Application of Synchrotron Radiation to Application of Synchrotron Radiation to Chemical Dynamics ResearchChemical 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. 21, 2010
OutlineOutline
IntroductionSynchrotron facilityCrossed molecular-beam apparatusPhotodissociation of propene (CH3-CH=CH2)Crossed-beam reaction of O + C2H4
Conclusion
IntroductionIntroduction
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 selectivity
- Limited detection efficiency, especially for TOF measurement, because of space charge problem
Photo-ionization by Direct VUV Ionization
Advantage- Universal
- Small dissociative ionization- 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+ + 2e-
d[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)
IIe e t =t = 1 1 ×× 10 10-5-5
Detection efficiency for a typical VUV Ionizer:
* l = 1 mm
I nsrrc = 1016 photons / s
A = 1 mm2 = 0.01 cm2
srrc = 1018 photons /cm2 s
= 10-17 cm2/photon
* Ionization probability of a molecule per second
pi = srrc ×× = 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)
pi t = 1 × 10-5
Harmonics Suppressor (Gas filter)
Employed Medium: He, Ne, Ar, Kr, Xe
noble gas
pump
pump pump
pump
pump
SR
Performance of Harmonic Suppressor
0 2 4 6 8 10 12 14
0.01
0.1
1
10
100
Ring current 146.5mAGap 60mmGas Ar
I/I0 = 10-3 @ 10 Torr
Ph
oto
cu
rre
nt
/ A
Ar Pressure / Torr
15 20 25 30 35 40 45 50
5
10
15
20
25
30
35
F
unda
men
tal P
hoto
n E
nerg
y / e
V
Undulator Gap / mm
Fundamental Photon Energy vs U9-GAPFundamental Photon Energy vs U9-GAP
U9 White Light Beamline at NSRRC
Light Source (U9 undulator)
Undulator period : 9 cm
Number of period (N): 48
Energy range : 5 ~ 50 eV
Energy resolution : E / E ~ 4 %
Photon flux: ~ 1016 photons/sec
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
C3H4 + H2
C3H3 + H2 + H
C2H4 + CH2
C2H3 + CH3
C2H2 + CH4
C2H2 + CH3 + H
Procedure:
1. Measure product time-of-flight spectra
2. Do simulation using a trial P(Et)
3. Fit experimental data to the best
4. Obtain kinetic energy distribution P(Et)
0
90
180
270
0
90
180
270
-80 -60 -40 -20 0 20 40 60 80 100
270
180
90
0() ()
MAV
A = M
BV
B
VB
VA A BAB+h
Velocity (arb. units)
= 0 (isotropic) = -1 (v ) = 2 (v // )
Velocity distributions of products after photodissociation
Three typical angular distributions of products after photodissociation
I(Et ,) = 1/4P(Et)[1+(Et)p2(cos)]
0 50 100 150 200 250
C3H
5 (IP = 8.2 eV)
Ion c
ount
Flight time (s)
m/z = 41
@ 30o, 8.8 eV
0 50 100 150 200 250 300
m/z = 41
@ 5o, 8.8 eV
0 10 20 30 40 500
100
200
300
400
500
600
H (IP = 13.6 eV)
Ion
coun
t (a.
u.)
Flight time (s)
m/z = 1
@ 30o, 14 eV C
3H
5+H
C3H
4+H+H
C3H
4+H+H
& C3H
3+H
2+H
& C2H
2+CH
3+H
20 40 60 800.0000
0.0005
0.0010
0 20 40 60 800.00
0.02
0.04
0.06
0.08
0.10
H
P(E
t)
Et (kcal/mol)
C3H
5+H (0.014)
C3H
4+H+H (0.073)
C3H
4+H+H (0.073)
& C3H
3+H
2+H (0.19)
& C2H
2+CH
3+H (0.65)
Only the leading part of H-atom correlates with C3H5 and most H atoms are attributed to triple dissociations.
Good S/N ratio!
(EI will cause severe dissociative ionization)
0 5 10 15 20 25 300
200
400
600
800
1000
1200
1400
H2 (IP = 15.4 eV)
Ion
coun
t (a.
u.)
Flight time (s)
m/z = 2
@ 30o, 17 eV
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.
Good S/N ratio!
0 20 40 60 80 1000.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
C3H
4+H
2
P(E
t)
Et (kcal/mol)
C3H
4+H
2
C3H
5 cracking
C3H
4+H+H
C3H
4 (IP = 9.5~10.4 eV)
m/z = 40
@ 10o, 9.5 eV
m/z = 40
@ 10o, 11.5 eV
0 50 100 150 200 250
Flight time (s)
Ion c
ount
m/z = 40
@ 30o, 9.5 eV
0 50 100 150 200 250 300
m/z = 40
@ 30o, 11.5 eV
Two components due to H2 and 2H eliminations are observed notably at lab angle 30o and 9.5 eV.
