Name: photoelectron imaging of anions illustrated by 310 nm detachment of f&#8722;
Uploaded: 2018-07-27T14:45:00
Description: Watch this Scientific Journal Video about Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of FÃ¢Ë†' at JoVE.com

This material is based upon work supported by the National Science Foundation under CHE - 1566157

NOTE: A general experimental protocol is presented here, specific to the WUSTL instrument. Specific instrument settings for the F&#8722; image presented in Figure 4a can be found in Table 1-2.
1. Ion Generation
<ol>
	<li>To generate anions, apply a backing gas or gas mixture (for F&#8722;, 40 psig. of O2) behind the pulsed nozzle and operate the nozzle at 10 Hz.
	<ol>
		<li>Set the nozzle duration on digital dela...

Two factors are particularly critical to the success of the described protocol. The best possible velocity mapping conditions must be determined and more crucially, a sufficient and relatively time invariant yield of the desired anion must be produced. Regarding the VMI focusing steps, steps 5.2 and 5.3 should be repeated in tandem with image analysis to determine the condition which gives the sharpest (narrowest) image features. Fine tuning of the electrode voltages (V5 and V6) is influenced by the size and location of ...

Anion photoelectron imaging1 is a variant on photoelectron spectroscopy and represents a powerful probe of atomic/molecular electronic structure and the interactions between electrons and neutral species. The information obtained is essential in developing the understanding of bound and metastable (electron-molecule scattering resonances) negative ion states, doorway states for chemical reduction, dissociative attachment processes and ion-molecule interactions. Furthermore, the results provide vital tests of high level ab initio theoretical methods, particularly those designed to deal with highly correlated systems and/or non-stationary states.
The technique combines ion production, mass spectrometry and charged particle imaging2,3,4 to sensitively probe electronic (and for small molecules, vibrational) structure. Working with anionic species allows good mass selectivity via time of flight mass spectrometry (TOF-MS). Visible/near ultraviolet (UV) photons are sufficiently energetic to remove the excess electron, allowing the use of table top laser sources. An additional benefit of the use of anions is the ability to photoexcite low-lying, unstable anionic states which represent energy regimes under which the electrons and neutral atoms/molecules strongly interact. The use of velocity mapped imaging5 (VMI) affords uniform detection efficiency, even at low electron kinetic energies, monitors all ejected photoelectrons and simultaneously reveals the magnitude and direction of their velocities.
The experimental results are photoelectron images which contain photoelectron spectra (details of parent anion internal energy distributions and the energies of daughter neutral internal states) and photoelectron angular distributions (related to the electron orbital prior to the detachment). A particularly interesting application of the technique is found in fs time-resolved studies. An initial ultrafast laser pulse (pump) excites to a dissociative anion electronic state, and a second temporally delayed ultrafast pulse (probe) then detaches electrons from the excited anion. The control of the pump-probe time difference follows the evolution of energy states of the system and the changing nature of the orbitals of the system on the timescale of the atomic motion. Examples include the photodissociation of I2&#8722; and other interhalogen species6,7,8,9, the fragmentation and/or electron accommodation in I&#8722;&#183;uracil10,11,12,13, I&#8722;&#183;thymine13,14, I&#8722;&#183;adenine15, I&#8722;&#183;nitromethane16,17 and I&#8722;&#183;acetonitrile17 cluster anions and the revelation of the hitherto unexpectedly long timescale for the production of Cu&#8722; atomic anions after the photoexcitation of CuO2&#8722;18.
