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Slow Electron Velocity-map Imaging of Negative Ions: Applications to Spectroscopy and Dynamics Slow Electron Velocity-map Imaging of Negative Ions: Applications.

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Presentation on theme: "Slow Electron Velocity-map Imaging of Negative Ions: Applications to Spectroscopy and Dynamics Slow Electron Velocity-map Imaging of Negative Ions: Applications."— Presentation transcript:

1 Slow Electron Velocity-map Imaging of Negative Ions: Applications to Spectroscopy and Dynamics Slow Electron Velocity-map Imaging of Negative Ions: Applications to Spectroscopy and Dynamics Columbus June 2012

2 Spectroscopy and dynamics of free radicals, transition states, clusters Reactive free radicals play key role in combustion, planetary atmospheres, interstellar chemistry – Map out electronic and vibrational structure, with special focus on vibronic coupling Spectroscopy of potential energy surfaces for chemical reactions – Pre-reactive van der waals complexes – Transition state spectroscopy Clusters: evolution of properties of matter with size – Semiconductor clusters, metal oxides, water clusters, He droplets How do we do this? Anion photoelectron spectroscopy (PES) and its variants – Anion slow electron velocity-map imaging (SEVI), a high resolution version of PES – Combine with ion trapping and cooling to maximize resolution

3 How to improve energy resolution of photoelectron spectroscopy? Photoelectron spectroscopy – Very general, limited to 5-10 meV ZEKE (zero electron kinetic energy) spectroscopy – High resolution (0.1-0.2 meV) – Experimentally challenging – restricted to s-wave detachment SEVI – Resolution comparable to ZEKE without expt’l complications – Versatile structural probe Fixed h Tunable h ZEKESEVI The ZEKE Queen

4 SEVI apparatus Adaptation of ideas by Chandler, Houston, Parker Energy and angular distributions Electrons with 300-500 meV fill detector Very high resolution for the slow electrons

5 Slow electron velocity-map imaging Low VMI voltages, long flight tube – Photoelectrons with 4500 cm -1 (0.5 eV) or 2500 cm -1 (0.3 eV) fill the detector Optimized VMI conditions – Collinear geometry, pulsed detector –  -metal shielding, large VMI electrodes, DC voltages only – Small interaction region, finely adjustable extraction voltage Best resolution for the slower electrons (E  R 2 ) – Tune photon energy closer to a given transition threshold Flight tube: 50 cm -350V -255V GND μ-metal shielding (2 layers) Mass-selected anion beam -200V -146V GND 1024x1024 Pulsed MCP detector

6 SEVI of Cl -  Cl( 2 P 3/2 ), Cl*( 2 P 1/2 ) Quadrant symmetrized SEVI image Inverse Abel transformed image 2 P 1/2 2 P 3/2 Cl Cl*

7 SEVI of NeSˉ NeSˉ: D 0 =79 cm -1 NeS: D 0 =34 cm -1 X2-I1 splitting (A-B)=9 cm -1 Sˉ (m -1 )

8 SEVI of ArSˉ, KrSˉ ArSˉ: D 0 =409 cm -1 ArS: D 0 =120 cm -1 A, B, E are X2, I1, II0 origins KrSˉ: D 0 =630 cm -1 KrS: D 0 =163 cm -1 A, B, G are X2, I1, II0 origins

9 SEVI of S - (D 2 ) S D D Progressions in hindered rotor, S-D 2 stretch

10 SEVI of C n H¯ anions anions and neutrals seen in interstellar medium even n: closely spaced 2  +, 2  states in neutral odd n: evidence for linear and cyclic isomers in anion, neutral Taylor, 1998 PE spectra

11 C 4 H - ( 1  + )  C 4 H ( 2  + and 2  ) 2+2+ 22 2  + - 2  splitting is only 213 cm -1 Progressions in bending modes  vibronic coupling Zhou, 2007 B, C have different PAD’s Zhou, 2007

12 SEVI of C n Hˉ, odd n Direct measurement of S-O splitting in X state of C 5 H (25 cm -1 ) and T 0 for a state (1.309 eV) FC simulations show anion has linear X 3  g ˉ ground state Garand, Chem. Sci. 2010