C3H
3 (IP = 8.7~10.8 eV)
m/z = 39
@ 10o, 9.5 eV
C3H
3+H
2+H
C3H
4 cracking
m/z = 39
@ 10o, 11.5 eV
0 50 100 150 200 250
Flight time (s)
Ion c
ount
m/z = 39
@ 30o, 9.5 eV
0 50 100 150 200 250 300
m/z = 39
@ 30o, 11.5 eV
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.
C2H
3+CH
3
C2H
2+CH
3 +H
CH3 (IP = 9.8 eV)
Ion c
ount
m/z = 15
@ 30o, 11 eV
C2H
3 (IP = 8.3 eV)
m/z = 27
@ 30o, 10 eV
0 50 100 150
Flight time (s)
m/z = 15
@ 60o, 11 eV
0 50 100 150 200 250 300
m/z = 27
@ 60o, 10 eV
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.
CH2 (IP = 10.4 eV)
Ion c
ount
m/z = 14
@ 30o, 11 eV
C2H
4 (IP = 10.5 eV)
m/z = 28
@ 30o, 12 eV
0 50 100 150
Flight time (s)
m/z = 14
@ 60o, 11 eV
0 50 100 150 200 250 300
m/z = 28
@ 60o, 12 eV
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.
CH4 (IP = 12.6 eV)
Ion c
ount
m/z = 16
@ 30o, 14 eV
C2H
2+CH
4
C2H
2+CH
3+H
C2H
2 (IP = 11.4 eV)
m/z = 26
@ 30o, 11.5 eV
0 50 100 150
Flight time (s)
m/z = 16
@ 60o, 14 eV
0 50 100 150 200 250 300
m/z = 26
@ 60o, 11.5 eV
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.
0 10 20 30 40 500.00
0.01
0.02
0.03
0.04
0.05
0.06
C2H
2+CH
3+H
P(E
t)
Et (kcal/mol)
0 10 20 30 40 500.00
0.01
0.02
0.03
0.04
0.05
0.06
C2H
4+CH
2
P(E
t)
Et (kcal/mol)
0 10 20 30 40 500.00
0.01
0.02
0.03
0.04
0.05
0.06
C2H
3+CH
3
P(E
t)
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.
0 20 40 60 80 1000.00
0.01
0.02
0.03
C2H
2+CH
4
P(E
t)
Et (kcal/mol)
Averaged kinetic energy release, kinetic fraction and branching ratio.
Product channel
Eavail
(kcal/mol)
<Et>
(kcal/mol)
ft
(%)
Branching(%)
1st 2nd
C3H5+H 93.3 49.7 0 53.3 1
C3H4+H+H 37.8 16.5 ~7 b ~62 7
C3H4+H2 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
C2H2+CH3+H 44.7 11.6 ~7 b ~42 60
0
90
180
270
0
90
180
270270
180
90
0() ()
= 0 (isotropic) = -1 (v ) = 2 (v // )
Three typical angular distributions of products after photodissociation
I(Et ,) = 1/4P(Et)[1+(Et)p2(cos)], p2(cos) = (3cos2-1)/2
(Et) = 2 I(Et ,) = 3/4P(Et)cos2
(Et) = 0 I(Et ,) = 1/4P(Et)
(Et) = -1 I(Et ,) = 3/8P(Et)sin2
I(Et ,//) = 1/4P(Et)[1+(Et)] @ = 0o
I(Et ,) = 1/4P(Et)[1-(Et)/2] @ = 90o
(Et) = 2[I(Et ,//)–I(Et ,)] / [I(Et ,//)+2I(Et ,)]
0 30 60 90 120 1500
300
600
900
C2H
2+CH
3+H
C2H
2+CH
4
m/z = 26 (C2H
2)
@ 30o, 11.5 eV //
Flight time / s
0 20 40 60 80 1000
50
100
150
200
250
Ion
co
un
t (a
rb. u
nits
)
m/z = 16 (CH4)
@ 30o, 13.8 eV //
0 20 40 60 800.00
0.05
0.10
0.15
0.20
0.25
0.30
(E
t)
Et / kcal mol-1
CH4+C
2H
2
(Et) = 2[I(Et ,//)–I(Et ,)] / [I(Et ,//)+2I(Et ,)]
Averaged angular-anisotropy parameters for various dissociation channelsin photolysis of CH3CHCH2 and CD3CHCH2 at 157 nm
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
a from C2H2 due to triple dissociation
Photo-excited state of propene at 157 nmPhoto-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.
(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
Va
lve In
su
lato
r
Inn
er
ele
ctro
de
Ins
ula
to
r Ad
ap
te
r
Ou
ter
ele
ctro
de
12 13 14 150
50
100
150
I = (3P)p(3P) + (1D)p(1D)
p(3P):p(1D) = 96:4
O(1D)
O(3P)
O+ io
n si
gnal
s (a
. u.)
