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The following information pertains to experiments presently being conducted in the laser collisions lab of the above laboratory. In these experiments atoms and molecules excited by electron impact are studied when they are in a metastable state following the collision. Many different processes can occur in this reaction, depending upon the energetics and kinetics of the reaction. The experiments described here look at the process of inelastic scattering, where the target is left in a metastable state. The experiments are precursors to measuring stepwise laser excitation of metastable atoms and molecules.
An electron which has well defined energy Einc, and momentum Pei is incident on either an atom or a molecular target which is in its ground state.
The velocity of the gas is constrained to a narrow distribution by the supersonically expanding shock wave.
The target beam enters a second vacuum chamber where it travels ~ 100mm prior to interacting with a well defined beam of electrons. These electrons are produced in an electron gun that controls both the direction and energy of the electrons.
Only a very small number of these electrons will interact with the target beam, the majority passing through the interaction region and being collected by a Faraday cup 200mm downstream. The majority of the target beam is also unaffected during the reaction, and passes through the interaction region. The majority undergo an elastic collision where only the direction of the incident electron may change and the target remains in its ground state. A small number of targets will be excited, and depending upon the energy that can be exchanged between the electrons and the target, excitation to various states can occur. Depending upon the cross section for the state, which is a function of both the energy and the angle through which the electron is scattered, a small number of these targets will be excited to the metastable state.
Since these metastable excited targets cannot decay by photon emission, and since there are very few collisions between targets in the gas beam (their relative velocity is close to zero in the beam and therefore their relative position to each other does not change markedly), most of these will remain in the excited state as they pass out of the interaction region.
Scattered Atoms (Molecules) in a Metastable State with internal energy Eexc and which carry momentum Paf away from the reaction. Studying the momentum of the targets that have been deflected by the electron collision, since these are related to the electron momentum by conservation of momentum. It should be noted that in conventional electron scattering experiments which measure excitation cross sections, the target momentum is not usually considered.
There are a number of complications that can arise in these experiments where the target momentum is exploited for measurement. If the incident electron is of sufficient energy to excite not only the metastable state but also higher lying states which through photon emission can decay to the metastable state, there will be a contribution to the target signal due to these cascades. For a significant number of targets there is more than one metastable state, and these may be sufficiently close in excitation energy to be unresolvable when exploiting their individual momentum transfer contributions. This effect is particularly significant when the target is a molecule since the metastable vibrational and rotational states will be much closer together than can be resolved by even the highest resolution electron impact studies.
The difficulties associated with cascading can be overcome by confining the study to the region close to threshold for excitation of the metastable state.
For studies of excitation at energies where contributions from cascading become significant, a time resolved coincidence experiment between the scattered electrons and the deflected targets becomes necessary. Unlike conventional coincidence studies, the direction of the coincident scattered electron is known since the momentum of the excited target is known, and hence the collection rate of the experiments can be enhanced by placing the electron detector at the appropriate point in space.
The difficulties associated with state selectivity can be overcome by employing a further stepwise laser excitation step from the metastable state to an upper state using a high resolution laser probe. Since the laser radiation transfers information about the metastable state to this upper state, measurement of the upper state can in principal yield information about the lower state once the laser excitation mechanism is well understood. Allows the electron excitation mechanism to be studied by analysing the stepwise excited state. Field Ionisation if the laser excited state has sufficiently high principal quantum number (high yields, in principal 100% efficient).
The latter Field Ionisation technique is adopted in these experiments, although photon detection can be easily installed into the vacuum system if this proves advantageous to measurement of the signal. Molecular Vibrational & Rotational states can be studied in detail for a wide range of targets. A detailed analysis of the laser excitation process is required to fully characterise the electron excited state.
The processes where the linear momentum transferred from the electron to the target is exploited to determine cross sections for excitation of the state are now considered. The square root in the above expression shows the origin of the two different momenta at any given deflection angle of the targets as described in figure 4.
