Atomic Spectroscopy:Atomic Spectroscopy:

Atomic Emission SpectroscopyAtomic Emission SpectroscopyAtomic Absorption SpectroscopyAtomic Absorption Spectroscopy

Atomic Fluorescence SpectroscopyAtomic Fluorescence Spectroscopy

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

Atomic Emission SpectroscopyAtomic Emission Spectroscopy

Ingle and Crouch,Ingle and Crouch,Spectrochemical AnalysisSpectrochemical Analysis

Electronic Levels for Individual ElectronsElectronic Levels for Individual Electrons

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Electronic Configuration of AtomsElectronic Configuration of Atoms

Al: 1sAl: 1s22 2s 2s22 2p 2p66 3s 3s22 3p 3p11

Electronic Configuration of AtomsElectronic Configuration of Atoms

l = 0l = 0s-orbitals-orbitalmmll = 0 = 0

l = 1l = 1p-orbitalp-orbital

mmll = 0, = 0, ±±11

l = 2l = 2d-orbitald-orbital

mmll = 0, ± = 0, ± 1, 1, ±±22

1s1s2s 2p2s 2p3s 3p 3d3s 3p 3d4s 4p 4d 4f4s 4p 4d 4f5s 5p 5d 5f5s 5p 5d 5f6s 6p 6d 6f6s 6p 6d 6f

Aufbau OrderAufbau Order

Electronic Configuration of AtomsElectronic Configuration of Atoms

Al: 1sAl: 1s22 2s 2s22 2p 2p66 3s 3s22 3p 3p11

Electronic States of AtomsElectronic States of Atoms

Quantum numbers for electronsQuantum numbers for electrons Quantum numbers for many-electron atomsQuantum numbers for many-electron atoms

ll: orbital angular momentum quantum: orbital angular momentum quantum LL: orbital angular momentum quantum number: orbital angular momentum quantum number number (0,1, … n-1 number (0,1, … n-1 e.g., for 2 e-: e.g., for 2 e-: LL = = ll11++ll22, , ll11++ll22 -1, -1, ll11++ll22 -2, …,| -2, …,| ll11--ll22 | |

where 0=s, 1=p, 2=d, 3=f)where 0=s, 1=p, 2=d, 3=f) 0 = S, 1 = P, 2 = D, 3 = F0 = S, 1 = P, 2 = D, 3 = F

mmll: orbital magnetic quantum number: orbital magnetic quantum number MMLL: orbital magnetic quantum number (: orbital magnetic quantum number (mmll))

((ll, , ll-1, …, 0, …, --1, …, 0, …, -l l )) 2 2LL+1 possible values+1 possible values

ss: electron spin quantum number (1/2) : electron spin quantum number (1/2) SS: total spin quantum number: total spin quantum number S = sS = s11++ss22, , ss11++ss2 2 -1, …,| -1, …,| ss11--ss2 2 ||

S = 0 singlet, S = 1 doublet, S = 2 tripletS = 0 singlet, S = 1 doublet, S = 2 triplet

mmss: spin magnetic quantum number: spin magnetic quantum number MMSS: spin magnetic quantum number (: spin magnetic quantum number (mmss))

(+1/2, -1/2)(+1/2, -1/2) 2 2SS+1 possible values+1 possible values

JJ: total angular quantum number: total angular quantum number J = L+S, L+S-J = L+S, L+S-11, …, , …, | | L-SL-S||

Spectroscopic Description of Spectroscopic Description of Atomic Electronic States – Term SymbolsAtomic Electronic States – Term Symbols

Multiplicity (2S +1)Multiplicity (2S +1) describes the number of possible orientations describes the number of possible orientations of total spin angular momentum whereof total spin angular momentum where S is the resultant spin S is the resultant spin quantum numberquantum number (1/2 x # unpaired electrons)(1/2 x # unpaired electrons)

Resultant Angular Momentum (L)Resultant Angular Momentum (L) describes the coupling of the describes the coupling of the orbital angular momenta of each electron (add the morbital angular momenta of each electron (add the mLL values for values for

each electron)each electron)

Total Angular Momentum (J)Total Angular Momentum (J) combines orbital angular momentum combines orbital angular momentum and intrinsic angular momentum (i.e., spin).and intrinsic angular momentum (i.e., spin).

To Assign J Value:To Assign J Value: if less than half of the subshell is occupied, take the minimum if less than half of the subshell is occupied, take the minimum

value J = | L − S | ; value J = | L − S | ; if more than half-filled, take the maximum 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.if the subshell is half-filled, L = 0 and then J = S.

