Near-Threshold Photodetachment Cross Section of (SF6)nâ Cluster

Subscriber access provided by NORTHERN KENTUCKY UNIV STEELY

Article

Near Threshold Photodetachment Cross Section of (SF)¯ Cluster Anions: The Ion Core Structure 6

n

Itamar Luzon, Maoz Nagler, Vijayanand Chandrasekaran, Oded Heber, and Daniel Strasser J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b09967 • Publication Date (Web): 15 Dec 2015 Downloaded from http://pubs.acs.org on December 16, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Near Threshold Photodetachment Cross Section of (SF6)n¯ Cluster Anions: The Ion Core Structure Itamar Luzon1, Maoz Nagler1, Vijayanand Chandrasekaran1, Oded Heber2 and Daniel Strasser1,* 1

Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel.

2

Department of Particle Physics, Weizmann Institute of Science, Rehovot 76100, Israel.

Corresponding author: [email protected] KEYWORDS: Photodetachment cross section, anion clusters, electrostatic ion trap, ion core ABSTRACT Photodetachment cross sections as a function of photon energy are measured for cold (SF6)n¯ cluster anions, stored in the electrostatic ion beam trap (EIBT). Absolute photodetachment crosssections near the adiabatic limit are reported. The strong dependence of SF6¯ absolute photodetachment cross section on the anion equilibrium bond length allows concluding that the excess charge is localized on a SF6¯ ion core, only subtly perturbed by the neighboring cluster units.

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 20

Introduction The study of atomic and molecular cluster systems offers a bridge between our understanding of isolated gas phase molecules, nano-particle systems and bulk condensed phase.1 With the advent of mass spectrometric methods, charged clusters can be studied with exact size selectivity allowing systematic examination of different phenomena as a function of cluster size. Changes in cluster properties as a function of size can exhibit monotonic behavior, reflecting for example the systematic change of volume or surface area.2 However, special "magic number" clusters often exhibit abrupt changes in cluster properties due to the geometric arrangement of the finite number of cluster units. Such magic number clusters are typically identified by their enhanced abundance in ion mass spectra,3 or in more elaborate spectroscopic measurements that provide direct insight into the cluster energetics.4,5 For example, in (SF6)n clusters, n=13 magic number displays enhanced stability associated with an icosahedral structure, closing a first solvation shell around a central unit.6 A particularly interesting aspect of charged clusters is the degree of charge localization on one or more units within the cluster. Interestingly, even rare gas cluster cations exhibit a so called molecular “ion core”, in which the excess charge is delocalized over a small cation molecule in the center of atomic rare gas cluster. More specifically, trimer ion cores, forming RG3+RGn type structures (where RG is a rare gas atom), are identified based on photoabsorption cross section variation with cluster size.7 Considering charge delocalization in anionic clusters we can tentatively divide cluster anions into two categories based on the way the monomer unit binds an excess electron. In one category, molecules such as carbon dioxide, water or nitrogen that have negative electron


Page 3 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


affinities and cannot bind an electron as a monomer.8 But in clusters, the extra electron can be stabilized in a solvated state9 or form a covalently bonded ion core as found in CO2.10–13 In the second category, molecules such as anthracene,14,15 pyrene,16,17 or CS218–24 that have positive electron affinities and can bind an electron in a localized valance state. Nevertheless, in clusters the extra electron can still be delocalized over two or more units, forming for example dimer or trimer ion cores. Photoelectron spectroscopy was successfully applied to study such anionic clusters, deducing ion core structure and identifying ion-core switching by comparing photoelectron spectra with calculated electron binding energies.9,14,25 In most cases, quantum chemical calculations are needed to reliably interpret changes in the photoelectron spectra in terms of the ion-core structure. However, in anions of second category with a significant geometry change between the neutral and anion species, the near threshold photodetachment cross section itself can be directly sensitive to the ion core structure. Small geometrical shift towards the neutral geometry due to charge delocalization can strongly increase the FrankCondon overlap of the anion ground state with low lying neutral states. A particularly suitable cluster systems for exploring such photodetachment cross section sensitivity to the ion core are SF6 based cluster anions. The inertness and large electron attachment cross section of SF6 make it an efficient electron scavenger used in many scientific as well as industrial applications.26,27 Electron attachment into the a1g antibonding orbital of the Oh symmetry neutral SF6, increases the S-F bond length from 1.57Å to 1.73 Å.28 The S-F bond elongation stabilizes the electron, such that the vertical detachment energy (VDE) determined by photoelectron spectroscopy amounts to ~3.1eV.29 Recently we reported sensitive SF6¯ photodetachment cross section measurements near the adiabatic threshold, showing more than 3 orders of magnitude cross section drop between ~3eV and 2eV photon energies.30 The measured 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 20

