Analysis beam

In this equation, L is the thickness of the sample along the analysis beam. From Equations 3A.3 and 3A.4 written for the photostationary state, the equation is as follows ... 

Experiments are performed with mixtures that are diluted several hundred fold with inert gas in order to control the temperature increase in the reaction cell caused by the exothermic recombination event. The magnitude of the thermal effects depends upon the fraction of molecules dissociated by the initial flash, the energy of the average absorbed quantum, the fraction of the cell volume sampled by the analysis beam, and the length of the observation period. Many of the difficulties may be overcome by recording data for a short time interval following the initiating flash [30]. 

Grehl,T., Mollers, R., Niehuis, E. (2003) Low energy dual beam depth profiling influence of sputter and analysis beam parameters on profile performance using TOF-SIMS. Applied Surface Science, 203-204,277-280. 

The third major limitation is that the analysis process itself can change the composition of the surfaces to be analyzed. The two most common t5 es of this problem are analysis beam-induced damage and inhomogeneities caused by the sputtering process, which is used often to probe past the topmost surface of the sample. 

Fig. 4.13 Demonstration of the high mass range capabilities of a TOF SIMS with mass spectra of a polyethylene oxide self-assembled monolayer on Si (molecular weight = 5,000) recorded using a 2 nA, 15 keV Ga analysis beam in a PHI TRIFT III instrument...

At this point, it is worth noting that a 50 kHz analysis beam cannot contain more than 1000 ions per pulse (the remainder is discarded). This translates into a current of 80 pA striking the sample (1000 ions times 50 x 10 ions/s all divided by 6.28 X 10 cps, where the latter equates to 1 pA of current) when using an initial... 

Although 80 pA appears ideal for Static SIMS applications, this is much too low to satisfy the sputter rates required in Dynamic SIMS. To counter this, a second primary ion beam of much higher current is used in such instruments to etch some volume of the sample s surface between analysis beam cycles. This beam, commonly referred to as the sputter beam, is pulsed at the same frequency but out of phase with the analysis beam. 02 or Cs beams are most commonly used for this purpose, as these can also induce a significant enhancement of respective secondary ion yields (see Section 3.3.2) owing to the small fraction implanted into the substrates surface. The downside of this approach is that the sensitivity and detection limit gains resulting from simultaneous ion detection are lost. This is due to the fact that the sputtered population cannot be recorded (only ions produced by the analysis beam are recorded). 

Pulsed primary ion beams used for generating the secondary ion signal (this beam is henceforth referred to as the analysis beam) are pulsed at frequencies of between 10 and 50 kHz. Each pulse then lasts from a few nanoseconds to several hundred nanoseconds with the time between pulses ranging from tens to hundreds of microseconds. These times are adjusted to control ... 

The mass range. This is determined by the time between analysis beam pulses with longer times translating to larger mass ranges... 

The mass resolution. This is determined by the analysis beam pulse width, i.e. a smaller width translates to higher mass resolution... 

The secondary ion signal intensity. This can also be controlled by adjusting the analysis beam pulse duration... 

In addition, the primary ion analysis beam focusing/pulse sequencing can be tailored to optimize the specific information required. As an example, the pulse width, which defines the mass resolution, can be adjusted by pulsing the electrodynamic fields within the primary ion column such as to accelerate slower ions/decelerate faster ions within a specific pulse, such that they all arrive at the sample surface at the same time. This is referred to as beam bunching. The downside is that spatial resolution is lost when operating in the micro-probe mode. 

As a lower current density of the analysis beam reaches the sample surface per unit time (relative to instruments utilizing continuous primary ion beams), increased sputter rates can only be realized through the irradiation of the analyzed area of the sample by a second pulsed primary ion beam. These beams are thus referred to as the sputter beam. Note Both beams must be operated in an interleaved manner with respect to each other, i.e. only one can be sticking the sample at a time. 

To ensure effective sputtering, the sputter beam is operated such that the dose is significantly higher (tens of nanoampere) than that of the analysis beam (< 1 picoampere). The sputter rate is then contiolled through the adjustment of the sputter beam pulse width, between tens to hundreds of microseconds, as opposed to the adjustment of the ion optics as used in continuous primary ion beams. The extraction ion optics is also switched off over the interval the sputter beam is directed at the sample (this also aids in providing for additional charge compensation). Depth resolution will then become a function of the sputter beam... 

The interlaced or interleaved mode describes a situation in which the sputter beam is switched on during the flight time of the secondary ions produced by the analysis beam. To avoid interferences between the analysis and sputter beams, a delay and lead-off time of several ps is implemented. This represents the fester and hence, more commonly used mode for depth profiling. This mode is also better for interface analysis and allows for reduced adsorption of gas phase species. 

In the case of primary ion pulsed Time-of-Fhght-based SIMS instruments, crater edge effects can be removed by rastering the primary ion analysis beam over a smaller region centered within the middle of the primary ion sputter beam raster pattern. 

One approach is to use longer analysis beam pulses in conjunction with pulsed extraction, in order to optimize resolution in both space and mass. In this case, the switch-on of the extraction voltage will become the time reference. 

Fig. 8.5 90°-geometric layout of the excitation and analysis beams for detection of transient absorbance (/ = pathlength, D = base of the analysed cylindrical volume)... 

Fig. 8.14 Pump beam and analysis beam are overlapped on the sample...

In the systems for femto-picosecond transient spectroscopy it is used a special detection technique, known as pump and probe . The idea behind this technique is to use the same laser source to generate the excitation pulse (PUMP), and the analysis beam (PROBE). The path of the PROBE beam is varied in length by a delay line, i.e. a mobile platform on which are mounted mirrors that reflect the laser beam with high efficiency. The change in the optical path allows the control of the temporal distance between excitation and analysis (Fig. 8.14). 

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