Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F−

Summary

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.

Abstract

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⁻), 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.

Introduction

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 imaging^2,3,4 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⁻ and other interhalogen species^6,7,8,9, the fragmentation and/or electron accommodation in I⁻·uracil^10,11,12,13, I⁻·thymine^13,14, I⁻·adenine¹⁵, I⁻·nitromethane^16,17 and I⁻·acetonitrile¹⁷ cluster anions and the revelation of the hitherto unexpectedly long timescale for the production of Cu⁻ atomic anions after the photoexcitation of CuO₂⁻¹⁸.

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 10⁻⁵ 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 10⁻⁸ Torr). Ion detection and probing takes place in the detection region (pressure of 10⁻⁹ 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 θ, (relative to the photon polarization direction) represents the direction of an electron'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 (ψ) an image represents |ψ|² for the photoelectron²³.

The 3D electron probability density is cylindrically symmetric about the polarization of the electric vector (ε_p) of the radiation with consequent scrambling of information. Reconstruction of the original distribution is achieved mathematically^24,25,26,27. 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⁻ and imaging of its photoelectron distribution. Several similar versions of the design are used in various research laboratories^{6,29,30,31,32,33,34,35,36,37,38,39,40,41,42}, but no two instruments are exactly alike. Additionally, instrument settings are strongly interdependent and highly sensitive to small changes in conditions and instrument dimensions.

Protocol

NOTE: A general experimental protocol is presented here, specific to the WUSTL instrument. Specific instrument settings for the F⁻ image presented in Figure 4a can be found in Table 1-2.

1. Ion Generation

1. To generate anions, apply a backing gas or gas mixture (for F⁻, 40 psig. of O₂) behind the pulsed nozzle and operate the nozzle at 10 Hz.
   1. Set the nozzle duration on digital delay generator 1 (DDG1), channel A (A1) and trigger the pulsed nozzle driver to inject the gas into discharge.
   2. Apply a high voltage discharge pulse V1. The timing and duration of the pulse are controlled by channel C (C1) on DDG1.
   3. As the escape of O₂ gas can lead to increased laboratory fire risk, ensure that all gas lines are leak tight. Since high gas pressures can lead to failure of gas lines, ensure that the pressure is kept below maximum operating pressure. Ensure that power supplies are properly grounded and switched off when cables are being attached or removed.

2. Ion Extraction, Separation and Detection

1. To extract anions from the source, apply a high voltage extraction pulse (V2) to the ion extraction plate.
   1. Set the timing and duration of the ion extraction pulse using DDG1 channel D (D1).
2. To monitor the anion mass spectrum, put the instrument into ion mode.
   1. Connect the detector voltage divider to the imaging detector MCPs.
   2. Apply voltage V11 to the detector anode (phosphor screen).
   3. Connect the ion detector voltage divider output to the oscilloscope channel 1 input.
   4. Connect the MCP power supply to the voltage divider input and gradually increase voltage. An input voltage V9 provides V7 to the entry side and V8 to the exit side of the MCP.
      CAUTION: Do not exceed maximum allowable voltage for MCPs.
3. Separate the anions by TOF-MS.
   1. Set the acceleration stack voltage V3.
   2. Using DDG1 channel E (E1), set the timing and duration for the potential switch high voltage pulse (V3).
   3. Externally trigger the oscilloscope from DDG1 channel F (F1) to set the TOF-MS time scale.
4. Adjust the discharge and extraction pulse magnitudes (V1-V2), discharge, extraction, potential switch and nozzle timing and duration through channels A-E on DDG1 to produce ion signal on the oscilloscope.

3. Ion Yield and Resolution Optimization.

NOTE: Steps 3.1 and 3.2 should be repeated iteratively to obtain the optimum resolution and ion yield. (Tables 1-2 show the settings used to generate the F⁻ image shown in the results section).

1. To optimize the number of anions of a given species, adjust the ion source settings.
   1. Adjust the pressure of O₂ gas behind the nozzle using the regulator on the gas cylinder.
   2. Adjust the pulsed nozzle duration of operation (A1).
   3. Adjust the magnitude of the discharge pulse voltage (V1).
   4. Adjust the timing and duration of the discharge pulse voltage (C1).
   5. Adjust the timing and duration of the ion extraction pulse (D1).
   6. Adjust the duration the potential switch is at high voltage (E1).
   7. Adjust the voltage on central element of the einzel lens (V4). The ion peaks on the oscilloscope should increase in intensity.
      CAUTION: Ensure O₂ pressure is kept below maximum operating pressure.
2. Adjust the TOF-MS settings to optimize the mass spectral resolution and ion separation
   1. Adjust the ion extraction voltage (V2) to achieve Wiley-McLaren focusing. The ion peaks on the oscilloscope should narrow.
   2. Adjust the acceleration stack voltage V3.