Spectroscopic Description of Spectroscopic Description of Ground Electronic States – Term SymbolsGround Electronic States – Term Symbols

Term Symbol Form: Term Symbol Form: 2S+12S+1{L}{L}JJ

2S+1 – multiplicity2S+1 – multiplicityL – resultant angular momentum quantum numberL – resultant angular momentum quantum numberJ – total angular momentum quantum numberJ – total angular momentum quantum number

Ground state has maximal S and L values. Ground state has maximal S and L values.

Example: Ground State of Sodium – 1sExample: Ground State of Sodium – 1s222s2s222p2p663s3s11

Consider only the one valence electron (3sConsider only the one valence electron (3s11))L = l = 0, L = l = 0, S = s = ½, S = s = ½, J = L + S = ½J = L + S = ½so, the term symbol is so, the term symbol is 22SS½½

Are you getting the concept?Are you getting the concept?

Write the ground state term symbol for fluorine.Write the ground state term symbol for fluorine.

C – 1sC – 1s222s2s222p2p22

Step 1:Consider two valence p electronsStep 1:Consider two valence p electrons11stst 2p electron has n = 2, l = 1, m 2p electron has n = 2, l = 1, m ll = 0, = 0, ±1, m±1, mss = ±½ → 6 possible sets of = ±½ → 6 possible sets of

quantum numbersquantum numbers22ndnd 2p electron has 5 possible sets of quantum numbers (Pauli Exclusion 2p electron has 5 possible sets of quantum numbers (Pauli Exclusion

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

are indistinguishableare indistinguishable

Spectroscopic Description of Spectroscopic Description of All Possible Electronic States – Term SymbolsAll Possible Electronic States – Term Symbols

Step 2: Draw all possible Step 2: Draw all possible microstates. Calculate Mmicrostates. Calculate ML L and and

MMSS for each state. for each state.

C – 1sC – 1s222s2s222p2p22

Step 3: Count the number of microstates for each MStep 3: Count the number of microstates for each MLL—M—MSS possible possible

combinationcombination

Spectroscopic Description of Spectroscopic Description of All Possible Electronic States – Term SymbolsAll Possible Electronic States – Term Symbols

Step 4: Extract smaller tables representing each possible termStep 4: Extract smaller tables representing each possible term

C – 1sC – 1s222s2s222p2p22

Step 5: Use Hund’s Rules to determine the relative energies of all Step 5: Use Hund’s Rules to determine the relative energies of all possible states.possible states.1. The highest multiplicity term within a configuration is of lowest 1. The highest multiplicity term within a configuration is of lowest energy.energy.2. For terms of the same multiplicity, the highest L value has the 2. For terms of the same multiplicity, the highest L value has the lowest energy (D < P < S).lowest energy (D < P < S).3. For subshells that are less than half-filled, the minimum J-value 3. For subshells that are less than half-filled, the minimum J-value state is of lower energy than higher J-value states.state is of lower energy than higher J-value states.4. For subshells that are more than half-filled, the state of maximum 4. For subshells that are more than half-filled, the state of maximum J-value is the lowest energyJ-value is the lowest energy..

Based on these rules, the ground electronic configuration for carbon has Based on these rules, the ground electronic configuration for carbon has the following energy order: the following energy order: 33PP00 < < 33PP11 < < 33PP22 < < 11DD22 < < 11SS00

Spectroscopic Description of Spectroscopic Description of All Possible Electronic States – Term SymbolsAll Possible Electronic States – Term Symbols

Write term symbols in analogous manner except consider the Write term symbols in analogous manner except consider the orbital to which an electron is promoted.orbital to which an electron is promoted.

For example, excitation of Na promotes one valence electron For example, excitation of Na promotes one valence electron into the 3p orbital. In this case, n = 3, S = ½, 2S+1 = 2, L into the 3p orbital. In this case, n = 3, S = ½, 2S+1 = 2, L = 1 (P term), J = 3/2, 1/2.= 1 (P term), J = 3/2, 1/2.

There are two closely spaced levels in the excited term of There are two closely spaced levels in the excited term of sodium with term symbols sodium with term symbols 22PP1/21/2 and and 22PP3/23/2

Spectroscopic Description of Spectroscopic Description of Excited States – Term SymbolsExcited States – Term Symbols

This type of splitting (same L but This type of splitting (same L but different J) is called different J) is called fine structure.fine structure.

