Application of Synchrotron Radiation to
Chemical Dynamics Research
Shih-Huang Lee（李世煌）
National Synchrotron Radiation Research Center (NSRRC)
國家同步輻射研究中心
Oct. 9, 2014
Outline

Introduction
 Synchrotron facility
 Crossed molecular-beam apparatus
 Photodissociation of propene (CH3-CH=CH2)
 Crossed-beam reactions of O(3P, 1D) + C2H4
 Conclusion
Introduction
Ionization detection of reaction products is
ideal for molecular beam experiments in
chemical reaction dynamics research.
 Electron Impact Ionization
 Photoionization
Electron Impact Ionization
Advantage
- Universal
- Cheap
Disadvantage
- Severe dissociative ionization
- No quantum state and species (e.g., CO/C2H4)
selectivity
- Limited detection efficiency, especially
for TOF measurement, because of space
charge problem
Photo-ionization by Direct VUV Ionization
Advantage
- Universal
- Small dissociative ionization (major)
- Somewhat state selective / species selective
- Low detector background for low IP products
- Potentially higher detection efficiency
Disadvantage
- Low photon fluxes in the VUV region
- low availability and expensive
Detection efficiency for a typical electron impact ionizer:
* l = 1 cm
Ie = 1 mA (~ 1016 electrons / cm2 s)
M + e-  M+ + 2ed[M+]/dt = Ie  [M]
* Probability of a molecule to be ionized in one second
 = 1×10-16 cm2/electron  pi = Ie  = 1016 × 10-16 = 1 s-1
* For a molecule with 1.0 × 105 cm/s (1000 m/s), the
probability to be ionized (resident time t = 1 × 10-5 s)
Ie  t = 1 × 10-5
Detection efficiency for a typical VUV Ionizer:
*
l = 1 mm
I nsrrc = 1016 photons / s
A = 1 mm2 = 0.01 cm2
 nsrrc = 1018 photons /cm2 s
 = 10-17 cm2/photon
* Ionization probability of a molecule per second
pi =  nsrrc ×  = 10 s-1
* For a molecule with 1.0 × 105 cm/s (1000 m/s), the
probability to be ionized (resident time t = 1 × 10-6 s)
 nsrrc t = pi t = 1 × 10-5
Chemical Dynamics Beamline
Synchrotron at NSRRC, Taiwan
Chemical Dynamics Beamline (U9 White Light Beamline)
U9-undulator (U9-聚頻磁鐵)
Undulator (聚頻磁鐵)
1st 3
rd
4th
2nd
noble gas
pump
pump
SR
pump pump
pump
Harmonics Suppressor (Gas filter)
Employed Medium: He, Ne, Ar, Kr, Xe
Dynamic tuning-U9
Au-coated/Cu
@7mm Gas-cell-M2(P~10 torr)
300
He
Photocurrent /A
250
Ne
200
150
100
Ar
Kr
50
Xe
0
20
25
30
35
U9-Gap /mm
40
45
Performance of Harmonic Suppressor
100
Ring current 146.5mA
Gap 60mm
Gas Ar
-3
I/I0 = 10 @ 10 Torr
Photocurrent / A
10
1
0.1
0.01
0
2
4
6
8
Ar Pressure / Torr
10
12
14
Fundamental (first-harmonic) photon energy vs U9 gap
35
Fundamental Photon Energy / eV
30
25
20
15
10
5
15
20
25
30
35
Undulator Gap / mm
40
45
50
U9 White Light Beamline at NSRRC
Light Source (U9 undulator)
Undulator period : 9 cm
Number of period (N): 48
Energy range : 5 ~ 50 eV (but limited by filter gas)
Energy resolution : E / E ~ 4 %
Photon flux: ~ 1016 photons/sec (@ first harmonics)
Liquid nitrogen
液態氮 (77 K)
He refrigerator
氦冷頭 (14 K)
Daly ion detector
Quadrupole mass filter
四極質譜儀
Crossed-Molecular-Beam Apparatus
交叉分子束系統
How to increase detection sensitivity
 Neutral flight distance is shorten as 10 cm (15 cm
in Berkeley). Sensitivity gains about 2.3 times.