Note that for each atom detection angle qa there are 2 contributing momenta Paf1, Paf2 excepting the case of Forward & Backward scattered electrons, as shown in figure 5. Hence with sufficient time resolution and detection angle resolution the experiments should reveal 2 distinct peaks at any given detection angle. It should be noted that the lifetimes of the low l highly excited helium Rydberg targets, typically only a few microseconds, exclude them from being detected in the present experiments where the targets drift for around 300 microseconds from the interaction region to the metastable detector.
The chamber is lined with a double layer of m-metal which reduces external magnetic fields to < 5mG at the interaction region.
Fast output deflector circuit only allows electrons to exit electron gun, preventing any ions created in field free region or other high potential region from reaching interaction region. Angle selectivity is achieved using 1mm apertures 1 & 2 spaced 60mm apart prior to a 719BL CEM metastable detector at end of detector (LHS). Electrostatic deflection plates remove unwanted electrons and ions from entering the detector and possibly reaching the detectors.
Visible Laser Diodes (VLD's) align the detector to the interaction region formed by electron beam, gas beam and rotation axis of detector. A control pulse (0 - 150Hz) drives the gas nozzle HT pulse & the electron gun Grid following an appropriate delay. The Turbo-MCS is gated to either sweep over ~100ms for each gas pulse or is set to count events for a set number of gas pulses at each detection angle.


The following graphs show the metastable angular distribution when the target gas is excited using a continuous electron beam, rather than a pulsed electron beam for four different excitation energies from 20eV to 50eV. As the incident energy increases forward scattering starts to dominate over backscattering as a collision mechanism as might be expected. The evolution from single atom momentum at q'a = 4A°through double momentum peaks at intermediate energies then back to a single peak at the highest scattering angles can clearly be seen as is discussed in figure 4.
Gaussian profiles have been fitted to the peaks to establish the amplitude, width and position of each peak as a function of the deflection angle.
Note that a 4ms electron excitation peak is visible at higher detection angles, which is thought to be due to either photon or stray electron contributions. This accurately yields the time difference between excitation of the target and collection of the metastables. For these experiments the atoms excited by the incident electrons are counted and fitted to a Gaussian of width w and height h as shown in figure 14.
Figure 15 shows the results of these least squares fit calculations at 40eV Incident Energy, showing the amplitude h and width w together with the associated peak positions. Note that this method allows the DCS to be determined over the complete set of scattering angles from forward to 180A° backward scattering of the electrons.
Note that some discrepancies exist between the calculations from the fast and the slow atoms. This effectively acts as a high resolution timing probe of the target gas beam which for helium has a time spread of the order of 6ms from the interaction region to the detector. Estimated delivery dates - opens in a new window or tab include seller's handling time, origin ZIP Code, destination ZIP Code and time of acceptance and will depend on shipping service selected and receipt of cleared payment - opens in a new window or tab. International Shipping - items may be subject to customs processing depending on the item's declared value. Your country's customs office can offer more details, or visit eBay's page on international trade. By clicking Confirm bid, you commit to buy this item from the seller if you are the winning bidder. By clicking Confirm bid, you are committing to buy this item from the seller if you are the winning bidder and have read and agree to the Global Shipping Program terms and conditions - opens in a new window or tab.
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The electron impact occurs when a well controlled electron beam interacts with a well defined gas beam comprising ground state atoms or molecules under investigation.
Here the electrons do not lose any energy in the reaction, however the momentum of the electrons may change. Here the incident electron loses energy in the reaction to the target, which is subsequently excited to one of its (usually discrete) states. In these experiments the target is prepared in some way so that it has internal energy prior to interaction with the electron beam.
