R-dependent strong R-dependent strong field ionization from a field ionization from a neutral ground state neutral ground state diatomic moleculediatomic molecule

G. N. Gibson, H. Chen*, V. G. N. Gibson, H. Chen*, V. Tagliamonti*Tagliamonti*

University of ConnecticutUniversity of Connecticut

*Also giving talks in this session*Also giving talks in this session

DAMOP 2012Anaheim, CAJune 6, 2012

Funding from the National Science

Foundation

R-dependent R-dependent ionizationionization

• Important test of any strong-field theory of ionization• Simple tunneling models do not agree with

experiments on Iodine• Rcritical in even charged molecules?• Lochfrass – implies very strong dependence of

ionization rate on R near Re: Γ(R)

• » Want to measure Γ(R) directly from the ground state over a large range in R – should see a huge effect

• Chen: ionization from the B-state vs. R• Tagliamonti: 1ω2ω ionization from the B-state vs.

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Pump-dump-probePump-dump-probe

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4 5 6 7 8 9 10 11 120

1

2

3

10

12

14

16

18

0

5

10

15

20

25

31.0

31.5

32.0

(2,0)u

(2,1)

B u

+

I2

I+

2

R (a.u.)

I2+ 2 p

oten

tial

ene

rgy

(eV

)

X g,3/2

(1,1)

(2,0)g

I2+

2

X g

+

A u,3/2

I 2, I+ 2 p

oten

tial

ene

rgy

(eV

)

Not to scale

Pum

p

Pro

be

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Depletion at 45 fsDepletion at 45 fs

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00.00

0.02

0.04

0.06

(2,1

) S

igna

l

3600

3650

3700

3750

3800

3850

3900

Delay [ps]

TO

F [

ns]

(2,1)

(2,1)

(2,0)

Depletion of B-state into X-state

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Wavepacket monitored Wavepacket monitored in (2,0) channelin (2,0) channel

(20% excitation)*(75% de-excitation) = 15% in high-v X-stateDAMOP 2012 6

Fourier Transform Fourier Transform SpectroscopySpectroscopy

0 2 4 6 8 10 12 14 16 18 20 22 243600

3650

3700

3750

3800

3850

3900

Frequency [ps-1]

TO

F [

ns]

X-state v=0"Lochfrass"

X-state v= 33!Returning wavepacket

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What about IWhat about I22++ signal? signal?

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What happened to What happened to expected modulation?expected modulation?

• NO modulation at 5 THz in the I2+ signal

• However, we do see strong 6.3 THz modulation – this is due to Lochfrass in the ground state near Re

• We know that Γ(R) must be a strongly increasing function of R, because of Lochfrass, which should lead to a huge modulation of the ionization rate, because the range of R is much bigger.

• However, Lochfrass does not give the final state, only the rate out of the ground state Γtotal(R) = Γ→X-state(R) + Γ→(1,0)(R)since (1,0) is the only other 1-e ionization signal.

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ΓΓ(R) has strong (R) has strong dependence, just not dependence, just not

to Ito I22++

• (1,0) represents ionization from an inner-orbital or into strongly mixed states.

• HOMO is antibonding - ionization won’t lead to (1,0).

• Inner orbitals are bonding – ionization will weaken bond.

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Branching ratio Branching ratio between Ibetween I22

++ and (1,0) and (1,0) channelschannels

• Even single pulse branching ratio to (1,0) is very large ~67%.

• Branching ratio increases with R, as measured from the B state.

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Orbital structureOrbital structure

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ConclusionsConclusions• Ionization projects ion into mixed states

o Asymmetric work of Tagliamontio Branching ratio to (1,0)o At large R, molecule falls apart

• Hard to claim we are cleanly removing an electron from one orbital of another – everything gets highly mixed by the field

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