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

Synchrotron at NSRRC, TaiwanC

he

mic

al

Dy

na

mic

s

Be

am

lin

e

Chemical Dynamics Beamline (U9 White Light Beamline)

U9-undulator (U9-聚頻磁鐵 )

Undulator (聚頻磁鐵 )

1st 3rd

2nd

4th

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 

Crossed-Molecular-Beam Machine

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

Layout of the transient high-voltage discharge circuit

Discharge current on an oscilloscope

300 mV on the scope → 30 mA discharge current

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)

O(1D) + C2H4

(ethylene oxide)

(x)

(o)(o)

(o)

(45.4)

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.