This is here defined as a state that cannot decay to a lower state of the target via a single photon emission (dipole allowed) transition. State selectivity is obtainable by exploiting momentum transferred to the atom by the electron impact. This is brought about in these experiments by employing a supersonically expanded jet of target gas which is skimmed by an expontially flared aperture in a source chamber. These ground state targets eventually enter the high vacuum pump located at the top of the interaction chamber from where they are ejected. They can then drift for many hundreds of microseconds before a collisional de-excitation occurs either with each other or with a surface or stray molecule inside the vacuum chamber. Note that if the targets are molecules they may share this internal energy between rotational & vibrational states in the metastable electronic manifold. This can be achieved using conventional electron energy analysers that select electrons of a specific energy and scattering angle for measurement. As the possible target deflection angles following the collision is confined to a much smaller range than the possible scattering angles of the electrons (which range from 0A° through to 180A° in space), in principle the cross section for excitation of the state can be completely mapped out over all possible electron scattering angles. In these experiments only the momentum of the electron is measured, both for the incident beam and for the scattered electrons using energy and angle selective analysers. The signal will then be an incoherent summation of the contributions fom these excited states.
Of particular interest in this region is the study of temporary negative ion states that show up as resonances. Measurements of energy selected scattered electrons in coincidence with measurement of the excited targets then exclude any contribution from upper states, since the electrons which excite these states are excluded from the coincidence measurement. Tunable laser radiation has an energy resolution typically 1,000 to 1,000,000 times better than is possible using electron spectrometers, and so resonant excitation can individually select the metastable state for further excitation following electron impact excitation. The coplanar momentum transfer scheme, showing the relationship between the extrema of the electron scattering angles together with the corresponding extrema of the target deflaction angles. The graphs show the effects as a function of the momentum, velocity and deflection angle of the target helium atoms. Variation of the maximum and minimum target deflection angle as a function of the incident energy of the electrons.
This aperture together with the 1mm exit orifice of the nozzle are aligned using visible laser diodes. All are aligned using Visible Laser Diodes apertured to accurately define their individual axes.
This is important as the detector may measure ions created as the electron beam interacts with the gas beam.
The calibrated angle plate which accurately measures the deflection angle of the target beam. This additionally allows the zero angle to be calibrated wrt the apertures in the source chamber which define the ground state target beam direction. Since the interaction region to detector distance is accurately known, this immediately yields the value of final atom momentum Paf(qa) to substitute into the appropriate equations. Gaussian fitting parameters as a function of the Detection Angle for the time resolved metastable atom deflections at 40eV Incident Energy.


It can be seen that the DCS is forward peaked as expected, and that the DCS has been determined at the complete set of scattering angles from forward to backward scattering. This may be attributable to errors associated with the solid angle of detection of the analyser which has not been considered in the above calculation. Import charges previously quoted are subject to change if you increase you maximum bid amount. Thus the electrons may be scattered throughout space from the reaction zone, the distribution of these electrons then giving information on the elastic differential cross section for the target in question. These states may be below the ionisation limit of the target, in which case the target usually remains neutral, or higher order excitation processes can occur where more than one electron in the target is excited. The interaction then removes energy from the target and so the electrons emerge from the reaction with greater energy than they had prior to the reaction. The excited target therefore has a very long lifetime, since the probability of 2 photon decay is much smaller than for single photon decay processes. For light atoms and molecules (He & H2 for example) the metastable targets are deflected through angles from ~5A° for forward electron scattering to in excess of 30A° for backward scattering depending upon the impact energy.
An electron of well defined momentum Pei collides with a target atom or molecule in its ground state that has a well defined initial momentum Pai. These experiments are conventionally limited to a range of scattering angles that does not include either the forward scattering region of the backward scattering region.
These experiments do not have the spatial or temporal resolution to define either the incident or the final target momentum, since they mostly produce a target beam via continuous effusive flow from a capillary.
These resonances occur since the scattered electron following excitation can remain in the region of the excited target for sufficient time to form a pseudo-stable state which has its own unique properties. As before the electron of incident momentum Pei is scattered inelastically from the target of incident momentum Pai, leaving the target in a metastable state. Electrons incident along the z-axis have a well defined energy Einc and thus a well defined momentum Pei. Electrons incident along the z-axis have a well defined energy Einc and a well defined momentum Pei.
