Post on 06-Jan-2016
description
Femtosecond Dynamics of Molecules in Intense Laser
Fields
CPC2002T.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
» 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
• 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?
109 Wcm-21010 1011 1012 1013 1014 1015 1016
1 VÅ-1 10 VÅ-1
Unfocussed ns dye laser
Focussed ns dye laser
Focussed re-gen fs laser
Fusion + Fissionresearch
Focussed ns Nd:YAG
Perturbative Non-perturbativeIt’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, = e2E2/4m2=Up - ponderomotive
energy
»How about intermediate states? e.g. Low Rydberg
»Test out the Ã2+ X2r transition of NO...
Experimental Scheme
60000
40000
20000
En
ergy
(cm
-1)
1.0 1.2 1.4 1.6 1.8 2.00
B()
X()
A (3s)
C (3p)D
RNO/Å
» 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.
Experimental Setup
Ar+ laser fs oscillator
Weak 90 fs, 800 nm pulses (80 MHz)
PC
0.1 m lens
PMT
M/C
scope
Nd: YAG laser Amplifier
Intense 140 fs, 800 nm pulses (10 Hz)
KDP xtal
/2 plate
M400 nm
Static cell, NO 1.6 Torr
Intense 100 fs, 400 nm pulses (10 Hz)
0.2 m lens
M400 nm
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
21800 22890 23980 25070 26160 27250
Sp
atia
lly in
tegr
ated
SF
E0 (a.u.)0.000
0.030
Frequency/cm-1
»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).
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...
2448024960
2544025920
26400
0.0000.005
0.0100.015
0.0200.025
0.030
0.00.20.40.60.81.0
frequency (cm-1)
|aA(2) |2
E0 (a.u.)
… in comparison
24480 24960 25440 25920 264000.000
0.0100.020
0.030
00.10.20.30.40.50.60.7
frequency (cm-1 )
E0 (a.u.)
00.20.40.60.8
11.2
frequency (cm-1 )
E0 (a.u.)0.000
0.0100.020
0.030
24416 24852 25288 25724 26160
4 - state model
16 - state model
Results10000
Peak Intensity (1013Wcm-2)
400 nm
0 1 2 3 4 5 6
410 nm
405 nm
S F (
arb.
uni
ts)
»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.
16 state model4 state modelexperimental
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 nmprobe
Stark pulse delay
stat
e en
ergy
Unperturbed A state
A state shifted intoresonance by Stark pulse
A state shifted out of resonanceby Stark pulse (strong probe)
Ground state
Experimental Setup
Ar+ laser fs oscillator
Nd:YAG laser Regen. Amp.
PC
scope
800 nm 10 Hz
delay stageNO/Ar mixture
to rotary pump
PMT
M/CMB
400 nm
Results… shifting the state into resonance
-1.0 -0.5 0.0 0.5 1.0time delay (ps)
fluo
resc
ence
(ar
b. u
nits
)
I400nm = 5.3 TWcm-2
2.4 TWcm-2
3.4 TWcm-2
5.8 TWcm-2
7.9 TWcm-2
9.9 TWcm-2
IStark
shifting the state out of resonance
-2.0 -1.0 0.0 1.0 2.0time delay (ps)
fluo
resc
ence
(ar
b. u
nits
)
I400nm = 27 TWcm-2
3.3 TWcm-2
2.5 TWcm-2
1.8 TWcm-2
Semiclassical Models...
-400-200
200400 0.006
0.0070.008
0.0090.010
0.011
0.004
0.0050.006
0.0070.008
ES (a.u.)
ES (a.u.)
0
D (fs)
D (fs)
-400-200
200400
0
ES (a.u.)
ES (a.u.)
D (fs)
D (fs)
400-200
0200
400 0.006
0.007
0.008
0.009
0.010
0.011
0.004
0.005
0.006
0.007
0.008
400-200
0200
400
4 -
stat
e m
odel
3 -
stat
e m
odel
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 NO2
NO2
NO2*
(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) pump-probe delay (ps)
-1.0 0.0 1.0 2.0-1.0 0.0 1.0 2.0
v’ =
0 f
luor
esce
nce
v’ =
1 f
luor
esce
nce
pump = 400 nmprobe = 800 nm
Ipump 5.3 TWcm-2.Iprobe 0.5; 1.0; 2.0; 4.0 TWcm-2.
0.5 TWcm-2
1.0 TWcm-2
2.0 TWcm-2
4.0 TWcm-2
0.5 TWcm-2
1.0 TWcm-2
2.0 TWcm-2
4.0 TWcm-2
Consider the coupled photon-molecule system
Ground state moleculeand n photons
|X,n>
Excited state moleculeand n photons
|A,n>
Excited state moleculeand 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 moleculeand n photons
|X,n>
Excited state moleculeand 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. AX then 2 photon absorption
»3. AX, XA 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 t0
»Will produce more v = 0?
80100
120140
160180 0.5
0.70.9
1.11.3
1.51.7
1.92.1
ON---O Bondlength (Ångströms)O-N-O Angle (degrees)
0
-30000-25000-20000-15000-10000
-50000
5000|X 2A1,n
|3s,n-3
2. AX 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. AX , XA 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.
80100
120140
160180 0.5
0.70.9
1.11.3
1.51.7
1.92.1
05000
1000015000200002500030000
ON---O Bondlength (Ångströms)O-N-O Angle (degrees)
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