 Quadrupole rod assembly is enlarged by a factor of
1.7 (1.25〃 v.s.  0.75〃). Transmission is ~ 2.8 times
larger.
 In comparison with the Berkeley ALS endstation.
The sensitivity is ~ 6.5 times better.
 He refrigerator is used to evacuate the ionization
region to an ultrahigh vacuum (< 5×10-12 torr).
S/N gains 10 times than before for H2 detection.
(I) Photodissociation of propene at 157 nm
CH3-CH=CH2 + 157 nm  C3H5 + H
 C3H4 + H + H
Procedure:
 C3H4 + H2
1. Measure product time-of-flight spectra
 C3H3 + H2 + H
2. Do simulation using a trial P(Et)
 C2H3 + CH3
3. Fit experimental data to the best
 C2H2 + CH3 + H
4. Obtain kinetic energy distribution P(Et)
 C2H4 + CH2
 C2H2 + CH4
Velocity distributions of products after photodissociation
VA
AB+h
A
B
VB
MAVA = MBVB
-80
-60
-40
-20
0
20
40
Velocity (arb. units)
60
80
100
Three typical angular distributions of products after photodissociation
0 
270
90
180
 = 2 (v // )
0 ()
0()
90
270
180
 = 0 (isotropic)
270
90
180
 = -1 (v  )
I(Et ,) = 1/4P(Et)[1+(Et)p2(cos)], p2(cos) = (3cos2-1)/2
(EI will cause severe dissociative ionization)
C3H5 (IP = 8.2 eV)
m/z = 41
o
@ 5 , 8.8 eV
Only the leading part
of H-atom correlates
with C3H5 and most H
atoms are attributed
to triple dissociations.
Ion count
m/z = 41
o
@ 30 , 8.8 eV
0
50 100 150 200 250
0
50 100 150 200 250 300
Flight time (s)
H (IP = 13.6 eV)
Good
S/N ratio!
600
m/z = 1
o
@ 30 , 14 eV
C3H5+H
C3H4+H+H
C3H4+H+H
& C3H3+H2+H
& C2H2+CH3+H
400
300
200
C3H5+H (0.014)
C3H4+H+H (0.073)
C3H4+H+H (0.073)
& C3H3+H2+H (0.19)
& C2H2+CH3+H (0.65)
0.0010
0.08
0.06
P(Et)
Ion count (a.u.)
500
H
0.10
0.0005
0.04
0.02
100
0.0000
20
40
60
80
0.00
0
0
10
20
30
Flight time (s)
40
50
0
20
40
Et (kcal/mol)
60
80
H2 (IP = 15.4 eV)
m/z = 2
o
@ 30 , 17 eV
0.030
Good S/N ratio!
0.020
Ion count (a.u.)
1200
1000
800
0.035
600
0.015
400
0.010
200
0.005
0
0
5
10
15
20
Flight time (s)
25
C3H4+H2
0.025
P(Et)
1400
30
0.000
0
20
40
60
80
Et (kcal/mol)
The detection for atomic and molecular hydrogen is very tough due
to the short resident time (high speed) in the ionization region. The
increase of detection sensitivity and the decrease of detector
background improve the S/N ratio of atomic and molecular hydrogen
products. The condition is better than the ALS machine.
100
C3H4 (IP = 9.5~10.4 eV)
m/z = 40
o
@ 10 , 9.5 eV
m/z = 40
o
@ 10 , 11.5 eV
Ion count
C3H4+H2
C3H5 cracking
C3H4+H+H
m/z = 40
o
@ 30 , 9.5 eV
0
50
100
150
200
250
m/z = 40
o
@ 30 , 11.5 eV
0
50
100
150
200
250
300
Flight time (s)
Two components due to H2 and 2H eliminations are observed
notably at lab angle 30o and 9.5 eV.
C3H3 (IP = 8.7~10.8 eV)
m/z = 39
o
@ 10 , 11.5 eV
m/z = 39
o
@ 10 , 9.5 eV
C3H3+H2+H
Ion count
C3H4 cracking
0
m/z = 39
o
@ 30 , 9.5 eV
50
100
150
200
250
m/z = 39
o
@ 30 , 11.5 eV
0
50
100
150
200
250
300
Flight time (s)
The dissociative ionization of C3H4 becomes severe as detected with
electron impact ionization. The selective photoionization (9.5 eV) can
avoid completely dissociative ionization of C3H4.
