Excitation processes during Excitation processes during strong-field ionization and strong-field ionization and dissociatation of moleculesdissociatation of molecules

Grad Grad students:students:Li Fang, Brad Li Fang, Brad

MoserMoser

FundingFunding::NSF-AMONSF-AMONovember 29, 2006November 29, 2006

ISUILS5ISUILS5Lijiang, ChinaLijiang, China

George N. George N. GibsonGibsonUniversity of University of ConnecticutConnecticut

Department of Department of PhysicsPhysics

MotivationMotivation Excitation of molecules by strong laser Excitation of molecules by strong laser

fields is not well-studied.fields is not well-studied. Excitation can have positive benefits, Excitation can have positive benefits,

such as producing inversions in the VUV such as producing inversions in the VUV and providing spectroscopy of highly and providing spectroscopy of highly excited states of molecules. excited states of molecules. Excited Excited states of Hstates of H22

++ have have nevernever been studied been studied before!before!

Can be detrimental to certain Can be detrimental to certain applications, such as quantum applications, such as quantum tomography of molecular orbitals.tomography of molecular orbitals.

OutlineOutline

Ionization of IIonization of I22 to the A to the A 22 state of I state of I22++..

Excitation of HExcitation of H22++ from 2p from 2puu to 2s to 2sgg via a via a

7-photon resonance at 800 nm.7-photon resonance at 800 nm. Tuning experiment showing resonant Tuning experiment showing resonant

excitation from the 1sexcitation from the 1sgg and 2p and 2puu states states to 2pto 2puu..

Theory of high-order multiphoton Theory of high-order multiphoton excitation.excitation.

Conclusions.Conclusions.

Inner-orbital ionizationInner-orbital ionization In TOF, we measure IIn TOF, we measure I22 → I → I22

++. If the I. If the I22++

does not dissociate, we have no idea what does not dissociate, we have no idea what electronic state it is left in.electronic state it is left in.

This is a general problem with TOF This is a general problem with TOF spectroscopy, except for one exception:spectroscopy, except for one exception:(I(I22

2+2+)* → I)* → I2+2+ + I + I0+0+, i.e. asymmetric , i.e. asymmetric dissociation.dissociation.

Can look for fluorescence, but more on Can look for fluorescence, but more on that later.that later.

In TOF, we have found the we can identify In TOF, we have found the we can identify excited states by their vibration signature excited states by their vibration signature in pump-probe experiments.in pump-probe experiments.

Laser SystemLaser System

• Ti:Sapphire 800 nm OscillatorTi:Sapphire 800 nm Oscillator• Regenerative or Multipass Regenerative or Multipass

AmplifierAmplifier• 750 750 J pulses @ 1 KHzJ pulses @ 1 KHz• Transform Limited, 25 fs pulsesTransform Limited, 25 fs pulses• Can double to 400 nmCan double to 400 nm• Have a pump-probe setupHave a pump-probe setup

Ion Time-of-Flight Ion Time-of-Flight SpectrometerSpectrometer

Laser

Drift Tube MCPConical Anode

Parabolic Mirror

AMP

DiscriminatorTDCPC

II22 potential potential energy energy curvescurves

2 3 4 5 60

1

15

20

10

15

20

30

(1,1)

(2,0)

I2

2+

I2: X

g

I2

+: A u

Ene

rgy

(ev)

Internuclear separation (Å)

II2+2+ pump-probe data pump-probe data

Simulation of A stateSimulation of A state

Conclusions from IConclusions from I22

Can identify excited molecular states Can identify excited molecular states from vibrational signature.from vibrational signature.

Can learn about the strong-field Can learn about the strong-field tunneling ionization process, tunneling ionization process, especially details about the angular especially details about the angular dependence.dependence.

Could be a major problem for Could be a major problem for quantum tomography.quantum tomography.

VUV Detection VUV Detection SchemeScheme

• Vertical Focal Volume Vertical Focal Volume • ~ 45 ~ 45 m diameterm diameter• ~ 10~ 101515 W/cm W/cm22

• Iridium Imaging OpticIridium Imaging Optic• 2 inch F/1.332 inch F/1.33• R R ~ ~ 20%20%

Timed Correlated Timed Correlated Photon Counting Photon Counting (TCPC) – (TCPC) – 5 ns resolution5 ns resolution

60 70 80 90 100 110 120 1300.000

0.005

0.010

0.015

0.020

0.025

0.030

130 140 150 160 170 180 190 2000.000

0.002

0.004

0.006

0.008

0.010

N V

: 2s

- 2

p

N I:

2s2 2p

3 - 2

s2 2p2 (1

D)3

s

N I:

2s2 2p

3 - 2

s2 2p2 (3

P)3s

N I:

