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Halaman
Hukum
Interaksi Cahaya dan
Materi (2)Sifat cahaya pada tingkat atom (at the atomic
level)
Penjelasan tentang interaksi cahaya dengan
materi
Teori Gelombang tidak dapat menjelaskan
fenomena fisika pada tingkat atau skala atomik
contoh pada efek fotolistrik ( the photoelectric
effect)
Page 127
Emitter Plate
(Cathode)Collector
Plate
(Anode)
Elektron
dipancarkan dari
plat logam
Aliran Arus
Efek Fotolistrik
Cahaya dipancarkan
ke katoda yang
terbuat dari logam
dalam tabung
vakum
menyebabkan
elektron-elektron
terlepas dan pergi
menuju anoda
Arus mengalir
dalam rangkaian
VACUUM TUBE
Ammeter
+
Efek Fotolistrik
Teori Gelombang Menerangkan bahwa:
a) Energi yang terkumpul dari cahaya yang diserap logam
akan menyebabkan elektron terlepas (“sunbathing” effect)
b) Setiap panjang gelombang dari cahaya akan menghasilkan
emisi terpicu/stimulasi elektron selagi energi total yang
datang ke logam cukup.
c) Semakin kuat energi cahaya yang datang, semakin besar
energi yang dipunyai elektron yang terlepas.
(a) Energi yang terkumpul dari cahaya yang diserap logam
akan menyebabkan elektron terlepas (“sunbathing”
effect)
Efek Fotolistrik
(c) Semakin kuat energi cahaya yang datang, semakin besar
energi yang dipunyai elektron yang terlepas.
Teori Kuantum membuktikan bahwa terori gelombang salah:
Elektron terlepas secara instant selagi energi foton
yang datang melebihi energi ikat elektron dengan inti
Hanya panjang gelombang cahaya dengan energi >
dari energi ikat yang dapat melepaskan elektron
Energi Kinnetik dari elektron yang terlepas tidak
bergantung pada Intensitas cahaya yang datang
(b) Setiap panjang gelombang dari cahaya akan
menghasilkan emisi terpicu/stimulasi elektron selagi
energi total yang datang ke logam cukup.
Tingkat Energi Atom:
(yang tidak bisa dijelaskan dengan Teori
Gelombang)
Pada Tingkat atom, cahaya digambarkan
sebagai paket-paket energi (foton)
Setiap foton mempunyai tingkat – tingkat
energi yang diskrit.
Elektron yang mengorbit inti atom berada pada
tingkat-tingkat energi yang diskrit.
E = energy per photon
h = Planck’s constant (6.62 10-34
joule sec)
= photon frequency
c = velocity of light in a vacuum
= photon wavelength
Energi Foton
chhE
Frekuensi (Panjang Gelombang) vs.
Energi Foton
joulesch
hE
nmGreen
19
9
834
1038.3106.587
1031062.6:)6.587(
Perbandingan energi yang dibawa oleh foton cahaya
hijau (Green - 587.6 nm) dan foton dari cahaya biru
(Blue - 400 nm) :
joulesch
hE
nmBlue
19
9
834
1097.410400
1031062.6:)400(
Defenisi dari electron Volt (eV)
jouleseVvoltelectron 1910602.1)(1
Untuk menghindari pengunaan pangkat negatif yang
besar (pada range 10-19 ) untuk energi foton (joule)
maka didefinisikan satuan energi yang lebih
mudah :
1 electron volt = energi yag diproleh oleh sebuah
foton ketika ia dipercepat melalui sebuah beda
potensial 1 volt (J/C)
Energi Foton dalam eV
Cahaya Biru (400 nm): E = 3.10 eV
Cahaya Hijau (587.6 nm): E = 2.11 eV
Cahaya Merah (700 nm): E = 1.77 eV
photoelectron
Tingkat Energi atom Ketika terjadi
Efek Fotolistrik
Electron absorbs
photon and jumps to
higher energy level
Tingkat-Tingkat Energi: Peristiwa
Penyerapan Foton
Dengan menyerap
foton dengan energi
yang cukup, sebuah
elektron dapat
meloncat ke tingkat
energi yang lebih
tinggi.
Elektron memancarkan
(emisi) foton dan
meloncat ke tingkat
energi yang lebih
rendah.
