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Tutorial on how to assemble a Photofragment and - attofel

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Tutorial on how to assemble a
Photofragment and Photoelectron Spectrometer
Theofanis N. Kitsopoulos
Department of Chemistry, University of Crete
and Institute of Electronic Structure and Laser
Foundation for Research and Technology-Hellas
ATTOFEL Summer School, May 5th 2011
Basic Vacuum Technology*
dV (Volume) l
S : Pumping Speed :
dt
s
dV (Volume) l
C:Conductance :
dt
s
dV
l
Q:Throughput : ∆P
torr
dt
s
Vacuum Chamber
Q = C∆P = C (P2 − P1 )
1 1
пЈґ1
+
Q = P2 S P
пЈЅ =
S SP C
пЈґ
Q = P1S P
пЈѕ
* Building Scientific Apparatus, Cambridge Fourth Edition
Master
Equation
Air: made up of “molecules” with weights 30 amu
Mean Free path: Distance a particle needs to travel before colliding with
another particle
Viscous Flow: λ is much SMALLER than dimensions of vacuum “tubing”
Molecular Flow: λ is much LARGER than dimensions of vacuum “tubing”
Conductance of a Cyclic Aperture
Viscous Flow
C ≈ 15d 2 (l / s )
d в‰Ў diameter (O/ ) in cm
Molecular Flow
1/ 2
пЈ«T пЈ¶
C ≈ 3.7  A(l / s )
пЈ­mпЈё
A в‰Ў area = 0.25ПЂd 2 ,
d в‰Ў diameter (O/ ) in cm
T в‰Ў Temperature in K
m в‰Ў mass in a.m.u.
Conductance of a Cylindrical Tubes
D: Diameter in cm
L: Length in cm
Conductance of a Cylindrical Tube Network
Example
1. Caper ≈ 15d 2 = 15(0.01) 2
в€’4
= 15 Г—10 l / s
100 torr
P=?
2. Qaper = Caper ∆P ≈ Caper Psample
= 100torr в‹…15 Г—10 в€’ 4 l / s
в€…0,01cm
30 cm
l
= 0.15torr
s
3
3. CTube
в€…25 cm
3
D
25
≈ 12
= 12
L
30
= 6250l / s
1 1
1
1
1
4. =
+
=
+
S S P CTube 3000 6250
S = 2027l / s
Pump
Pump
SSP=3000l/s
P=3000l/s
Qaper
0.15
5. P =
=
torr
S
2027
= 7.4 Г—10 в€’5 torr
в€…0.2 cm
Differential Pumping
1
1
1
1
1.
=
=
+
S1 S 2 S P CTube
1
1
=
+
1500 6250
S1 = S 2 = 1210l / s
100 torr
P1=?
Q2
P2=?
в€…0,01cm
Pump
Pump
SSP=1500l/s
=1500l/s
Pump
Pump
SSP=1500l/s
=1500l/s
P
P
1/ 2
пЈ«T пЈ¶
0.15
2. P1 =
=
torr 3. Cskimmer ≈ 3.7 m  A(l / s )
пЈ­ пЈё
S
1210
1/ 2
2
в€’4
l
= 1.3 Г—10 torr
пЈ« 300 пЈ¶ ПЂ 0.2
= 3.7пЈ¬
= 0.37
пЈ·
s
4
пЈ­ 30 пЈё
l
l
в€’4
в€’5
4. Q2 ≈ Cskimmer P1 = 0,37 1.3 × 10 torr = 4.5 × 10 torr
s
s
l
в€’5
4.5 Г—10 torr
Q2
s = 4 Г—10 в€’8 torr
5. P2 ≈
=
S2
1210l / s
Qaper
Using Pulsed Nozzle the “load” is reduced and the densities increased
QPulsed = QCW ⋅ f ⋅ ∆t
∆t: Time duration (typically 10-500 µs)
f: repetition rate (10-2000 Hz)
Newport BV 100
E. Grant & coworkers
Rev. Sci. Instrum. 51, 1469 (1981)
http://www.even-lavie-valves.com/
D. Proch and T. Trickl,
Rev. Sci. Instrum. 60, 713 (1989).
http://www.tu-chemnitz.de/physik/ION/Technology/Piezo_Valve/index.html
http://www.beamdynamicsinc.com/skimmer_description.htm
Charged Particle Optics
Potential Energy
Potential Energy Surfaces
AО’* +О“
• Product Branching Ratios
AО’+О“*
• Product Energy Distributions
AB+О“
О‘О’О“
RA-B
• Control Chemical Reactivity
Product Detection
by
Photoionization and Mass Spectrometry
Potential Energy
AB+
Direct Photoionization
AB*
AB
RA-B
Resonance
Enhanced
Multi
Photon
Ionization
Imaging Spectrometer
Photolysis
Laser
MCP’s
Phosphor
Imaging Detector
CCD
Pulsed
Nozzle
Ionization
Laser
Ion Imaging: D.W. Chandler and P.L. Houston, J. Chem. Phys. 87, 1445 (1987).
Laser
∆v
v
Inverse Abel
Molecular Beam
Homogeneous Extraction
Velocity Mapping: A.T.J.B. Eppink and D.H. Parker Rev. Sci. Instrum. 68, 3477 (1997)
Ion Lens
∆v
v
Molecular Beam
Inhomogeneous Extraction
Inverse Abel
The inverse Abel Trnasform
Symmetry Axis
200
Intensity
?