13 Longer chains

14 Next generation of SEVI experiments: Peak widths in SEVI spectra of polyatomic molecular anions are typically 20-30 cm -1 wide (i.e. spin-orbit splitting in C n H ground state) Why is resolution worse than for atomic species? Ion temperature limits resolution – Unresolved rotational contours, incomplete vibrational cooling Implement anion trapping and cooling Lai-Sheng Wang

15 Modified SEVI apparatus

16 Another view Buffer gas: H 2 (35 K) or He (5K) Trapping time: 49 ms (20 Hz rep rate) Gas density:  3*10 13 cm -3

17 Determination of Ion Temperature SEVI spectrum of C 5 ¯ Population of anion spin-orbit states (splitting 26.5 cm -1 ) serves as temperature probe. Distribution corresponds to 30K. Taken with He at 5K.  =1/2 3/2

18 Comparison of SEVI spectra recorded with ions that come straight from the Even-Lavie Valve and ions that have been thermalized in the rf trap at 35K. For S 3 ¯, the choice of buffer gas plays a crucial role. Both spectra were recorded at trap temperatures of 35K with very similar H 2 and He densities inside the ion trap. Impact of ion cooling on SEVI spectrum of S 3 ¯ (bent anion and neutral)

19 Indenyl Radical Combustion intermediate – acetylene-oxygen-argon flames Intermediate in the formation of PAHs Marinov, N. M.; Castaldi, M. J.; Melius, C. F.; Tsang, W. Combust. Sci. Technol. 2007, 128, 295.

20 Calculations Anion: 1 A 1 0.0 E rel (eV) Radical: 2 A 2 Radical: 2 B 1 1.7 2.7 hvhv B3LYP/ aug-cc-pVTZ Harmonic frequencies C 2v geometry

21 Overview Cooled to 35 K with H 2 buffer gas in ion trap FC simulation, 130 cm -1 FWHM EA = 1.802(1) eV T 0 ≈ 0.86 eV 220 cm -1 FWHM

22 Closer look 20 cm -1 FWHM 11 cm -1 FWHM

23 Compare to simulation Non-FC allowed transitions Mix of s- and p-wave Vibronic coupling to 2 B 1 state?


25 Spectroscopy of reactive potential energy surfaces?

26 Czako et al, JCP 2009. F + CH 4 reaction F - CH 4 has a C 3v structure short F - —HCH 3 bond – Near transition state of F + CH 4 reaction Cheng et al. JCP 2011. K. Liu et al: evidence for reactive resonances in correlated product distributions (PRL, 2004) E assympt

27 Comparison to Recent Published Results Cheng, M.; Feng, Y.; Du, Y. K.; Zhu, Q. H.; Zheng, W. J.; Czako, G.; Bowman, J. M. J. Chem. Phys. 2011, 134. SEVI overview F( 2 P 3/2 )CH 4 F( 2 P 1/2 )CH 4 Cheng et al.

28 SEVI of F¯ CH 4 Structure below E asympt is from bound states Structure at higher eBE is from transition state region Partially-resolved features; combination of internal rotor and C- F stretch expected Bound van der Waals states E asympt

29 Cold, near threshold F¯ CD 4 See structure above E asympt associated with TS region Considerably less signal from vdW region Progression(s) at 115 cm -1 Assignment in progress (new data!) Distance between vertical lines 115 cm -1

30 Summary SEVI offers “next generation” of anion photodetachment experiments – First technique that systematically improves resolution of anion PES without sacrificing (much) generality Where are we headed? – Cold ions via trapping/cooling – Bare and complexed metal/semiconductor clusters – Pre-reactive complexes and transition states (in progress) – Theory needed to simulate TS spectra, vibronic coupling

31 Many thanks: Etienne Garand Tara Yacovitch Jongjin Kim Christian Hock $$$ AFOSR Andreas Osterwalder Matt Nee Jia Zhou

32 … and the rest of the group!

33 Why is SEVI spectrum of H 2 Fˉ so sensitive to photon energy? Detachment occurs by p-wave (l=1) Wigner threshold law comes into play Features at low eKE are less intense h 1 h 2

34 F + CH 4 ground state Tentative assignments : no TS simulations yet Large geometry differences Isotope effects Bound van der Waals states Hindered methyl rotation or intermolecular bend narrow: resonances? Intermolecular stretch

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