3.3
Ion beam formation and mass separation
Once the ions have been created in the discharge, the resulting soup of ions needs to be transformed into a stable pulsed beam, so that the ion of interest may be mass isolated for photodetachment. First, the ions are drawn through the skimmer, which separates the source chamber from the other vacuum chambers. The skimmer has a flat conical shape with a 2mm aperture diameter, and directs the ions from the coldest part of the gas expansion into a well defined beam, with low divergence. It also allows differential pumping between the two chambers. Behind the skimmer are a series of 3-element electrostatic lenses, which focus the ion beam, while accelerating it to a beam energy of 500 eV. This provides a collimated, constant energy beam, which may be used for mass separation.
3.3.1 Simultaneous gating, bunching, and re-referencing
To prepare the above ion beam for mass separation, three transformations need to be applied. First, the beam must be gated to from discrete ion packets before it can enter a mass spectrometer[57]. Even though the ANU employs a pulsed source, the duration of the pulse is longer then the desired ion packet duration, so a method to extract only a short packet of ions from the longer pulse is required. Secondly, it is desirable to re-reference the ion beam to ground, in order to simplify the transport optics along the remainder of the beamline, as otherwise successively higher lens voltages will be required. This is easily achieved by using a potential switch, whereby a cylinder along the path is rapidly changed from 500V to 0V so that the ions inside the cylinder during the switch will exit at ground potential, but with unchanged kinetic energy[19].
The third requirement arises from the low photodetachment count rates that have proved problematic in previous spectrometers. Maximising the photoelectron count rate is especially important for difficult to measure species, such as vinylidene, where the total ion count rate from the source is relatively low or unstable. One way to address this is to introduce axial bunching, where a long gated ion packet (80mm) may be bunched to match the dimensions of the laser beam (2mm), resulting in an increase in the ion density at the interaction region by a factor of 40. This is achieved by applying a variable voltage across the ion packet, essentially providing a kick voltage to the ions at the end of the packet, so that they catch up to the front of the ion packet by the time they reach the interaction region. Because the ANU spectrometer is in-line, introducing an axial velocity spread will not effect the VMI resolution, as only off-axial components are mapped into the image.
Previous spectrometers have used three separate electrostatic devices to achieve each of these beam transformations, however the HR-PEI spectrometer at the ANU employs one single device to simultaneously accomplish each of these tasks. A schematic diagram of the novel gating/bunching/re-referencing unit employed in the ANU spectrometer, is shown in Fig. 3.9. Ions from the foreoptics enter the unit from the left at +500V, where a fast MOSFET (metal-oxide-semi-conductor field-effect transistor) switch switches the cylinder potential (at 30 Hz) from +500 V to +0 V, re-referencing the ions inside the cylinder to ground[58]. This simultaneously gates the ions into a discrete 80 mm long packet, with ions outside of the cylinder deflected. Instead of using a continuous cylinder, a series of closely spaced aluminium rings are used, intersected with insulating macor spacers. These rings are biased through a resistor chain, producing a uniform axial electric field. In order to bunch the ion packet, when the front of the cylinder (A) is switched to ground, the
back of the cylinder (B) is switched to a variable kick voltage−Vbunch. This produces and additional voltage gradient, accelerating the ions at the back of the packet so that by the time they reach the interaction region, they have caught up to the ions at the front. This bunches the 80 mm packet to 2 mm, increasing the ion density by a factor of 40. The kick voltage required to bunch an ion packet of length` over a drift lengthL is given by,
∆E = 2E`
L , (3.2)
where E is the energy of the beam, and ∆E is the kick energy. Therefore, bunching an 80 mm packet at 500 eV over a ∼2 m flight tube requires a kick voltage of∼40 V.
Fast MOSFET switch
+500V +500V
+0V
80mm
Figure 3.9: Gating/bunching/re-referencing unit employed in the ANU spectrometer. Ions from the foreoptics enter the unit from the left at +500V, where a fast MOSFET switch switches the unit potential from +500V to +0V, re-referencing the ions inside the cylinder to ground. A series of closely spaced aluminium rings, intersected with insulating macor spacers, are biased through a resistor chain, producing a uniform axial electric field. When the front ring is switched to ground, the back of the cylinder switches to a kick voltage
−Vbunch introducing a voltage gradient to bunch the ion packet[58,59].
3.3.2 Time-of-flight mass spectroscopy
A time-of-flight (TOF) mass spectrometer may be easily implemented into the HR-PEI spectrometer due to the pulsed, in-line nature of the apparatus. Mass separation enables the target ion of interest to be selected from the packet of ions, resulting in a pure, one- species measurement at the interaction region. After the gating/bunching/re-referencing unit the ions, with a kinetic energy of 500 eV and 0 V potential, enter a 2 m long high vacuum drift tube. Two Heddle pairs of Einzel lenses transport the ions down the flight tube, with the first pair of Einzel lenses focusing the ion packet through an aperture which defines the packet size at the interaction region, while the second pair of Einzel lenses focuses the ion packet through a second aperture to define the beam divergence[60]. As the ions all have the same kinetic energy, heavier ions will have a longer flight time, as given by t= L √ m √ 2E, (3.3)
where m is the mass of the ion, E the beam energy (500 eV), and L the flight distance (2 m). A retractable 18 mm diameter micro channel plate (MCP) can be lowered into the