Modern Optical Spectroscopy
Introduction of AES, NMR, and Laser Shu-Ping Lin, Ph.D. Institute of Biomedical Engineering E-mail: [email protected] Website: http://web.nchu.edu.tw/pweb/users/splin/
Nuclear Magnetic Resonance (NMR) Spectroscopy Molecular Spectroscopy
Nuclear magnetic resonance (NMR) spectroscopy: A spectroscopic technique that gives us information about the number and types of atoms in a molecule, for example, about the number and types of 1 hydrogen atoms using H-NMR spectroscopy. 13 carbon atoms using C-NMR spectroscopy. 31 phosphorus atoms using P-NMR spectroscopy. Nuclear Spin States
An electron has a spin quantum number of 1/2 with allowed values of +1/2 and -1/2.
This spinning charge has an associated magnetic field.
In effect, an electron behaves as if it is a tiny bar magnet and has what is called a magnetic moment. The same effect holds for certain atomic nuclei.
Any atomic nucleus that has an odd mass number, an odd atomic number, or both, also has a spin and a resulting nuclear magnetic moment.
The allowed nuclear spin states are determined by the spin quantum number, I, of the nucleus.
The Chemistry of Life - Atoms
The basic unit of each chemical element is the atom. Atoms have a large nucleus, composed of protons and neutrons held together by the Strong Force. The electrons "orbit" the nucleus, attracted by the Electrical Force.
The number of protons determines the chemical element, and the number of neutrons determines the isotope of the element.
No net electrical charge Ions -- when atoms gain or lose electrons
http://cass.ucsd.edu/public/tutorial/scale.html
Carbon Orbitals around a nucleus
Figure1.1 Periodic Table
Periodic table indicating the atomic properties of all elements found on earth.
Periodic Table Explorer is a simple Periodic Table software. http://www.technosamrat.com/freewares/periodic-table-explorer/ Nuclear Spin States
A nucleus with spin quantum number I has 2I + 1 spin states; if I = 1/2, there are two allowed spin states.
Spin quantum numbers and allowed nuclear spin states for atoms common to organic compounds.
Element 1H 2H 12C 13C 14N 15N 16O 19F 31P 32S Nuclear spin quantum 1/2 1 0 1/2 1 1/2 0 1/2 1/2 0 number (I )
Number of 2 3 1 2 3 2 1 2 2 1 spin states Nuclear Spins in H0
1 13 Within a collection of H and C atoms, nuclear spins are completely random in orientation.
When placed in a strong external magnetic
field of strength H0, however, interaction between nuclear spins and the applied magnetic field is quantized. The result is that only certain orientations of nuclear magnetic moments are allowed. Nuclear Spins in H0
1 13 for H and C, only two orientations are allowed. Nuclear Spins in H0
In an applied field strength of 7.05T the difference in energy between nuclear spin states for 1 H is approximately 0.120 J (0.0286 cal)/mol, which corresponds to a frequency of 300 MHz (300,000,000 Hz). 13 C is approximately 0.030 J (0.00715 cal)/mol, which corresponds to a frequency of 75MHz (75,000,000 Hz). Nuclear Spin in H0
The energy difference between allowed spin states increases linearly with applied field strength. 1 Values shown here are for H nuclei. 在較強磁場中，二自旋狀態之能量差△E，比在較
弱磁場中之能量差更大。其能量差△E與磁場強度Ho成 正比，即 h EH (12-1) 2 o
其中 △E=α及β自旋狀態之能量差
h=蒲郎克常數
Ho=外磁場強度，高斯 (gauss)
γ=迴轉磁係數 (gyromagnetic ratio)，每一質子為 26,753-1高斯-1 當一質子在合適磁場接受一光子時，可由α-自旋 狀態跳至β-自旋狀態，稱此原子核是共振 (resonance)。 一光子的能量以E=hν表示，此式與式 (12-1) 合併可得
h E h H (12-2) 2 o
解出為ν 1 H 2 o (12-3) 一質子之26,753-1高斯-1
1 (26,753)H 4257.8 s11高斯 H (高斯 ) (12-4) 2 oo
質子共振頻率發生於光譜的無線電頻率區。對
於共振所需無線電頻率可由磁場Ho求出。
最常用的NMR操作頻率為60~300MHz(60~300百 萬赫 ) 相當於14,092 高斯的磁場。對於較高解析度的 NMR，則使用300~600MHz之頻率操作。 Nuclear Magnetic Resonance
When nuclei with a spin quantum number of 1/2 are placed in an applied field, a small majority of nuclear spins are aligned with the applied field in the lower energy state.
