compounds


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Separation Techniques


Thin Layer

Chromatography

Differential rates of

migration of analytes

through a stationary

phase by movement of

a liquid or gaseousmobile phase

Qualitative analysis of

mixtures

Gas Chromatography Quantitative and

qualitative analysis ofvolatile compounds

High Performance

Liquid

Chromatography

Quantitative and

qualitative analysis of

nonvolatile compounds

Electrophoresis Differential rates ofmigration of analytes

through a buffered

medium

Quantitative and

qualitative analysis of

ionic compounds


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Beers Law


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Absorption Law

Known as Beer-Lamberts Law Commonly known as Beers Law

Relates factors that affect the attenuation of

monochromatic radiation

Po P

b

Absorbing solution

of concentration, c


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Transmittance

Refers to the amount of radiation that passes

through the medium

Often expressed as % T


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Absorbance

Refers to radiation taken in by the sample

Transfers energy to absorbing molecules

Leads to decrease in intensity of radiation


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Beers Law

a = absorptivity or extinction coefficient, k

A = absorbance, unitless quantity

b = pathlength

c = concentration

= molar absorptivity or molar extinction coefficient

c = mole/L


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For Mixtures


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Assumptions

Incident radiation is monochromatic

Absorbers (molecules, atoms etc.) are

independent of each other

Incident radiation are of parallel rays,

perpendicular to surface of absorbing medium Absorbing medium is homogeneous and does

not scatter radiation

Path length is uniform over the cross-section

of beam. Incident flux will not lead to saturation effects


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Limitations

Beers Law is a limiting Law

Only true for dilute solution ( 0.01 M) Affected by extent of interaction between molecules

Affected by charge distribution Affected by electrostatic interaction

Affected by high concentration of electrolytes/analyte

change in refractive index

shift in chemical equilibrium

Li h d M


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Limitations of Beers Law

Scattering of light due to particulates in the sample

Stray light

Few exceptions to linear relationship of A and b at

fixed c

More exception between A and c at fixed b

real deviation

instrumental deviations

chemical deviations

Li ht d M tt


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Chemical Deviations

absorption, dissociation or reaction with solvent

monomer-dimer formation

metal complex formation

acid-base interaction

Li ht d M tt


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Instrumental Deviations

A. Strictly applies only to monochromatic radiation

Polychromatic radiation differs when of each varies Select where sample absorbs the most

B. Stray Light

Results from scattering and reflection off

gratings, mirrors, lenses, filters and windows

More significant at high absorbance values

C. Mismatched Cells

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3-Light _Matter_s.ppt