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:
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
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
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 ( 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
SEVI apparatus Adaptation of ideas by Chandler, Houston, Parker Energy and angular distributions Electrons with meV fill detector Very high resolution for the slow electrons
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
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*
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 )
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
SEVI of S - (D 2 ) S D D Progressions in hindered rotor, S-D 2 stretch
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
C 4 H - ( 1 + ) C 4 H ( 2 + and 2 ) 2+2+ 22 2 splitting is only 213 cm -1 Progressions in bending modes vibronic coupling Zhou, 2007 B, C have different PAD’s Zhou, 2007
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
Next generation of SEVI experiments: Peak widths in SEVI spectra of polyatomic molecular anions are typically 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
Modified SEVI apparatus
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
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
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)
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.
Calculations Anion: 1 A E rel (eV) Radical: 2 A 2 Radical: 2 B hvhv B3LYP/ aug-cc-pVTZ Harmonic frequencies C 2v geometry
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
Closer look 20 cm -1 FWHM 11 cm -1 FWHM
Compare to simulation Non-FC allowed transitions Mix of s- and p-wave Vibronic coupling to 2 B 1 state?
Spectroscopy of reactive potential energy surfaces?
Czako et al, JCP 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 K. Liu et al: evidence for reactive resonances in correlated product distributions (PRL, 2004) E assympt
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.
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
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
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
Many thanks: Etienne Garand Tara Yacovitch Jongjin Kim Christian Hock $$$ AFOSR Andreas Osterwalder Matt Nee Jia Zhou
… and the rest of the group!
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
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