The nucleus begins to precess and traces out a cone-shaped surface, in much the same way a spinning top or gyroscope traces out a cone- shaped surface as it precesses in the earth’s gravitational field.
Nuclear Magnetic Resonance
If the precessing nucleus is irradiated with electromagnetic radiation of the same frequency as the rate of precession,
the two frequencies couple
energy is absorbed
the nuclear spin is flipped from spin state +1/2 (with the applied field) to -1/2 (against the applied field). Nuclear Magnetic Resonance
(a) Precession and (b) after absorption of
electromagnetic radiation. Bo=Ho Nuclear Magnetic Resonance
Resonance: In NMR spectroscopy, resonance is the absorption of energy by a precessing nucleus and the resulting “flip” of its nuclear spin from a lower energy state to a higher energy state.
The precessing spins induce an oscillating magnetic field that is recorded as a signal by the instrument.
Signal: A recording in an NMR spectrum of a nuclear magnetic resonance. Nuclear Magnetic Resonance
1 If we were dealing with H nuclei isolated from all other atoms and electrons, any combination of applied field and radiation that produces a signal for one 1H would produce a signal for all 1H. The same is true of 13C nuclei.
Hydrogens in organic molecules, however, are not isolated from all other atoms. They are surrounded by electrons, which are caused to circulate by the presence of the applied field.
The circulation of electrons around a nucleus in an applied field is called diamagnetic current and the nuclear shielding resulting from it is called diamagnetic shielding. Nuclear Magnetic Resonance
The difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small.
The difference in resonance frequencies for
hydrogens in CH3Cl compared to CH3F under an applied field of 7.05T is only 360 Hz, which is 1.2 parts per million (ppm) compared with the irradiating frequency. 360 Hz 1.2 = 6 = 1.2 ppm 300 x 106 Hz 10 Nuclear Magnetic Resonance
Signals are measured relative to the signal of the reference compound tetramethylsilane (TMS).
CH3
CH3 Si CH3
CH3 Tetramethylsilane (TMS)
1 For a H-NMR spectrum, signals are reported by their shift from the 12 H signal in TMS. 13 For a C-NMR spectrum, signals are reported by their shift from the 4 C signal in TMS.
Chemical shift (): The shift in ppm of an NMR signal from the signal of TMS. Chemical Shift - 1H-NMR
Chemical shift δ is usually expressed in parts per million (ppm) by frequency, because it is calculated from:
Since the numerator is usually in hertz, and the denominator in megahertz, delta is expressed in ppm. NMR signal that absorbs at 300 Hz lower than does TMS at an applied frequency of 300 MHz has a chemical shift of:
Although the frequency depends on the applied field the chemical shift is independent of it. On the other hand the resolution of NMR will increase with applied magnetic field resulting in ever increasing chemical shift changes NMR Spectrometer
Schematic diagram of a nuclear magnetic resonance spectrometer. NMR Spectrometer
Essentials of an NMR spectrometer are a powerful magnet, a radio-frequency generator, and a radio-frequency detector. The sample is dissolved in a solvent, most commonly CDCl3 or D2O, and placed in a sample tube which is then suspended in the magnetic field and set spinning. Using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in about 2 seconds. NMR Spectrum 1 H-NMR spectrum of methyl acetate.
High frequency: The shift of an NMR signal to the left on the chart paper.
Low frequency: The shift of an NMR signal to the right on the chart paper. Equivalent Hydrogens
Equivalent hydrogens: Hydrogens that have the same chemical environment.
