To try and replicate the O− + C2H4 → H2CC− + H2O pathway which has provided successful vinylidene production in past studies, a dual jet source was developed and trailed in this work, based on the design of Lineberger[56]. However as highlighted above, the fragility of H2CC−production ensures that developing an optimised source is no easy task.
Initially, a second Parker pulsed jet valve was installed in the source chamber, per- pendicular to the primary jet, as shown in Fig. 3.4. This new jet was connected to a secondary gas line, allowing for different gas mixtures and pressures to be used in each gas line, to try and produce different ions in cross beam expansion. However, despite extensive testing with various gas and experimental parameters, this simple dual source design was not able to improve on the single jet performance. In fact, it was found that adding the secondary jet dramatically increased the ion source temperature, decreasing the ion yield, limiting the spectral quality. This is likely due to collision effects between the perpendicular expansions. In usual operation, the skimmer is positioned inside the cone of silence of the gas expansion so as to only sample from the cold, directional part of the flow. However, when two expansions intersect, this changes the shape of the flow and collapses the cone of silence. Consequently, the skimmer samples hotter, less directional ions with non-zero perpendicular velocities. The result of this can be seen in Fig. 3.4 where O−2 photoelectron spectra at 1064 nm, measured using a single jet and a dual jet source, are shown for comparison. The dual jet source produced hotter ions (T=528K cf. 212K), demonstrated by the presence of hot bands, and the differing ratio of the fine structure components in the dual jet spectrum.
Primary gas line
4-pin discharge
Pulsed jet 1 Farady cup
Skimmer
Pulsed jet 2 Secondary gas line
(a) First iteration of a dual jet source. The primary gas line enters the chamber from the left, and the gas expands through a 4 pin discharge. The primary expansion can be crossed by a secondary expansion using a sec- ond jet, perpendicular to the first. Different gases may be used in each gas line in an at- tempt to produce the desired target ion.
0 2000 4000 6000 8000 10000 Binding Energy (cm 1) 0.0 0.2 0.4 0.6 0.8 1.0 Normalized Intensity (0, 0) (1, 0) (2, 0) (3, 0) (1, 1) (0, 1) (0, 2) (0, 3)
Ion Source O
2Temperatures
Dual jet:T=528K Single jet:T=212K
(b) Photoelectron spectrum of O−2 at 1064 nm
measured using a single jet source (−) and a dual jet source (−). The dual jet source has a much higher temperature, resulting in more intense hot bands.
Figure 3.4: Design and performance of the initial dual jet source. The cross beam expan- sion introduces turbulence to the flow, destroying the cone of silence and resulting in a hotter source temperature.
To overcome these issues, the Lineberger[56] source design configuration was imple- mented, as shown in Fig. 3.5. One of the major limitations in the first source iteration was the lack of x,y,z adjustment for the secondary pulse-valve jet. By mounting the second valve directly to the first valve, correct transverse (x) alignment could be assured, while sliding poles created mobility in the vertical (y) and longitudinal (z) directions. The size of the pulse-valve front flange was also reduced, so that the primary pulse-valve could be moved closer to the skimmer. Furthermore, the specifications for the discharge and pulse-valve jet faces were matched to the values of Lineberger’s design, in order to recre- ate the results from their lab. This involved moving from a 4-pin discharge on the primary (horizontal) jet to a two plate discharge on the secondary (vertical), where a thin Teflon insulator separates 2 stainless steel disks with a small hole (for the gas expansion) in the centre. A small 1mm aperture is machined into the jet/discharge interface in order to improve the adiabatic cooling, including a 40◦ conical nozzle to improve the directionality of the expansion. The pulse-valve front flange on the primary valve is also modified to include a 40◦ conical nozzle and an even smaller 0.5mm aperture.
Primary gas line
New pulsed jet face
Skimmer
Pulsed jet 1: 0.5mm aperture
Secondary gas line
z-translation y-translation Discharge plate Teflon insulator Ground plate Pulsed jet 2: 1mm aperture
Figure 3.5: Modified dual jet source, involving a two-plate discharge based on the Lineberger[56] design. Custom jet faces were machined, with 1mm/0.5mm apertures and a 40◦ conical nozzle, to improve the adiabatic cooling of the gas during the expansion.
With this arrangement, the source conditions of Linberger’s laboratory in Colorado should be able to be replicated exactly within the ANU spectrometer. However this still failed to produce a stable vinylidene anion yield. As such, various other gas combinations were tried, with the resulting mass spectra shown in Fig. 3.5. The best results were achieved with a mixture of ethylene, oxygen, and argon in the discharge line, where the argon is used as a buffer gas to assist with cooling. Yet, the vinylidene ion signal produced was still insufficient for a photoelectron signal to be measured with a low signal to noise ratio, while the ions that were produced were still hotter then ions produced in a single jet source. Surprisingly, a large fluoride ion signal was also present in the mass spectra in Fig. 3.6. This was traced to the Teflon insulator, which was replaced with a Macor disc to remove the F− signal.