4. Photoelectron Production and Detection

1. Switch the spectrometer to the imaging mode.
   1. Reduce the voltage applied to the ion detector voltage divider (V9) to zero.
   2. Disconnect the ion detector voltage divider from the MCPs.
   3. Connect the MCP and imaging power supplies to the imaging high voltage pulse.
   4. Connect the imaging high voltage pulse to the imaging MCPs
2. Apply a permanent voltage to phosphor screen (V11) and MCPs (V9).
3. Synchronize the arrival time of laser pulses from the nanosecond (ns) dye laser with the arrival time of the ion of interest within the VMI lens.
   1. Connect the fast photodiode to oscilloscope channel 2.
   2. Externally trigger the Nd:YAG laser flash lamps and Q switch using DDG2 channels H (H2) and G (G2). Adjust the timing of the laser trigger (H2) until the photodiode output is close to but preceding the ion signal of interest.
   3. Apply voltage to the imaging repeller (V5) and extractor (V6) electrodes.
   4. Set the camera to long exposure and adjust the laser trigger timing (H2) to maximize the number of electron detection events observed on the PC screen.
      CAUTION: Class IV laser radiation will permanently damage eyesight. Wear appropriate eye protection. Do not look directly into the beam even when wearing eye protection. Avoid specular reflections.
4. Apply a high voltage pulse to the MCP timed to coincide with arrival of the photon pulse to amplify electron signal within the photoelectron production window.
   1. Set the imaging pulse voltage (V10).
   2. Set the imaging pulse timing and duration using DDG2 channel F (F2) such that the imaging pulse is centered on the arrival time of the photon pulse.

5. Image Focusing

1. Set the camera to short exposure.
   1. Trigger the CCD camera to open at the start of an experimental cycle using DDG2 channel E (E2).
2. Collect a background-subtracted image
   1. Collect several frames with the laser pulse coincident with the anion of interest.
   2. Collect several frames with the laser pulse not coincident with any anion.
   3. Subtract the frames collected off coincidence from the frames collected on coincidence.
   4. Repeat step 5.2 and accumulate an image.
3. Adjust the imaging repeller (V5) and extraction (V6) electrode voltages. Generate a new image by repeating step 5.2. The best focusing condition is achieved when the image features are at their narrowest.

6. Image Collection

1. With the camera in short exposure mode, switch to centroided collection.
2. Repeat step 5.2 at the optimum focusing condition to accumulate a sub-pixel resolution image.

7. Data Extraction

NOTE: The data manipulations performed in this section are performed using specifically written programs in the MatLab platform.

1. Locate the center of the image by determining the center of mass (intensity) of the image, using the inherent symmetry of the image to find the center of inversion, or (in the case of low signal to noise) iteratively minimizing the width of the transitions in the spectrum by selecting different trial centers.
   1. Inverse Abel transform the image to recover the 3D velocity distribution.
2. Generate photoelectron spectra
   1. Integrate the intensity as a function of angle for all radii (this is the spectrum in the radial and hence momentum or velocity domain). In practice this is acheived by summation over all radii.
      
      where I(r) is the radial intensity and I(r,θ) is the intensity at point r, θ.
   2. Calibrate the spectrum for electron kinetic energy by comparison with an image recorded under the same conditions with transitions of known eBE.
      eKE = eKEcal×(r/r_cal)²
      where eKE_ref is the kinetic energy of a known transition in the reference spectrum, r_ref is the radius of the ring in the reference image corresponding to this transition and eKE is the kinetic energy associated with radius r in the experimental image.
   3. Convert the radial spectrum to the energy domain via Jacobian transformation. The energy corresponding to a given r is determined as in 7.2.2. The intensity I(r) is divided by √eKE.
3. Angular Distribution of electrons.
   1. Select a transition in the spectrum.
   2. For different small angular ranges, integrate over the radial range associated with the transition and plot against θ. In practice the integration is acheived by summation over all radii in the range r_{0 -}FWHM/2 to +FWHM/2.
      
      where I(θ) is the angular intensity, r0 is the radial value of the transition maximum and FWHM is the full width at half maximum across the radial range of the transition.

Results

By centroiding⁴³ the data recorded on the 640×480 pixel CCD array of the camera, a grid resolution of 6400×4800 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 "binned" by summing n×n blocks of points. Similar treatment is also necessary for the display of imaging re...

Discussion

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 ...

Materials

Name	Company	Catalog Number	Comments
Digital Delay Generators	Berkeley Nucleonics Corp.	565-8c	DDG1
Digital Delay Generators	Berkeley Nucleonics Corp.	577-8c	DDG2
HV Power Supplies	Stanford Research Systems	PS325	V3
HV Power Supplies	Stanford Research Systems	PS325	V2
HV Power Supplies	Stanford Research Systems	PS325	V5
HV Power Supplies	Burle Inc.	PF1053	V9
HV Power Supplies	Burle Inc.	PF1053	V4
HV Power Supplies	Burle Inc.	PF1053	V10
HV Power Supplies	Burle Inc.	PF1054	V9,V11
HV Power Supplies	Bertan	205B-05R	V6
HV Pulsers	Directed Energy Inc.	PVX-4150	V2
HV Pulsers	Directed Energy Inc.	PVX-4140	V1
HV Pulsers	Directed Energy Inc.	PVX-4140	V11
HV Pulsers	Directed Energy Inc.	PVX-4140	V3
Pulsed Nozzle Driver	Parker Hannifin (General Valve)	Iota-One
Pulsed Nozzle	Parker Hannifin (General Valve)	Series 9
Camera	Imperx	VGA120
Imaging Detector	Beam Imaging Systems	BOS40
Oscilloscope	LeCroy	Wavejet 334
Photodiode	ThorLabs	DET10A
Diffusion Pump	Leybold	DIP 8000
2×Turbo Pump	Leybold	TMP361
Rotary Pump	Leybold	D40B
2×Rotary Pump	Leybold	D16B
Oxygen Gas	Praxair	OX 5.0RS
Tunable Laser	Spectra Physics Sirah Dye Laser	Cobra-Stretch
Pump laser for Dye Laser	Sepctra Physics Nd:YAG	INDI-10