A molecule with 1 set of equivalent hydrogens gives 1 NMR signal.
O H3 C CH3 CH3 CCH3 ClCH 2 CH2 Cl C C H3 C CH3 Propanone 1,2-Dichloro- Cyclopentane 2,3-Dimethyl- (Acetone) ethane 2-butene Equivalent Hydrogens
A molecule with 2 or more sets of equivalent hydrogens gives a different NMR signal for each set.
Cl Cl CH3 CH3 CHCl O C C H H 1,1-Dichloro- Cyclopent- (Z)-1-Chloro- Cyclohexene ethane anone propene (3 signals) (2 signals) (2 signals) (3 signals) Signal Areas
Relative areas of signals are proportional to the number of H giving rise to each signal, Modern NMR spectrometers electronically integrate and record the relative area of each signal. Chemical Shift - 1H-NMR Type of Chemical Type of Chemical Hydrogen Shift () Hydrogen Shift () ( CH3 ) 4 Si 0 (by definition) O RCH 0.8-1.0 3 RCOCH3 3.7-3.9 Chemical RCH2 R 1.2-1.4 O Shifts R3 CH 1.4-1.7 RCOCH2 R 4.1-4.7 1H-NMR R C= CRCHR 2 2 1.6-2.6 RCH2 I 3.1-3.3 RC CH 2.0-3.0 RCH2 Br 3.4-3.6 ArCH 3 2.2-2.5 RCH2 Cl 3.6-3.8 ArCH R 2 2.3-2.8 RCH2 F 4.4-4.5 ROH 0.5-6.0 ArOH 4.5-4.7 RCH2 OH 3.4-4.0 R2 C= CH2 4.6-5.0 RCH2 OR 3.3-4.0 R2 C= CHR 5.0-5.7 R2 NH 0.5-5.0 ArH 6.5-8.5 O O RCCH3 2.1-2.3 RCH 9.5-10.1 O O RCCH2 R 2.2-2.6 RCOH 10-13 Chemical Shift
Chemical shift depends on the (1) electronegativity of nearby atoms, (2) hybridization of adjacent atoms, and (3) diamagnetic effects from adjacent pi bonds.
Electronegativity Electroneg- Chemical CH3 -X ativity of X Shift ()
CH3 F 4.0 4.26 CH3 OH 3.5 3.47 CH3 Cl 3.1 3.05 CH3 Br 2.8 2.68 CH3 I 2.5 2.16 (CH3 ) 4 C 2.1 0.86 (CH3 ) 4 Si 1.8 0.00 Chemical Shift
Hybridization of adjacent atoms.
Type of Hydrogen Name of Chemical (R = alkyl) Hydrogen Shift ()
RCH3 , R2 CH2 , R3 CH Alkyl 0.8 - 1.7
R2 C=C(R)CHR2 Allylic 1.6 - 2.6 RC CH Acetylenic 2.0 - 3.0
R2 C=CHR, R2 C=CH2 Vinylic 4.6 - 5.7 RCHO Aldehydic 9.5-10.1 Chemical Shift
Diamagnetic effects of pi bonds
A carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal to lower frequency (to the right) to a smaller value.
A carbon-carbon double bond deshields vinylic hydrogens and shifts their signal to higher frequency (to the left) to a larger value.