Photon energy / eV
O atoms from 3% O2/He by discharge
0
50
100
150
200
Rel
ativ
e io
niza
tion
cros
s se
ctio
ns (
a. u
.)
rel
(3P)
rel
(1D)
Primary beam (0o source): Discharge media @ 100 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 @ 50 psiVelocity = 880 m/s
Collision energy Ec = 3.0 kcal/mol
0
200
400
600
0 50 100 150 2000
200
400
600
0 50 100 150 200 0 50 100 150 200 0 50 100 150 200
0
200
400
600
800
data O(3P)+C
2H
4CH
2CHO+H
O(3P)+C2H
4CH
3+HCO
total
-18° -10° 10° 20°
Rel
ativ
e io
n si
gnal
(ar
b. u
nits
)
30° 40° 50° 60°
70°
Flight time / s
80° 100°
m/z = 15 for the sample 20% O2/He
108°
PI @ 12.8 eV O(1D) = 0.17%
0
200
400
600
0 50 100 150 2000
200
400
600
0 50 100 150 200 0 50 100 150 200 0 50 100 150 200
0
200
400
600
800
-18° data O(3P)+C
2H
4CH
2CHO+H
O(3P)+C2H
4CH
3+HCO
O(1D)+C2H
4CH
3+HCO
total
-10° 10° 20°
Rel
ativ
e io
n si
gnal
(ar
b. u
nits
)
30° 40° 50° 60°
70°
Flight time / s
80° 100°
m/z = 15 for the sample 3% O2+13% Ar/He
108°
PI @ 12.8 eV O(1D) = 3.5%
0 5 10 15 200
2
4
6
8
10
0 5 10 15 20 0 5 10 15 20 0 45 90 135 180
0
2
4
6
8
10 0° 30° 60° 90°
P(
)
P(E
t)
120°
Et / kcal mol-1
150° 180°
O(3P) + C2H
4 CH
2CHO + H
/ degree
0 3 6 9 120
2
4
6
8
10
0 3 6 9 12 0 3 6 9 12 0 45 90 135 180
0
2
4
6
8
10 0° 30° 60° 90°
P(E
t)
120°
Et / kcal mol-1
150° 180°
O(3P) + C2H
4 CH
3 + HCO
P(
)
/ degree
0 5 10 15 20 250
2
4
6
8
10
0 5 10 15 20 25 0 5 10 15 20 25 0 45 90 135 180
0
2
4
6
8
10 0° 30° 60° 90°
P(E
t)
120°
Et / kcal mol-1
150° 180°
O(1D) + C2H
4 CH
3 + HCO
P(
)
/ degree
0 50 100 150 200
0
20
40
60
80
100
0 50 100 150 200 0 50 100 150 200 0 50 100 150 200 250
0
20
40
60
80
100
-10°
data
O(1D)+C2H
4CH
2CO+2H
O(1D)+C2H
4CH
2CO+H
2
O(3P)+C2H
4CH
2CO+H
2
total
10° 20° 30°
Re
lativ
e io
n s
ign
al (
arb
. un
its)
40°
Flight time / s
50° 60°
m/z = 42 for the sample 20% O2/He
70°
PI @ 11.1 eV O(1D) = 0.17%
0 50 100 150 200
0
100
200
300
400
500
0 50 100 150 200 0 50 100 150 200 0 50 100 150 200 250
0
100
200
300
400
500
data
O(1D)+C2H
4CH
2CO+2H
O(1D)+C2H
4CH
2CO+H
2
O(3P)+C2H
4CH
2CO+H
2
total
-10°
m/z = 42 for the sample 3% O2+13% Ar/He
10° 20° 30°
Rel
ativ
e io
n si
gnal
(ar
b. u
nits
)
40°
Flight time / s
50° 60° 70°
O(1D) = 3.5%PI @ 11.1 eV
0 20 40 60 800
2
4
6
8
10
0 20 40 60 80 0 20 40 60 80 0 45 90 135 180
0
2
4
6
8
10 0° 30° 60° 90°
P(E
t)
120°
Et / kcal mol-1
150° 180°
O(3P) + C2H
4 CH
2CO + H
2
P(
)
/ degree
0 20 40 60 800
2
4
6
8
10
0 20 40 60 80 0 20 40 60 80 0 45 90 135 180
0
2
4
6
8
10 0° 30° 60° 90°
P(E
t)
120°
Et / kcal mol-1
150° 180°
O(1D) + C2H
4 CH
2CO + H
2
P(
)
/ degree
0 5 10 15 20 250
2
4
6
8
10
0 5 10 15 20 25 0 5 10 15 20 25 0 45 90 135 180
0
2
4
6
8
10 0° 30° 60° 90°
P(E
t)
120°
Et / kcal mol-1
150° 180°
O(1D) + C2H
4 CH
2CO + 2H
P(
)
/ degree
(x)
(o)
(o)
(o)
(-1.9)
(-8.7) CH2(3B1)+H2CO
(x)
(?)
T.L. Nguyen, L. Vereecken, X.J. Hou, M.T. Nguyen, and J. Peeters, J. Phys. Chem. A 109, 7489 (2005)
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, and angular distributions in chemical reactions have been successfully measured in this endstation.
• This endstation is an important site for studying complicated chemical reactions.