2s2 2p

3 - 2

s2p4

N I

I: 2

s2 (1S)

2p2 -

2s(

2S)2

p3

N I

II:

2s2 (1

S)2p

- 2

s2p2

N I

II:

2s2p

2 - 2

p3 [25

.2 e

V]

N I

V:

2s(2

S)2p

- 2

p2

N II

: 2s2 (1

S)2p

2 - 2

s(2S

)2p3

N II

I: 2

s2p2 -

2p3

N I

II:

2s2 (1

S)2p

- 2

s2p2

N II

: 2s2 (1

S)2p

2 - 2

s2 2p(

2P°)

3s

N I

II:

2s2 (1

S)2p

- 2

s2p2

N II

: 2s2 (1

S)2p

2 - 2

s2 2p(

2P°)

3s

N II

: 2s2 (1

S)2p

2 - 2

s(2S

)2p3

VUV Fluorescence Spectrum of N2

Inte

nsity

[a.

u.]

UnidentifiedMolecular Lines?

Uni

dent

ifie

d

N I:

2s2 2p

3 - 2

s2 2p2 (3

P)3s

N I

V:

2s(2

S)2p

- 2

p2 [23

.4 e

V]

Uni

dent

ifie

d

N I:

2s2 2p

3 - 2

s2 2p2 (3

P)3s

Uni

dent

ifie

d

Wavelength [nm]

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.015

20

25

30

35

40

Bond softening

n=

n=2

Ato

mic

lim

its

3pu

3dg

2sg

4fu

2pu

3dg

2pu

Ene

ry [

eV]

R [nm]

H+2

1sg

n=1

H+ + H+

Pressure Dependence of Pressure Dependence of LL

1 10 100 100010-5

10-4

10-3

10-2

10-1

60 J/shot

30 J/shot

Slope =1.34

Slope = 0.91

Cou

nts/

shot

Pressure [mTorr]

Slope = 0.94

Fluorescence quenchingFluorescence quenching

0 10 20 30 40 500

100

200

300

400

500

-1000 -750 -500 -250 0 250 500 750 1000300

400

500

600

Cou

nts(

106 S

hot)

Delay (fs)Dep_T

Cou

nts/

(106 s

hot)

Probe Energy [J]

Pump+Probe Probe Alone

Quenching with circular Quenching with circular polarization – rules out polarization – rules out

rescatteringrescattering

0 10 20 30 40 500

10

20

30C

ount

s/(1

06 sho

t)

Probe Power [J]

We have a We have a direct direct excitation of excitation of HH22

++ which is which is not due to not due to rescattering.rescattering.

The excitation The excitation efficiency is efficiency is about 1%.about 1%.

0 1 2 3 4 5 6 7 8 9 10

0

5

10

15

20

25

30

35

40

45

Probeionization

7- resonance

1- bond softening

Ionization

H + H

H+ + H(1s)

H+ + H(2l)

H+ + H+

2pu

2sg

3pu

Pot

enti

al E

nerg

y [e

V]

Internuclear separation [Angstroms]

1sg

1+

g

Wavelength tuning Wavelength tuning experimentsexperiments

Little work done on strong-field Little work done on strong-field processes with a tunable laser. The processes with a tunable laser. The assumption is that we are generally in assumption is that we are generally in the tunneling regime where the photon the tunneling regime where the photon energy is not significant.energy is not significant.

However, we have shown that high-However, we have shown that high-order resonant processes can be quite order resonant processes can be quite strong. If true, there should be a strong. If true, there should be a resonant signature.resonant signature.

Recently brought online an optical Recently brought online an optical parametric amplifier – a TOPAS.parametric amplifier – a TOPAS.

Resonances in HResonances in H22++

There is a problem with There is a problem with looking for resonances looking for resonances in Hin H22

++: ion is : ion is dissociating when it dissociating when it reaches the region of reaches the region of strong-field coupling. strong-field coupling. Curves are all changing, Curves are all changing, so resonances will be so resonances will be smeared out.smeared out.

However, the 2pHowever, the 2puu state state is quite parallel to the is quite parallel to the ground states, so we ground states, so we might see a resonance.might see a resonance.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.015

20

25

30

35

40

Bond softening

n=

n=2

Ato

mic

lim

its

3pu

3dg

2sg

4fu

2pu

3dg

2pu

Ene

ry [

eV]

R [nm]

H+2

1sg

n=1

H+ + H+

Tuning data in HTuning data in H22..

Figure B.3: Wavelength scans of L signal. Red and black show runs from two d ifferent days.

Conclusions from VUV Conclusions from VUV datadata

We see Lyman-We see Lyman- radiation following strong- radiation following strong-field ionization of Hfield ionization of H22..

