© Imperial College LondonPage 1 Probing molecular structure and dynamics using laser driven electron recollisions 30 th April 2009 Sarah Baker Quantum

© Imperial College LondonPage 1

Probing molecular structure and dynamics using laser driven electron

recollisions

30th April 2009

Sarah Baker

Quantum Optics and Laser Science group Blackett Laboratory


Introduction to our groupWe conduct a variety of experiments aiming to study molecular structure and dynamics, through both high-order harmonic generation, and electron rescattering.

Femtolasers compactPRO;

1 kHz, 30 fs, 1 mJ

Argon filled hollow core fibreHHG

7 fs

0.25 mJChirped mirrors for compression

Coherent Legend-HE-USP

1 kHz, 50 fs, 2 mJElectron

rescattering in aligned molecules

14 fs

0.4 mJ

1.2 mJ

0.8 mJ

KM Labs Red Dragon

1 kHz, 25 fs, 6 mJ Two-colour HHG

<10 fs

~1 mJ

2 mJ

4 mJ


Introduction to our group

Electron rescattering in aligned molecules

HHG

Two-colour HHG

Underlying process is laser driven electron recollision…

Returning electron wavepacket can have energy components up to ~ 150 eV, or

wavelength 1 angstrom

Recollision lasts ~ 1 fs

We conduct a variety of experiments aiming to study molecular structure and dynamics, through both high-order harmonic generation, and electron rescattering.


Outline of talkIntroduction

1 Laser driven electron recollisions

2. HHG and electron rescattering

Experiments

3. Velocity map imaging of high energy rescattered electrons: towards

harnessing these high energy electrons to obtain structural information

4. Probing attosecond dynamics by chirp encoded recollision (PACER):

recent developments

5. HHG in larger molecules with 1300nm driving field: experiment

conducted at Rutherford Appleton Laboratory, to search for signatures of

the orbital structure in HHG signal

Conclusions, outlook, acknowledgments, adverts…


Laser driven electron recollisions

Returning electron wavepacket can have energy components up to ~ 150 eV, or

wavelength 1 angstromsRecollision lasts ~ 1 fs

Tunnel ionisation of

atom

picks up K.E.

Acceleration of electron in laser field

Recollision of electron with

parent

Ionisation can occur for a range of times around the peak of the electric field.

Parts of the electron wavepacket born at different times follow different trajectories, and gain varying amounts of energy from the field.


Propagation in the laser field… o spreads the electron wavepacket in time and momentum

o chirps the electron wavepacket

Laser driven electron recollisions

Continuous range of return energies 0 – 3.17Up

At 800 nm, 5 x 1014 Wcm-2: Up = 30 eV

Min. electron wavelength 1.2 angstroms

Sign of chirp reverses during the recollision, at the time corresponding to recollision of maximum energy electrons


On recollision of the electron wavepacket with the parent ion…

HHG and electron rescattering

Recombination can occur, resulting in emission of high

frequency photons

High-order harmonic generation

Scattering can occur, either elastically or inelastically.

The scattered electron will then once again experience

acceleration/deceleration in the laser field


HHG and electron rescatteringTypical harmonic spectrum Typical electron spectrum

Li et al, PRA 39, 5751 (1989) Grasbon et al, PRL 91, 173003 (2003).

plateau

rapid fall

cut-off3.17Up+Ip

10Up


Electron rescattering

Grasbon et al, PRL 91, 173003 (2003).

10Up

Direct + rescattered

electrons

Rescattered electrons only

ATI peaks


Structural information through electron rescattering

The angular distribution of rescattered electrons may exhibit diffraction peaks/minima; a signature of the molecular internuclear separation [Lein et al, Phys. Rev. A 66, 051404 (2002)].

Spanner et al, J. Phys. B 37, L243

(2004).Best information found at

high electron energies

5-6 a.u: 340-490 eV

(Up ~ 42eV)Measuring angular

distribution of such high energy electrons is

experimentally difficult!

TOF, scan angle

Velocity map imaging

COLTRIMS

Okunishi et al [J. Phys. B 41, 201004 (2008)] measured ATI in O2, N2 up to electron energy 120 eV – but angle scans time consuming







Measuring angular distribution of such high

energy electrons is experimentally difficult!

5-6 a.u: 340-490 eV

(Up ~ 42eV)

TOF, scan angle


COLTRIMS

AMOLF group recently investigated ionisation of O2, N2, CO2 by XUV using VMI [JMO 55, 2693 (2008)] up to electron energy 60 eV.







Measuring angular distribution of such high

energy electrons is experimentally difficult!

5-6 a.u: 340-490 eV

(Up ~ 42eV)

TOF, scan angle


COLTRIMS

Meckel et al [Science 320, 1478 (2008)] used COLTRIMS to detect diffraction signatures in O2, N2 at ~ 100 eV


A velocity map imaging spectrometer for high energy electrons

We have been developing a VMI spectrometer capable of detecting few-hundred eV electrons.

