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 »Introduction to intense field phenomena »Huge ac-Stark shifts in NO »Time resolved ac-Stark shift experiments »Intense field manipulation of NO 2 photodissociation dynamics

Intense field phenomena Characterized by non-perturbative phenomena Large ac-Stark shifts Multiphoton phenomena predominate A bove T hreshold I onization O ver- t he- B arrier Ionization Tunnel-Ionization L ight- I nduced P otentials

OK, just how Intense is Intense? 10 9 Wcm VÅ VÅ -1 Unfocussed ns dye laser Focussed ns dye laser Focussed re-gen fs laser Fusion + Fission research Focussed ns Nd:YAG Perturbative Non-perturbative It’s the end of spectroscopy as we know it...

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,  = e 2 E 2 /4m  2 =U p - ponderomotive energy »How about intermediate states? e.g. Low Rydberg »Test out the à 2  +  X 2  r transition of NO...

Experimental Scheme Energy (cm -1 ) B(  ) X(  ) A (3s  ) C (3p  ) D R NO /Å » Ã (v = 2)  X (v = 0) 2-photon resonance is at nm » Sit above resonance and crank up intensity! (monitor fluorescence) » Interpret results using semiclassical model of light matter interaction.

Experimental Setup

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

Semiclassical Models Spatially integrated S F E 0 (a.u.) Frequency/cm -1 »Sixteen state model includes v = 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).

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 frequency (cm -1 ) |a A (2) | 2 E 0 (a.u.)

… in comparison frequency (cm ) E 0 (a.u.) frequency (cm ) E 0 (a.u.) state model 16 - state model

Results Peak Intensity (10 13 Wcm -2 ) 400 nm nm 405 nm S F (arb. units) »Upper state is shifted into bandwidth of 400 nm laser at about 2×10 13 Wcm -2. »16 state semiclassical model not perfect, but confirms intepretation »state shifts at approximately 50% of ponderomotive energy. 16 state model 4 state model experimental

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?

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

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

Results… shifting the state into resonance time delay (ps) fluorescence (arb. units) I 400nm = 5.3 TWcm TWcm TWcm TWcm TWcm TWcm -2 I Stark

shifting the state out of resonance time delay (ps) fluorescence (arb. units) I 400nm = 27 TWcm TWcm TWcm TWcm -2

Semiclassical Models E S (a.u.) E S 0 D (fs) D E S (a.u.) E S D (fs) D state model 3 - state model

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?

Doing it to NO 2 NO 2 NO 2 * (X) NO + O (A) 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

Results? pump-probe delay (ps) v’ = 0 fluorescence v’ = 1 fluorescence pump = 400 nm probe = 800 nm I pump 5.3 TWcm -2. I probe 0.5; 1.0; 2.0; 4.0 TWcm TWcm TWcm TWcm TWcm TWcm TWcm TWcm TWcm -2

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

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

The Interpretation 1 2 |A,n> |3s ,n-2> |3s ,n-3> 3 »1. Direct 3 photon absorption »2. A  X then 2 photon absorption »3. A  X, X  A dynamics, then 2 photon absorption |X,n>

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 t 0 »Will produce more v = 0?

2. A  X then 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...

3. A  X, X  A dynamics, 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.

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?

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