Figure 1 shows the Washington University in St. Louis (WUSTL) anion photoelectron imaging spectrometer19. The instrument consists of three differentially pumped regions. Ions are produced in the source chamber which operates at a pressure of 10&#8722;5 Torr and contains a discharge ion source20, and electrostatic ion extraction plate. Ions are separated by mass in a Wiley-McLaren TOF-MS21 (the pressure in the TOF-Tube is 10&#8722;8 Torr). Ion detection and probing takes place in the detection region (pressure of 10&#8722;9 Torr) which contains a VMI lens5 and a charged particle detector. The main components of the instrument are schematically illustrated in Figure 1b where the shaded region represents all the elements contained within the vacuum system. Gas is introduced through the pulsed nozzle into the discharge. To offset the high inlet pressure, the source chamber is maintained under vacuum using an oil-based diffusion pump. The discharge region is illustrated in more detail in Figure 2a. A high potential difference is applied between the electrodes, which are insulated from the face of the nozzle by a series of Teflon spacers. In fact, the Teflon acts as the source of fluorine atoms for the results shown later.
The discharge produces a mixture of anions, cations and neutral species. The ion extraction plate, ion acceleration stack, potential switch and microchannel plate (MCP) detector (Figure 1b) form the 2 m long Wiley McLaren TOF-MS. Ions are extracted by the application of a (negative) voltage pulse to the ion extraction plate and then all ions are accelerated to the same kinetic energy. Variation of the extraction pulse magnitude focuses the arrival time in the VMI lens while the einzel lens reduces the spatial cross section of the ion beam. Anions are re-referenced to ground using a potential switch22, the timing of which acts as a mass discriminator. Anion selection is achieved by synchronizing the arrival of a visible/near uv photon pulse with the arrival time of the anion in the VMI lens. The ion separation and detection regions use oil free turbopumps to protect the imaging detector.
Anions and photons interact to produce photoelectrons throughout the spatial volume of the Steinmetz solid, representing the overlap between the ion and laser beams. The VMI lens (Figure 2b) consists of three open electrodes, the purpose of which is to ensure that all photoelectrons reach the detector and that the momentum space distribution of the photoelectrons is maintained. To achieve this, different voltages are applied to the extractor and repeller such that, regardless of the spatial point of origin, electrons with the same initial velocity vector are detected at the same point on the detector. The detector consists of a set of chevron-matched MCPs which act as electron multipliers. Each channel has a diameter on the order of a few microns, localizing the gain and preserving the initial impact position. A phosphor screen behind the MCPs indicates the position via the amplified electron pulse as a flash of light which is recorded using a charge coupled device (CCD) camera.
The timing and duration of the various voltage pulses required are controlled using a pair of digital delay generators (DDG, Figure 3). The whole experiment is repeated on a shot by shot basis with a repetition rate of 10 Hz. For each shot, several ions and photons interact producing a few detection events per camera frame. Several thousand frames are accumulated into an image. The image center represents the momentum space origin and hence the distance from the center (r) is proportional to the speed of an electron. Angle &#952;, (relative to the photon polarization direction) represents the direction of an electron&#39;s velocity. An image contains the distribution of detection event densities. Thus, it can also be viewed as representing the probability density for detection (at a given point) of an electron. Invoking the Born interpretation of the wave function (&#968;) an image represents |&#968;|2 for the photoelectron23.
The 3D electron probability density is cylindrically symmetric about the polarization of the electric vector (&#949;p) of the radiation with consequent scrambling of information. Reconstruction of the original distribution is achieved mathematically24,25,26,27. The radial distribution (of electrons) in the reconstruction is the momentum (velocity) domain photoelectron spectrum which is converted into the energy domain via application of the appropriate Jacobian transformation.
The anion photoelectron imaging spectrometer (Figure 1) used in these experiments is a custom-built instrument28. The settings in Table 1 and Table 2 for the protocol are specific to this instrument for the production of F&#8722; and imaging of its photoelectron distribution. Several similar versions of the design are used in various research laboratories6,29,30,31,32,33,34,35,36,37,38,39,40,41,42, but no two instruments are exactly alike. Additionally, instrument settings are strongly interdependent and highly sensitive to small changes in conditions and instrument dimensions.

