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

This method can help answer key questions in the electron scattering molecular spectroscopy and reaction dynamics field, such as the nature of molecular orbitals, molecular electronic structure, vibrational energy levels, and the nature of scattering and auto-detachment resonances. The main advantage of this technique is that it’s highly efficient, recording an entire photoelectron spectrum and angular distribution in a single image. To begin generating anions, apply a backing gas or gas mixture behind the pulsed nozzle.

Operate the nozzle at 10 hertz. Set the nozzle duration on the digital delay generator Channel A.Then, trigger the pulsed nozzle driver to inject the gas into discharge. Using Channel C on the digital delay generator, apply the high voltage discharge pulse V1.To monitor the anion mass spectrum, first put the instrument into ion mode.

Connect the detector voltage divider into the imaging detector microchannel plates. Apply voltage V11 to the detector anode. After this, connect the ion detector voltage divider output to the oscilloscope channel one input.

Connect the microchannel plate power supply to the voltage divider, and gradually increase the voltage as outlined in Table 2. In order to separate the anions by time of flight mass spectrometry, set the acceleration stack voltage to V3 as outlined in the text protocol. Use Channel E on the digital delay generator to set the timing and duration for the potential switch high voltage pulse.

First, reduce the voltage applied to the ion detector voltage divider to zero. Switch the spectrometer into imaging mode. Then, disconnect the ion detector voltage divider from the microchannel plates.

Connect the microchannel plate power supply and the imaging power supply to the imaging high voltage pulse, and connect the imaging high voltage pulse to the imaging microchannel plates. Turn on the HV pulser. After this, apply a permanent voltage to the phosphor screen and the microchannel plates.

Connect the fast-photo diode to oscilloscope channel two. Using Channels H and G, externally trigger the ak laser flash lamps and key switch. Adjust the timing of the laser trigger until the photodiode output is close to, but preceding, the ion signal of interest.

Next, apply voltage to the imaging repeller and extractor electrodes. Set the camera to long exposure. Using Channel H, adjust the laser trigger timing to maximize the number of electron detection events observed on the PC screen.

Use Channel F to set the pulse timing and duration such that the imaging pulse is centered on the arrival time of the photon pulse. To begin, use Channel E to trigger the charge coupled device camera to open at the start of an experimental cycle. Collect several frames with the laser pulse coincident with the anion of interest.

Then collect several frames with the laser pulse not coincident with any anion. Subtract the frames collected off coincidence from the frames collected on coincidence. Repeat this process of collecting and subtracting frames to accumulate a background subtracted image.

After this, adjust the imaging repeller and extraction electrode voltages. Repeat the frame collection and subtraction process again to generate a new background subtracted image. To begin image collection, switch the camera to centrated collection.

Repeat the frame collection and subtraction process at the optimum focusing condition to accumulate a sub-pixel resolution image. Here is a representative image resulting from the photo detachment at fluoride ions at a photon energy of four electron volts. The left half of this image is the experimentally measured image, while the right half is an inverse able transformation of the data displayed at the same resolution.

The two concentric circles correspond to the two narrow transitions seen in the photoelectron spectrum. The integrated intensities over all angles for each radial distance from the center are scaled by the appropriate Jacobian transformation to generate the photoelectron spectrum. The two transitions reflect the existence of two low lying electronic states of neutral fluorine.

The difference in the transition kinetic energies reveals that the first excited state is just 50 milli electron volts higher than the ground state, the measure of the strength of the spin orbit interaction. The photoelectron angular distributions for each transition show that the electron distribution is polarized perpendicular to the electric vector of the radiation. These angular distributions can be seen to be almost identical when scaled relative to their respective maxima.

The optimal focusing condition is seen to be a ratio of 0.700 between the repeller and extractor. Even small alterations to this ratio are seen to be detrimental to the velocity resolution. The photoelectron imaging technique has paved the way for researchers in the field of chemical reaction dynamics, photoelectron spectroscopy, and electron scattering.

This allows for a researcher to probe the nature of electron wave functions from both stable ions and from dynamic systems, such as the evolution of a chemical reaction from reactants to products. In addition, interaction between electrons and neutral species can be examined, ranging from the simple atom all the way to the more complex molecular clusters. Remember, working with high-pressured gases, high voltages, and Class 4 laser radiation is hazardous, and precautions must be taken.

Be careful to avoid leaks from gas lines and do not exceed recommended pressure ratings. Switch off voltage supplies when exchanging cables. Also, never look directly into the laser beam, and avoid special reflections.

These may result in a permanent loss of vision. Appropriate eye protection is required, but will not protect from looking directly into the laser beam.