CH3 (IP = 9.8 eV)
C2H3 (IP = 8.3 eV)
m/z = 15
o
@ 30 , 11 eV
m/z = 27
o
@ 30 , 10 eV
Ion count
C2H3+CH3
C2H2+CH3 +H
m/z = 27
o
@ 60 , 10 eV
m/z = 15
o
@ 60 , 11 eV
0
50
100
150
0
50
100
150
200
250
300
Flight time (s)
These two radicals are hard to be detected using EI ionization owing to
severe dissociative ionization. Because all reaction products are
measured, we know most CH3 arises from C2H2+CH3+H dissociation.
CH4 (IP = 12.6 eV)
C2H2 (IP = 11.4 eV)
m/z = 16
o
@ 30 , 14 eV
m/z = 26
o
@ 30 , 11.5 eV
Ion count
C2H2+CH4
C2H2+CH3+H
m/z = 26
o
@ 60 , 11.5 eV
m/z = 16
o
@ 60 , 14 eV
0
50
100
150
0
50
100
150
200
250
300
Flight time (s)
The formation of methane (CH4) occurs rarely in photodissociation of
hydrocarbons. In this work methane is observed in the photolysis of
propene at 157 nm. Most C2H2 arises from triple dissociation.
Ion count
CH2 (IP = 10.4 eV)
0
50
100
C2H4 (IP = 10.5 eV)
m/z = 14
o
@ 30 , 11 eV
m/z = 28
o
@ 30 , 12 eV
m/z = 14
o
@ 60 , 11 eV
m/z = 28
o
@ 60 , 12 eV
150
0
50
100
150
200
250
300
Flight time (s)
Apparently only a dissociation channel contributes to CH2 and C2H4
because they can be fitted satisfactorily using single P(Et). CH2 is identified
to be from the methyl moiety via the photolysis of isotopic variant CD3C2H3.
0.06
0.03
C2H3+CH3
0.05
C2H2+CH4
0.04
P(Et)
P(Et)
0.02
0.03
0.02
0.01
0.01
0.00
0.00
0
20
40
60
80
0
100
10
30
40
50
Et (kcal/mol)
Et (kcal/mol)
0.06
0.06
C2H4+CH2
0.05
C2H2+CH3+H
0.05
0.04
0.04
P(Et)
P(Et)
20
0.03
0.03
0.02
0.02
0.01
0.01
0.00
0
10
20
30
Et (kcal/mol)
40
50
0.00
0
10
20
30
40
Et (kcal/mol)
C2H4+CH2, C2H3+CH3, and C2H2+(CH3+H) channels have similar P(Et).
It is difficult to distinguish them using electron impact ionization.
50
Averaged kinetic energy release, kinetic fraction and branching ratio.
Product
channel
Eavail
(kcal/mol)
<Et>
(kcal/mol)
1st
2nd
ft
(%)
Branching
(%)
C3H5+H
93.3
49.7
0
53.3
1
C3H4+H+H
37.8
16.5
~7 b
~62
7
142.0
25.4
0
17.9
0.2
C3H3+H2+H
52.7
25.4
~7 b
~61
17
C2H4+CH2
80.4
11.1
0
13.8
6
C2H3+CH3
79.5
11.3
0
14.2
4
C2H2+CH4
149.7
26.3
0
17.6
5
44.7
11.6
~7 b
~42
60
C3H4+H2
C2H2+CH3+H
Three typical angular distributions of products after photodissociation
0
270
90
180
 = 2 (v // )
0()
0()
90
270
270
180
 = 0 (isotropic)
90
180
 = -1 (v  )
I(Et ,) = 1/4P(Et)[1+(Et)p2(cos)], p2(cos) = (3cos2-1)/2
(Et) = 2  I(Et ,) = 3/4P(Et)cos2
(Et) = 0  I(Et ,) = 1/4P(Et)
(Et) = -1  I(Et ,) = 3/8P(Et)sin2
I(Et ,//) = 1/4P(Et)[1+(Et)] @  = 0o
I(Et ,) = 1/4P(Et)[1-(Et)/2] @  = 90o
(Et) = 2[I(Et ,//)–I(Et ,)] / [I(Et ,//)+2I(Et ,)]
250
200
m/z = 16 (CH4)
150
