Femtosecond dynamics of molecules in intense laser fields
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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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Femtosecond dynamics of molecules in intense laser fields

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


Talk structure

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


Intense field phenomena

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


Ok just how intense is intense

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


Non perturbative phenomena huge ac stark shifts in no

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...


Experimental scheme

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


Experimental setup

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


Semiclassical models

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


Semiclassical models1

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


Semiclassical models2

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)


In comparison

… 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

)


Results

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)


The next step

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

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


Experimental setup1

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


Results shifting the state into resonance

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)


Shifting the state out of resonance

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)


Semiclassical models3

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)


Conclusions

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

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


Results1

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)


Consider the coupled photon molecule system

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>


Femtosecond dynamics of molecules in intense laser fields

  • 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>


The interpretation

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>


1 direct 3 photon absorption

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?


2 a x then 2 photon absorption

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...


3 a x x a dynamics then 2 photon absorption

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.


Conclusions and questions

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

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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