atomic spectroscopy

Atomic Spectroscopy

An Introduction of Spectrometric Methods

Properties of Electromagnetic Radiation

Electromagnetic Radiation– energy radiated in the form of a WAVE caused

by an electric field interacting with a magnetic field

– result of the acceleration of a charged particle– does not require a material medium and can

travel through a vacuum


vi = i

where vi => velocity => frequency i => wavelength


in vacuum, velocity is independent of frequency,

maximum value

c =

= 2.998 X 108 m/s


"Representation of a beam of monochromatic, plane polarized radiation. The arrows represent the electrical vectors."


"Effect of change of medium on a monochromatic beam of radiation."


"Regions of the electromagnetic spectrum"


"Common Spectroscopic Methods Based on electromagnetic Radiation"


Wave Equationfor a single wave

y = A sin (2t + )


for n waves of same frequency superimposed of phase angle y = A1 sin (2t + 1 )

+ A2 sin (2t + 2 )+ A3 sin (2t + 3 )+ + An sin (2t + n )


"Superposition of sinusoidal waves: (a) same frequency, small difference in phase (b) same frequency, large difference in phase (c) difference frequencies, starting in phase.


for n waves of different frequencies superimposed of phase angle y = A1 sin (2

t + 1 )+ A2 sin (2

t + 2 )+ A3 sin (2

t + 3 )+ + An sin (n t + n )


"Superposition of sine waves to form a square wave: (a) combination of three sine waves, and (b) combination of three, as in (a), and nine sine waves."

Diffraction

"Propagation of waves through a slit: (a) xy >(b) xy ."

Diffraction

"Diffraction of monochromatic radiation by slits."

Diffraction

= CF = BC sin

n

= CF = BC sin

DE = OD sin

DiffractionBC DE BC DE

n = ---------- = ------------OD OE

where OE => distance from screen to plane of slitsBC => slit spacingDE => band spacing

EXAMPLE: Suppose that the screen in the figure shown is 2.00 m from the plane of the slits and that the slit spacing is 0.300 mm. What is the wavelength of radiation if the fourth band is located 15.4 mm from the central band?

EXAMPLE: Suppose that the screen in the figure shown is 2.00 m from the plane of the slits and that the slit spacing is 0.300 mm. What is the wavelength of radiation if the fourth band is located 15.4 mm from the central band?

BC DE BC DEn

= ------------ = ------------

OD OEn = 4

___OE = 2.00 m___BC = 0.300 mm___DE = 15.4 mm

EXAMPLE: Suppose that the screen in the figure is 2.00 m from the plane of the slits and that the slit spacing is 0.300 mm. What is the wavelength of radiation if the fourth band is located 15.4 mm from the central band?

n = 4 OE = 2.00 m___ ___BC = 0.300 mm DE = 15.4 mm

(0.300 mm)(15.4 mm) = ----------------------------

4 (2 * 1000)mm= 5.78 X 10-4 mm or 578 nm

Transmission of RadiationTransmission

– rate at which radiation passes through a transparent material is less than through a vacuum

– depends upon the kinds and concentrations of atoms, ions, and molecules in the medium

– radiation must interact with material– interaction must not undergo permanent energy

transfer

Transmission of Radiation

Transmissionni = c/vi

where ni => refractive index, measure of interactionc => speed of light in vacuumvi => speed of light in medium

Transmission of Radiation

Dispersion– variation in refractive index of a substance with

wavelength or frequencyRefraction of Radiation

– Snell’s Law

sin

n2 v1

sin

n1 v2

Reflection of Radiation

• reflection always occurs when radiation passes from one medium to another

• reflection greatest when two materials have the greatest difference on their refractive indices

Scattering of Radiation

Types of Scattering– Rayleigh Scattering

• scattering involving molecules which are considerably smaller than the wavelength of radiation

• blue sky results from greater scattering of shorter wavelength visible light


Types of Scattering– Scattering by Large Molecules

• can be measured• a function of the size and shape of molecule


Types of Scattering– Raman Scattering

• part of the radiation undergoes quantized frequency changes

Quantum-Mechanical Properties of Radiation

E = h

= hc/

Emission of Radiation

• Line spectra• Band spectra• Continuum spectra

Line spectra

“Emission spectrum of a brine obtained with an oxy-hydrogen flame”

Line spectra

“X-ray emission spectrum of Molybdenum metal”

Band spectra

“Energy-level diagram for (a) a sodium atom showing the source of a line spectrum and (b) a simple molecule showing the source of a band spectrum.”

Continuum spectra“Blackbody radiation curves”

Absorption of Radiation

energy of exciting photon must equal energy difference between ground state and 1 excited state for absorption to occur

Absorption of Radiation

"Some typical ultraviolet absorption spectra."

Atomic Absorption

• absorption occurs with only a few well- defined frequencies

• electronic excitation

Molecular Absorption

E = Eelectronic + Evibrational + Erotational

Molecular Absorption

"Partial energy level diagram for a fluorescent organic molecule."

Emission of Radiation

• radiation resulting from the relaxation to lower energy states of excited particles

Absorption Methods, Transmittance

T = P/Po

where T => transmittanceP => power of transmitted radiationPo => power of incident

radiation%T = (P/Po )*100

where %T => percent transmittance

Absorption Methods, Absorbance

A = - log10 T = - log10 (P/Po )

where A => absorbance

Absorption Methods, Beer’s Law

A = abc = bcwhere a => absorptivity

b => path lengthc => concentration

=> molar absorptivity

Spectroscopic Methods

“Major classes of spectroscopic methods”

Absorption Methods

“Attenuation of a beam of radiation by an absorbing species”

Absorption Methods“Single-beam photometer for absorption measurements in the visible region.”

Absorption Methods“Readout for an inexpensive photometer.”

transmittance scale is linearabsorbance scale is exponentialthus, read transmittance, then calculate absorbance

atomic spectroscopy

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