@ 30 , 13.8 eV
//
o

0.30
0.25
50
0.20
0
0
20
40
900
60
80
100
 (Et)
Ion count (arb. units)
100
CH4+C2H2
0.15
0.10
m/z = 26 (C2H2)
o
@ 30 , 11.5 eV
//
600
0.05

0.00
C2H2+CH3+H
0
300
20
40
Et / kcal mol
60
-1
C2H2+CH4
0
0
30
60
90
120
150
Flight time / s
(Et) = 2[I(Et ,//)–I(Et ,)] / [I(Et ,//)+2I(Et ,)]
80
Averaged angular-anisotropy parameters for various dissociation channels
in photolysis of CH3CHCH2 and CD3CHCH2 at 157 nm
a
Channel
<>
Channel
<>
Channel
<>
C3H5+H
~0
C3H2D3+H
~0
C2H3+CD3
0.05
C3H4+H2
-0.03
C3H3D2+D
~0
C2H2D+CHD2
0.03
C2H4+CH2
0.05
…
…
C2HD2+CH2D
0.03
C2H3+CH3
0.06
C3HD3+H2
-0.07
C2D3+CH3
0.03
C2H2+CH4
0.12
C3H2D2+HD
-0.03
…
…
C2H2+CH3+H
0.05a
C3H3D+D2
~0
C2HD3+CH2
0.08
from C2H2 due to triple dissociation
Photo-excited state of propene at 157 nm
Electronic states of propene nearby 157 nm:
-3s(11A"), -3p(21A'), -3p(21A"), -3p(31A")
The photo-excited state of propene at 157 nm is
-3p(21A') that produces a transition dipole
moment lying in the C-C=C plane (i.e., parallel
transition).
(II) Crossed-beam reaction of O(3P, 1D) +
C2H4 @ Ec = 3 kcal/mol

O(3P) + C2H4 → CH2CHO + H
→ CH3 + HCO
→ CH2CO + H2

O(1D) + C2H4 → CH2CO + 2H
→ CH3 + HCO
→ CH2CO + H2
Components of the discharge device
Outer electrode
Adapter
Insulator
Inner electrode
Insulator
Valve
Layout of the transient high-voltage discharge circuit
Discharge current on an oscilloscope
300 mV on the scope → 30 mA discharge current
Relative ionization cross sections (a. u.)
200
150
100
1
rel( D)
0
150
O atoms from 3% O2/He by discharge
3
100
3
1
1
I = ( P)p( P) + ( D)p( D)
3
1
p( P):p( D) = 96:4
3
O( P)
50
+
O ion signals (a. u.)
3
rel( P)
50
1
O( D)
0
12
13
14
Photon energy / eV
15
Primary beam (0o source):
Discharge media @ 104 psi
1. 20% O2 + 80% He (1D:3P = 0.0017)
2. 3% O2 + 13% Ar + 85% He (1D:3P = 0.035)
Velocity = 1285 m/s
Secondary beam (90o source):
Sample: neat ethylene @ 55 psi
Velocity = 880 m/s
Collision energy Ec = 3.0 kcal/mol
3
O( P) + C2H4 CH2CHO + H
80°
70°
60°
50°
40°
30°
20°
100° 90°
10°
0°
108°
V C2
=8
H4
2
1
=
80
s
m/
VCM
Newton diagram
VO
8
/s
m
5
-10°
-18°
m/z = 15 for the sample 20% O2/He
800
-18°
600
Relative ion signal (arb. units)
400
200
-10°
data
3
O( P)+C2H4CH2CHO+H
3
O( P)+C2H4CH3+HCO
10°
PI @ 12.8 eV
20°
O(1D) = 0.17%  0
total
0
30°
40°
50°
60°
70°
80°
100°
108°
600
400
200
0
600
400
200
0
0
50
100 150 200 0
50
100 150 200 0
50
Flight time / s
100 150 200 0
50
100 150 200
m/z = 15 for the sample 3% O2+13% Ar/He
800
-18°
-10°
600
data
3
O( P)+C2H4CH2CHO+H
400
O( P)+C2H4CH3+HCO
3
10°
PI @ 12.8 eV
20°
O(1D) = 3.5%
1