<table border="0" cellpadding="0" cellspacing="0" width="746">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Digital Delay Generators</td>
			<td style="width:244px;">Berkeley Nucleonics Corp.</td>
			<td style="width:119px;">565-8c</td>
			<td style="width:167px;">DDG1</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Digital Delay Generators</td>
			<td style="width:244px;">Berkeley Nucleonics Corp.</td>
			<td style="width:119px;">577-8c</td>
			<td>DDG2</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">HV Power Supplies</td>
			<td style="width:244px;">Stanford Research Systems</td>
			<td style="width:119px;">PS325</td>
			<td>V3</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">HV Power Supplies</td>
			<td style="width:244px;">Stanford Research Systems</td>
			<td style="width:119px;">PS325</td>
			<td>V2</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">HV Power Supplies</td>
			<td style="width:244px;">Stanford Research Systems</td>
			<td style="width:119px;">PS325</td>
			<td>V5</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">HV Power Supplies</td>
			<td style="width:244px;">Burle Inc.</td>
			<td style="width:119px;">PF1053</td>
			<td>V9</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">HV Power Supplies</td>
			<td style="width:244px;">Burle Inc.</td>
			<td style="width:119px;">PF1053</td>
			<td>V4</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">HV Power Supplies</td>
			<td style="width:244px;">Burle Inc.</td>
			<td style="width:119px;">PF1053</td>
			<td>V10</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">HV Power Supplies</td>
			<td style="width:244px;">Burle Inc.</td>
			<td style="width:119px;">PF1054</td>
			<td>V9,V11</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">HV Power Supplies</td>
			<td style="width:244px;">Bertan</td>
			<td style="width:119px;">205B-05R</td>
			<td>V6</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">HV Pulsers</td>
			<td style="width:244px;">Directed Energy Inc.</td>
			<td style="width:119px;">PVX-4150</td>
			<td>V2</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">HV Pulsers</td>
			<td style="width:244px;">Directed Energy Inc.</td>
			<td style="width:119px;">PVX-4140</td>
			<td>V1</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">HV Pulsers</td>
			<td style="width:244px;">Directed Energy Inc.</td>
			<td style="width:119px;">PVX-4140</td>
			<td>V11</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">HV Pulsers</td>
			<td style="width:244px;">Directed Energy Inc.</td>
			<td style="width:119px;">PVX-4140</td>
			<td>V3</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Pulsed Nozzle Driver</td>
			<td style="width:244px;">Parker Hannifin (General Valve)</td>
			<td style="width:119px;">Iota-One</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Pulsed Nozzle</td>
			<td style="width:244px;">Parker Hannifin (General Valve)</td>
			<td style="width:119px;">Series 9</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Camera</td>
			<td style="width:244px;">Imperx</td>
			<td style="width:119px;">VGA120</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Imaging Detector</td>
			<td style="width:244px;">Beam Imaging Systems</td>
			<td style="width:119px;">BOS40</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Oscilloscope</td>
			<td style="width:244px;">LeCroy</td>
			<td style="width:119px;">Wavejet 334</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Photodiode</td>
			<td style="width:244px;">ThorLabs</td>
			<td style="width:119px;">DET10A</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Diffusion Pump</td>
			<td style="width:244px;">Leybold</td>
			<td style="width:119px;">DIP 8000</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">2&#215;Turbo Pump</td>
			<td style="width:244px;">Leybold</td>
			<td style="width:119px;">TMP361</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Rotary Pump</td>
			<td style="width:244px;">Leybold</td>
			<td style="width:119px;">D40B</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">2&#215;Rotary Pump</td>
			<td style="width:244px;">Leybold</td>
			<td style="width:119px;">D16B</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Oxygen Gas</td>
			<td>Praxair</td>
			<td>OX 5.0RS</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Tunable Laser</td>
			<td style="width:244px;">Spectra Physics Sirah Dye Laser</td>
			<td style="width:119px;">Cobra-Stretch</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Pump laser for Dye Laser</td>
			<td style="width:244px;">Sepctra Physics Nd:YAG</td>
			<td style="width:119px;">INDI-10</td>
			<td></td>
		</tr>
	</tbody>
</table>

photoelectron imaging of anions illustrated by 310 nm detachment of f&#8722;

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound electrons with neutral molecules/atoms. State-of-the-art in vacuo anion generation techniques allow application to a broad range of atomic, molecular, and cluster anion systems. These are separated and selected using time-of-flight mass spectrometry. Electrons are removed by linearly polarized photons (photo detachment) using table-top laser sources which provide ready access to excitation energies from the infra-red to the near ultraviolet. Detecting the photoelectrons with a velocity mapped imaging lens and position sensitive detector means that, in principle, every photoelectron reaches the detector and the detection efficiency is uniform for all kinetic energies. Photoelectron spectra extracted from the images via mathematical reconstruction using an inverse Abel transformation reveal details of the anion internal energy state distribution and the resultant neutral energy states. At low electron kinetic energy, typical resolution is sufficient to reveal energy level differences on the order of a few millielectron-volts, i.e., different vibrational levels for molecular species or spin-orbit splitting in atoms. Photoelectron angular distributions extracted from the inverse Abel transformation represent the signatures of the bound electron orbital, allowing more detailed probing of electronic structure. The spectra and angular distributions also encode details of the interactions between the outgoing electron and the residual neutral species subsequent to excitation. The technique is illustrated by the application to an atomic anion (F&#8722;), but it can also be applied to the measurement of molecular anion spectroscopy, the study of low lying anion resonances (as an alternative to scattering experiments) and femtosecond (fs) time resolved studies of the dynamic evolution of anions.