Transition from Transition from 22PP1/21/2 → → 22SS1/21/2

To calculate the energy of a To calculate the energy of a single electron atomsingle electron atom with with quantum numbers L, S, and J:quantum numbers L, S, and J:

EEL,S,JL,S,J = ½ hc = ½ hc[J(J+1) - L(L+1) – S(S+1)][J(J+1) - L(L+1) – S(S+1)]

where where is the spin-orbit coupling constantis the spin-orbit coupling constant

Calculating Energies for TransitionsCalculating Energies for Transitions

Atomic emission spectra show a doublet in the Na spectrum Atomic emission spectra show a doublet in the Na spectrum due to spin-orbit coupling of the due to spin-orbit coupling of the 22P state. Given that P state. Given that = = 11.4 cm11.4 cm-1-1, find the energy spacing (in cm, find the energy spacing (in cm-1-1) between the ) between the upper upper 22PP3/23/2 and and 22PP1/21/2 states. states.

Are you getting the concept?Are you getting the concept?

22PP3/23/2

22PP1/21/2

22SS1/21/2

1 eV = 8065.5 cm1 eV = 8065.5 cm-1-1

Allowed and Forbidden TransitionsAllowed and Forbidden Transitions

Only a fraction of all possible transitions are observed.Only a fraction of all possible transitions are observed.Allowed transitionsAllowed transitions

-high probability, high intensity, electric dipole -high probability, high intensity, electric dipole interactioninteractionForbidden transitionsForbidden transitions

-low probability, weak intensity, non-electric dipole -low probability, weak intensity, non-electric dipole interactioninteraction

Selection rules for allowed transitions:Selection rules for allowed transitions:* The parity of the upper and lower level must be * The parity of the upper and lower level must be different. (The parity is even if different. (The parity is even if llii is even. The parity is odd is even. The parity is odd

if if llii is odd.) is odd.)

* * ll = ±1 = ±1* * JJ = 0 or ±1, but = 0 or ±1, but JJ = 0 to = 0 to JJ = 0 is forbidden. = 0 is forbidden.

Additional Splitting EffectsAdditional Splitting Effects

•Hyperfine splittingHyperfine splitting due to magnetic coupling of spin and orbital due to magnetic coupling of spin and orbital motion of electrons with the nuclear spin.motion of electrons with the nuclear spin.

•Isotope shift. Sufficient to determine isotope ratios.Isotope shift. Sufficient to determine isotope ratios.

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

•Splitting in a magnetic field (Zeeman effect):Splitting in a magnetic field (Zeeman effect):

* In absence of a magnetic field, states that differ * In absence of a magnetic field, states that differ only by their only by their MMJJ values are degenerate, i.e., they values are degenerate, i.e., they have have equivalent energies.equivalent energies.

* In presence of a magnetic field, this is not true * In presence of a magnetic field, this is not true anymore.anymore.

* Can be used for background correction.* Can be used for background correction.

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

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

Stark SplittingStark Splitting

www.wikipedia.orgwww.wikipedia.org

For H:For H:split split E E

For others:For others:split split (E) (E)22

Zeeman SplittingZeeman Splitting

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

MMJJ – Resultant total – Resultant total

magnetic quantummagnetic quantumnumbernumber

MMJJ = J, J-1, …, -J = J, J-1, …, -J

2J +1 possible values2J +1 possible values

NormalNormal AnomalousAnomalous

Sample Introduction and AtomizationSample Introduction and Atomization

Atomization:Atomization:Convert solution Convert solution → vapor-phase free atoms→ vapor-phase free atoms

Measurements usually made in hot gas or enclosed furnace:Measurements usually made in hot gas or enclosed furnace:

•flamesflames•plasmasplasmas•electrical discharges (arcs, sparks)electrical discharges (arcs, sparks)•heated furnacesheated furnaces

Free Free AtomsAtoms

Free Free AtomsAtoms

IonsIonsIonsIonsMole-Mole-culescules

Mole-Mole-culescules

NebulizationNebulization

DesolvationDesolvation

VolitalizationVolitalization

Adapted from Ingle and CrouchAdapted from Ingle and Crouch

Atomic Emission Spectroscopy (AES)Atomic Emission Spectroscopy (AES)

See also: Fundamental reviews in See also: Fundamental reviews in Analytical ChemistryAnalytical Chemistry e.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C. e.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C. Anal. Anal. Chem. Chem. 20022002, , 7474, 2691-2712 (“Atomic Spectroscopy”), 2691-2712 (“Atomic Spectroscopy”)