cross section drop was reproduced using calculated Frank-Condon overlaps, vanishing for the ground state to ground state transition at the adiabatic detachment energy (ADE).28,30 Although a 0.7eV increase of the ADE compared to ~1eV values obtained from kinetic modeling was required.31,32 The strong dependence of the photodetachment cross section on photon energy can be intuitively understood by considering the vertical photodetachment probability from the exponentially decaying tail of the anion ground state wave function: vanishing as the bond length approaches the neutral equilibrium distance. Figure 1 shows a simplified 1D model of two shifted harmonic oscillator potentials, for which the Gaussian ground state wave function of the anion results in an exponentially decaying cross section as a function of energy. The photodetachment cross section is therefore exponentially sensitive to small changes of the equilibrium bond length shift ∆R0 squared.

Figure 1: One dimensional near threshold photodetachment model scheme. The anion and neutral potentials are represented by one dimensional harmonic oscillators, shifted by ∆R0 and ADE, respectively on the distance and energy scales. The dashed lines show the anion ground state wave function and the corresponding Frank-Condon overlap as a function of photon energy hv.


Page 5 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In this work, we present photodetachment cross-section measurements of size selected small (SF6)n¯ clusters. The magnitude of the photodetachment cross-section is found to be nearly identical for the investigated clusters, indicating that the charge is localized on one SF6¯ monomer. Subtle changes in the photodetachment cross section curve as a function of cluster size are discussed. Experimental section The experimental setup has been described in detail by Luzon et al.30 Briefly, cold SF6¯ and (SF6)n¯ clusters are produced using a pulsed Even-Lavie ion source, directing ~100eV electrons into a supersonic expansion of argon carrier gas seeded with ~1% SF6 .33

Figure 2: Schematic representation of the EIBT setup for sensitive photodetachment cross section measurements. Inset shows the capacitive pickup signal from the off-center ring electrode, indicating the number of trapped ions and the bunch forward (FB) or backward (BB) direction.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 20

The produced anions are accelerated up to 4.2 keV and injected into the electrostatic ion beam trap (EIBT),34 shown schematically in figure 2. Constant potentials are applied to the EIBT mirror electrodes except the entrance Vp electrode which is lowered during ion beam injection, to allow 4.2keV ions to enter the trap. As soon as the ions of interest are in the trap, the Vp potential is raised and the selected ions are trapped, oscillating back and forth between two mirrors. A small sinusoidal RF potential of ~4V, at the oscillation frequency of a particular cluster of interest is applied to the VRF electrode in order to bunch the ion beam.35 As the electrostatic trap trajectories are mass independent, more than a single cluster size can be simultaneously injected into the trap. However, only the selected mass is bunched and synchronized with the applied RF potential. Unwanted ion species are actively removed from the trap by applying a synchronized pulsed kick-out potential of 100V to the VKO deflector located in the center of the trap. Applying the kick out pulse in the first 50 oscillations is sufficient for removing all the cluster anions oscillating out of synchronization with VRF and purifying the selected cluster anion sample.36 It is important to note that n=4 clusters oscillate at exactly half the frequency of the monomer anion, making it challenging to reliably remove the predominant monomer anions from the n=4 size clusters. Therefore, in this paper we report experiments performed with cluster sizes n=1-3 as well as the “magic number” n=13 cluster. The mass selected cluster bunch oscillates back and forth between the two mirrors. Each time the ion bunch passes through a pickup ring electrode, it induces an image charge, recorded by digital oscilloscope as shown in figure 2 inset. The image charge is proportional to the number of trapped ions, thus the pickup allows non-destructive monitoring of the number of trapped ions.35 Furthermore, as the pickup electrode is positioned off-center, closer to the exit mirror, the short and long half oscillations allow unambiguous identification of the ion bunch direction inside the 6 ACS Paragon Plus Environment