Electrons incident along the z-axis with well defined energy Einc and well defined momentum Pei can scatter throughout the range of angles from 0A° (forward scattering) through to 180A° (backward scattering) as shown in figures (a) and (b). Amplitude h, peak position and peak width w are all calculated using a least squares fit to the data, a sample of which is presented in figure 14.
If you reside in an EU member state besides UK, import VAT on this purchase is not recoverable. Site Map has been snapped in Brisbane - but were is the york recumbent rowerbike model YOR05676 a magnetic model How do I know design 4 cheap download Purchase Audition 3 cheapest pinnacle studio 14 ultimate Where To Buy windows 7 base units Discount. The energy required to excite these processes is usually above the ionisation limit, and so these excitation mechanisms often lead to ionisation via photon emission or through autoionisation. In these experiments the initial target state is almost always prepared by excitation with a laser beam from the ground state, since very high yields (theoretically up to 50% for CW laser excitation) of target excitation can be achieved prior to electron collision. This long lifetime (which can be up to seconds if there are no other mechanisms for de-excitation) is exploited in these experiments, which measure the quantity of metastable targets as a function of the target deflection angle following electron collision. The correspondence (via momentum and energy conservation equations) between the deflection angle and arrival time at the detector of the excited target with the electron scattering angle therefore allows differential cross sections to be measured over the full range of possible scattering geometries from forward to backward scattering.
The target is excited to a metastable state, the electron losing energy Eexc during the reaction. Any variation in cross section due to variations in the target momentum is therefore averaged out in the signal that is collected. The target is then further excited using resonant laser radiation to a higher lying state which is then studied. These electrons interact with a beam of target atoms or molecules prepared with a well defined momentum Pai travelling along the x-axis with an initial thermal energy Eai.
These electrons interact with a beam of target atoms or molecules prepared with a well defined momentum Pai travelling along the x-axis with an initial energy Eai. Since momentum must be conserved in the reaction, the change of electron momentum delta Pe along the z-axis for these cases must be balanced by a corresponding change in momentum Delta Pa of the targets along this axis. The graph shows the effect discussed in figure 4, where for each deflection angle there are two distinct atom momenta corresponding to unique electron scattering directions. These are normally pinched off from exiting the region of the filament using a -15V grid potential wrt the tip of the filament. Some experiments have also been conducted where the target beam is prepared in a metastable state using electron bombardment, but these experiments are very preliminary. The final momentum of the inelastically scattered electron is given by Pef, whereas the final momentum of the target is given by Paf.
The high resolution of the laser excitation allows details of the electron excitation to be studied in higher detail than is possible using electron spectroscopy alone. When requested the grid potential is rapildly raised so that the positive anode potential field reaches the filament to extract electrons which are accelerated and focussed onto the interaction region using 2 triple aperture electrostatic lenses. The scattered electrons leave the interaction region with momentum Pef at scattering angles (thetae, phie), whereas the deflected excited targets leave the interaction region with linear momentum Paf at polar angles (thetaa, phia). The scattered electrons are confined to the xz plane and leave the interaction region with momentum Pef at a scattering angles thetae. In general as shown in figure (c), for a particular target deflection angle between these two extrema there are two possible electron scattering angles that can result in targets deflected to this angle, as shown.
A further set of output deflectors pulsed simultaneously with the grid allows the final focussed beam to pass through the exit aperture and onto the target beam. The deflected excited targets therefore must also leave the interaction region in the xz-plane and are deflected to the angle thetaa. In the example shown, electrons scattered in the lower xz-plane experience a momentum change larger than electrons scattered into the upper xz-plane. This set of deflectors also removes any ions created inside the electron gun in the high potential regions from reaching the interaction chamber, since their flight times are much slower than the associated electron pulse.
Observation of the targets at any intermediate deflection angle will therefore display two different momenta for these targets.



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