Transcript No Slide Title

Quantum Optics and Laser Science group
Blackett Laboratory
Probing molecular structure and
dynamics using laser driven electron
recollisions
Sarah Baker
30th April 2009
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Introduction to our group
We conduct a variety of experiments aiming to study molecular structure and
dynamics, through both high-order harmonic generation, and electron rescattering.
Femtolasers
compactPRO;
Argon filled hollow core fibre
Chirped mirrors
for compression
1.2 mJ
1 kHz, 50 fs, 2 mJ
14 fs
0.4 mJ
0.8 mJ
KM Labs Red Dragon
2 mJ
1 kHz, 25 fs, 6 mJ
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Electron
rescattering
in aligned
molecules
<10 fs
~1 mJ
4 mJ
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HHG
0.25 mJ
1 kHz, 30 fs, 1 mJ
Coherent Legend-HE-USP
7 fs
Two-colour
HHG
Introduction to our group
We conduct a variety of experiments aiming to study molecular structure and
dynamics, through both high-order harmonic generation, and electron rescattering.
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
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HHG
Electron
rescattering
in aligned
molecules
Two-colour
HHG
Outline of talk
Introduction
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…
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Laser driven electron recollisions
Returning electron wavepacket can have
energy components up to ~ 150 eV, or
wavelength 1 angstroms
Tunnel
ionisation of
atom
Recollision lasts ~ 1 fs
Acceleration of
electron in
laser field
Recollision of
electron with
parent
picks up
K.E.
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.
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Laser driven electron recollisions
Propagation in the laser field… o spreads the electron wavepacket in time and
momentum
o chirps the electron wavepacket
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
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HHG and electron rescattering
On recollision of the electron wavepacket with the parent ion…
Recombination can occur,
resulting in emission of high
frequency photons
High-order harmonic generation
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Scattering can occur, either
elastically or inelastically.
The scattered electron will then
once again experience
acceleration/deceleration in the
laser field
HHG and electron rescattering
Typical harmonic spectrum
Typical electron spectrum
rapid fall
plateau
10Up
cut-off
3.17Up+Ip
Li et al, PRA 39, 5751 (1989)
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Grasbon et al, PRL 91, 173003 (2003).
Electron rescattering
10Up
Direct +
rescattered
electrons
Rescattered
electrons
only
Grasbon et al, PRL 91, 173003 (2003).
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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
Best information found at
(2004).
high electron energies
5-6 a.u: 340-490 eV
Measuring angular
distribution of such high
energy electrons is
experimentally difficult!
TOF, scan angle
Velocity map imaging
(Up ~ 42eV)
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
COLTRIMS
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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
Best information found at
(2004).
high electron energies
5-6 a.u: 340-490 eV
Measuring angular
distribution of such high
energy electrons is
experimentally difficult!
TOF, scan angle
Velocity map imaging
COLTRIMS
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(Up ~ 42eV)
AMOLF group recently investigated ionisation
of O2, N2, CO2 by XUV using VMI [JMO 55,
2693 (2008)] up to electron energy 60 eV.
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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
Best information found at
(2004).
high electron energies
5-6 a.u: 340-490 eV
Measuring angular
distribution of such high
energy electrons is
experimentally difficult!
(Up ~ 42eV)
TOF, scan angle
Velocity map imaging
COLTRIMS
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Meckel et al [Science 320, 1478 (2008)] used
COLTRIMS to detect diffraction signatures in
O2, N2 at ~ 100 eV
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A velocity map imaging spectrometer for
high energy electrons
We have been developing a VMI spectrometer capable of detecting fewhundred eV electrons.
