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

Atomic Emission Spectroscopy Ingle and Crouch, Spectrochemical Analysis

Electronic Levels for Individual Electrons Ingle and Crouch, Spectrochemical Analysis

Electronic Configuration of Atoms Al: 1s2 2s2 2p6 3s2 3p1

Electronic Configuration of Atoms Aufbau Order 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 6f l = 0 s-orbital ml = 0 l = 1 p-orbital ml = 0, ±1 l = 2 d-orbital ml = 0, ± 1, ±2

Electronic Configuration of Atoms Al: 1s2 2s2 2p6 3s2 3p1

Electronic States of Atoms Quantum numbers for electrons Quantum numbers for many-electron atoms l: orbital angular momentum quantum L: 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 number ML: orbital magnetic quantum number (Sml) (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 number MS: spin magnetic quantum number (Sms) (+1/2, -1/2) 2S+1 possible values J: total angular quantum number J = L+S, L+S-1, …, | L-S|

Spectroscopic Description of Atomic Electronic States – Term Symbols Multiplicity (2S +1) describes the number of possible orientations of total spin angular momentum where S 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 mL values 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.

Spectroscopic Description of Ground Electronic States – Term Symbols Term Symbol Form: 2S+1{L}J 2S+1 – multiplicity L – resultant angular momentum quantum number J – total angular momentum quantum number Ground state has maximal S and L values. Example: Ground State of Sodium – 1s22s22p63s1 Consider only the one valence electron (3s1) L = l = 0, S = s = ½, J = L + S = ½ so, the term symbol is 2S½

Are you getting the concept? Write the ground state term symbol for fluorine.

Spectroscopic Description of All Possible Electronic States – Term Symbols C – 1s22s22p2 Step 1:Consider two valence p electrons 1st 2p electron has n = 2, l = 1, ml = 0, ±1, ms = ±½ → 6 possible sets of quantum numbers 2nd 2p 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 MS for each state.

Spectroscopic Description of All Possible Electronic States – Term Symbols C – 1s22s22p2 Step 3: Count the number of microstates for each ML—MS possible combination Step 4: Extract smaller tables representing each possible term

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

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 2P1/2 and 2P3/2 This type of splitting (same L but different J) is called fine structure. Transition from 2P1/2 → 2S1/2

Calculating Energies for Transitions To calculate the energy of a single electron atom with quantum numbers L, S, and J: EL,S,J = ½ hcl[J(J+1) - L(L+1) – S(S+1)] where l is the spin-orbit coupling constant

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 l = 11.4 cm-1, find the energy spacing (in cm-1) between the upper 2P3/2 and 2P1/2 states. 2P3/2 2P1/2 2S1/2 1 eV = 8065.5 cm-1

Allowed and Forbidden Transitions Only a fraction of all possible transitions are observed. Allowed transitions -high probability, high intensity, electric dipole interaction Forbidden transitions -low probability, weak intensity, non-electric dipole interaction Selection rules for allowed transitions: * The parity of the upper and lower level must be different. (The parity is even if Sli is even. The parity is odd if Sli is odd.) * Dl = ±1 * DJ = 0 or ±1, but J = 0 to J = 0 is forbidden.

Additional Splitting Effects Hyperfine splitting due 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 MJ values 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.

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

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

Stark Splitting For H: split a E For others: split a (E)2 www.wikipedia.org

Zeeman Splitting MJ – Resultant total magnetic quantum number MJ = J, J-1, …, -J 2J +1 possible values Normal Anomalous Ingle and Crouch, Spectrochemical Analysis

Sample Introduction and Atomization Convert solution → vapor-phase free atoms Measurements usually made in hot gas or enclosed furnace: flames plasmas electrical discharges (arcs, sparks) heated furnaces Free Atoms Ions Mole-cules Nebulization Desolvation Volitalization Adapted from Ingle and Crouch

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)

Atomic Emission Spectroscopy (AES) 2D3/2, 5/2 2S1/2 At RT, nearly all electrons in 3s orbital Excite with flame, electric arc, or spark Common electronic transitions http://raptor.physics.wisc.edu/data/e_sodium.gif

Example AE Spectra H2 Hg He http://www.technology.niagarac.on.ca/lasers/Chapter2.html

An Ideal AES Source complete atomization of all elements controllable excitation energy sufficient excitation energy to excite all elements inert chemical environment no background accepts solutions, gases, or solids tolerant to various solution conditions and solvents simultaneous multi-element analysis reproducible atomization and excitation conditions accurate and precise analytical results inexpensive to maintain ease of operation

Flame AES Background signals due to flame fuel and oxidants – line spectra: OH• 281.1, 306.4, 342.8 nm from O + H2  H + OH H + O2  O + OH O2 250, 400 nm CH 431.5, 390.0, 314.3 nm CO bands between 205 to 245 nm CN, C2, CH, NH bands between 300 to 700 nm Unlike bands of atomic origin, these molecular bands are fairly broad. Continuum emission from recombination reactions e.g. H + OH  H2O + h CO + O  CO2 + h   Flames used in AES nowadays only for few elements. Cheap but limited. {Flame AES often replaced by flame AAS.} Ingle and Crouch

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 desolvation and volatilization rate High electron density suppresses ionization interference effects Background: Ar atomic lines and, in hottest plasma region, Bremsstrahlung (continuum radiation from slowing of charged particles) Price > $ 50 k Operating cost relatively high due to Ar cost (10–15 mL/min) and training. www.wikipedia.org, Ingle and Crouch

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

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