Relative ion signal (arb. units)
O( D)+C2H4CH3+HCO
200
total
0
30°
40°
50°
60°
70°
80°
100°
108°
600
400
200
0
600
400
200
0
0
50
100 150 200 0
50
100 150 200 0
50
100 150 200 0
Flight time / s
50
100 150 200
3
O( P) + C2H4 CH2CHO + H
10 0°
30°
60°
150°
180°
90°
8
6
4
0
10 120°
8
6
P()
P(Et)
2
4
2
0
0
5
10 15 20 0
5
10 15 20 0 5 10 15 20 0
-1
Et / kcal mol
45
90 135 180
 / degree
3
O( P) + C2H4 CH3 + HCO
10 0°
30°
60°
150°
180°
90°
8
6
4
0
10 120°
8
P()
P(Et)
2
6
4
2
0
0
3
6
9 12 0
3
6
9 12 0 3 6
-1
Et / kcal mol
9 12 0
45
90 135 180
 / degree
1
O( D) + C2H4 CH3 + HCO
10 0°
30°
60°
90°
150°
180°
8
6
4
0
10 120°
8
6
P()
P(Et)
2
4
2
0
0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0
Et / kcal mol
-1
45
90 135 180
 / degree
m/z = 42 for the sample 20% O2/He
100
-10°
80
data
1
O( D)+C2H4CH2CO+2H
60
10°
20°
PI @ 11.1 eV
30°
O(1D) = 0.17%  0
Relative ion signal (arb. units)
1
O( D)+C2H4CH2CO+H2
3
O( P)+C2H4CH2CO+H2
40
total
20
0
100
40°
80
50°
60°
70°
60
40
20
0
0
50 100 150 200 0
50 100 150 200 0
50 100 150 200 0
Flight time / s
50 100 150 200 250
m/z = 42 for the sample 3% O2+13% Ar/He
500
-10°
10°
20°
30°
400
data
1
O( D)+C2H4CH2CO+2H
300
Relative ion signal (arb. units)
1
O( D)+C2H4CH2CO+H2
PI @ 11.1 eV
O(1D) = 3.5%
3
O( P)+C2H4CH2CO+H2
200
total
100
0
500
40°
50°
60°
70°
400
300
200
100
0
0
50 100 150 200 0
50 100 150 200 0
50 100 150 200 0
Flight time / s
50 100 150 200 250
3
O( P) + C2H4 CH2CO + H2
10 0°
30°
60°
90°
150°
180°
8
6
4
0
10 120°
8
6
P()
P(Et)
2
4
2
0
0 20 40 60 80 0 20 40 60 80 0 20 40 60 80 0
Et / kcal mol
-1
45
90 135 180
 / degree
1
O( D) + C2H4 CH2CO + H2
10 0°
30°
60°
150°
180°
90°
8
6
4
P(Et)
2
0
10 120°
8
P()
6
4
2
0
0
20 40 60 80 0
20 40 60 80 0 20 40 60 80 0
-1
Et / kcal mol
45
90 135 180
 / degree
1
O( D) + C2H4 CH2CO + 2H
10 0°
30°
60°
90°
150°
180°
8
6
4
P(Et)
2
0
10 120°
8
P()
6
4
2
0
0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0
-1
Et / kcal mol
45
90 135 180
 / degree
Intersystem crossing
(x)
(-1.9)
(-8.7)
CH2(3B1)+H2CO
(?)
(o)
(x)
(o)
(o)
T.L. Nguyen, L. Vereecken, X.J. Hou, M.T. Nguyen, and J. Peeters, J. Phys. Chem. A 109, 7489 (2005)
O(1D) + C2H4
(45.4)
H2CCO + 2H
18.9
(o)
(x)
(o)
(o)
(ethylene oxide)
Conclusions
• Universal detection has been really achieved using
the powerful chemical dynamics endstation
associated with the U9 white light beamline.
• Product branching ratios, kinetic energy distributions,
and angular distributions in chemical reactions have
been successfully measured in this endstation.
• This endstation is an important site for studying
complicated chemical reactions.