4-1

Chap. 7 (Optical Instruments), Chap. 8 (Optical Atomic Spectroscopy)

• General design of optical instruments• Sources of radiation• Selection of wavelength• Sample containers• Radiation Transducers• Instruments

• Optical instruments fundamental methods Absorption Fluorescence Phosphorescence Scattering Emission Chemical Luminenscence

4-2

Optical methods• Similarities for differing methods over wavelength

range Stable source of radiation Transparent sample holder Isolation of region of interest Radiation detector

Transducer• Signal processor• Variations in setup depend upon detection of light

Linear for absorbance 90 degrees for fluorescence Emission and chemiluminescence source and

sample are same

4-3

Apparatus

4-4

Sources of radiation

• Materials Transparent

windows

4-5

Sources of Radiation

• Continuum source Emission over a

large range Intensity can vary

with wavelength

• Line Source Intense emission of

discrete lines

4-6

Light Sources

4-7

Laser Sources

• Laser properties light amplification by stimulated emission

of radiation

High intensity

Narrow wavelength

Coherent* Can very pulse energy, wavelength* Combined with laser system

electronics for short lifetime measurements

4-8

Laser Process

4-9

Laser Process• Pumping

Excitation of lasing materialCrystal (ruby)Semiconducter (GaAs)DyeGas (Ar)

Spontaneous EmissionEmission of radiation in random direction

Stimulated EmissionExcited laser species interact with emitted radiation* Deexcitation of excited species

Photon emission energy same as spontaneous emitted photonCoherent emission

4-10

Laser Dyes

4-11

Population Inversion and Amplification

Need to highly populate excited state

4-12

Three and four level transitions

Excitation to high state, transition to metastable state

4-13

0

10

20

30

Wav

enu

mb

er (

103 c

m-1

)Absorption and fluorescence process of Cm3+

Optical Spectra

HGF

7/2A

Z 7/2

Fluorescence Process

Excitation

EmissionlessRelaxation

FluorescenceEmission

4-14

4-15

Wavelength Selectors

• Quality of selected wavelength based on full with at half maximum

4-16

Filters

• Absorption filter Visible region Colored glass

or dye act as the filter

4-17

Filters

• Interference filters Combination of constructive and

destructive interference Filter wavelength based on properties of

filter

Dielectric layer determines wavelength

4-18

Filters• Constructive interference equations

n = 2dsin 90°, sin 1 n = 2d air = glass × = refractive index

n is order of interference

n

d 2

4-19

Monochromators

• Allow selection of specific wavelengths over a scanned range IR, Visible, Ultraviolet

• Similar components Entrance slit

Rectangular optical image Collimating lens

Parallel beam of radiation Prism or grating

Selection of wavelength Focus element

Reforms image and places on focal plan Exit slit

Isolates desired wavelength

4-20

Monochromators

Grating are more common in modern equipment Linear dispersion= variation in along plane ABD=Fdr/d, F= focal lengthD-1=d/nF=[nm/mm]

4-21

Monochromator

• Can calculate i is incident r is reflection

• i is known

• d is from grating in nm i.e., 2000

lines/mm needs to be converted to nm/line

• n is generally 1

• Angle r must be defined to find

)sin(sin ridn

4-22

Monochromator Slit

• Parameter that can be set • Controls light input• Resolution can be affected by slit width

Wavelength to be examined is considered Wider slits less resolution but may have

better signal

4-23

Monochromator Slit

• Can calculate slit width based on experimental consideration Resolution difference of

wavelength to be examined

• Theoretical calculation Actually need narrower slit

width due to imperfections

11

)(*5.0

DDw resolutioneff

4-24

Radiation Transducers

• Photon Transducers Photovoltaic cells Phototubes

e- emission from phosphor Photomultiplier

Cascade of electrons Photoconductors Photodiodes Charge-transfer

Si crystal collects charge due to absorption

4-25

Phototube and Photomultiplier

105-107 electrons/photon

4-26

Optical Atomic Spectroscopy• Optical Atomic Spectroscopy• Atomization Methods• Sample Introduction

• Optical Spectroscopy Elements converted to gaseous atoms or ions Measurements of atomic species

FluorescenceUV-Visible absorptionEmission

• Calculations can be made based on electron energy diagrams Transition between states

4-27

Na and Mg energy levels

4-28

Electronic Energy Symbols

• 2S+1LJ

• S is spin from unpaired e-

+ ½ L is written as S, P, D J=L+S

• Li= 1s22s1

L=0, S =+ ½ 2S1/2

4-29

Atomic Emission Spectra

• Excitation of electrons Short lived Relaxation to ground state

Emission of photon* Visible range* Possible multiple lines

• Absorption spectroscopy Resonance due to transitions from ground

to excited state• Fluorescence can also occur

4-30

Atomic Line Widths

• Broadening due to differing effects Uncertainty

vt• Line width due to Hg with lifetime of 2E-8s at

253.7 nm

4-31

Line Widths

• Doppler Atom moves during radiation interaction

4-32

Thermal effects

• Boltzmann equation

• Calculate Na atoms in 3p excited states to ground as 2500 K

• 3s to 3p transition is 3.37E-19J• P based on quantum states

3s has 2, 3p has 6

472.1)2500*2338.1

)1937.3exp(

2

61

E

KJKE

JE

N

N

o

j

4-33

4-34