Chapter : Atomic Spectroscopy, PPT, Semester, Engineering - Electronics and Communication Engineering (ECE) PDF Download

Atomic Spectroscopy:
Atomic Emission Spectroscopy
Atomic Absorption Spectroscopy
Atomic Fluorescence Spectroscopy

* Elemental Analysis
* Sample is atomized
* Absorption, emission or fluorescence of atoms or ions in the gas phase

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Atomic Emission Spectroscopy

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Electronic Levels for Individual Electrons

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Electronic Configuration of Atoms

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Electronic Configuration of Atoms

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Electronic Configuration of Atoms

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Electronic States of Atoms

Quantum numbers for electronsQuantum numbers for many-electron atoms
l: orbital angular momentum quantumL: orbital angular momentum quantum number
number (0,1, … n-1 e.g., for 2 e-: L= l1+l2, l1+l2-1, l1+l2-2, …,| l1-l2|
where 0=s, 1=p, 2=d, 3=f)0 = S, 1 = P, 2 = D, 3 = F
ml: orbital magnetic quantum numberML: orbital magnetic quantum number (Σml)
(l, l-1, …, 0, …, -l )2L+1 possible values
s: electron spin quantum number (1/2) S: total spin quantum number
S = s1+s2, s1+s2 -1, …,| s1-s2 |
S = 0 singlet, S = 1 doublet, S = 2 triplet
ms: spin magnetic quantum numberMS: spin magnetic quantum number (Σms)
(+1/2, -1/2)2S+1 possible values
J: total angular quantum number
J = L+S, L+S-1, …, | L-S|

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Spectroscopic Description of Atomic Electronic States –Term Symbols

Multiplicity (2S +1)describes the number of possible orientations of total spin angular momentum whereS is the resultant spin

quantum number(1/2 x # unpaired electrons)Resultant Angular Momentum (L)describes the coupling of the orbital angular momenta of each electron (add the mLvalues for each electron)Total Angular Momentum (J)combines orbital angular momentum and intrinsic

angular momentum (i.e., spin).To Assign J Value:if less than half of the subshell is occupied, take the minimum value J = | L − S | ; if more than half-filled, take the maximum value J = L + S; if the subshell is half-filled, L = 0 and then J = S.

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Spectroscopic Description of Ground Electronic States –Term Symbols

Term Symbol Form: 2S+1{L}J2S+1 –multiplicityL –resultant angular momentum quantum numberJ –total angular momentum quantum numberGround state has maximal S and L values. Example: Ground State of Sodium –1s22s22p63s1Consider only the one valence electron (3s1)L = l = 0, S = s = ½, J = L + S = ½so, the term symbol is 2S½

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Are you getting the concept?

Write the ground state term symbol for fluorine.

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Spectroscopic Description of All Possible Electronic States –Term Symbols

C –1s22s22p2

Step 1:Consider two valence p electrons

1st2p electron has n = 2, l = 1, ml= 0, 1, ms= ½ → 6 possible sets of quantum numbers

2nd2p electron has 5 possible sets of quantum numbers (Pauli Exclusion Principle)

For both electrons, (6x5)/2 = 15 possible assignments since the electrons are indistinguishable

Step 2: Draw all possible microstates. Calculate ML and MSfor each state.

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Spectroscopic Description of All Possible Electronic States –Term Symbols

C –1s22s22p2

Step 3: Count the number of microstates for each ML—MSpossible combination

Step 4: Extract smaller tables representing each possible term

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Spectroscopic Description of All Possible Electronic States –Term Symbols

C –1s22s22p2

Step 5: Use Hund’s Rules to determine the relative energies of all possible states.1. The highest multiplicity term within a configuration is of lowest energy.2. For terms of the same multiplicity, the highest L value has the lowest energy (D < P < S).3. For subshells that are less than half-filled, the minimum J-value state is of lower energy than higher J-value states.4. For subshells that are more than half-filled, the state of maximum J-value is the lowest energy.Based on these rules, the ground electronic configuration for carbon has the following energy order: 3P0< 3P1< 3P2< 1D2< 1S0

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Spectroscopic Description of Excited States –Term Symbols

Write term symbols in analogous manner except consider the orbital to which an electron is promoted.
For example, excitation of Na promotes one valence electron into the 3p orbital. In this case, n = 3, S = ½, 2S+1 = 2, L = 1 (P term), J = 3/2, 1/2.
There are two closely spaced levels in the excited term of sodium with term symbols 2P1/2and 2P3/2

This type of splitting (same L but different J) is called fine structure.
Transition from 2P1/2→ 2S1/2

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Calculating Energies for Transitions

To calculate the energy of a single electron atomwith quantum numbers L, S, and J:EL,S,J= ½ hcλ[J(J+1) -L(L+1) –S(S+1)]where λ is the spin-orbit coupling constant

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Are you getting the concept?