Chemical Type of H Name Shift ()
RCH3 Alkyl 0.8- 1.0 RC CH Acetylenic 2.0 - 3.0
R2 C=CH2 Vinylic 4.6 - 5.7 Chemical Shift
Magnetic induction in the bonds of a carbon- carbon triple bond shields an acetylenic hydrogen and shifts its signal lower frequency. Chemical Shift
Magnetic induction in the bond of a carbon- carbon double bond deshields vinylic hydrogens and shifts their signal higher frequency. Chemical Shift
The magnetic field induced by circulation of electrons in an aromatic ring deshields the hydrogens on the ring and shifts their signal to higher frequency. 常見各類型質子化學位移的典型值
質子類型 近似值δ(ppm)
－CH3 0.9
－CH2－ 1.3 ──CH 1.4 | O 2.1 ∥
──C CH3 ──CCH 2.5
R─ CH2 ─ X(X: 鹵素 , O) 3～4 ＼／ 5~6 CC＝ ／ ＼ H ＼／ 1.7 CC＝ ／ ＼ CH3 Ph─ H 7.2
Ph─ CH3 2.3 R─ CHO 9~10 R─ COOH 10~12 R─ OH 2~5 Ar─ OH 4~7
RNH─ 2 1.5~4
RNH2 ─ 1~1.5 R─ SH 1~1.5 Ar─ SH 3~4 3.4~4 Ar── NH2 , Ar NHR
─SOH3 11~12 Signal Splitting; the (n + 1) Rule
Peak: The units into which an NMR signal is split; doublet, triplet, quartet, multiplet, etc.
Signal splitting: Splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens.
(n + 1) rule: If a hydrogen has n hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s), its 1H-NMR signal is split into (n + 1) peaks. Signal Splitting (n + 1)
1 H-NMR spectrum of 1,1-dichloroethane.
For these hydrogens, n = 1; For this hydrogen, n = 3; their signal is split into CH3 - CH- Cl its signal is split into (1 + 1) = 2 peaks; a doublet Cl (3 + 1) = 4 peaks; a quartet Signal Splitting (n + 1)
Problem: Predict the number of 1H-NMR signals and the splitting pattern of each.
O (a) CH3 CCH2 CH3
O (b) CH3 CH2 CCH2 CH3
O (c) CH3 CCH( CH3 )2 Origins of Signal Splitting
Signal coupling: An interaction in which the nuclear spins of adjacent atoms influence each other and lead to the splitting of NMR signals.
Coupling constant (J): The separation on an NMR spectrum (in hertz) between adjacent peaks in a multiplet.
A quantitative measure of the spin-spin coupling with adjacent nuclei. Origins of Signal Splitting
Illustration of spin-spin coupling that gives rise to signal splitting in 1H-NMR spectra. Origins of Signal Splitting
1 The quartet-triplet H-NMR signals of 3- pentanone with the original trace and an expansion to show the signal splitting clearly. Coupling Constants
Coupling constant (J): The distance between peaks in a split signal, expressed in hertz.
The value is a quantitative measure of the magnetic interaction of nuclei with coupled spins.
Ha Ha Ha Hb Ha C C Hb Hb
Hb 6-8 Hz 8-14 Hz 0-5 Hz 0-5 Hz
Ha Ha Hb Ha Ha C C C C C C Hb Hb Hb 11-18 Hz 5-10 Hz 0-5 Hz 8-11 Hz Origins of Signal Splitting
The origins of signal splitting patterns. Each arrow represents an Hb nuclear spin orientation. Signal Splitting
Pascal’s triangle.
As illustrated by the highlighted entries, each entry is the sum of the values immediately above it to the left and the right. Physical Basis for (n + 1) Rule
Coupling of nuclear spins is mediated through intervening bonds.
H atoms with more than three bonds between them generally do not exhibit coupling.
For H atoms three bonds apart, the coupling is called vicinal coupling. Physical Basis for (n + 1) Rule
Coupling that arises when Hb is split by two different nonequivalent H atoms, Ha and Hc.
Coupling Constants
An important factor in vicinal coupling is the angle a between the C-H sigma bonds and whether or not it is fixed.
Coupling is a maximum when a is 0° and 180°; it is a minimum when a is 90°. More Complex Splitting Patterns
Complex coupling that arises when Hb is split by Ha and two equivalent atoms Hc.
結構之質子偶合常數J的典型值
* 註：烷基中的值7 Hz是繞鍵快速旋轉的平均值，若旋轉受到環或大的基所阻礙，可能看 到其他的分裂常數。 More Complex Splitting Patterns
Since the angle between C-H bond determines the extent of coupling, bond rotation is a key parameter.
