Flight times calculation

For example, if each bin represents 0.3 nsec and bin number 200 has been affected by an ion arrival, then the flight time must have been 200 x 0.3 = 60 nsec. Knowing the length of the drift tube, the ion drift velocity can be calculated, and from that calculation its m/z value can be deduced. 

To simulate the particle-particle collision, the hard-sphere model, which is based on the conservation law for linear momentum and angular momentum, is used. Two empirical parameters, a restitution coefficient of 0.9 and a friction coefficient of 0.3, are utilized in the simulation. In this study, collisions between spherical particles are assumed to be binary and quasi-instantaneous. The equations, which follow those of molecular dynamic simulation, are used to locate the minimum flight time of particles before any collision. Compared with the soft-sphere particle-particle collision model, the hard-sphere model accounts for the rotational particle motion in the collision dynamics calculation thus, only the translational motion equation is required to describe the fluid induced particle motion. In addition, the hard-sphere model also permits larger time steps in the calculation therefore, the simulation of a sequence of collisions can be more computationally effective. The details of this approach can be found in the literature (Hoomans et al., 1996 Crowe et al., 1998). 

Fig. 2.15 (a) In low temperature pulsed-laser field evaporation of silicon, ions formed are all doubly charged. The energy distributions are very narrow, similar to those found in low temperature field evaporation of metals. However, the onset flight times are always shorter than the calculated values, indicating a photo-excitation effect as will be discussed in Sec.2.2.6. 

Fig. 3.19 Mass separated energy distributions of pulsed-laser field desorbed 4He+ and ions obtained with the Penn State pulsed-laser ToF atom-probe when the flight path length was 778 cm. Their onset flight times are separated by 34 ns, exactly that calculated from the system constants and their critical ion energy deficits at 5.5 kV.

The same measurement is repeated by assuming different values of a. The coefficient of determination or correlation factor r2 is calculated for each value of a. The best value of a is the one with its value of r2 closest to one. In a calibration done by Ren et al.71 for the Penn State flight-time-focused atom-probe, the following values were obtained ... 

If the parameters in parentheses remain constant, then m/z can be calculated from the ions flight times. 

Figure 4.50 Calculated surfaces of equal flight times (isochrones, broken curves) intersecting the electron paths behind the exit slit of an electrostatic parallel plate analyser ( 0 = 30°). An individual isochrone may be labelled by its distance L from the exit slit together with its angle of inclination y with respect to the principal trajectory. From [VSa83] where numerical data for y at preselected distances L are also given.

Noxon calculated the rate constant of 0(1D) quenching by 02 on the basis of unit quantum yield and of the equilibrium concentration of O (lD) atoms. His value of 6 x 10 11 cm3 molec"1 sec-1 agrees well with 7 x 10 11 cm3 molec 1 sec 1 obtained independently (456), indicating that the assumption of unit quantum yield may be justified. Below 1332 A the production of O( S) is energetically possible. Filseth and Welge (348) have observed an emission at 5577 A due to the transition Of SJ- Ol f)) in the flash photolysis of 02 below 1340 A. The intensity is so weak that Xe has to be added to induce the transition. No quantum yield of O( X) production has been measured. Recently Stone et al. (937) have measured the flight time of O atoms produced in the flash photolysis of the molecular beam of 02 in the vacuum ultraviolet. The O atoms are detected by the chemiionization reaction with samarium. The technique is similar to the one described in Section II 4.1. 

It has been seen from Figs. 2 and 4, and from Eq. (4), that the flight time of the ions to the detector is of the order of 0.1 ms. Therefore, the lifetimes of all of the species observed must be of this same order of magnitude. Obviously the 0 ion observed from CO cannot be that state with a lifetime of 5 x 10 s calculated by Herrick and Stillinger l Since the early work of Stuckey and Kiser, other investigators also have observed doubly-charged negative ions. Their observations and experiments will be described and discussed later however, their results indicate that the lifetimes of the X species are > 10 s. 

The time-of-flight (ToF) analyzer is the most widely used analyzer in static SIMS. As its name indicates, for this analyzer the flight time of an ion is the parameter for measurement. When ions are obtained with a constant kinetic energy from an acceleration potential (V) of 3-8 kV, the flight time of ions through a distance (L) of flight tube to reach a detector is calculated. 

Of course an intense laser pulse will be absorbed by the surface and may cause a significant temperature rise, leading to photothermal reactions. Since the main interest is in the photochemistry it is necessary to separate the two contributions. Fortunately expressions for the temperature rise at the surface for a given laser pulse shape have been derived.18,19 Thus the temperature rise can be calculated and, if the thermal activation energy of the process under study is known or measured, the expected thermally equilibrated values of flight time and internal... 

In the time-of-flight technique, calculate the increase in signal-to-noise ratio (sensitivity) for a subject of 50 cm diameter and a timing resolution... 

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