J.P. Brichta, S. Walker, X. Sun, J.H. SandersonDepartment of Physics, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada

Laser induced coincidence Coulomb imaging

1. Recent significant results 2. Goals and future prospects3. Experimental setup at Waterloo4. kHz Laser system at Waterloo

1. Recent significant results

2 Goals and Future prospects

1. With long 40fs pulses: to investigate the effects of laser deformation of small molecules, in particular CO2 and OCS. Initial dissociative channels and how they influence higher charge state channels.2. With sub 10fs pulses. improve the spatial and temporal resolution achieved so far by using shorter pulses. In addition to the hollow fiber apparatus we will investigate the possibility of using 1-2fs sources to initiate Coulomb explosion3. Investigate the use of attosecond pulses to initiate Coulomb explosion.

Event 1

Event 2

Event 3

Electrostatic charge induced by film anode

YX

YX

XY

YX

5.6 cm

Repeller = 3.05kVGrid = 50V

12 cm

=14mm MCPfront = 0VMCPmid = 1.1kVMCPback = 2.2kV

Film anode = 2.6kV

MBWCanode = 0V

Above: Four-channel current trace from MBWC anode

Above: Time-of-flight spectrometer schematicMBWC anode schematic

YXYXYXXY

YXXYYXXY

IIIIII

IIY

I

IIX

where

,

A fragment’s positional data can be computed by analyzing the current signal output from each of four channels [1].

[1] T. Mizogawa et al, Nucl. Inst. Meth. Phys. Res. A312 (1992) 547-552

3. Experimental setup at Waterloo

0

0

2 201

2

x

y

rep grid

z

m x xp

Tm y y

pT

U U T Tp q

d T

A particle with mass m, charge q and zero initial momentum will strike the detector at time T0, with coordinates (x0, y0). The momentum of an ion of identical mass and charge can therefore be computed [2]:

[2] A. Hishikawa et al, J. Elec. Spec. Rel. Phenom. 141 (2004), 195-200

1 2

cos

O O O

O C

O C

p p p

p p

p p

CO2 3+ CO2+ + O+ C+ + O+ + O+ (seq.)CO2 3+ C+ + O+ + O+ (concerted)

Above: Momentum vectors of ionic fragments reveal much about the molecular geometry prior to Coulomb explosion. θ is the angle between the oxygen momentum vectors and can be related to the bond angle. χ is used to determine the reaction pathway (concerted or sequential).

90

90

Seq

Con

Only Coulomb explosion events where all the fragments are recorded and momentum is conserved (p ~ 0) are retained for geometry analysis.

Triatomic Coulomb explosions will occur along a sequential or concerted reaction pathway. In a sequential reaction, one ion predissociates, leaving a rotor that may take ~ps to dissociate. In the case of a molecule like CO2, the oxygen ions will have very different final energies. However, in a concerted reaction, structural deformation may occur by simultaneous stretching of the C-O bonds, or bending of the molecule. In such a case, the oxygen ions will have similar energies upon dissociation. The so-called “chi parameter” is used to determine the reaction pathway.

3. Experimental setup at Waterloo

pC

pO1

pO2

χ

θv

Above: A coincidence map showing which species were ionized by the same laser shot. Left: Chi distributions for (1,1,1) and (2,2,2) ionization channels.

3. Experimental setup at Waterloo

The experimental setup is now being tested out with the laser system, preliminary results with 40fs pulses show I=5x1015 W/cm2 is possible.

Which can be solved with an iterative algorithm, matching the final positions and momenta.

3. Experimental setup at Waterloo

Using the recovered position and time information, the geometry of triple coincidence Coulomb explosion events can be calculated with classical mechanics. For three charged particles, the Hamiltonian is:

3 2

3 2

3 1

3 1

2 1

2 12

33

2

22

2

112

1

2

1

2

1

r r

q qk

r r

q qk

r r

q qk p

mp

mp

mH

3

( )

i

ii p

i

i j i ji

j ii j

pr H

m

q q r rp k

r r

1

2

3

The equations of motion of the ith particle are described by:

3. Experimental setup at Waterloo

Above: A surface plot showing recovered geometry for the (1,1,1) and (2,2,2) ionization channels. The equilibrium bond angle in CO2 is 177°, and the C-O bond length is 1.16 Å. The recovered geometry distributions have a similar bond angle, and about twice the equilibrium bond length, consistent with the theory of enhanced ionization.

-0.5 0 0.5 1 1.5-5

0

5

x [Å]

y [Å

]

(1,1,1)

-0.5 0 0.5 1 1.5-5

0

5

x [Å]

y [Å

]

(2,2,2)

4. kHz Laser system at Waterloo

The stretcherless regen isSeeded by a Femtolasers <10fs oscillator, and use regenerative pulse shaping with two Pellicle etalons. Angle tuning allows the band with of the output pulse to be increased from 30nm to 50nm or more the pulse stretches to 10fs during the 16 roundtrips

Above: Calculation and measurement of the spectrum from the amplifier including the two pelicals

System Component Total Amount GVD (fs^2) TOD (fs^3)

TOD Mirrors 90 Reflections 9.00E+03 1.96E+05Faraday

Rotator (M-18) 56 mm 6.44E+03 -2.42E+03Pockels Cell

(K*DP) 680 mm 2.30E+04 2.13E+04Polarizing

Beam Splitter 145 mm 6.45E+03 4.68E+03Ti:Saph 256 mm 1.17E+04 8.49E+03

Other Material 40 mm 1.78E+03 1.29E+03Totals 5.83E+04 2.30E+05

4. kHz Laser system at Waterloo

In order to compress the pulse the dispersion of every element in the amplifier must be considered and the prism compressor separation and chirped mirror bounces set. Currently 40fs pulses are possible, and deformable mirror compensation can bring these down to 20fs

Above: Schematic of the prism compressor Above: Autocorrelation of compressed pulse

Above: GVD of the amplifier and compressor

4. kHz Laser system at Waterloo

The pulses from the amplifier will be used to seed a hollow fiber apparatus which can produce sub6fs pulses.

The apparatus consists of a differentially pumped chamber two 1mm thick windows a femtolasers hollow fiber with inner core 250um and Ar gas, and air. The system parameters have been set such that the chirped mirrors have adequate compensating capacity.