2 AB AB + + e AB* AB +* + e n h or n 1 h 1 + n 2 h 2 + : -absorption 1h  n h  -ionization Energy

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

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

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

Typically: 1xh 2xh 3xh VUVUV nxh Visible Visible..IR Technically more feasable to increase n + 3

1xh 2xh 3xh nxh A A** A+A+ AB AB + + e AB** 4 Increasing probability of resonance intermediate states. Hence spectra complications -

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

6 II. Electronic angular momentum changes in molecules: z 2xh 3xh  ´=3(  )  ´=2(  )  ´=1(  )  ´=0(  )  ´´=0(  )  i  1xh 2xh 3xh

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

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 +

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

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

REMPI-TOF

LASERS REMPI TOF

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  S Dye laser Control G-Valve External Pellin Broca prism SHG Control Box In Out +HV

REMPI-TOF: Mass spectrum of HCl, showing: H +, 35 Cl +,H 35 Cl +, 37 Cl +,H 37 Cl + For  = nm LASER radiation

LASER Radiation wavenumbers LASER Radiation frequency TOF/ion mass Number of ions Mass spectra vs LASER radiation:

Ion formations vs LASER radiations cm cm -1 mass 3537

Ion formations vs LASER radiations mass intensity cm cm -1 1 REMPI- TOF (H + ) spectrum

..AND REMPI – Current spectrum: LASER wavenumbers (1/ (cm -1 )) Intensity

HCl REMPI-Current Spectrum /cm -1 Intensity

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

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 

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

2xh   =(E 1 -E 0 )/h A A** A+A+ |i1> |i4> |i3> |i2> : AB AB + + e AB** 2xh - absorption:

3xh   =(E 1 -E 0 )/h AB AB + + e AB** |i1> |i4> |i3> |i2> : 3xh - absorption:

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:

....approximations:...3) assume virtual intermediate states: 3xh   “i”“i” “i”“i” 3) ,2,3), hence: -for s i (J,  ),  i 2 =  e 2

13 1xh 3xh 2xh        i 2 ´s are treated as variables in simulation procedures

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

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

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