150
?
100
50
0
-1.0
-0.5
0.0
0.5
1.0
Projection axis
Inverse Abel Transform
3D
Slice
Kinematics of Conventional Imaging using DC fields
Back-scattered
Forward-scattered
П„=0
∆t~ 50 ns
Kinematics using Pulsed Extraction Fields
Back-scattered
Forward-scattered
П„=0
τ ≥ 50ns
∆t>500 ns
TIME LAG FOCUSING
W.C.Wiley and I.H. McLaren Rev. Sci. Instrum. 26 1150(1955)
Slice Imaging Extraction Schematic
Gebhardt et al Rev. Sci. Instum. 72 3848 (2001)
Very Homogeneous field produced by using very high quality grids (>750 lpi,
25 Вµm spacing, ~50% transmittance)
Einzel lens is used for velocity mapping and zooming.
Pulse Field rise time becomes critical for very fast particles like H-atoms or e–
Pulsed Homogeneous
Extraction
Fast Gating
Detector
Molecular
Beam
300
400
500
Time (ns)
600
15ns
Einzel Lens
Slicing Using Weak Extraction Fields (DC Slicing)
J. J. Lin, J. Zhou, W. Shiu,and K. Liu, Rev. Sci. Instrum.74, 2495 (2003)
D. Townsend, M.P. Minitti ,and A.G. Suits, Rev. Sci. Instrum 74, 2530 (2003)
Laser Slicing
K. Tonokura and T. Suzuki Chem. Phys. Lett. 1, 224 (1994)
D.A. Chestakov et al. J. Phys. Chem. A, 108, 8100 (2004)
New Ion Optics Design allows both SLICINGur and
Velocity Mapping to occur using a SINGLE E FIELD
Intensity
0,8
0,4
0,0
250
Rev. Sci. Instum. 77 83101 (2006)
Beam
Beam and Grid
Gas
260
270
280
290
300
310
e speed (Pixels)
320
330
340
350
Cl2 Cl(2P3/2)+Cl(2P3/2)
"Doppler Free" Slice Imaging
Measure and slice image with both photolysis and laser polarizations parallel to the TOF
axis (Geometry ZZ), without scanning the probe laser wavelength
Measure slice images using 3 more polarization geometries ZX,XX and XZ
Normalize angular distributions of ZX,XX and XZ to the angular distribution of ZZ
1,0
ZZ
Intensity
0,8
0,6
ZX
0,4
ZX Raw
ZZ
ZX Normalized
0,2
0,0
0
20
40
60
80 100 120 140 160 180
Scattering Angle (deg)
Advantages
• Corrects for errors caused by detector inhomogeneities and laser
fluctuations during Doppler scanning of probe laser
• Reduces signal averaging time as probe laser is not scanned
Extracting Scattering Information for Slice Images
I R ( ρ ) ≈ I RA ( ρ ) dx ⇒ I RA ( ρ ) ∝ I R ( ρ )
dφ
Tim
e
of
Fli
ght
dОё
Ax
is
dx
Hence we conclude that the Pixel
Intensity of a Slice Image is
equivalent to the Inverse-Abel
result of conventional imaging
∆θ
Angular Distributions
N (Оё )
I (Оё ) =
∆θ
Оё
x-axis
Speed Distributions
P (u ) в€ќ I R ( x) x sin Оё
Works for Very Fast and Very Slow particles
HBr H+Br/Br*
VERY FAST
H-atom, 16000 m/s
VERY SLOW
Br-atom, 290 m/s
Resolutions effects of the REMPI process
2+1
via
2s state
3+1
via
2p state
HBr в†’ H+Br(2P3/2)
H+Br*(2P1/2)
Photoelectron Images
3+1 via the 2p state
KER≈
≈0,01 eV
H+ recoil from the photoelectron
KEe (eV)
Ve (m/s)
VRion (m/s)
∆V/V (%)
0,01
59174
32
0
1
591737
325
4
2+1 via the 2s state
KEe=1,7 eV
m H VH = meVe
1,5
724727
398
5
1,712
774181
425
5
2
836842
459
6
The recoil splitting is minimal at 90o as the H-atom velocity and that
of the photoelectron are perpendicular to each other
θ = 0°°
0
6000
12000
18000
θ = 90°°
0
6000
12000
18000
Speed of H-atoms / ms-1
The these results indicate that our resolution is ∆v/v<1.5% and
limited by the ion recoil
Chem. Phys. 301, 209, (2004)
Micro-Channel Plate Particle Detectors
Electron Detection (TOF)
Ground Shield
MCP FRONT
MCP BACK
SOLID ANODE
100KΩ
100 V
100KΩ
1000 V
100KΩ
1800 V
100KΩ
2000 V
1 nF
Signal OUT / SCOPE
50Ω
ION Detection
Ground Shield
MCP FRONT
MCP BACK
SOLID ANODE
100KΩ
-2000 V
100KΩ
-1100 V
100KΩ
-200 V
100KΩ
0V
Signal OUT / SCOPE
50Ω
Imaging Particle Detection
Ground Shield
MCP FRONT
MCP BACK
100KΩ
100KΩ
0 ( -800 V)
100KΩ
900 ( 200 V)
100KΩ
1800 ( 1200 V)
Phosphor ANODE
1 nF
6000 V
Signal OUT / SCOPE
50Ω
Continetti and coworker Rev. Sci. Instrum. 70, 2268 (1999)
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