Using COLTRIMS for pump-probe studies of molecular dynamics

Report
I. Bocharova
L. Cocke, I. Litvinyuk, A. Alnaser, C. Maharjan, D. Ray
Outline
 Motivation
 Coulomb explosion imaging.
 Experiment requirements.
 Experimental setup.
 H2 and D2 experiments.
 N2 and O2 experiments.
 C2H2 experiment.
 Future plans.
Motivation
To study the structure and its time
evolution of different gas
molecules, using Coulomb
explosion imaging
Coulomb explosion imaging
Accelerator Coulomb Explosion
Imaging
MeV
beam
Exploding
molecule
Ultrathin
foil
Laser Coulomb Explosion Imaging
Detector
Detector
Detector
Originally
developed
for
investigation of a static structure:
collision of molecular ions beam
and thin foil
Now using laser short pulses interacting with
molecules in gas phase
KER
Why Coulomb explosion imaging?
+
+
D +D
+
D2
pump
D2
1400
internuclear distance
D2 8 fs pulse
D2 30 fs pulse
1200
6
D2
4
Theory: 4 fs
||
2
counts
1000
800
600
Exp. 8fs
Exp. >40fs
2
400
200
0
0.5
0
5
10
15
Energy, eV
20
+
D +D
potential energy
Direct method which allows
for best time resolution : can
use short pulses  Possible
to observe molecules with
fast dynamics such as D2
1.0
1.5
2.0
2.5
25
R / Angstrom
3.0
3.5
Requirements
 Laser impulse shorter than vibration period of
molecule.
 High intensity to produce highly charged states, so
explosion potential can be approximated by Coulomb
potential.
 Minimize the thickness of molecular target beam, so
that interaction volume is minimal.
Experimental setup
Gas jet
y
z
Piezoelectric
slit
x
Laser pulse
Recoil detector
M
Recoil side of
spectrometer
Energy, eV
Looking for explosion fragments in coincidence
Magnitude of vector sum of all fragments momenta (a.u.)
Pump-probe
Weak pump pulse
excites molecule
Probe
E
Pump
t
Delay allows
time evolution
E
Probe
Pump
t
Intense probe
pulse ionizes
molecule
Molecule explodes
E
Pump-probe setup
d
d
d
pump-pulse
Pump pulse
d
probe-pulse
Probe pulse
θ
d
pump-pulse
θ
 sin  

 n 
  arcsin
x
probe-pulse
OP
 
c
  1

cos(θ  φ) 


ΔOP  d  n
 1  1 

cos(
φ
)
cos(
φ
)




(CR)EI – (Charge Resonance) Enhanced Ionization
e-
R0
Rc
 Diatomic molecule: double
well potential.
 Picture is asymmetric in
laser field.
 R0 is an interatomic distance
for neutral molecule.
 Distance R between two
centers increases.
 At some critical distance Rc
enhanced ionization occurs.
D2 experment
KER at fixed delays
30
25
S(E,t)
counts
20
15
10
5
delay 0_10fs
0
2
4
6
8
10
12
10
12
8
10
12
8
10
12
10
12
KER, eV
delay 10_20fs
(2)
D++D+
2
4
6
8
delay 20_30fs
2
pump
t
D2+(X2Sg+)
4
delay 30_40fs
(1)
2
D2 (X1Sg+)
6
4
6
delay 40_50fs
2
4
6
8
D2 KER vs Delay spectrum
counts
R, a.u.
Long pulse (30fs) CREI
Laser parameters: pump 8fs 3x1014 W/cm2
probe 8fs 9x1014 W/cm2.
KER, eV
D2: theory and experiment
Theoretical calculation: Xiao-Min Tong, C.D. Lin
H2 experiment
KER (eV)
KER (eV)
20
20
10
10
DELAY (fs)
0
0
0
50
100
0
DELAY (fs)
50
100
3000
O+O+
TOF 2 (ns)
N2 and O2 experiment
PIPICO
O2+O+
2000
O2+O2+
O3+O2+
O3+O3+
O2+5
1000
O2
+4
2000
1000
2800
3000
O2+3
O2 +
N+ N+
TOF 2
O2+2
1000
100
2000
N2+N+
O2
N2+N2+
10
N3+N+
N4+N3+
N3+N2+
N3+N3+
N4+N2+
1000
1
1000
2000
TOF 1
2800
KER Spectra for Oxygen
O3+ + O2+ Pair
O2++ O2+ Pair
80
100
1000
KER (eV)
600
600
50
40
400
200
200
0
0
0
80
DELAY (fs)
150
0
80
DELAY (fs)
150
KER Spectra for Nitrogen
N2++N2+ pair
PIPICO
3000
80
N3++N2+ pair
70
1000
40
40
TOF 2
KER (eV)
N+N+
100
2000
N2+N+
N2+N2+
N3+N+
10
N3+N2+
N3+N3+
0
60
1000
1000
N4+N3+
DELAY (fs)
0
N4+N2+
120
60
2000
TOF 1
2800
120
1
DELAY (fs)
C2H2 : polyatomic molecule
C2H2 : isomerization of acetylene to vinylidene
Time scale? the upper limit established is 60 fs1
C
acetylene
H
C
C
H
C
H
vinylidene
H
H-CC-H  [H-CC-H]2+  CH+ + CH+
 C+ + CH2+
Idea: With short pulses pump-probe technique can be applied to
follow the dynamics of isomerization process.
1 T.
Osipov, C. L. Cocke, M. H. Prior, A. Landers, Th. Weber, O. Jagutzki,
L. Schmidt, H. Schmidt-Böcking, and R. Dörner, Phys. Rev. Lett. 90,
233002 (2003).
TOF2
px (a.u.)
C2H2 acetylene and vinylidene channels separation
TOF1
pz (a.u.)
Momentum-imaging investigations of the dissociation of D2+ and the isomerization of
acetylene to vinylidene by intense short laser pulses. A. S. Alnaser, I. Litvinyuk, T. Osipov, B.
Ulrich, A. Landers, E.Wells, C. M. Maharjan, P.Ranitovic, I. Bocharova, D.Ray and
C.L.Cocke. Journal of Physics B: Atomic, Molecular & Optical Physics. (accepted)
Future plans
 C2H2 experiment.
 Continue experiments with N2 and O2.
 CO2: triatomic molecule.

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