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Femtosecond Dynamics of Molecules in Intense Laser Fields

Femtosecond Dynamics of Molecules in Intense Laser Fields. CPC2002 T.W. Schmidt 1 , R.B. López-Martens 2 , G.Roberts 3 University of Cambridge, UK 1. Universität Basel, Confoederatio Helvetica 2. Lunds Universitet, Sverige 3. University of Newcastle, UK. Talk Structure.

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Femtosecond Dynamics of Molecules in Intense Laser Fields

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  1. Femtosecond Dynamics of Molecules in Intense Laser Fields CPC2002 T.W. Schmidt1, R.B. López-Martens2, G.Roberts3 University of Cambridge, UK 1. Universität Basel, Confoederatio Helvetica 2. Lunds Universitet, Sverige 3. University of Newcastle, UK

  2. Talk Structure • Introduction to intense field phenomena • Huge ac-Stark shifts in NO • Time resolved ac-Stark shift experiments • Intense field manipulation of NO2 photodissociation dynamics

  3. Intense field phenomena • Characterized by non-perturbative phenomena • Large ac-Stark shifts • Multiphoton phenomena predominate • Above Threshold Ionization • Over-the-Barrier Ionization • Tunnel-Ionization • Light-Induced Potentials

  4. OK, just how Intense is Intense? Fusion + Fission research Unfocussed ns dye laser Focussed ns dye laser 1 VÅ-1 10 VÅ-1 109 1010 1011 1012 1013 1014 1015 1016 Wcm-2 Focussed ns Nd:YAG Focussed re-gen fs laser It’s the end of spectroscopy as we know it... Perturbative Non-perturbative

  5. Non-perturbative phenomena:Huge ac-Stark shifts in NO • Depends on state, can be positive or negative. • Ground state always negative (energy goes down). • Excited states depends on neighbouring states &c. • Rydberg states, De = e2E2/4mw2=Up- ponderomotiveenergy • How about intermediate states? e.g. Low Rydberg • Test out the Ã2S+ X2Pr transition of NO...

  6. Experimental Scheme D C (3pp) A (3ss) 60000 • Ã (v = 2)  X (v = 0) 2-photon resonance is at 409.8 nm • Sit above resonance and crank up intensity! (monitor fluorescence) • Interpret results using semiclassical model of light matter interaction. B(P) 40000 Energy (cm-1) X(P) 20000 0 1.0 1.2 1.4 1.6 1.8 2.0 RNO/Å

  7. Weak 90 fs, 800 nm pulses (80 MHz) fs oscillator Ar+ laser PC scope Nd: YAG laser Amplifier Intense 140 fs, 800 nm pulses (10 Hz) PMT KDP xtal M400 nm M/C l/2 plate 0.2 m lens 0.1 m lens M400 nm Static cell, NO 1.6 Torr Intense 100 fs, 400 nm pulses (10 Hz) Experimental Setup

  8. Semiclassical Models Choose basis set Calculate eigenstates as function of field strength Propagate time dependent Schrödinger equation by projecting onto time dependent eigenstates Interpolate eigenstates and eigenvalues from calculations Evaluate final population in excited state

  9. Semiclassical Models • Sixteen state model includes v = 0 - 5 for A,C,D states, v = 0 for X state. • Schrödinger Equation propagated by projecting wavefunction onto time dependent eigenstates. • Matrix elements from literature (experimental). Spatially integrated SF 0.030 E0(a.u.) 0.000 27250 26160 25070 23980 22890 21800 Frequency/cm-1

  10. Semiclassical Models • Four state model includes v = 2 for A,C,D states, v = 0 for X state. • Schrödinger Equation propagated as per 16 state model • Results simpler to interpret... 1.0 0.8 0.6 2 | a | A (2) 0.4 0.2 0.0 0.030 0.025 26400 0.020 25920 0.015 E0(a.u.) 25440 0.010 24960 0.005 24480 0.000 frequency (cm-1)

  11. … in comparison 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 4 - state model 0.030 0.020 E (a.u.) 0.010 26400 0 25920 25440 0.000 24960 24480 -1 frequency (cm ) 1.2 1 0.8 0.6 0.4 0.2 16 - state model 0 0.030 0.020 E (a.u.) 0.010 0 26160 25724 0.000 25288 24852 24416 -1 frequency (cm )

  12. Results • Upper state is shifted into bandwidth of 400 nm laser at about 2×1013Wcm-2. • 16 state semiclassical model not perfect, but confirms intepretation • state shifts at approximately 50% of ponderomotive energy. 410 nm 10000 405 nm SF(arb. units) 400 nm 16 state model 4 state model 0 1 2 3 4 5 6 experimental Peak Intensity (1013Wcm-2)

  13. The Next Step... • We want to know exactly what we’re doing to the NO molecules… • Can we time resolve the shifting states? • Can we utilise the shift to effect dynamics?