In molecules with free rotation about C-C sigma bonds, H atoms bonded to the same carbon in CH3 and CH2 groups are equivalent.
If there is restricted rotation, as in alkenes and cyclic structures, H atoms bonded to the same carbon may not be equivalent.
Nonequivalent H on the same carbon will couple and cause signal splitting.
This type of coupling is called geminal coupling. More Complex Splitting Patterns
In ethyl propenoate, an unsymmetrical terminal alkene, the three vinylic hydrogens are nonequivalent. More Complex Splitting Patterns
Tree diagram for the complex coupling seen for the three alkenyl H atoms in ethyl propenoate. More Complex Splitting Patterns
Cyclic structures often have restricted rotation about their C-C bonds and have constrained conformations.
As a result, two H atoms on a CH2 group can be nonequivalent, leading to complex splitting. More Complex Splitting Patterns
A tree diagram for the complex coupling seen for the vinyl group and the oxirane ring H atoms of 2-methyl-2-vinyloxirane. More Complex Splitting Patterns
Complex coupling in flexible molecules.
Coupling in molecules with unrestricted bond rotation often gives only m + n + I peaks.
That is, the number of peaks for a signal is the number of adjacent hydrogens + 1, no matter how many different sets of equivalent H atoms that represents.
The explanation is that bond rotation averages the coupling constants throughout molecules with freely rotation bonds and tends to make them similar; for example in the 6- to 8-Hz range for H atoms on freely rotating sp3 hybridized C atoms. More Complex Splitting Patterns
Simplification of signal splitting occurs when coupling constants are the same. More Complex Splitting Patterns
Peak overlap occurs in the spectrum of 1-chloro- 3-iodopropane.
Hc should show 9 peaks, but because Jab and Jbc are so similar, only 4 + 1 = 5 peaks are distinguishable. Stereochemistry & Topicity
Homotopic atoms or groups
H H Cl Substitute Cl Substitution does not C one H by D C produce a stereocenter; Cl Cl therefore hydrogens H D are homotopic. Dichloro- Achiral methane (achiral)
Homotopic atoms or groups have identical chemical shifts under all conditions. Stereochemistry & Topicity
Enantiotopic groups
H Substitute Substitution produces a Cl H one H by D Cl stereocenter; C therefore, hydrogens are F C H F enantiotopic. Both D hydrogens are prochiral; one is pro-R-chiral, the Chlorofluoro- Chiral methane other is pro-S-chiral. (achiral)
Enantiotopic atoms or groups have identical chemical shifts in achiral environments.
They have different chemical shifts in chiral environments. Stereochemistry & Topicity
Diastereotopic groups
H atoms on C-3 of 2-butanol are diastereotopic.
Substitution by deuterium creates a chiral center.
Because there is already a chiral center in the molecule, diastereomers are now possible. H OH Substitute one H OH H on CH2 by D
H H D H 2-Butanol Chiral (chiral)
Diastereotopic hydrogens have different chemical shifts under all conditions. Stereochemistry & Topicity
The methyl groups on carbon 3 of 3-methyl-2- butanol are diastereotopic.
If a methyl hydrogen of carbon 4 is substituted by deuterium, a new chiral center is created.
Because there is already one chiral center, diastereomers are now possible. OH
3-Methyl-2-butanol
Protons of the methyl groups on carbon 3 have different chemical shifts. Stereochemistry and Topicity
1 H-NMR spectrum of 3-methyl-2-butanol.
The methyl groups on carbon 3 are diastereotopic and appear as two doublets. 13C-NMR Spectroscopy
13 Each nonequivalent C gives a different signal 13 1 A C signal is split by the H bonded to it according to the (n + 1) rule.
