Introduction to Spectroscopy pdf

Lecture Date: January 18 th , 2007 Introduction to Spectroscopy What is Spectroscopy?. matter covers applications of spectroscopy to chemical analysis... History of Analytical Spectrosco

Lecture Date: January 18 th , 2007 Introduction to Spectroscopy

What is Spectroscopy?

matter

covers applications of spectroscopy to chemical

analysis

History of Analytical Spectroscopy

can be dispersed into constituent colors, and coins the

term “spectrum”

– Newton also produced the first “spectroscope” based on lenses,

a prism, and a screen

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

spectrum contains a number of dark lines, developed

the diffraction grating

explains the sun’s spectrum

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

History of Analytical Spectroscopy

of electricity and magnetism

spectroscopists, including:

…

Introduction to Spectroscopy

Figures from NASA (www.nasa.gov)

spectrum

specific (narrow) range of

frequencies in this

spectrum

The Electromagnetic Spectrum

around the EM spectrum!

Properties of Electromagnetic Radiation

components

– E = electric field

– B = magnetic field

– Wavelength (frequency)

– Amplitude

– Phase

-1 -0.5

0.5 1

-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



Note – this figure shows polarized radiation!

Interference of Radiation

when two waves reinforce

each other

two waves cancel each other

The Interaction of Radiation and Matter

can interact in a number of ways

– Diffraction

– Refraction

– Scattering

– Polarization

– Absorption

Transmission of Radiation

through a medium is dependent on the medium itself

undergo a frequency change, it cannot be undergoing a

permanent energy transfer

– Radiation, an EM field, polarizes the electron clouds of

atoms in the medium

– Polarization is a temporary deformation of the electron

clouds

Transmission and Refraction

i i

c n





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

 i= the velocity of the radiation in the medium in m/s

n i = the refractive index at the frequency i

between the radiation and the medium

– Liquids: n i ~ 1.3 to 1.8

– Solids: n i ~ 1.3 to 2.5

substances

media with different refractive indices, it can abruptly

change direction

1

2

2

1

2

1

sin

sin

v

v n

n









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

n 1= the refractive index in medium 1

of the change in velocity in

the media

an interface Its extent

depends on the refractive

indices of the media

 1

 2

Medium 1

Medium 2

Diffraction

– Also known as far-field diffraction, parallel beam

diffraction

– Important in optical microscopy

– Also known as near-field diffraction

– Widely used in

spectroscopic instruments

to separate frequencies

(can be made precisely)



 2d sin

http://www.astro.virginia.edu/research/observatories/40inch/fobos/images/grating.jpg

– Important for instrument design, crystallography

Scattering

– Scattering of small amounts of radiation by molecules

and atoms (whose size is near to the wavelength of

the radiation)

scattering in different directions

– Practical use in particle size analysis

4

1



scattering 

Figure from Sears, et al., “University Physics”, 7 th Ed., 1988

Coherent Radiation

conditions: (1) it has the

same frequency or set of

frequencies, and (2) it has a

well-defined and constant

phase relationship

– Coherent radiation is

“cross-corelated” in that the

properties of one beam can

be used to predict those of the

other beam

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)

Incoherent Radiation

emission, e.g individual

atoms in a large sample

emitting photons

to a tiny (undetectable)

extent

radiation

radiation:

– Incandescent light bulbs

– Filament sources

– Deuterium lamps

Incoherent radiation: different frequencies (colors) combined to produce continuous radiation with varying phase, frequency and

amplitude

Diagram from wikipedia.org (public domain)

More Properties of Electromagnetic Radiation

waves and particles

– Quantum mechanics developed around the concept of

the photon, the elementary unit of radiation

 E is 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

h

Absorption and Emission

change

– involves energy transfer of EM radiation to a

substance, usually at specific frequencies

corresponding to natural atomic or molecular energies

of radiation (photons

E = h

Higher energy

Lower energy

Energy Levels

occur in nature:

– Electronic

– Rotational

– Vibrational (including phonons and heat)

– Nuclear

energy-driven transitions between these “states” can be

studied (as opposed to a continuous range of energies)

The Uncertainty of Measurements

short periods, it can be difficult to measure their energies

accurately

uncertainty”:

t



The Uncertainty Principle

the location and the momentum of a particle exactly – a

fundamental limit on all measurements

position affects its momentum, and vice versa

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

then you can specify its momentum along x to p x

– As the uncertainty in x increases (x ), that of p x decreases (x

), and vice versa

p

x  x  12



Spectra and Spectrometers

– 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

of radiation with matter, so the properties of such

interactions can be studied

Spectroscopy in Analytical Chemistry

from chemical physics to biology, from individual atoms to

the largest molecules

– UV-Visible spectroscopy

– IR spectroscopy

– Raman spectroscopy

– X-ray spectroscopy

– NMR spectroscopy

– EPR spectroscopy

Further Reading

P W Atkins and R S Friedman, Molecular Quantum

R P Feynman, R B Leighton, M Sands, The Feynman

Lectures on Physics, Addison-Wesley, Reading MA (1977)

Any good physics text!