  14. Time-Resolved ac Stark Effect A state shifted out of resonance by Stark pulse (strong probe) A state shifted into resonance by Stark pulse state energy 400 nm probe Unperturbed A state Ground state Stark pulse delay

  15. Experimental Setup + Ar laser fs oscillator PC scope Regen. Amp. Nd:YAG laser 400 nm PMT 800 nm 10 Hz M/C MB to rotary pump delay stage NO/Ar mixture

  16. Results… shifting the state into resonance IStark 2.4 TWcm-2 3.4 TWcm-2 5.8 TWcm-2 I400nm = 5.3 TWcm-2 fluorescence (arb. units) 7.9 TWcm-2 9.9 TWcm-2 -1.0 -0.5 0.0 0.5 1.0 time delay (ps)

  17. shifting the state out of resonance 3.3 TWcm-2 fluorescence (arb. units) 2.5 TWcm-2 1.8 TWcm-2 I400nm = 27 TWcm-2 -2.0 -1.0 0.0 1.0 2.0 time delay (ps)

  18. Semiclassical Models... 4 - state model 0.008 0.007 0.008 0.007 0.006 -400 400 -200 0.006 -200 E (a.u.) 0 0.005 0 S 0.005 200 E (a.u.) 200 400 S D (fs) 0.004 400 0.004 D (fs) 3 - state model 0.011 0.010 0.011 0.009 0.010 -400 400 0.009 0.008 -200 E (a.u.) -200 0.008 0 S 0 0.007 E (a.u.) 0.007 200 200 S 400 0.006 D (fs) 400 0.006 D (fs)

  19. Conclusions... • AC Stark effect is time resolvable • Can use one laser to shift, another to populate • Ionization is important • Is it possible to influence photodissociation dynamics in this way?

  20. Doing it to NO2 NO2* (A) NO* + O (X) NO + O • Same experimental setup as before • 400 nm acts as 3 photon pump • monitor fluorescence from particular vibronic state of NO as function of delay between pump and probe NO2

  21. Results? lpump = 400 nm lprobe = 800 nm Ipump 5.3 TWcm-2. Iprobe 0.5; 1.0; 2.0; 4.0 TWcm-2. 0.5 TWcm-2 0.5 TWcm-2 1.0 TWcm-2 1.0 TWcm-2 v’ = 0 fluorescence v’ = 1 fluorescence 2.0 TWcm-2 2.0 TWcm-2 4.0 TWcm-2 4.0 TWcm-2 -1.0 0.0 1.0 2.0 -1.0 0.0 1.0 2.0 pump-probe delay (ps) pump-probe delay (ps)

  22. Consider the coupled photon-molecule system energy Excited state molecule and n photons |A,n> Ground state molecule and n photons |X,n> Excited state molecule and n-1 photons |A,n-1>

  23. Excitation process becomes a curve crossing • Franck-Condon Principle applies itself through normal curve crossing rules • Intense laser causes avoided crossing energy Ground state molecule and n photons |X,n> Excited state molecule and n-1 photons |A,n-1>

  24. The Interpretation |A,n> • 1. Direct 3 photon absorption • 2. AX then 2 photon absorption • 3. AX, XA dynamics, then 2 photon absorption 2 |3ss,n-2> 1 3 |3ss,n-3> |X,n>

  25. 1.Direct 3 photon absorption • Direct 3 photon absorption is FC weak at 400 nm. • Increased avoided crossing by 800 nm will lessen its importance • Channel only important at t0 • Will produce more v = 0?

  26. 2.AXthen 2 photon absorption • A state populated on leading edge of laser pulse • Increased avoided crossing by 800 nm will trap population above and below seam. • Dynamics on A state may lead to preference for v = 0, enhanced by 800 nm irradiation 200 fs after peak of 400 nm pulse...

  27. 3.AX, XAdynamics, then 2 photon absorption • Channel is statistical • molecules cross as they trickle down from A state. • Channel important while 400 nm laser is on • Probably responsible for v = 1 signal.

  28. Conclusions and Questions... • Production of v’ = 1 takes approximately 400 fs. • Is the second channel responsible for enhanced v’ = 0 at t = 200 fs? • Other wavelengths produce consistent results • Need better photoproduct diagnostics to fully understand dynamics • Theoretical results would be interesting! • Can intense fields be used to control photodissociation?

  29. Acknowledgements • Research Studentship, Churchill College, Cambridge • Eleanora Sophia Wood Travelling Scholarship, University of Sydney • EPSRC • Royal Society of London …. and these guys

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