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

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

•1877: Gouy designs pneumatic nebulizer1877: Gouy designs pneumatic nebulizer

•1920s: Arcs and sparks used for AES1920s: Arcs and sparks used for AES

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

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

Atomic Emission Spectroscopy (AES)Atomic Emission Spectroscopy (AES)2S1/2

22DD3/2, 5/23/2, 5/222PP3/23/2

22PP1/21/222SS1/21/2

At RT, nearly allAt RT, nearly allelectrons in 3selectrons in 3sorbitalorbital

Excite with flame, Excite with flame, electric arc, or electric arc, or sparkspark

Common electronicCommon electronictransitionstransitions

http://raptor.physics.wisc.edu/data/e_sodium.gifhttp://raptor.physics.wisc.edu/data/e_sodium.gif

Example AE SpectraExample AE Spectra

http://www.technology.niagarac.on.ca/lasers/Chapter2.htmlhttp://www.technology.niagarac.on.ca/lasers/Chapter2.html

HH22

HgHg

HeHe

An Ideal AES SourceAn Ideal AES Source

1. complete atomization of all elements2. controllable excitation energy3. sufficient excitation energy to excite all elements4. inert chemical environment5. no background6. accepts solutions, gases, or solids7. tolerant to various solution conditions and solvents8. simultaneous multi-element analysis9. reproducible atomization and excitation conditions10. accurate and precise analytical results11. inexpensive to maintain12. ease of operation

Flame AESFlame AES

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

•OHOH•• 281.1, 306.4, 342.8 nm 281.1, 306.4, 342.8 nm from O + Hfrom O + H22 H + OH H + OH

H + OH + O22 O + OH O + OH

•OO22 250, 400 nm250, 400 nm

•CHCH 431.5, 390.0, 314.3 nm431.5, 390.0, 314.3 nm•COCO bands between 205 to 245 nmbands between 205 to 245 nm•CN, CCN, C22, CH, NH bands between 300 to 700 nm, CH, NH bands between 300 to 700 nm

Unlike bands of atomic origin, these molecular bands are fairly broad.Unlike bands of atomic origin, these molecular bands are fairly broad.

•Continuum emission from recombination reactionsContinuum emission from recombination reactions e.g. H + OH e.g. H + OH H H22O + hO + h CO + O CO + O CO CO22 + h + h

Flames used in AES nowadays only for few elements. Cheap but Flames used in AES nowadays only for few elements. Cheap but limited. {Flame AES often replaced by flame AAS.}limited. {Flame AES often replaced by flame AAS.}

Ingle and CrouchIngle and Crouch

Inductively Coupled Plasma AESInductively Coupled Plasma AES•Spectral interference more likely for plasma than for flame due to larger Spectral interference more likely for plasma than for flame due to larger population of energetically higher states.population of energetically higher states.

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

•4000 to 10,000 K: Very few molecules4000 to 10,000 K: Very few molecules

•Long residence time (2–3 ms)Long residence time (2–3 ms) results in high desolvationresults in high desolvation and volatilization rateand volatilization rate

•High electron density suppresses High electron density suppresses ionization interference effectsionization interference effects

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

•Price > $ 50 kPrice > $ 50 k

•Operating cost relatively high dueOperating cost relatively high due to Ar cost (10–15 mL/min) andto Ar cost (10–15 mL/min) and training.training. www.wikipedia.org,www.wikipedia.org, Ingle and CrouchIngle and Crouch

Microwave Plasma AESMicrowave Plasma AES•Power 25 to 1000 W Power 25 to 1000 W (ICP 1000–2000 W)(ICP 1000–2000 W)

•Frequency 2450 MHz Frequency 2450 MHz (ICP 4 to 50 MHz)(ICP 4 to 50 MHz)

•Argon, helium or nitrogenArgon, helium or nitrogen

•Temperature estimated to be 2000 - 3000 KTemperature estimated to be 2000 - 3000 K

•Low temperature causes problems with liquidsLow temperature causes problems with liquids

•Useful for gases: Useful for gases: GC–microwave plasma AESGC–microwave plasma AES

Arcs and SparksArcs and Sparks•Arc = Arc = An electrical discharge between two or more An electrical discharge between two or more conducting electrodes (1-30 A)conducting electrodes (1-30 A)•Spark = Spark = An intermittent high-voltage discharge (few An intermittent high-voltage discharge (few sec)sec)

•Limited to qualitative and semi-quantitative use (arc Limited to qualitative and semi-quantitative use (arc flicker)flicker)•Particularly useful for solid samples (pressed into Particularly useful for solid samples (pressed into electrode)electrode)•The burn takes > 20 sec (need multichannel detector)The burn takes > 20 sec (need multichannel detector)

Ingle and CrouchIngle and Crouch