Tingkat-Tingkat Energi: Peristiwa
Emisi Foton
Tingkat-Tingkat Energi: Peristiwa
Emisi Foton
When an electron
drops to a lower energy
level, a photon is
emitted
Atomic Energy Levels Ground state (E0): lowest, most stable energy level in
an atom:
strongest electrostatic attraction between nucleus and
electron
lowest electron kinetic energy
Excited states (E1 E2 etc.): with elevation to higher
energy levels, electrons become less stable:
weaker electrostatic attraction
higher electron kinetic energy
farther (on average) from nucleus
Page 128
Tingkat energi Level:
Atom Hidrogen
voltelectoneV
stateexcitationmwherem
eVEm
)(2,1,0
)1(
6.132
Page 129
Tingkat energi Level:
Atom Hidrogen
eVeV
E 6.131
6.130
eVeV
E 4.34
6.131
eVeV
E 51.19
6.132
eVeV
E 85.016
6.133
)(2,1,0
)1(
6.132
stateexcitationm
m
eVEm
Tanda Negatif indicates
electrostatic attraction to
nucleus (Binding Energy)
Consider negative electron
energy as the “electrostatic hill”
that the electron must climb to
be freed from the atom
Zero energy free electron
Atomic Energy Diagram
Shows all the valid energy levels for the atom.
Energy required for an electron to jump to a
higher level
Photon energy released as an electron drops to a
lower level
Hydrogen Energy Diagram
-14
-13
-7
-9
-8
-10
-11
-12
-2
0
-1
-3
-4
-5
-6
E0
E1
E2
E3E4
E5
0 eV
10.2 eV
Tra
nsi
tion L
evel
above
Gro
und S
tate
(eV
)
Ele
ctro
n E
ner
gy
(eV
)
13.6 eV
12.74 eV
12.09 eV
Ionization State
91 nm
121 nm
486 nm656 nm
HYDROGEN ENERGY
LEVEL DIAGRAM
103 nm
97 nm
Fig 78, p 128
Hydrogen Balmer
series encompasses
transitions up or
down to/from the first
excited state (E1).
Note the hydrogen F
line (E3 E1) and C
line (E2 E1)
Energy Levels: Hydrogen Atom
After absorbing energy, the electron remains in an
excited state for an extremely short period (~ 10 nsec)
Spontaneous emission: as the unstable, excited electron
drops to a lower energy level, it emits a photon.
Photon energy is equal to the difference in atomic
energy levels
Energy Levels: Hydrogen Atom
eVeVeVEEE 89.1)4.3(51.112
Photon energy for the transition from E2 to E1:
nmeV
nmeV
E
chchE 3.656
89.1
239,1 1
Hydrogen C-line
Energy Levels: Hydrogen Atom
nmeV
nmeV
E
chchE
eVeVeVEEE
877,166.0
239,1
66.0)51.1(85.0
1
23
A single energy jump from a higher excited state
causes a smaller energy transition (e.g. from E3 to
E2):
Energy Levels: Hydrogen Atom
eVeVeVEEE 55.2)4.3(85.013
Multi-level energy transitions can also occur (e.g.
from E3 to E1):
The greater the energy transition as an electron jumps
to a lower energy level, the shorter the wavelength of
the emitted photon
nmeV
nmeV
E
chchE 486
55.2
239,1 1
Hydrogen F-line
Atomic Spectra: Hydrogen
All energy transitions (single-level and multi-level)
are possible for the hydrogen atom.
Photons corresponding to all possible transitions are
emitted:
gives rise to characteristic discrete spectral lines of
low pressure H2 gas
Atomic Spectra: Hydrogen
Discrete hydrogen spectral lines “fingerprint”
for hydrogen.
Discrete hydrogen spectra used extensively in
astronomy
Characteristic atomic spectra in a gas best seen at
low pressure - at higher pressures, spectra begin to
change
Atomic Spectra
Bunsen burned salts containing various elements in a
flame:
placed a series of slits in front of the flame
directed the light through a prism to disperse ’s
This allowed him to view the line spectra of elements
)1( glassnd
1n
Spectroscope
Absorption vs. Emission Spectra
Absorption Spectra
sample
sample
400 nm 500 nm 600 nm 700 nm
www.chem.uidaho.edu
Solar Spectrum
www.chem.uidaho.edu
Atomic Spectra
The colors of fireworks are created by atomic line spectra
Atomic Spectra: Hydrogen
Increasing gas temperature excites a greater
proportion of H atoms more atoms
spontaneously emitting photons
Explains why gas discharge lamps glow brighter as
they warm up
Atomic Spectra
Increasing gas pressure changes atomic spectra
collisions between molecules also cause energy
exchanges.
As pressure increases, collision frequency increases
discrete spectra gradually give way to a
continuous spectrum
QQ1. Which transition in the hydrogen atom would result in
emission of the shortest wavelength photon?
(A) E0 E1
(B) E3 E1
(C) E4 E1
(D) E5 E2
wrong direction
3.4 – 0.85 = 2.55 eV
3.4 – 0.54 = 2.86 eV
1.51 – 0.38 = 1.13 eV
QQ2. What type of atomic spectrum would most likely be seen
for a hydrogen gas cloud in space?