From the linear pressure dependence and the From the linear pressure dependence and the pump-probe quenching experiment, we know pump-probe quenching experiment, we know the excitation is a direct strong-field effect.the excitation is a direct strong-field effect.

From the tuning experiment, we know the From the tuning experiment, we know the excitation is resonant.excitation is resonant.

From this, we know that we have 4, 5, and 7 From this, we know that we have 4, 5, and 7 photon transitions in Hphoton transitions in H22

++ driven by the laser driven by the laser field.field.

What is so special about What is so special about charged diatomic charged diatomic

molecules?molecules?Ground state is a Ground state is a

far off-far off-resonant resonant covalent state.covalent state.

Above this is a Above this is a pair of pair of strongly strongly coupled ionic coupled ionic states.states.

Only a weak Only a weak coupling coupling between them.between them.

If the ionic If the ionic states are states are populated, populated, there will there will probably be probably be inversions.inversions.

3-Level Model System3-Level Model System

This system can be solved exactly for the n-photon Rabi frequency!

But, what is the physics?But, what is the physics?First, consider just the coupling between levels 2 and 3:

This gives:

and let

Expand in Bessel Expand in Bessel Functions:Functions:

The eigenvalues are linear in the field, so their time average is zero, so there is no AC Stark shift.The Fourier components are at ±n, exactly.

““Floquet Ladder”Floquet Ladder”

The pair of degenerate states are strongly modulated by the laser field and create a complete Floquet ladder of states – with no ac Stark shift!

The ground state couples to this through a 1-photon process which only produces a small Stark shift.

Key is a linear response to Key is a linear response to an external field – flux an external field – flux

qubits?!qubits?!

20-photon transitions seen!20-photon transitions seen!

En

ergy

leve

ls

5-photon transition 5-photon transition with a pulsed laserwith a pulsed laser

-600 -400 -200 0 200 400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0

-PulseF = 0.18

Popu

latio

n

Time

-600 -400 -200 0 200 400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0

No sweepF = 0.30

Popu

latio

n

Time

-600 -400 -200 0 200 400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0

SweepF = 0.30

Popu

latio

n

Time

Adiabatic passage Adiabatic passage on a 10-photon on a 10-photon

transitiontransition

0.15 0.20 0.25 0.300.0

0.2

0.4

0.6

0.8

1.0

dE/dt = 6/Tn

2

Popu

latio

n

Field Strength [a.u.]

ConclusionsConclusions

We see Lyman-We see Lyman- radiation produced through radiation produced through the direction excitation of Hthe direction excitation of H22

++.. We propose that the excitation comes from We propose that the excitation comes from

a resonant 7-photon transition driven by the a resonant 7-photon transition driven by the degenerate strongly coupled states in Hdegenerate strongly coupled states in H22

++.. This mechanism will also produce equal This mechanism will also produce equal

amounts of metastable hydrogen.amounts of metastable hydrogen. Hydrogen is not the best candidate for Hydrogen is not the best candidate for

excitation because of the high ionization excitation because of the high ionization rate of the excited state. However, it is rate of the excited state. However, it is easier to analyze.easier to analyze.

Ion TOF pump/probe Ion TOF pump/probe experimentexperiment

HH22 → H → H22+ + → H→ H++ + H(1s) → H + H(1s) → H++ + H(2p). + H(2p).

But, we have no hope of seeing HBut, we have no hope of seeing H++ + + H(2p)H(2p)on top of Hon top of H++ + H(1s). + H(1s).

However, a weak pulse can ionize However, a weak pulse can ionize H(2p) but not H(1s). HH(2p) but not H(1s). H++ + H(2p) → H + H(2p) → H++ + H+ H++ we can see through coincidence we can see through coincidence measurements.measurements.

So, we are looking for a low energy HSo, we are looking for a low energy H++ + H+ H+ + correlation.correlation.

Correlated HCorrelated H++ ion ion spectrumspectrum

580 590 600 610 620 630 6400

50

100

150C

ount

s/(1

06 sho

ts)

Ion TOF [ns]

Intensity dependence of Intensity dependence of correlated signalcorrelated signal

1013 10140.1

1

10

100C

ount

s/(1

06 sho

ts)

Intensity [W/cm2]

Conclusions from TOF Conclusions from TOF measurementmeasurement

Can detect a low energy HCan detect a low energy H++ + H + H++ correlation.correlation.

Energy is ~2.7 eV, exactly as Energy is ~2.7 eV, exactly as expected from the proposed expected from the proposed excitation pathway.excitation pathway.

Population of H(2p) relative to H(1s) Population of H(2p) relative to H(1s) is ~1%.is ~1%.

Ionization competes with excitation.Ionization competes with excitation.