Basic design of grid assembly

All electrodes electro-polished stainless steel

- 15kV

-11.7 kV

High voltages required

Large aperture

MCP

Flight tube with large diameter:

length ratio

Pulsed molecular beam inOccasional

breakdown at > 13 kV

Interaction region (laser into/out of page)



(Very) recently obtained electron images up to -10kV…

Coherent Legend-HE-USP

1 kHz, 50 fs, 2 mJ

14 fs

0.1 mJ

0.8 mJ

1.2 mJ

45 deg OAP

f=20 cm

Skimmed molecular beam (axis out of page) delivered

to interaction region

Gas jet 1 kHz, backed with 250 mbar Xe

CCD

Vertical polarisation

1. Peaked structure confined to axis of polarisation

3. Continuous distribution extending to larger radii, emitted more isotropically

2. Weaker emission perpendicular to polarisation



Recently tested up to -10kV…

Weak low order ATI peaks

Separation 1.9 +/- 0.5 eV

14 fs, 2.2 x 1014 Wcm-2

Peak of spectrum at 766 nm = 1.6 eV

Spectrum at hollow fibre exit

For short pulse, high intensity, do not expect clear ATI rings [Grasbon

et al, PRL 91, 173003 (2003)]




To obtain angular distribution need to account for varying shift

of rescattered electron momentum in different directions

Energy distribution along polarisation direction

Up computed from spot size (60 x 68 microns) , energy

(100 uJ at chamber entrance), and pulse

duration (14 fs) measurements:

Up = 13.1 eV




Energy distribution along polarisation direction

Up computed from spot size (60 x 68 microns) , energy

(100 uJ at chamber entrance), and pulse

duration (14 fs) measurements:

Up = 13.1 eV

Observe plateau and cut-off at 10Up as expected.

ATI peaks just visible on log plot

Maximum energy electron detected ~140 eV – but limited by laser intensity, not performance of VMI

spectrometer.

Promising progress towards measuring angular distribution of 200-300 eV electrons


High order harmonic generationTypical harmonic spectrum

Li et al, PRA 39, 5751 (1989)

rapid fall

plateau

cut-off3.17Up+Ip

dteekazd tiikz )(||)( 0

Spectral amplitude within strong field approximation described by

• Usually assumes single-active electron • Assumes continuum states are

approximated as plane waves.• Ignores influence of laser field upon

molecular bound state

Within SFA there is a simple (Fourier) relationship between harmonic amplitude and orbital wavefunction0


Effect of nuclear dynamics on HHGFor a diatomic molecule, ionisation also launches a vibrational (expanding) nuclear wavepacket on the ionic PES.

Time interval in which this wavepacket can evolve before electron returns 0-2.6 fs.

For light nuclei significant motion can occur in this time window.


Vibrational wavefunctions (nuclear contribution) Electronic

ground states

Effect of nuclear dynamics on HHG

Electron travel time

For a diatomic molecule, ionisation also launches a vibrational (expanding) nuclear wavepacket on the ionic PES.

Time interval in which this wavepacket can evolve before electron returns 0-2.6 fs.

For light nuclei significant motion can occur in this time window.

dRkRRekRRkv ikx ,000

assuming superposition of plane waves for continuum electron wavefunction:

. Harmonic signal proportional to .22 )()( kvka dkeka ikx

c )(

At the recollision, recombination amplitude given by


A single harmonic spectrum contains information about nuclear dynamics occurring on ionic PES during electron travel time, through amplitude vs frequency information.

However, amplitude vs frequency also depends on a(k): 22 )()( kvka

Probing attosecond dynamics by chirp encoded recollisionPACER:

Compare harmonic yield in two isotopes and assume invariance of a(k)

Ratio vs frequency information allows nuclear dynamics to be extracted.

First proposed by M. Lein, Phys. Rev. Lett. 94, 053004 (2005).

Harmonic emission is chirped: one-to-one mapping between electron travel time and harmonic frequency.

Recombination amplitude decreases in time as nuclear wavepacket evolves.


Probing attosecond dynamics by chirp encoded recollision

Laser electric field

Time

PACER:

H2+


Probing attosecond dynamics by chirp encoded recollisionPACER:

H2+

Laser electric field

Time

D2+

Detecting that motion occurs between emission of successive harmonic orders.

Signal Ratio D2/H2

Return time

1

Signature of the slower nuclear dynamics of D2


PACER experimental set-up

Gas jet rep rate 2 Hz (limited by pumping speed).

Detect 17th harmonic and beyond.

Focus 9mm before jet to isolate short electron trajectories.

CCD

OAP f=40 cm

Gas jet variable z

Variable aperture

250 uJ 8 fs

800 nm 1kHz

MCP

Experimental set-up: gas jet position and confocal parameter/focal size can be continuously varied.


