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. 21, 2010

Outline Introduction Synchrotron facility Crossed molecular-beam apparatus Photodissociation of propene (CH 3 -CH=CH 2 ) Crossed-beam reaction of O + C 2 H 4 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 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 I e = 1 mA (~ electrons / cm 2 s) M + e -  M + + 2e - d[M + ]/dt = I e  [M] * Probability of a molecule to be ionized in one second ××  = 1 × cm 2 /electron  p i = I e  = × = 1 s -1 × * For a molecule with 1.0 × 10 5 cm/s (1000 m/s), the × probability to be ionized (resident time  t = 1 × s) I e   t = 1 × 10 -5

 Detection efficiency for a typical VUV Ionizer: * l = 1 mm I nsrrc = photons / s A = 1 mm 2 = 0.01 cm 2  srrc = photons /cm 2 s  = cm 2 /photon * Ionization probability of a molecule per second × p i =  srrc ×  = 10 s -1 × * For a molecule with 1.0 × 10 5 cm/s (1000 m/s), the × probability to be ionized (resident time  t = 1 × s) p i  t = 1 × 10 -5

Synchrotron at NSRRC, Taiwan Chemical Dynamics Beamline

Chemical Dynamics Beamline (U9 White Light Beamline)

U9-undulator (U9- 聚頻磁鐵 )

Undulator ( 聚頻磁鐵 )

1 st 3 rd 2 nd 4 th

Harmonics Suppressor (Gas filter) Employed Medium: He, Ne, Ar, Kr, Xe noble gas pump SR

Performance of Harmonic Suppressor

Fundamental 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: ~ 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× torr). S/N gains 10 times than before for H 2 detection.

(I) Photodissociation of propene at 157 nm CH 3 -CH=CH nm  C 3 H 5 + H  C 3 H 4 + H + H  C 3 H 4 + H 2  C 3 H 3 + H 2 + H  C 2 H 4 + CH 2  C 2 H 3 + CH 3  C 2 H 2 + CH 4  C 2 H 2 + CH 3 + H Procedure: 1.Measure product time-of-flight spectra 2.Do simulation using a trial P(E t ) 3.Fit experimental data to the best 4.Obtain kinetic energy distribution P(E t )

I(E t,  ) = 1/4  P(E t )  [1+  (E t )  p 2 (cos  )]

Only the leading part of H-atom correlates with C 3 H 5 and most H atoms are attributed to triple dissociations. Good S/N ratio! (EI will cause severe dissociative ionization )

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!

Two components due to H 2 and 2H eliminations are observed notably at lab angle 30 o and 9.5 eV.

The dissociative ionization of C 3 H 4 becomes severe as detected with electron impact ionization. The selective photoionization (9.5 eV) can avoid completely dissociative ionization of C 3 H 4.

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 CH 3 arises from C 2 H 2 +CH 3 +H dissociation.

Apparently only a dissociation channel contributes to CH 2 and C 2 H 4 because they can be fitted satisfactorily using single P(E t ). CH 2 is identified to be from the methyl moiety via the photolysis of isotopic variant CD 3 C 2 H 3.

The formation of methane (CH 4 ) occurs rarely in photodissociation of hydrocarbons. In this work methane is observed in the photolysis of propene at 157 nm. Most C 2 H 2 arises from triple dissociation.

C 2 H 4 +CH 2, C 2 H 3 +CH 3, and C 2 H 2 +(CH 3 +H) channels have similar P(E t ). It is difficult to distinguish them using electron impact ionization.

Averaged kinetic energy release, kinetic fraction and branching ratio. Product channel E avail (kcal/mol) (kcal/mol) f t (%) Branching (%) 1 st 2 nd C 3 H 5 +H C 3 H 4 +H+H ~7 b ~627 C 3 H 4 +H C 3 H 3 +H 2 +H ~7 b ~6117 C 2 H 4 +CH C 2 H 3 +CH C 2 H 2 +CH C 2 H 2 +CH 3 +H ~7 b ~4260

I(E t,  ) = 1/4  P(E t )  [1+  (E t )  p 2 (cos  )], p 2 (cos  ) = (3cos 2  -1)/2  (E t ) = 2  I(E t,  ) = 3/4  P(E t )  cos 2   (E t ) = 0  I(E t,  ) = 1/4  P(E t )  (E t ) = -1  I(E t,  ) = 3/8  P(E t )  sin 2  I(E t,//) = 1/4  P(E t )  [1+  (E t  = 0 o I(E t,  ) = 1/4  P(E t )  [1-  (E t  = 90 o  (E t ) = 2[I(E t,//)–I(E t,  )] / [I(E t,//)+2  I(E t,  )]

Averaged angular-anisotropy parameters for various dissociation channels in photolysis of CH 3 CHCH 2 and CD 3 CHCH 2 at 157 nm Channel <><> <><> <><> C 3 H 5 +H ~ 0 C 3 H 2 D 3 +H ~ 0 C 2 H 3 +CD C 3 H 4 +H C 3 H 3 D 2 +D ~ 0 C 2 H 2 D+CHD C 2 H 4 +CH …… C 2 HD 2 +CH 2 D 0.03 C 2 H 3 +CH C 3 HD 3 +H C 2 D 3 +CH C 2 H 2 +CH C 3 H 2 D 2 +HD …… C 2 H 2 +CH 3 +H 0.05 a C 3 H 3 D+D 2 ~ 0 C 2 HD 3 +CH a from C 2 H 2 due to triple dissociation

Photo-excited state of propene at 157 nm Electronic states of propene nearby 157 nm:  -3s(1 1 A"),  -3p(2 1 A'),  -3p(2 1 A"),  -3p(3 1 A") The photo-excited state of propene at 157 nm is  -3p(2 1 A') that produces a transition dipole moment lying in the C-C=C plane.

(II) Crossed-beam reaction of O( 3 P/ 1 D) + C 2 H E c = 3 kcal/mol O( 3 P) + C 2 H 4 → CH 2 CHO + H → CH 3 + HCO → CH 2 CO + H 2 O( 1 D) + C 2 H 4 → CH 2 CO + 2H → CH 3 + HCO → CH 2 CO + H 2

Components of the discharge device Valve InsulatorInner electrode Insulator AdapterOuter electrode

Layout of the transient high-voltage discharge circuit

Discharge current on an oscilloscope 300 mV on the scope → 30 mA discharge current

Primary beam (0 o source): Discharge 100 psi 1.20% O % He ( 1 D: 3 P = ) 2.3% O % Ar + 85% He ( 1 D: 3 P = 0.035) Velocity = 1285 m/s Secondary beam (90 o source): Sample: neat 50 psi Velocity = 880 m/s Collision energy E c = 3.0 kcal/mol

eVO( 1 D) = 0.17%

eVO( 1 D) = 3.5%

eVO( 1 D) = 0.17%

O( 1 D) = 11.1 eV

(x) (o) (-1.9) (-8.7) CH 2 ( 3 B 1 )+H 2 CO (x) (?) T.L. Nguyen, L. Vereecken, X.J. Hou, M.T. Nguyen, and J. Peeters, J. Phys. Chem. A 109, 7489 (2005)

O( 1 D) + C 2 H 4 (ethylene oxide) (x) (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.