Angle- and internuclear Angle- and internuclear separation- resolved strong separation- resolved strong

field processes in field processes in moleculesmolecules

Grad student: Grad student: Li Li FangFang

FundingFunding: : NSF-NSF-AMOAMO

May 26, 2010May 26, 2010DAMOPDAMOP

Houston, TXHouston, TX

George N. George N. GibsonGibsonUniversity of University of ConnecticutConnecticut

Department of Department of PhysicsPhysics

IntroductionIntroduction A standard sample of molecules will be in their A standard sample of molecules will be in their

equilibrium configuration and randomly equilibrium configuration and randomly oriented.oriented.

However, strong field molecular processes However, strong field molecular processes depend on the depend on the orientation and alignmentorientation and alignment of of the molecule and the the molecule and the inter-nuclear inter-nuclear separationsseparations..

We start with this:We start with this: We would like this:We would like this:

Control MethodsControl Methods One can control One can control inter-nuclearinter-nuclear

separation by ionizing to dissociating separation by ionizing to dissociating states. However, several states are states. However, several states are usually populated, one must work in an usually populated, one must work in an ion, and the dissociation happens ion, and the dissociation happens quickly. Also, one can’t study the quickly. Also, one can’t study the neutral molecule.neutral molecule.

AlignmentAlignment can be controlled through can be controlled through adiabatic fields or impulsive techniques, adiabatic fields or impulsive techniques, but often the degree of alignment in not but often the degree of alignment in not very high, unless multiple pulses are very high, unless multiple pulses are used, or the sample is not field-free.used, or the sample is not field-free.

Resonant excitation Resonant excitation provides an interesting provides an interesting

alternativealternativeUsing pump-probe Using pump-probe

techniques, we can techniques, we can control R.control R.

Resonant excitation Resonant excitation follows a cos(follows a cos())22 pattern, producing a pattern, producing a well-aligned and well-well-aligned and well-defined sample.defined sample.

This gives:This gives:

<cos(<cos())22> = 0.6> = 0.6at room at room temperature with temperature with one laser pulse.one laser pulse.

[For unaligned samples [For unaligned samples <cos(<cos())22> = 0.33]> = 0.33]

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(2,1)

B u

+

I2

I+

2

R (a.u.)

I2+ 2 p

oten

tial

ene

rgy

(eV

)X

g,3/2

(1,1)

(2,0)

I2+

2

X g

+

A u,3/2

I 2, I+ 2 p

oten

tial

ene

rgy

(eV

)

Not to scale

2

g3

u4

g2

g4

u3

g

2

g4

u3

g1

u

Pum

p

Probe

2

g4

u4

g

Wavepacket motion Wavepacket motion independent of angleindependent of angle

Ionization to IIonization to I22++

0.0 0.5 1.0 1.5 2.0 2.50.00

0.02

0.04

0.06

0.08

0.10

Cou

nts/

shot

Time Delay [ps]

PolarizationAngle

0 6 12 18 24 30 36 42 48 54 60 66 74 82 90

Ionization vs. RIonization vs. R We know <R(t)> from the motion on the We know <R(t)> from the motion on the

B state.B state. Can convert from time to R(t).Can convert from time to R(t).

RRcc of a neutral excited state of a neutral excited state

RRcc is at 8.6 a.u. is at 8.6 a.u.

Appears to Appears to increase with increase with angle or angle or decreasing field decreasing field along the axis.along the axis.

Ionization Ionization potential potential increasesincreases with with R in contrast to R in contrast to HH22

++, which , which decreases with decreases with R.R.

PRA 59, 4843 (1999).

Hydrogen curvesHydrogen curves

0 1 2 3 4 5 6 7 8 9 1015

20

25

30

35

40

45 1sg 2pu 2sg 3pu 3dg 2ppu Ion

Ene

rgy

[eV

]

R [Angstroms]

Polar plots of ionization Polar plots of ionization from the Ifrom the I22 B B 33uu

++ state state to Ito I22

++

0.000

0.005

0.010

0.015

0.020

0

30

60

90

120

150

180

210

240

270

300

330

0.000

0.005

0.010

0.015

0.020

Requilibrium

cos()2

0.00

0.01

0.02

0.03

0.04

0.05

0

30

60

90

120

150

180

210

240

270

300

330

0.00

0.01

0.02

0.03

0.04

0.05

cos()2

Rintermediate

0.00

0.02

0.04

0.06

0.08

0.10

0

30

60

90

120

150

180

210

240

270

300

330

0.00

0.02

0.04

0.06

0.08

0.10

cos()2

Rcritical

Shows Shows uu

symmetrsymmetryy

Polar plots of ionization Polar plots of ionization from the Ifrom the I22 B B 33uu

++ state state to Ito I22

++

0.00

0.02

0.04

0.06

0.08

0.10

0

30

60

90

120

150

180

210

240

270

300

330

0.00

0.02

0.04

0.06

0.08

0.10

Req

(5 a.u.)

Rint

(8 a.u.)

Rc (8.6 a.u.)

Ionization to IIonization to I222+2+

0.0 0.5 1.0 1.5 2.0 2.5

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

PolarizationAngle

0 6 12 18 24 30 36 42 48 54 60 66 74 82 90

Cou

nts/

shot

Time Delay [ps]

I2+

2

Polar plots of ionization Polar plots of ionization to Ito I22

2+2+

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0

30

60

90

120

150

180

210

240

270

300

330

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Requilibrium

cos( R

intermediate

cos( R

critical

cos(

I2+

2

ConclusionsConclusionsResonant short-pulse excitationResonant short-pulse excitation Provides high degree of alignmentProvides high degree of alignment Provides controlled internuclear motionProvides controlled internuclear motion Allows us to measure ionization rates as Allows us to measure ionization rates as

a function of angle and Ra function of angle and R Possible coupling between angle and RPossible coupling between angle and R Mechanism for RMechanism for Rcc in an excited neutral? in an excited neutral?

Is it just 1 electron in a double well, or Is it just 1 electron in a double well, or do the ionic states play a role?do the ionic states play a role?