Atomic beam dosing

Figure 1 Schematic of the atomic beam dosing source used with REMPI detection by Murphy et al. to study the recombination of H [36, 37] and N [38] at metal single crystal surfaces. A single crystal surface is supported on a manipulator in the path of a collimated molecular beam. The beam supplies reactant molecules or atoms, produced using a microwave discharge in the glass nozzle, which react and recombine at the surface. The reaction products are ionised by the laser, which is focused in front of the surface (inset), and the resulting ions are timed into a microchannel plate detector.

Fig. 6.4 Numbers (per some unit beam dose) of the Sg atoms surviving at the column exit as a function of temperature [30].

Other useful methods for the quantitative destmction of thiolate monolayers on the gold surface include metastable atom °° or kiloelectronvolt ion bombardment and electron beam damage techniques . These methods are successfully used to prepare patterned monolayers. Quantitative removal of the thiol by an electron beam requires an e-beam dose of 10-100 mCcm . Recently, X-rays were shown to damage gold-thiol monolayers . ... 

Two additional atoms of Cn were detected in later experiments conducted at FLNR. The hrst atom, Cn, was observed using the same setup, reaction, and beam parameters target thickness 1.4 mg Cm , beam dose 3.4x10, " Ca... 

The time necessary for removing one monolayer during a SIMS experiment depends not only on the sputter yield, but also on the type of sample under study. We will make an estimate for two extremes. First, the surface of a metal contains about 1015 atoms/cm2. If we use an ion beam with a current density of 1 nA/cm2, then we need some 150 000 s - about 40 h - to remove one monolayer if the sputter yield is 1, and 4 h if the sputter rate is 10. However, if we are working with polymers we need significantly lower ion doses to remove a monolayer. It is believed [4] that one impact of a primary ion affects an area of about 10 nm2, which is equivalent to a circle of about 3.5 nm diameter. Hence if the sample consists, for example, of a monolayer film of polymer material, a dose of 10n ions/cm2 could in principle be sufficient to remove or alter all material on the surface. With a current density of 1 nA this takes about 1500 s or 25 min only. For adsorbates such as CO adsorbed on a metal surface, we estimate that the monolayer lifetime is at least a factor of 10 higher than that for polymer samples. Thus for static SIMS, one needs primary ion current densities on the order of 1 nA/cm2 or less, and one should be able to collect all spectra of one sample within a quarter of an hour. 

RBS spectra were obtained using a 2.120 MeV He+2 ion beam at a backscattering angle of 162. The spectra were accumulated for a total ion dose of 40 uC using a 10 nA beam current. The number of Ti atoms/cm2 in the sample was calculated by comparison to spectra for a standard Si wafer implanted with a known dose of Sb. 

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