First PACER measurement

Ratio H2/H2

D2

H2

8fs, 2 x 1014 Wcm-2, 800nm, pulsed gas jet source. Focus 9mm before jet to isolate short electron trajectories.

Baker et al., Science 312 p 424 (2006).

Ratio H2/H2



8fs, 2 x 1014 Wcm-2, 800nm, pulsed gas jet source. Focus 9mm before jet to isolate short electron trajectories.


Single molecule response, allowing for short trajectories only.

Includes two-centre interference effects.

Includes effect of reabsorption of generated XUV.

dRkRkRRkc )2/)cos(cos(,0

Blue curve:

averaged over randomly aligned angle distribution.

Use BO potentials for H2+ and D2

+.

Ratio H2/H2



Red: time evolution as reconstructed from harmonic spectra, different runs of genetic algorithm

Blue: time evolution calculated from known BO potential

The increasing ratio is a signature of the slower nuclear motion in D2+, and can

be used to gain information about the nuclear motion:

We have made a measurement with ~100 as time resolution, using 8 fs pulses:

launch of electron and nuclear wavepackets is synchronous by nature of the process.


R (

bo

hrs

)

Time (fs)


Dynamic interference through PACERUsing longer, higher intensity pulses we were able to observe dynamic interference in the PACER signal, as the nuclear wavepacket expands.

[Baker et al., PRL 101 053901 (2008)].

PACER can also be used to detect

signatures of the structure of the orbital


PACER in methaneOriginal experiment was also conducted in CH4 and CD4: a harmonic ratio that strongly increased with order was detected.


We postulated that this was evidence of very fast bond angle changes following ionisation


PACER in methaneRecent calculations support the very fast nature of conformational

changes in CH4

(Courtesy of Serguei Patchkovskii)

Analytical calculation including only nuclear contribution, within BO

approximation

An electric field lifts the degeneracy of the 3

branches of the Jahn-Teller distortion allowing

them to be treated independently.

Very promising result regarding applicability of PACER to larger systems


HHG for structural information

Can lead to complete reconstruction of 0 [Itatani et al. Nature 432, 867 (2004)]; so far in N2 only.

dteekazd tiikz )(||)( 0

Within SFA there is a simple (Fourier) relationship between harmonic amplitude and orbital wavefunction0

Larger molecules have low Ip, so harmonic cut-off at relatively low orders

Impulsively align molecules with pump pulse

Time delayed probe pulse for

HHG

Measure HHG spectrum as function of angle between pump and probe polarisation

We have been working towards retrieval of orbitals of more complecated organic molecules [Torres et al, PRL 98, 203007 (2007)].


HHG in larger molecules at 1300 nmLarger molecules have low Ip, so harmonic cut-off at relatively low orders

Recent experiment at ARTEMIS facility at Rutherford Appleton Laboratories

Use longer wavelength driving field to extend the harmonic cut-off at a fixed intensity: sampling orbital with larger range of momentum components.

KMLabs Red Dragon 1

kHz 10 mJ 80 fs 780 nm

TOPAS

Flat field spec

CCD

Imaging MCP85:15

1300 nm 50 fs

1 mJ

HR 1300 nm HT 780 nm

f = 30 cm

Gas jet (1 kHz) or continuous

flow


HHG in larger molecules at 1300 nmWe observed 27th – 67th harmonics in N2O, 27th – 71st in C2H2 and good alignment revivals in both gases:

N2O C2H2

Half revival, parallel polarisation of pump and probe

Angle scans should allow tomographic reconstruction within SFA to be tested for these polyatomic molecules.


ConclusionsPACER is a promising new technique for probing fast nuclear motion in molecules

Time resolution ~100 as

May be applicable to larger molecules…

We have developed a VMI spectrometer capable of measuring angular and energy distribution of few-hundred eV electrons: useful for obtaining structural information through electron rescattering.

PACER with larger range of molecules: C3H4?

Outlook

PACER with long trajectories: compare retrieved R(t) with that from short trajectories -> any detectable effect of the different field strengths at recombination?

Full analysis of long wavelength HHG experiment to test tomographic reconstruction for polyatomic molecules

Measurement of angular distribution of 200-300 eV electrons in diatomic molecule (N2…)


People

Adverts!

Delphine Darios; VMI of high energy electrons

Imma Procino; Retrieval of molecular axis alignment from Coulomb explosion imaging experiments without cylindrical symmetry

Two posters:

PACER work:

Joe Robinson Manfred Lein

Ciprian Chirila

Long wavelength HHG:

Tom Siegel Ricardo Torres

Leonardo Brugnera Imma Procino

Jonathan Underwood

Staff at ARTEMIS facility

E. Springate, I.C. E. Turcu, C. Froud

Jon Marangos John Tisch

Electron VMI:

Delphine Darios Marco Siano David Holland (STFC)

Documents

© Imperial College LondonPage 1 Probing molecular structure and dynamics using laser driven electron recollisions 30 th April 2009 Sarah Baker Quantum