Coupling constants of 100-250 Hz are common, which means that there is often significant overlap between signals, and splitting patterns can be very difficult to determine. 13 The most common mode of operation of a C-NMR spectrometer is a proton-decoupled mode. 13C-NMR Spectroscopy
In a proton-decoupled mode, a sample is irradiated with two different radiofrequencies, 13 one to excite all C nuclei.
a second broad spectrum of frequencies to cause all protons in the molecule to undergo rapid transitions between their nuclear spin states. 13 On the time scale of a C-NMR spectrum, each proton is in an average or effectively constant nuclear spin state, with the result that 1H-13C spin-spin interactions are not observed; they are decoupled. 13C-NMR Spectroscopy
13 Proton-decoupled C-NMR spectrum of 1- bromobutane. Chemical Shift - 13C-NMR
13C-NMR chemical shifts of representative groups Chemical Shift - 13C-NMR
Type of Chemical Type of Chemical Carbon Shift () Carbon Shift ()
RCH3 10-40 C R 110-160 RCH2 R 15-55 R CH 20-60 3 O RCH2 I 0-40 RCOR 160 - 180 RCH2 Br 25-65 O RCH Cl 35-80 2 RCNR2 165 - 180 R3 COH 40-80 O R3 COR 40-80 RCCOH 165 - 185 RC CR 65-85 O O R2 C=CR2 100-150 RCH, RCR 180 - 215 Interpreting NMR Spectra
Alkanes 1 H-NMR signals appear in the range of 0.8-1.7. 13 C-NMR signals appear in the considerably wider range of 10-60. Alkenes 1 H-NMR signals appear in the range 4.6-5.7. 1 H-NMR coupling constants are generally larger for trans-vinylic hydrogens (J= 11-18 Hz) compared with cis-vinylic hydrogens (J= 5-10 Hz). 13 2 C-NMR signals for sp hybridized carbons appear in the range 100-160, which is to higher frequency from the signals of sp3 hybridized carbons. Interpreting NMR Spectra
1 H-NMR spectrum of vinyl acetate. Interpreting NMR Spectra
Alcohols 1 H-NMR O-H chemical shift often appears in the range 3.0-4.0, but may be as low as 0.5. 1 H-NMR chemical shifts of hydrogens on the carbon bearing the -OH group are deshielded by the electron-withdrawing inductive effect of the oxygen and appear in the range 3.0-4.0. Ethers 1 A distinctive feature in the H-NMR spectra of ethers is the chemical shift, 3.3-4.0, of hydrogens on the carbons bonded to the ether oxygen. Interpreting NMR Spectra
1 H-NMR spectrum of 1-propanol. Interpreting NMR Spectra
Aldehydes and ketones 1 H-NMR: aldehyde hydrogens appear at 9.5-10.1. 1 H-NMR: a-hydrogens of aldehydes and ketones appear at 2.2-2.6. 13 C-NMR: carbonyl carbons appear at 180-215. Amines 1 H-NMR: amine hydrogens appear at 0.5- 5.0 depending on conditions. Interpreting NMR Spectra
Carboxylic acids 1 H-NMR: carboxyl hydrogens appear at 10-13, higher than most other types of hydrogens. 13 C-NMR: carboxyl carbons in acids and esters appear at 160-180. Interpreting NMR Spectra
Spectral Problem 1; molecular formula
C5H10O. Interpreting NMR Spectra
Spectral Problem 2; molecular formula
C7H14O. Laser Laser - Characteristics
Laser - is a special type of light sources or light generators. The word LASER represents Light Amplification by Stimulated Emission of Radiation
Characteristics of light produced by Lasers
Monochromatic (single wavelength)
Coherent (in phase)
Directional (narrow cone of divergence) Incandescent lamp • Chromatic • Incoherent The first microwave laser was • Non-directional made in the microwave region in 1954 by Townes & Shawlow using ammonia as the lasing medium. The first optical laser was Monochromatic light source constructed by Maiman in 1960, using ruby (Al2O3 doped with a dilute • Coherent +3 concentration of Cr ) as the lasing • Non-directional medium and a fast discharge flash- lamp to provide the pump energy. Laser - Stimulated Emission
When excited atoms/molecules/ions undergo de-excitation (from excited state to ground state), light is emitted
Types of light emission
E Spontaneous emission - chromatic & 4 E incoherent 3 excited E2 state