Basic design of grid assembly
Occasional
breakdown at
> 13 kV
Pulsed molecular beam in
Large
aperture
MCP
- 15kV
-11.7 kV
Flight tube with
large diameter:
length ratio
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High
voltages
required
All electrodes electropolished stainless steel
Interaction region (laser
into/out of page)
A velocity map imaging spectrometer for
high energy electrons
(Very) recently obtained electron images up to -10kV…
Coherent Legend-HE-USP
0.8 mJ
14 fs
1 kHz, 50 fs, 2 mJ
0.1 mJ
Vertical
polarisation
1.2 mJ
1. Peaked structure
confined to axis of
polarisation
2. Weaker emission
perpendicular to polarisation
3. Continuous distribution
extending to larger radii,
emitted more isotropically
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45 deg
OAP
f=20 cm
CCD
Skimmed molecular beam
(axis out of page) delivered
to interaction region
Gas jet 1 kHz, backed with
250 mbar Xe
A velocity map imaging spectrometer for
high energy electrons
Recently tested up to -10kV…
Spectrum at hollow fibre exit
14 fs, 2.2 x 1014 Wcm-2
Peak of spectrum at 766 nm
= 1.6 eV
Weak low order ATI peaks
Separation 1.9 +/- 0.5 eV
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For short pulse, high intensity, do
not expect clear ATI rings [Grasbon
et al, PRL 91, 173003 (2003)]
A velocity map imaging spectrometer for
high energy electrons
Recently tested up to -10kV…
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
To obtain angular distribution
need to account for varying shift
of rescattered electron
momentum in different directions
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Energy distribution along polarisation
direction
A velocity map imaging spectrometer for
high energy electrons
Recently tested up to -10kV…
Observe plateau and cutoff 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.
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
Promising progress towards measuring angular distribution of
direction
200-300 eV electrons
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High order harmonic generation
Typical harmonic spectrum
Spectral amplitude within strong field
approximation described by
rapid fall
d ( )    0 | z | a(k )eikz  eit dt
plateau
cut-off
3.17Up+Ip
•
•
•
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 wavefunction0
Li et al, PRA 39, 5751 (1989)
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Effect of nuclear dynamics on HHG
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.
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Effect of nuclear dynamics on HHG
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.
At the recollision, recombination amplitude given by
vk     0 R  0 R  k eikx0 R   R, k dR
Vibrational
wavefunctions
(nuclear
contribution)
Electron
travel time
Electronic
ground states
assuming superposition of plane waves for continuum electron wavefunction:
 c   a (k )eikxdk . Harmonic signal proportional to  2 a (k )v(k ) .
2
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PACER:
Probing attosecond dynamics
by chirp encoded recollision
Recombination amplitude
decreases in time as
nuclear wavepacket evolves.
Harmonic emission is chirped:
one-to-one mapping between
electron travel time and harmonic
frequency.
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):
Compare harmonic yield in
two isotopes and assume
invariance of a(k)
 2 a ( k )v ( k )
2
Ratio vs frequency information
allows nuclear dynamics to be
extracted.
First proposed by M. Lein, Phys. Rev. Lett. 94, 053004 (2005).
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PACER:
Probing attosecond dynamics
by chirp encoded recollision
H2+
Laser electric field
Time
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Probing attosecond dynamics
by chirp encoded recollision
PACER:
Signature of the slower nuclear dynamics of D2
H2
Signal Ratio
D2/H2
+
Laser electric field
Time
1
Return time
D2+
Detecting that motion occurs between emission of successive harmonic orders.
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PACER experimental set-up
Experimental set-up: gas jet position and confocal parameter/focal size can be
continuously varied.
Gas jet variable z
250 uJ
8 fs
800 nm
1kHz
MCP
Variable
aperture
OAP f=40 cm
Gas jet rep rate 2 Hz (limited by pumping speed).
Detect 17th harmonic and beyond.
Focus 9mm before jet to isolate short electron trajectories.
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CCD
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First PACER measurement
8fs, 2 x 1014 Wcm-2, 800nm, pulsed gas jet source. Focus 9mm before jet
to isolate short electron trajectories.
D2
H2
Ratio H2/H2
Ratio H2/H2
Baker et al., Science 312 p 424 (2006).
