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=s, p=1, 2=d, 3=f) e.g., for 2 e-: L = l1+l2, l1+l2 -1, l1+l2 -2, …,| l1-l2 | 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 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
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 Sli is odd.) * Dl = ±1 * DJ = 0 or ±1, but J = 0 to J = 0 is forbidden.