Page 7 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EIBT, allowing us to synchronize the 5ns photo detachment laser pulses such that the ions move towards the detector during laser-cluster interaction. Neutral photodetachment products from forward moving ions are not deflected by the exit mirror and can be detected downstream by a microchannel plate (MCP) detector. A 3mm hole in the center of the MCP allows optimal colinear overlap of the photodetachment laser beam with the trapped ion trajectories, while maintaining a >50% geometric detector acceptance.30,37 The 10Hz Optical Parametric Oscillator (OPO) laser is tuned in the 420-709 nm and 210-419 nm, scanning the respective signal and doubled signal wavelength ranges in ~10 meV steps. To insure stability and reliable measurement of the low cross sections, the computer controlled scan spends longer times at low signal wavelength range. The computer controlled scan automatically changes both wavelength and ion species to minimize any effect of possible systematic artifacts. Following Luzon et al,30 the photo detachment cross section is determined according to:

PD

(1) ℎ =

ions.

Where η includes the detection efficiency of the MCP and the geometric laser-ion overlap factor, NPD(hν) is the number of neutral products detected due to photodetachment laser pulse at hv photon energy, Nion is the number trapped ions determined from the image charge on the pickup electrode and Φ(hν) is the photon flux. Although absolute determination of the η factor is possible, we obtain absolute cross sections by normalization to a previous SF6¯ photodetachment measurement in the VDE region.30,38



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 20

Results and discussion Figures 3a-3d show typical photodetachment neutral product signal as a function of time after the laser pulse for cluster sizes n=1,2,3 and 13 respectively. As the neutral products retain the velocity of the parent anion, their time of flight is indicative of their source. The number of neutral photodetachment products per laser pulse (NPD) is obtained by integrating the counts in the indicated time of flight windows that correspond to neutrals produced at the center of the EIBT. The dotted lines show the background neutral signal, resulting predominantly from collisions of trapped ions with the residual gas in the EIBT. Due to the low residual gas pressure in the EIBT, the background neutrals signal is low and is multiplied by a factor of x25. As shown in figure 3a, the background practically vanishes for the SF6¯ monomer, allowing the sensitive background free measurements near the ADE limit. This periodic background signal, clearly seen in figures 3b-3d, varies slowly over the trapping time is obtained by recording the neutral signals arriving to the MCP before the laser pulse, averaging over 20 oscillations. This background signal, which is especially important at photon energies where cross sections are low, is subtracted from the total number of neutrals observed in the time window of the ion to obtain NPD(hv). The synchronized arrival of the neutrals from photodetachment and background neutrals indicates that the ion bunch is in the field free region at the center of the EIBT when irradiated by the laser pulse. As neutral SF6 molecules show strong absorption in the IR, large (SF6)n¯ clusters can also produce neutrals by a unimolecular channel of black body induced radiative decay (BIRD),5 leading to evaporation of neutral monomers at room temperature. For small cluster sizes n4) Cluster Anions. J. Chem. Phys. 2008, 129, 244309.

(22)

Matsuyama, Y.; Nagata, T. Structures of Molecular Anion: Photoelectron Spectroscopy and Theoretical Calculations. Chem. Phys. Lett. 2008, pp 31–35.

(23)

Habteyes, T.; Velarde, L.; Sanov, A. Effects of Isomer Coexistence and Solvent-Induced Core Switching in the Photodissociation of Bare and Solvated (CS2)2- Anions. J. Chem. Phys. 2009, 130, 124301.

(24)

Goebbert, D. J.; Wende, T.; Jiang, L.; Meijer, G.; Sanov, A.; Asmis, K. R. IR Spectroscopic Characterization of the Thermally Induced Isomerization in Carbon Disulfide Dimer Anions. J. Phys. Chem. Lett. 2010, 1, 2465–2469.

(25)

Sanov, A.; Carl Lineberger, W. Dynamics of Cluster Anions: A Detailed Look at Condensed-Phase Interactions. Phys. Chem. Comm. 2002, p 165.

(26)

Johnstone, W. R.; Newell, W. M. Absolute Elastic Differential Cross Sections for Electron Scattering from SF6. J. Phys. E At. Mol. Opt. Phys. 1991, 24, 473–487.

(27)

MIalik, N. H.; Qureshi, A. H. A Review of Electrical Breakdown in Mixtures of SF6 and Other Gases. IEEE Trans. Electr. Insul 1979, EI-14, 1.

(28)

Eisfeld, W. Highly Accurate Determination of the Electron Affinity of SF6 and Analysis of Structure and Photodetachment Spectrum of SF6-. J. Chem. Phys. 2011, 134, 054303.