- Excited e ’s when returning to ground states E emit light spontaneously (called spontaneous 1 Ep1 Ep2 emission). ground E p4 state Photons emitted when e-’s return from E different excited states to ground states have 0 Ep1=(E1 – E0) = different frequencies (chromatic) hv1 Ep2=(E2 – E0) = Spontaneous emission happens randomly and hv2 requires no event to trigger the transition Ep4=(E4 – E0) = (various phase or incoherent) hv4 Laser - Stimulated Emission Types of light emission (cont’d)
Stimulated emission - monochromatic & coherent
While an atom is still in its excited state, one E4 E can bring it down to its ground state by 3 E2 stimulating it with a photon (P1) having an energy equal to the energy difference of the E1 excited state and the ground state. In such
a process, the incident photon (P ) is not 1 Ep1=(E2– Ep1=(E2–E0)=hv2
absorbed and is emitted together with the E0)=hv2 Ep2=(E2–E0)=hv2
E photon (P2), The latter will have the same 0 frequency (or energy) and the same phase
(coherent) as the stimulating photon (P1). Laser uses the stimulated emission process to amplify the light intensity
As in the stimulated emission process, one incident photon (P1) will bring about the emission of an additional photon (P2), which in turn can yield 4 photons, then 8 photons, and so on…. Laser - Formation & Conditions
The conditions must be satisfied in order to sustain such a chain reaction:
Population Inversion (PI), a situation that there are more atoms in a certain excited state than in the ground state PI can be achieved by a variety means (electrical, optical, chemical or mechanical), e.g., one may obtain PI by irradiating the system of atoms by an enormously intense light beam or, if the system of atoms is a gas, by passing an electric current through the gas.
- Presence of Metastable state, which is the excited state that the excited e ’s can have a relatively long lifetime (>10-8 second), in order to avoid the spontaneous emission occurring before the stimulated emission
In most lasers, the atoms/molecules/ions in the lasing medium are not “pumped” directly to a metastable state. They are excited to an energy level higher than a metastable state, then drop down to the metastable state by spontaneous non-radiative de-excitation.
Photon Confinement (PC), the emitted photons must be confined in the system long enough to stimulate further light emission from other excited atoms This is achieved by using reflecting mirrors at the ends of the system. One end is made totally reflecting & the other is slight transparent to allow part of the laser beam to escape. Laser - Functional Elements
Output Feedback mechanism coupler
Lasing medium
High Energy Partially reflectance input transmitting mirror mirror Energy pumping mechanism Laser Action
Lasing medium at ground state Pump energy Population inversion Pump energy Start of stimulated emission Pump energy Stimulated emission building up Pump energy
Laser in full operation Types of Lasers
There are many different types of lasers
The lasing medium can be gas, liquid or solid (insulator or semiconductor)
Some lasers produce continuous light beam and some give pulsed light beam
Most lasers produce light wave with a fixed wave-length, but some can be tuned to produce light beam of wave-length within a certain range.
Laser type Physical form of lasing Wave length (nm) medium Helium neon laser Gas 633
Carbon dioxide laser Gas 10600 (far-infrared) Argon laser Gas 488, 513, 361 (UV), 364 (UV) Nitrogen laser Gas 337 (UV) Dye laser Liquid Tunable: 570-650 Ruby laser Solid 694 Nd:Yag laser Solid 1064 (infrared) Diode laser Semiconductor 630-680 Laser - Applications Laser can be applied in many areas
Commerce
Compact disk, laser printer, copiers, optical disk drives, bar code scanner, optical communications, laser shows, holograms, laser pointers
Industry
Measurements (range, distance), alignment, material processing (cutting, drilling, welding, annealing, photolithography, etc.), non-destructive testing, sealing
Medicine
Surgery (eyes, dentistry, dermatology, general), diagnostics, ophthalmology, oncology
Research
Spectroscopy, nuclear fusion, atom cooling, interferometry, photochemistry, study of fast processes
Military
Ranging, navigation, simulation, weapons, guidance, blinding