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2 AB AB + + e AB* AB +* + e n h or n 1 h 1 + n 2 h 2 + : -absorption 1h n h -ionization Energy
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Why multiphoton exitations(?); advantages/disadvantages One color experiments / data Experimental methods: Multiphoton ionization (MPI & REMPI) Data interpretations / theory: “What to see and what not to see(?)” Results / examples: - characterization of state properties / energies - (2+1) vs (3+1) REMPI - ”New” states observed - analysis of complicated spectra - state interactions - multi-photon absorption “mechanism” - energy distribution in molecules - polyatomic molecules Two color experiments / data
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Why multiphoton exitations(?); advantages/disadvantages One color experiments / data Experimental methods: Multiphoton ionization (MPI & REMPI) Data interpretations / theory: “What to see and what not to see(?)” Results / examples: - characterization of state properties / energies - (2+1) vs (3+1) REMPI - ”New” states observed - analysis of complicated spectra - state interactions - multi-photon absorption “mechanism” - energy distribution in molecules - polyatomic molecules Two color experiments / data
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Few- photon absorption in atoms and molecules: Excitation to high energy states of neutral species: 1xh 2xh 3xh nxh A A** A+A+ AB AB + + e AB** 2
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Typically: 1xh 2xh 3xh VUVUV nxh Visible Visible..IR Technically more feasable to increase n + 3
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1xh 2xh 3xh nxh A A** A+A+ AB AB + + e AB** 4 Increasing probability of resonance intermediate states. Hence spectra complications -
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5 Spectroscopic characteristics: I.Angular momentum quantum numbers (L) can change as: L = (0),.. n for nxh Thus electronic angular momentum changes in atoms as L = 0, 1 / l = 1 per “photon step” 1x3x
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6 II. Electronic angular momentum changes in molecules: z 2xh 3xh ´=3( ) ´=2( ) ´=1( ) ´=0( ) ´´=0( ) i 1xh 2xh 3xh
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III. Total angular momentum changes: z 7 I, II, III => More states can be excited as n increases J = 0, 1,.., n For nxh J:QJ-1;PJ+1;RJ-3;NJ-2;OJ+2;SJ+3;T J
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As n (in nxh ) increases: Technically feasable Larger number of excited states accessable. Hence more spectroscopic information Better consistancy / accuracy in determining spectroscopic parameters due to larger number of transitions More complicated spectra / more overlap of features + + - How to proceed?: 8 +
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Why multiphoton exitations(?); advantages/disadvantages One color experiments / data Experimental methods: Multiphoton ionization (MPI & REMPI) Data interpretations / theory: “What to see and what not to see(?)” Results / examples: - characterization of state properties / energies - (2+1) vs (3+1) REMPI - ”New” states observed - analysis of complicated spectra - state interactions - multi-photon absorption “mechanism” - energy distribution in molecules - polyatomic molecules Two color experiments / data
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I. Experimentally: a) Simply and inexpensively / for gas samples: + - LASER beam 9 AB AB + + e AB** 3 photon absorption; 1 photon ionization i.e.: (3+1)REMPI-Current LASER beam
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REMPI-TOF
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LASERS REMPI TOF
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Voltage divider HV - 2Kv supply HX Nozzle Turbo Pump TOF Tube Focus lens MCP Detector Oscilloscope Computer EXT Excimer Laser One Shot Cycle Input Output Dye Laser SHG Time Delay 200-1200 S Dye laser Control G-Valve External Pellin Broca prism SHG Control Box In Out +HV
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REMPI-TOF: Mass spectrum of HCl, showing: H +, 35 Cl +,H 35 Cl +, 37 Cl +,H 37 Cl + For = 477.795 nm LASER radiation
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LASER Radiation wavenumbers LASER Radiation frequency TOF/ion mass Number of ions Mass spectra vs LASER radiation:
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Ion formations vs LASER radiations 20930.0 cm -1 20929.5 cm -1 mass 3537
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Ion formations vs LASER radiations mass intensity 20929 cm -1 20930 cm -1 1 REMPI- TOF (H + ) spectrum
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..AND REMPI – Current spectrum: LASER wavenumbers (1/ (cm -1 )) Intensity
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HCl REMPI-Current Spectrum 2085020900209502100021050 /cm -1 Intensity
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Why multiphoton exitations(?); advantages/disadvantages One color experiments / data Experimental methods: Multiphoton ionization (MPI & REMPI) Data interpretations/theory: “What to see and what not to see(?)” Results / examples: - characterization of state properties / energies - (2+1) vs (3+1) REMPI - ”New” states observed - analysis of complicated spectra - state interactions - multi-photon absorption “mechanism” - energy distribution in molecules - polyatomic molecules Two color experiments / data
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Interpretation / Theory: Determine: 1)Signal intensity transition probabilities population in ground state E = E j -E i = photon energy x n 11 EjEj EiEi n x h n
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1xh - absorption: A/I fl Exp. Calc. E 10 E 20 E 30 1xh E h = E 10 =E 1 -E 0 =(E 1 -E 0 )/h E 10 E h
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2xh =(E 1 -E 0 )/h A A** A+A+ |i1> |i4> |i3> |i2> : AB AB + + e AB** 2xh - absorption:
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3xh =(E 1 -E 0 )/h AB AB + + e AB** |i1> |i4> |i3> |i2> : 3xh - absorption:
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1) :::::: rot. contribution electr. Contrib. vibr. Contrib. ; c v = fasti 2) |i1> AB AB + + e AB** |i3> |i2> 1)B-O approximation, 2) No resonance intermediate states, Approximations:
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....approximations:...3) assume virtual intermediate states: 3xh “i”“i” “i”“i” 3)......................................... 1,2,3), hence: -for s i (J, ), i 2 = e 2
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13 1xh 3xh 2xh i 2 ´s are treated as variables in simulation procedures
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Table I. s 1,s 3 J and dependent; s 1,s 3 fyrir ´´= 0: 14 Selection rules: = 0, 1,.., n; n = 1,2,3,.. J = 0, 1,.., n; n = 1,2,3,..; 0 J = 1,.., n; n = odd; = 0 J = 0, 2,.., n; n = even = 0
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1) Intensity 2) E i (B,D, 0 ) E (nxh ) Intensity vs (nxh ) / I( ) can be evaluated and compared with REMPI spectra: Spectra simulations 15
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Why multiphoton exitations(?); advantages/disadvantages One color experiments / data Experimental methods: Multiphoton ionization (MPI & REMPI) Data interpretations / theory: “What to see and what not to see(?)” Results / examples: - characterization of state properties / energies - (2+1) vs (3+1) REMPI - ”New” states observed - analysis of complicated spectra - state interactions - multi-photon absorption “mechanism” - energy distribution in molecules - polyatomic molecules Two color experiments / data
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