physics 203/204
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Physics 203/204. 10 Interaction of Light with Matter. Fluorescence and Phosphorescence Lasers Diatomic Molecules Rotational and Vibrational Motion of a Molecule Molecular Spectra. Fluorescence and Phosphorescence. Fluorescence Light emission ends instantly incident radiation does - PowerPoint PPT PresentationTRANSCRIPT
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Physics 203/204Physics 203/204
10 Interaction of Light with Matter
•Fluorescence and Phosphorescence•Lasers•Diatomic Molecules•Rotational and Vibrational Motion of a Molecule•Molecular Spectra
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Fluorescence and Phosphorescence
•Fluorescence
•Light emission ends instantly incident radiation does
• Phosphorescence
•Light emission continues after incident radiation is shut off
•LIFETIME of excited state shortLIFETIME of excited state short
•FLUORESCENCEFLUORESCENCE
•LIFETIME of excited state longLIFETIME of excited state long
•PHOSPHORESCENCEPHOSPHORESCENCE
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h
Spontaneous EmissionSpontaneous Emission
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•Emitted frequency does not have to equal incident frequency
•Must be less than or equal to incident frequency
•Incident blue light can produce red light
•Incident red light CANNOT produce blue light
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h
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STIMULATED STIMULATED EMISSIONEMISSION
h
h
h
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•Emitted light is
•In phase
•Going in same direction
•More intense
•same frequency
•with incident light
•Thus we get intense MONOCHROMATIC
•light emitted
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•Need long lived excited state•in order to have high probability•of Stimulated emission•META STABLE STATE
•If all light produced by stimulated emission
•is made to travel in same direction
•and not diverge (Attenuate)
•We would have a strong beam of parallel intense monochromatic light
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•L ight•A mplified by•S timulated•E mission of•R adiation
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Helium States Neon States
Pumping
Electric discharge
Metastable State
He-Ne collisions
E2
E1
E0
h E2-E1
Laser light
632.8 nm
rapid decay
ground state
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Diatomic Molecules
n1l1ml1ms1nNlNmlNms N
r1,r2, rN Antisymmetric combination of the products
n1l1ml1ms1
r1 n2l2ml2ms2
r2 nNlNmlNms Nr N
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Motion of an object can be analyzed into three
types
•translation of the center of mass•rotation (rigid) about the center of mass •vibration (elastic) about center of mass
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The energy of a molecule can be broken down
into four parts
Etot Eelec Etrans Erot EvibThe motion of the molecule is coupled to the
electronic standing wave patterns and effects
their energyfrequency by changing their
environment, i.e. different translational or
rotational or vibrational motions of the nuclei
produce electronic standing wave patterns
electronic stationary states of slightly different
energies.
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The rotational energy of a molecule is given by
Erot 1
2Icm2
1
2Icm
Icm 2 1
2Icm
Lcm2
Icm
m1m2m1 m2
r2 r 2
reduced mass, r inter atomic distance
The angular momentum of the molecule is quantized
Lcm
= J(J 1) ; J 0,1,2,3,
Erot
2
2IcmJ(J 1); J 0,1,2,3,
The frequency differences between these levels lie in
the microwave frequency range
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Near the equilibrium bond length,
the bonding force between the nuclei
can be approximated by a spring force
and a diatomic molecule simulates
a simple harmonic oscillator of frequency
f 1
2 k
; k spring constant
motion of reduced mass about spring whose
equilibrium length is the equilibrium bond
length
E vib v 1
2
h ; v 0 ,1, 2 ,
The frequency differences between these levels
lie in the infrared range
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Molecular spectra will have both vibrational and rotational levels. The spacing between rotational levels is much smaller.
Hence for each electronic level there will be a set of vibrational levels superimposed and for vibrational levels there will be a
set of rotational levels superimposed . The total energy for a fixed electronic standing wave pattern
labeled by "n"is the sum of the energies due to vibrational and rotational modes
EnvJEn 2J J1
2
2I v 1
2
h; J 0,1,2,;v=0,1,2,
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E000
E001
E010
E020
E100Molecules will absorb or emit photons that
have frequencies equal to the
DIFFERENCE of natural
frequencies
E101
E110
E120