Atomic emission spectra show a doublet in the Na spectrum due to spin-orbit coupling of the 2P state. Given that λ= 11.4 cm-1, find the energy spacing (in cm-1) between the upper 2P3/2and 2P1/2states.

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Allowed and Forbidden Transitions

Only a fraction of all possible transitions are observed.Allowed transitions-high probability, high intensity, electric dipole interactionForbidden transitions-low probability, weak intensity, non-electric dipole interactionSelection rules for allowed transitions:* The parity of the upper and lower level must be different. (The parity is even if Σliis even. The parity is odd if Σliis odd.)* Δl= ±1* Δj= 0 or ±1, but J= 0 to J= 0 is forbidden.

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Additional Splitting Effects

•Hyperfine splittingdue to magnetic coupling of spin and orbital motion of electrons with the nuclear spin.

•Isotope shift. Sufficient to determine isotope ratios.

•Splitting in an electric field (Stark effect): Relevant for arc and spark techniques.

•Splitting in a magnetic field (Zeeman effect):* In absence of a magnetic field, states that differ only by their MJvalues are degenerate, i.e., they have equivalent energies.* In presence of a magnetic field, this is not true anymore.* Can be used for background correction.

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Pretsch/Buhlmann/Affolter/Badertscher,Structure Determination of Organic Compounds

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Stark Splitting

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Zeeman Splitting

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Sample Introduction and Atomization

Atomization:Convert solution → vapor-phase free atomsMeasurements usually made in hot gas or enclosed furnace:

• flames

•plasmas

•electrical discharges (arcs, sparks)

•heated furnaces

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Atomic Emission Spectroscopy (AES)

See also: Fundamental reviews in Analytical Chemistry
e.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C. Anal. Chem.2002, 74, 2691-2712 (“Atomic Spectroscopy”)

•Beginning 19th century: alcohol flame (Brewster, Herschel, Talbot, Foucault)

•mid 1800s: Discovery of Cs, Tl, In, Ga by atomic spectroscopy (Bunsen, Kirchhoff)

•1877: Gouy designs pneumatic nebulizer

•1920s: Arcs and sparks used for AES

•1930s: First commercial AES spectrometer (Siemens-Zeiss)

•1960s: Plasma sources (commercial in 1970s)

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Atomic Emission Spectroscopy (AES)

At RT, nearly allelectrons in 3sorbitalExcite with flame, electric arc, or sparkCommon electronictransitions

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Example AE Spectra

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An Ideal AES Source

1.complete atomization of all elements

2.controllable excitation energy

3.sufficient excitation energy to excite all elements

4.inert chemical environment

5.no background

6.accepts solutions, gases, or solids

7.tolerant to various solution conditions and solvents

8.simultaneous multi-element analysis

9.reproducible atomization and excitation conditions

10.accurate and precise analytical results

11.inexpensive to maintain

12.ease of operation

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Flame AES

•Background signals due to flame fuel and oxidants –line spectra:

•OH•281.1, 306.4, 342.8 nmfrom O + H2⇒H + OHH + O2⇒O + OH

•O2250, 400 nm

•CH431.5, 390.0, 314.3 nm

•CObands between 205 to 245 nm

•CN, C2, CH, NH bands between 300 to 700 nmUnlike bands of atomic origin, these molecular bands are fairly broad.

•Continuum emission from recombination reactionse.g. H + OH ⇒H2O + hνCO + O ⇒CO2+ hν⇒⇒ Flames used in AES nowadays only for few elements. Cheap but limited. {Flame AES often replaced by flame AAS.}

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Inductively Coupled Plasma AES

•Spectral interference more likely for plasma than for flame due to larger population of energetically higher states.

•Modern ICP power: 1–5 kW (4 to 50 MHz)

•4000 to 10,000 K: Very few molecules

•Long residence time (2–3 ms)results in high desolvationand volatilization rate

•High electron density suppresses ionization interference effects

•Background: Ar atomic lines and,in hottest plasma region, Bremsstrahlung (continuum radiationfrom slowing of charged particles)

•Price > $ 50 k

•Operating cost relatively high dueto Ar cost (10–15 mL/min) andtraining.

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Microwave Plasma AES

•Power 25 to 1000 W (ICP 1000–2000 W)

•Frequency 2450 MHz (ICP 4 to 50 MHz)

•Argon, helium or nitrogen

•Temperature estimated to be 2000 -3000 K

•Low temperature causes problems with liquids

•Useful for gases: GC–microwave plasma AES

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Arcs and Sparks

•Arc = An electrical discharge between two or more conducting electrodes (1-30 A)

•Spark = An intermittent high-voltage discharge (few μsec)

•Limited to qualitative and semi-quantitative use (arc flicker)

•Particularly useful for solid samples (pressed into electrode)

•The burn takes > 20 sec (need multichannel detector)