Download photoelectric effect, atomic spectra, electron diffraction

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MatterMatter-Energy interactions are problematic
– Matter = collection of oscillating dipoles
– High ν oscillators “missing in action” at low T
Quantized Energy
Classical energy exchange
q1 q2
1
p2
PE =
KE =
m v2 =
R
2
2m
ΔE Æ 0
Quantized energy exchange
ΔE = h ν
Photoelectric Effect
Monochromatic
light ejects
electrons from
metal surface
Measure
#electrons
(current)
Measure
electron KE by
adjusting voltage
(QM if hν/kT not << 1)
Photoelectric Effect
Classical physics says
– Elight proportional to intensity (not frequency)
frequency)
Predicts
– If electrons bound to atoms,
Intensity threshold for current
Intensity and electron KE rise together
Current & KE do not depend on frequency
Observe the opposite!
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Photoelectric Effect
Atomic Spectra
Observations
– Current rises with intensity
no threshold
– KE = hν − φ
KE
Indep’t of intensity
Einstein (1879(1879-1955)
ν
– 1905
– light energy = photons
Atomic Spectra
Magic numbers predict H atom λ
1
1 y
j
z
−
= RH i
2
λ
n2 {
k n1
1
Why?
where n > n1 and are integers
Atomic Spectra
Bohr (1885 – 1962)
– 1913, H atom model
electron in stable circular orbit
Light emitted if electron jumps orbits
– hi E orbit Æ lo E orbit
– E change = hν
Angular momentum quantized = nh/2π
nh/2π
– n is (magic) quantum number
Rydberg
RH =
2 π 2 me q e 4
h3 c
2
Sporadic Progress
Heat capacity
Blackbody, 1900 Æ E quantized
Photoelectric, 1905 Æ E localized
Atomic spectra, 1913 Æ Connects
quantum theory to huge volume of data
What about matter?
de Broglie’s Proposal
de Broglie (1892(1892-1987)
1923
– photons
E = h ν = m c2
h / λ = m c = momentum
– matter
h/λ=mv=p
– de Broglie
λ=h/p
Why do atoms exist?
de Broglie’s wavelength
m = relativistic
photon mass
(rest mass = 0)
Measure λ by Diffraction
“quick” electron
me = 9.1 x 10-31 kg
v = 3 x 107 m/s
λ = 24 pm
ave Ar atom at 298 K
m = 6.63 x 10-26 kg
v = (3 k T / m)1/2 = 430 m/s
λ = 23 pm
Alan walking
(88 kg)(1 m/s)
m/s)
λ = 7.6 x 10-24 pm
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Interference
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