Application of Synchrotron Radiation to Chemical Dynamics Research

1. 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. 22, 2015

2. Outline • Introduction to ionization • Synchrotron facility • Crossed molecular-beam apparatus • Photodissociation of propene (CH3-CH=CH2) • Crossed-beam reactions of O(3P, 1D) + C2H4 • Conclusion

3. Introduction Ionization detection of reaction products is ideal for molecular beam experiments in chemical reaction dynamics research. Electron Impact Ionization Photoionization

4. 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

5. 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 flux in the VUV region - low availability and expensive

6. 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) Ie t =1 × 10-5

7. 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

8. Chemical Dynamics Beamline Synchrotron at NSRRC, Taiwan

9. Chemical Dynamics Beamline (U9 White Light Beamline)

10. U9-undulator (U9-聚頻磁鐵)

11. Undulator (聚頻磁鐵)

12. 1st 3rd 4th 2nd

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

16. Performance of Harmonic Suppressor

17. Fundamental (first-harmonic) photon energy vs U9 gap

19. 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)

21. 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

22. 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.

23. (I) Photodissociation of propene at 157 nm CH3-CH=CH2 + 157 nm  C3H5 + H  C3H4 + H + H  C3H4 + H2  C3H3 + H2 + H  C2H3 + CH3  C2H2 + CH3 + H  C2H4 + CH2  C2H2 + CH4 Procedure: • Measure product time-of-flight spectra • Do simulation using a trial P(Et) • Fit experimental data to the best • Obtain kinetic energy distribution P(Et)

24. I(Et ,) = 1/4P(Et)[1+(Et)p2(cos)], p2(cos) = (3cos2-1)/2

25. (EI will cause severe dissociative ionization) Only the leading part of H-atom correlates with C3H5 and most H atoms are attributed to triple dissociations. Good S/N ratio!

26. Good S/N ratio! 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.

27. Two components due to H2 and 2H eliminations are observed notably at lab angle 30o and 9.5 eV.

28. 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.

29. 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.

30. 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.

31. 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.

32. C2H4+CH2,C2H3+CH3, and C2H2+(CH3+H) channels have similar P(Et). It is difficult to distinguish them using electron impact ionization.

33. 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 Averaged kinetic energy release, kinetic fraction and branching ratio.

34. 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 ,)]

35. (Et) = 2[I(Et ,//)–I(Et ,)] / [I(Et ,//)+2I(Et ,)] Two momentum-matched fragments have the same  value

36. 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 Averaged angular-anisotropy parameters for various dissociation channels in photolysis of CH3CHCH2 and CD3CHCH2 at 157 nm a from C2H2 due to triple dissociation

37. 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).

38. (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

39. Components of the discharge device Valve Insulator Insulator Inner electrode Adapter Outer electrode

40. Layout of the transient high-voltage discharge circuit

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

43. Primary beam (0o source): Discharge media @ 104 psi • 20% O2 + 80% He (1D:3P = 0.0017) • 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

45. PI @ 12.8 eV O(1D) = 0.17%  0

46. PI @ 12.8 eV O(1D) = 3.5%