(29)

Bopp, J. C.; Roscioli, J. R.; Johnson, M. A.; Miller, T. M.; Viggiano, A. A.; Villano, S. M.; Wren, S. W.; Lineberger, W. C. Spectroscopic Characterization of the Isolated SF6and C4F8- Anions: Observation of Very Long Harmonic Progressions in Symmetric Deformation Modes upon Photodetachment. J. Phys. Chem. A 2007, 111, 1214–1221.

(30)

Luzon, I.; Nagler, M.; Heber, O.; Strasser, D. SF6- Photodetachment near the Adiabatic Limit. Phys. Chem. Chem. Phys. 2015, 17, 7670–7675. 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 20

(31)

Troe, J.; Miller, T. M.; Viggiano, A. A. Communication: Revised Electron Affinity of SF6 from Kinetic Data. J. Chem. Phys. 2012, 136, 121102.

(32)

Menk, S.; Das, S.; Blaum, K.; Froese, M. W.; Lange, M.; Mukherjee, M.; Repnow, R.; Schwalm, D.; von Hahn, R.; Wolf, a. Vibrational Autodetachment of Sulfur Hexafluoride Anions at Its Long-Lifetime Limit. Phys. Rev. A 2014, 89, 022502.

(33)

Even, U.; Jortner, J.; Noy, D.; Lavie, N.; Cossart-Magos, C. Cooling of Large Molecules below 1 K and He Clusters Formation. J. Chem. Phys. 2000, 112, 8068.

(34)

Dahan, M.; Fishman, R.; Heber, O.; Rappaport, M.; Altstein, N.; Zajfman, D.; van der Zande, W. J. A New Type of Electrostatic Ion Trap for Storage of Fast Ion Beams. Rev. Sci. Instrum. 1998, 69, 76.

(35)

Rahinov, I.; Toker, Y.; Heber, O.; Strasser, D.; Rappaport, M. Lifetime Measurements in an Electrostatic Ion Beam Trap Using Image Charge Monitoring. Rev. Sci. Instrum. 2012, 83, 033302.

(36)

Toker, Y.; Altstein, N.; Aviv, O.; Rappaport, M. L.; Heber, O.; Schwalm, D.; Strasser, D.; Zajfman, D. The Kick-out Mass Selection Technique for Ions Stored in an Electrostatic Ion Beam Trap. J. Instrum. 2009, 4, P09001.

(37)

Attia, D.; Strasser, D.; Heber, O.; Rappaport, M. L.; Zajfman, D. Transverse Kinematics of Ions Stored in an Electrostatic Ion Beam Trap. Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. 2005, 547, 279–286.

(38)

Mock, R. S.; Grimsrud, E. P. Electron Photodetachment of the Molecular Anions of SF6 and Several Perfluorinated Hydrocarbons. Chem. Phys. Lett. 1991, 184, 99–101.

(39)

Grumbling, E. R.; Sanov, A. Solvation Effects on Angular Distributions in H- (NH3)n and NH2- (NH3)n Photodetachment: Role of Solute Electronic Structure. J. Chem. Phys. 2011, 135, 164301.

(40)

Boutin, A.; Maillet, J.; Fuchs, A. H. Structure and Dynamics of Simulated (SF6)N Clusters in the Size Range N=7–55. J. Chem. Phys. 1993, 99, 9944.

(41)

Reed, T. M. The Ionization Potential and the Polarizability of Molecules. J. Phys. Chem. 1955, 59, 428–432. 18 ACS Paragon Plus Environment

Page 19 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(42)

Dao, D. B.; Mabbs, R. The Effect of the Dipole Bound State on AgF− Vibrationally Resolved Photodetachment Cross Sections and Photoelectron Angular Distributions. J. Chem. Phys. 2014, 141, 154304.

(43)

Khuseynov, D.; Blackstone, C. C.; Culberson, L. M.; Sanov, A. Photoelectron Angular Distributions for States of Any Mixed Character: An Experiment-Friendly Model for Atomic, Molecular, and Cluster Anions. J. Chem. Phys. 2014, 141, 124312.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 20

TOC figure :


Near-Threshold Photodetachment Cross Section of (SF6)nâ Cluster

Recommend Documents

Near-Threshold Photodetachment Cross Section of (SF6)nâ Cluster