By centroiding43 the data recorded on the 640&#215;480 pixel CCD array of the camera, a grid resolution of 6400&#215;4800 is possible. However, extraction of the spectra and angular distributions involves inverse Abel transformation of the data which requires the image intensity to vary relatively smoothly. As a compromise, the centroided data is &#34;binned&#34; by summing n&#215;n blocks of points. Similar treatment is also necessary for the display of imaging re...

Watch this Scientific Journal Video about Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of FÃ¢Ë†' at JoVE.com

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of FÃƒÆ’Ã‚Â¢Ãƒâ€¹Ã¢â‚¬Â '

Here, we present a protocol for photoelectron imaging of anionic species. Anions generated in vacuo and separated by mass spectrometry are probed using velocity mapped photoelectron imaging, providing details of anion and neutral energy levels, anion and neutral structure and the nature of the anion electronic state.

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F&#8722;

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound ...

Education

Research

JoVE Core

JoVE Journal

Physics

Chemistry

Unit 1

Periodic Properties of the Elements

Work and Kinetic Energy

SCIENTISTS IN ACTION

Formation of Ordered Biomolecular Structures by the Self-assembly of Short Peptides

In nature, complex functional structures are formed by the self-assembly of biomolecules under mild conditions. Understanding the forces that control ...

Confocal Imaging of Confined Quiescent and Flowing Colloid-polymer Mixtures

The behavior of confined colloidal suspensions with attractive interparticle interactions is critical to the rational design of materials for directed ...

Synthesis of Immunotargeted Magneto-plasmonic Nanoclusters

Magnetic and plasmonic properties combined in a single nanoparticle provide a synergy that is advantageous in a number of biomedical applications ...

Preparing Adherent Cells for X-ray Fluorescence Imaging by Chemical Fixation

X-ray fluorescence imaging allows us to non-destructively measure the spatial distribution and concentration of multiple elements simultaneously over ...

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb ...

Regioselective O-Glycosylation of Nucleosides via the Temporary 2',3'-Diol Protection by a Boronic Ester for the Synthesis of Disaccharide Nucleosides

Regioselective O-Glycosylation of Nucleosides via the Temporary 2',3'-Diol Protection by a Boronic Ester for the Synthesis of Disaccharide Nucleosides

Disaccharide nucleosides, which consist of disaccharide and nucleobase moieties, have been known as a valuable group of natural products having ...

Automation of a Positron-emission Tomography (PET) Radiotracer Synthesis Protocol for Clinical Production

Automation of a Positron-emission Tomography PET Radiotracer Synthesis Protocol for Clinical Production

The development of new positron-emission tomography (PET) tracers is enabling researchers and clinicians to image an increasingly wide array of biological ...

A Rhodopsin Transport Assay by High-Content Imaging Analysis

Rhodopsin misfolding mutations lead to rod photoreceptor death that is manifested as autosomal dominant retinitis pigmentosa (RP), a progressive blinding ...

Technical Aspect of the Automated Synthesis and Real-Time Kinetic Evaluation of [11C]SNAP-7941

Technical Aspect of the Automated Synthesis and Real-Time Kinetic Evaluation of [11C]SNAP-7941

Positron emission tomography (PET) is an essential molecular imaging technique providing insights into pathways and using specific targeted radioligands ...

Obtention of Giant Unilamellar Hybrid Vesicles by Electroformation and Measurement of their Mechanical Properties by Micropipette Aspiration

Giant vesicles obtained from phospholipids and copolymers can be exploited in different applications: controlled and targeted drug delivery, biomolecular ...