Topic 2 Introduction to Spectroscopy

7/29/2019 Topic 2 Introduction to Spectroscopy

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SKA6014

ADVANCED ANALYTICAL CHEMISTRY

TOPIC 2Introduction to Spectroscopy

Azlan Kamari, PhD

Department of ChemistryFaculty of Science and Mathematics

Universiti Pendidikan Sultan Idris


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What is Spectroscopy?

The study of the interaction between radiation

and matter

Analytical spectroscopy, as defined in this

class, covers applications of spectroscopy tochemical analysis


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History of Analytical Spectroscopy

1666: Isaac Newton (England) shows that white light

can be dispersed into constituent colors, and coins theterm spectrum

Newton also produced the first spectroscope based

on lenses, a prism, and a screen

1800: W. Herschel and J. W. Ritter show that infrared

(IR) and ultraviolet (UV) light are part of the spectrum

1814:Joseph Fraunhofer noticed that the suns

spectrum contains a number of dark lines, developedthe diffraction grating

1859: G. Kirchoff obtains spectra of the elements,

explains the suns spectrum


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The Visible Spectrum of the Sun

(Black lines are absorption by elements in the cooler outer region of the star)

Figure from National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation, http://www.noao.edu/image_gallery/html/im0600.html


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History of Analytical Spectroscopy

1870: J. C. Maxwell formalizes and combines

the laws of electricity and magnetism

1900 to present: More than 25 Nobel prizes

awarded to spectroscopists, including:

1902: H. A. Lorentz and P. Zeeman

1919: J. Stark

1933: P. A. M. Dirac and E. Schrodinger

1945: W. Pauli

.

1999: A. Zewail


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Introduction to Spectroscopy

Figures from NASA (www.nasa.gov)

The electromagnetic

spectrum Each color you see is a

specific (narrow) range of

frequencies in this

spectrum


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The Electromagnetic Spectrum

Modern life (not just analytical spectroscopy) revolves

around the EM spectrum!


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Properties of Electromagnetic Radiation

Wave/particle duality

PerpendicularEand B

components

E = electric field

B = magnetic field

Wave properties:

Wavelength (frequency) Amplitude

Phase1 2 3 4 5

-1

-0.5

0.5

1

1 2 3 4 5

-1

-0.5

0.5

1

Long wavelength

(low frequency)

Short wavelength

(high frequency)c = the speed of light (~3.00 x 108 m/s)

= the frequency in cycles/second (Hz)

= the wavelength in meters/cycle

c

Note this figure

shows polarized

radiation!


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Interference of Radiation

Monochromatic: radiation containing a single frequency

Polychromatic: radiation containing multiple frequencies

Constructive interference:

when two waves reinforce

each other

Destructive interference: when

two waves cancel each other


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The Interaction of Radiation and Matter

Electromagnetic radiation travels fastest in a vacuum

When not travelling in a vacuum, radiation and matter

can interact in a number of ways

Some key processes (for spectroscopy):

Diffraction

Refraction

Scattering

Polarization

Absorption


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Transmission of Radiation

The velocity at which radiation travels (or propagates)

through a medium is dependent on the medium itself

When radiation travels through a medium and does not

undergo a frequency change, it cannot be undergoing a

permanent energy transfer

However, radiation can still interact with the medium

Radiation, an EM field, polarizes the electron clouds of

atoms in the medium

Polarization is a temporary deformation of the electron

clouds


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Transmission and Refraction

The refractive index (ni) of a medium is given by:

i

icn

c = the speed of light (~3.00 x 108 m/s)

i = the velocity of the radiation in the medium in m/sni = the refractive index at the frequency i

Refractive index measures the degree of interaction

between the radiation and the medium

Liquids: ni~ 1.3 to 1.8 Solids: ni~ 1.3 to 2.5

Refractive index can be used to identify pure liquid

substances


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Refraction

When radiation passes through an interface between two

media with different refractive indices, it can abruptly

change direction Snells law:

1

2

2

1

2

1

sin

sin

v

v

n

n

1 = the velocity of the radiation in medium 1 in m/s

n1 = the refractive index in medium 1

Snells law is a consequence

of the change in velocity in

the media

Reflection always occurs at

an interface. Its extent

depends on the refractive

indices of the media

1

2

Medium 1

Medium 2


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Diffraction

Fraunhofer diffraction:

Also known as far-field diffraction, parallel beamdiffraction

Important in optical microscopy

Fresnel diffraction

Also known as near-field diffraction


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Diffraction

Diffraction gratings:

Widely used in

spectroscopic instruments

to separate frequencies(can be made precisely)

sin2dn

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Bragg diffraction multiple slit Fraunhofer diffraction:

Important for instrument design, crystallography


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Scattering

Rayleigh scattering (an elastic process): Scattering of small amounts of radiation by molecules

and atoms (whose size is near to the wavelength of

the radiation)

Mie scattering: applies to large particles, involvesscattering in different directions.

Practical use in particle size analysis

4

1

scattering


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Polarization

Polarization of EM radiation a simple classical picture:


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Coherent Radiation

Coherent radiation fulfils two

conditions: (1) it has the

same frequency or set offrequencies, and (2) it has a

well-defined and constant

phase relationship

Coherent radiation is cross-correlated in that the

properties of one beam can be

used to predict those of the

other beam

Examples of coherent

radiation:

Lasers

Microwave sources (masers)

Coherent radiation: different

frequencies (colors) with a defined

phase relationship interfere to produce

a pulse

Diagram from wikipedia.org (public domain)


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Incoherent Radiation

Produced by random

emission, e.g. individual

atoms in a large sampleemitting photons

Actually is coherent, but just

to a tiny (undetectable)

extent Also known as continuous

radiation

Examples of incoherent

radiation: Incandescent light bulbs

Filament sources

Deuterium lamps

Incoherent radiation: differentfrequencies (colors) combined to

produce continuous radiation with

varying phase, frequency and

amplitude

Diagram from wikipedia.org (public domain)


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More Properties of Electromagnetic Radiation

Wave and particle behaviour: photons behave asboth waves and particles

Quantum mechanics developed around the concept of

the photon, the elementary unit of radiation

Plancks law:

Eis the energy of the photon in joules

h is Planck's constant (6.624 x 10-34 joule seconds)

is the frequency of the radiation

hE


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Absorption and Emission

Absorption is a process accompanied by an energy

change involves energy transfer of EM radiation to a

substance, usually at specific frequencies

corresponding to natural atomic or molecular energies

Emission occurs when matter releases energy in the form

of radiation (photons)

E= h

Higher energy

Lower energy

Absorption Emission


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Energy Levels

Several types of quantum-mechanical energy

levels occur in nature: Electronic

Rotational

Vibrational (including phonons and heat)

Nuclear

For each of these, a discrete quantum state and

energy-driven transitions between these states can be

studied (as opposed to a continuous range of energies)


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The Uncertainty of Measurements

Because the lifetimes of quantum states canpersist for short periods, it can be difficult to

measure their energies accurately

This is usually stated in the form of an energy-

time uncertainty:

tE


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The Uncertainty Principle

The uncertainty principle: it is not possible to know both

the location and the momentum of a particle exactly afundamental limit on all measurements

In Heisenbergs terms, the act of measuring a particles

position affects its momentum, andvice versa

In equation form:

In other words, if you know the position of a particle to within x,

then you can specify its momentum alongxto px

As the uncertainty inxincreases (x), that ofpxdecreases (x

), and vice versa

px x 21


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Spectra and Spectrometers

Spectra are usually plotted as frequency vs. amplitude

Instead of energy, wavelength or energy (related

properties) can also be used

The choice of x- and y-axes is often dependent on the

particular technique, its history, etc

Key parameter is frequency/energy/wavelength

resolution

Spectrometers:

instruments that measure the interaction

of radiation with matter, so the properties of such

interactions can be studied


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Spectroscopy in Analytical Chemistry

Widely used approach for characterizing systems ranging

from chemical physics to biology, from individual atoms tothe largest molecules

Some of the most common techniques are:

UV-Visible spectroscopy

IR spectroscopy

Raman spectroscopy

X-ray spectroscopy NMR spectroscopy

EPR spectroscopy

Documents

Topic 2 Introduction to Spectroscopy