(A) Absorption spectrum
(B) Emission spectrum
(C) Continuous spectrum
(D) All of the above
Low pressure, cold gas
Page 129Fraunhofer Lines (Solar Absorption Spectrum
Page 129
Designation Element Wavelength (nm) Designation Element Wavelength (nm)
y O2 898.765 c Fe 495.761
Z O2 822.696 F H β 486.134
A O2 759.370 d Fe 466.814
B O2 686.719 e Fe 438.355
C H α 656.281 G' H γ 434.047
a O2 627.661 G Fe 430.790
D1 Na 589.594 G Ca 430.774
D2 Na 588.997 h H δ 410.175
D3 He 587.565 H Ca+
396.847
E2 Fe 527.039 K Ca+
393.368
b1 Mg 518.362 L Fe 382.044
b2 Mg 517.270 N Fe 358.121
b3 Fe 516.891 P Ti+
336.112
b4 Fe 516.751 T Fe 302.108
b4 Mg 516.733 t Ni 299.444
Major Fraunhofer Lines (Solar Spectrum)
Solar Spectrum Series of absorption lines produced when sunlight
emitted from the hotter solar chromosphere is
absorbed by the cooler outer solar photosphere
Hydrogen makes up 92.1% of the sun’s atoms,
helium 9.2%, and sodium, calcium, and iron 0.1%
Overall, several thousand solar Fraunhofer lines
representing 67 elements
Table 1 – Major Solar Fraunhofer Lines
Designation Wavelength (nm) Origin
A 759.4 terrestrial oxygen
B 686.7 terrestrial oxygen
C 656.3 hydrogen (Hα)
D1 589.6 neutral sodium (Na I)
D2 589.0 neutral sodium (Na I)
E 527.0 neutral iron (Fe I)
F 486.1 hydrogen (Hβ)
H 396.8 ionized calcium (Ca II)
K 393.4 ionized calcium (Ca II)
Fluorescence
Page 129
Fluorescence Fluorescence is an example of energy absorption
followed by spontaneous photon emission.
Many substances that can be raised by a stimulating
source from ground state to an excited state, then
spontaneously emit photons, will theoretically
fluoresce.
Page 129
Fluorescence
Typically a high frequency source (UV) is needed to
raise atoms from the ground state.
A fluorescent substance could undergo a single energy
level transition: E0 E1 for excitation, followed by
E1 E0 for spontaneous emission. This is rare.
Most fluorescent substances, after excitation, will
undergo a “non-radiative” transition (e.g. E2 E1)
followed by photon release (e.g. E1 E0)
Fluorescence (typical case) Fig 79, p 130
Fluorescence Most substances are not 100% efficient, emitting less
energy than they absorbed.
Thermal agitation (vibrational loss) is the most
common cause of the energy loss between absorption
and emission (non-radiative transition)
The emitted photon will therefore have lower energy
than the exciting photon
UV absorption
Fluorescence
emission
(photon)
Electron
elevated
to higher
energy
level
Electron spontaneously
returns to lower energy state
nanoseconds pass
Fluorescence & Stoke’s Reaction Electron energy
has decreased:
thermal loss
(< 1 nsec)
Fluorescence Thermal losses explain why fluorescent emission
usually has longer wavelength (different color) than the
excitation source.
e.g. blue light may be absorbed by a fluorescent
substance. Thermal agitation causes energy loss from
the excited atoms, leaving less energy for emission.
The emitted photon will have longer (e.g. green)
The difference in energy between the excited and
emitted photon is called Stoke’s shift
Explaining Stoke’s Shift
Molecules contain discrete electronic energy levels
(E0, E1, E2 , etc. …..).
Each energy level also consists of a series of
vibrational sub-levels (due to motion of the non-rigid
nucleus within the molecular “framework”)
Interaction with surrounding molecules causes rapid
loss of vibrational energy to the environment (this is
the “thermal agitation” loss; non-radiative transition)
The various types of atomic energy (electronic,
vibrational, spin angular momentum etc.) are
depicted in Jablonski diagrams
Jablonski Diagram for Fluorescence
Excited vibrational states
Vibrational relaxation
F
E0
E1
E2
En
Ground state
Ener
gy
Incident radiation excites the ground state
molecule (A)
The molecule is also excited to vibrational
levels of the excited state
Vibrational levels rapidly deactivate due to
collisions with the surrounding environment
until the molecule reaches its lowest excited
state (vibrational relaxation)
If the interaction between the excited molecule
and its surroundings is not enough to cause the
large energy transfer to return it to ground state,
the molecule fluoresces (F, radiative decay)
A
Ophthalmic Applications of
Fluorescence Page 131
Ophthalmic Applications of
Fluorescence
Sodium fluorescein has several
applications in ophthalmic practice:
tear film instillation to:
visualize ocular surface anomalies
evaluate tear film stability
Fluorescein angiography
Fluorescein A cobalt blue filter provides excitation energy (365 -
470 nm)
Thermal agitation decreases the energy available for
emission
522 nm green photons are emitted
Fluorescein: Ocular Surface Evaluation
Includes detection of corneal surface abrasions,
superficial punctate keratitis, foreign body tracks in
contact lens wearers etc.