Modification of the source continued, with some of the designs shown in Fig. 3.7. First, the original 4-pin discharge was temporarily mounted to the secondary (vertical) pulsed jet, so that it’s performance could be tested against the 2-plate design from Fig. 3.5. The 4-pin discharge was found to produce higher ion counts, however issues arose with gas leaking between the jet and discharge chamber, while a large fluoride signal dominated the mass spectrum due to the Teflon faceplate, used for mounting. Therefore, a new
§3.2 Ion production 35 15 20 25 30 35 40 Mass (amu) Ion Signal O OH F C2 C2H H2CC O2
Primary: 50%O2, 50%Ar
Secondary:10%C2H4, 90%Ar
Primary: 50%O2, 50%Ar
Secondary:50%C2H4, 50%Ar
Primary: 50%O2, 50%Ar
Secondary:100%C2H4
Primary: 100%O2
Secondary:10%C2H4, 90%Ar
Primary: 100%Ar
Secondary:10%C2H4, 50%O2, 40%Ar
Primary: 10%C2H4, 50%O2, 40%Ar
Secondary:10%C2H4, 50%O2, 40%Ar
Figure 3.6: Mass spectra from different gas mixtures using a dual valve source. An ethy- lene/oxygen mixture in the discharge line proved most successful at producing vinylidene, but still could not achieve high ion yields.
purpose-designed 4-pin chamber was machined out of Macor for a sealed fit to the gas jet. Next, 0.3mm and 0.5mm aperture masks were introduced between then discharge and jet to investigate the effect this may have on the adiabatic cooling. O−2 spectra measured with the mask in (0.3mm/0.5mm) vs mask out (1mm) confirmed that a smaller aperture did lead to a significant increase in the gas cooling. Finally, a new 4-pin discharge chamber was also produced to mount on the primary (horizontal) jet valve. This chamber featured a 0.3mm aperture and 4mm expansion chamber for efficient cooling, within a hemispherical disc shape, with the bottom cutaway in order to allow the two pulsed jets to be placed close together to achieve optimal expansion overlap. By having discharges on both the primary and secondary jets, the effect of having a vertical vs horizontal discharge could also be examined. Discharge Pins 0.5mm aperture Teflon face plate Original 4-pin discharge
(a) The original 4-pin dis- charge source was temporar- ily mounted to the secondary pulsed jet, in order to test the performance of a 4-pin vs 2- plate discharge. Discharge Pins 0.3mm aperture mask Macor Insulator 4mm expansion chamber
(b) A new 4-pin discharge de- signed for the secondary jet was machined using Macor. A 0.3mm aperture mask sits between the jet/Macor faces to improve adiabatic cooling.
Discharge Pins 0.3mm
aperture
Macor insulator
(c) A new 4 pin discharge was also machined for the primary jet, with a 0.3mm aperture and 4mm expansion chamber to help improve the adiabatic cooling.
Optimal conditions were found with the 4-pin discharge on the primary (horizontal) pulsed-valve jet firing, with a buffer gas expansion out of the secondary jet to help with ion cooling. The jet-to-jet and jet-to-skimmer proximities also proved important, with best results obtained when the skimmer is aligned with the expansion beam cross over point, as shown in Fig. 3.8. It is this configuration that lead to the vinylidene results that are presented in chapters 8-11.
From this study, a few key requirements for a optimum ion source can be defined,
The 4-pin discharge out performs a 2-plate design.
Having the main discharge in-line with the beam (horizontal jet) ensures the cold- est ions are selected by the skimmer and reduces the transverse velocity, which is important for high resolution VMI.
With a cross beam expansion, it is important for the skimmer to be close to both jet openings to ensure it samples from the coldest part of the beam.
Teflon is not a good choice of insulating material, as it may result in a large fluoride ion signal.
The discharge is controlled by a pair of home-built power supplies, one of which is held at constant voltage, while the second adds an additional high voltage pulse which is responsible for ionisation. Under typical operating conditions, the constant voltage is held at 800-1200V (depending on the target ion) with an additional pulse voltage of up to 1600V. The width of the discharge can also be varied, from ∼ 10−100µs, while the shape of the discharge can be modified using an inbuilt variable resistor. The discharge and pulsed jets operate at 30Hz, with the jet pulse and discharge fire times controlled by a BNC digital delay generator (DDG). The jet opening lengths are varied using a Parker valve driver, which can also add a delay between the primary and secondary jet times.
Primary gas line Skimmer 4-pin discharge 0.3mm aperture Secondary gas line 0.3mm aperture mask Cutaway, for closer jet proximity
Figure 3.8: Final dual-jet source design. A 4 pin discharge, with an 0.3mm aper- ture, is mounted to the primary (horizontal) jet. The main expansion is crossed by a buffer/reactive gas expansion from the secondary (vertical) jet. The proximity of the jets to each other and to the skimmer is important in ensuring a cold, directional, ion beam is obtained.