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First PACER measurement
8fs, 2 x 1014 Wcm-2, 800nm, pulsed gas jet source. Focus 9mm before jet
to isolate short electron trajectories.
Blue curve:
c k     0 R  R, k cos( kR cos(  ) / 2)dR
averaged over randomly aligned
angle distribution.
Use BO potentials for H2+ and D2+.
Single molecule response, allowing for
short trajectories only.
Ratio H2/H2
Includes two-centre interference effects.
Includes effect of reabsorption of generated
XUV.
Baker et al., Science 312 p 424 (2006).
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First PACER measurement
The increasing ratio is a signature of the slower nuclear motion in D2+, and can
be used to gain information about the nuclear motion:
R (bohrs)
WeRed:
have
made
a
time
evolution
as
measurement
reconstructedwith
from~100
asharmonic
time resolution,
spectra,
using
8 fs runs
pulses:
different
of genetic
algorithm
launch
of electron and
nuclear
wavepackets
Blue: time
evolution is
synchronous
by nature
calculated from
known
of BO
the potential
process.
Time (fs)
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Baker et al., Science 312 p 424 (2006).
Dynamic interference through PACER
Using 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
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PACER in methane
Original experiment was also conducted in CH4 and CD4: a harmonic ratio that
strongly increased with order was detected.
Baker et al.,
Science 312
p 424 (2006).
We postulated that
this was evidence
of very fast bond
angle changes
following ionisation
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PACER in methane
Recent calculations support the very fast nature of conformational
changes in CH4
Analytical calculation
including only nuclear
contribution, within BO
approximation
An electric field lifts the
degeneracy of the 3
branches of the JahnTeller distortion allowing
them to be treated
independently.
(Courtesy of Serguei Patchkovskii)
Very promising result regarding applicability of PACER to larger systems
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HHG for structural information
Within SFA there is a simple (Fourier) relationship between harmonic amplitude
and orbital wavefunction0
d ( )    0 | z | a(k )eikz  eit dt
Can lead to complete reconstruction of 0 [Itatani et al. Nature 432, 867 (2004)];
so far in N2 only.
We have been working towards retrieval of orbitals of more complecated
organic molecules [Torres et al, PRL 98, 203007 (2007)].
Impulsively
align molecules
with pump pulse
Time delayed
probe pulse for
HHG
Measure HHG spectrum
as function of angle
between pump and
probe polarisation
Larger molecules have low Ip, so harmonic cut-off at relatively low orders
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HHG in larger molecules at 1300 nm
Larger molecules have low Ip, so harmonic cut-off at relatively low orders
Use longer wavelength driving field to extend the harmonic cut-off at a fixed
intensity: sampling orbital with larger range of momentum components.
Recent experiment at ARTEMIS facility at Rutherford Appleton Laboratories
HR 1300 nm
HT 780 nm
Flat field
spec
CCD
f = 30 cm
85:15
TOPAS
KMLabs Red
Dragon 1
kHz 10 mJ
80 fs 780 nm
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1300 nm
50 fs
1 mJ
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Gas jet (1 kHz)
or continuous
flow
Imaging
MCP
HHG in larger molecules at 1300 nm
We 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.
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Conclusions
PACER 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.
Outlook
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…)
PACER with larger range of molecules: C3H4?
PACER with long trajectories: compare retrieved R(t) with that from short
trajectories -> any detectable effect of the different field strengths at
recombination?
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People
Jon Marangos John Tisch
PACER work:
Electron VMI:
Long wavelength HHG:
Joe Robinson
Manfred Lein
Ciprian Chirila
Delphine Darios
Marco Siano
David Holland (STFC)
Tom Siegel
Ricardo Torres
Leonardo Brugnera
Imma Procino
Jonathan Underwood
Staff at ARTEMIS facility
Adverts!
E. Springate, I.C. E. Turcu,
C. Froud
Two posters:
Delphine Darios; VMI of high energy electrons
Imma Procino; Retrieval of molecular axis alignment from Coulomb explosion
imaging experiments without cylindrical symmetry
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