Fluorescein is placed into the tear film and the green
emission pattern is viewed through a (stimulating)
cobalt blue filter (usually with a biomicroscope)
Superficial punctate keratitis
Cornea
Double AbrasionFluorescein
Fluorescein Angiography Fluorescein dye is injected into the blood (or may be
taken orally)
A retinal camera equipped with UV filter is used to
monitor the passage of fluorescein through the pre-
retinal vasculature
Any fresh hemorrhages, vascular leak etc. shows up as
fluorescent green patches outside the blood vessels
(which also fluoresce)
Page 132
Arterial Phase: NORMAL
Fluorescein Angiography: Eye
Early venous phase - Normal
Fluorescein Angiography: Eye
Complete Filling Arteries & Veins: NORMAL
Fluorescein Angiography: Eye
Late Phase - Fading of Dye: NORMAL
Fluorescein Angiography: Eye
Early Phase - Dye leakage: DIABETIC RETINOPATHY
Fluorescein Angiography: Eye
Fluorescein Angiography Fluorescein angiography is especially useful in diabetes
where microaneursyms, and “dot” and “blot”
hemorrhages show up.
Arterio-venous Phase - Neovascularization:
DIABETIC RETINOPATHY
Fluorescein Angiography: Eye
Late Phase - Hemorrhage from new vessels:
DIABETIC RETINOPATHY
Fluorescein Angiography: Eye
Fluorescein Angiography: Dot
Hemorrhages & Microaneurysms
Fluorescein Angiography: Dot
Hemorrhages & Microaneurysms
Fluorescence overlay antigen mapping (C) of a skin section using two
Mab’s.
A RITC-conjugate
B FITC-conjugate
C overlay image of A and B shows yellow fluorescence on sites
where both antigens are present (between arrows). (Bar, 50 microns)
From: Jonkman: J Clin Invest, Volume 95(3).March , 1995.1345-1352
Fluorescence Microscopy
PhosphorescencePage 132
Phosphorescence Same principle as fluorescence, but atoms of a
phosphorescent substance will remain in the excited
state for a much longer period of time.
This requires the existence of a metastable state - i.e. a
relatively stable excited state that the atom may remain
in from second to hours.
To understand the basis of a metastable state (also
applies to lasers) must look at sources of atomic energy
beyond electronic energy
Page 132
Most fluorescent substances emit a photon of different
wavelength than the absorbed photon. The reason for this is:
Practice Problem I
(A) spontaneous emission occurs in a random direction
with a random wavelength
(B) vibrational losses in the excited substance reduce the
energy available for emission
(C) thermal transfer increases the energy of the atom
resulting in an emitted photon of shorter wavelength
(D) metastable substances build up stored energy
resulting in emission of a higher energy photon
Fluorescence could be described as a process of:
Practice Problem II
(A) stimulated emission, where high energy photons cause an
atom to drop immediately to the ground state, emitting
photons of longer wavelength due to thermal loss
(B) thermal agitation of an atom from a metastable state to a
higher, unstable state, causing an immediate drop to ground
state, accompanied by photon emission
(C) spontaneous emission, where longer wavelength photons
are absorbed and shorter wavelength photons are emitted
due to thermal loss
(D) spontaneous emission, where higher energy photons are
absorbed, and lower energy photons are emitted
In the process of fluorescence, Stoke’s shift refers to:
Practice Problem III
(A) the almost immediate drop by an excited atom to a slightly lower
energy level prior to spontaneous emission of a fluorescent
photon of shorter wavelength than the exciting photon
(B) the increase in atomic energy level that occurs when the atom
absorbs a photon of sufficient energy to raise it to the required
state to allow subsequent fluorescent emission
(C) the difference in energy level between excitation and emission
energy that is responsible for the characteristic wavelength of the
emitted photon
(D) the tendency of atoms to vibrate and make translational
movements as they are raised to higher energy levels by exciting
photons
Compared with fluorescence, phosphorescence differs:
Practice Problem IV
(A) in that phosphorescent atoms must be raised to substantially
higher energy levels due to the much more significant thermal
losses that occur for the prolonged life of the excited state
(B) in that phosphorescence involves a combination of spontaneous
and stimulated emission, while fluorescence involves only
spontaneous emission
(C) only in the presence of a metastable state to maintain excited
phosphorescent atoms above ground state for prolonged time
periods
(D) only in the fact that phosphorescent atoms do not suffer any
vibrational (thermal) loss after initial excitation, so the
stimulating energy is equal to the transitional energy giving rise
to the emitted photon
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