Mechanisms of Excimer Laser induced Positive Ion Emission From Ionic Crystals
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15 Mat. Res. Soc. Symp. Proc. Vol. 388 01995 Materials Research Society
The ion energy distribution was estimated from TOF signals accompanying the first few laser pulses on previously unexposed surfaces. The energy distribution may be obtained by transforming the TOF distribution dN d--E
dN (dE)- I d--T-d{ it-
[t)
t3 md2
,(1
where E is the kinetic energy of an ion detected at time t after the laser pulse (E = mv 2/2 = md 2/2t 2 ), I(t) is the ion intensity as a function of time, m is the ion mass, and d is the distance between the sample and the detector. To facilitate numerical analysis, the TOF signal was smoothed by performing a least-squares fit of the TOF signal, where energy distribution E(t) was empirically expressed as a sum of Gaussians. To predict the ion energies associated with emission from various defect configurations, ion trajectory simulations were performed by summing the electrostatic forces due to the individual ions in a simulated MgO cluster and integrating the resulting equation of motion using a second order Taylor approximation.1 0 Computations with clusters of different sizes showed little improvement in the computed trajectory when the cluster size was increased beyond a 19 x 19 x 10 ion array. Trajectories computed without image charge corrections typically yielded ion energies within 2% of the (repulsive) Madelung binding energy of the initial defect configuration; this difference is a reasonable estimate of errors due to the finite cluster size and computational inaccuracies. The ion energy simulations reported below incorporated an image charge term to account for the dielectric response of the bulk. Since the image charge reduces the accelerations along the trajectory, the addition of the image charge term should not degrade the numerical accuracy of the trajectory simulation. RESULTS A typical set of TOF signals at a mass to charge ratio of 12 amu/e (Mg 2 +) accompanying the first five laser pulses (1.2 J/cm 2 per pulse) on previously unexposed, polished MgO appear in Fig. 1. The results of fitting Eq. 1 to the experimental data are also shown. The TOF signal accompanying the first pulse is slightly distorted relative to the signals accompanying the remaining pulses. In particular, the first pulse produces a number of high energy ions. At this fluence, the second and successive pulses on polished surfaces consistently produce two broad peaks which are well described by the model Gaussian energy distributions. The position and breadth of these peaks are remarkably consistent from pulse to pulse, until the decreasing signal intensities no longer provide adequate statistics. Typical TOF distributions at a mass to charge ratio of 24 amu/e (Mg+) accompanying the 2 first five pulses incident on a previously unexposed, polished surface at a fluence of 1.1 J/cm appear in Fig. 2. Again, two peaks were observed. Figure 2 also shows the results of a least squares fit of Eq. 2 to the corresponding experimental TOF distributions. As in the case of the Mg 2 + emissions, the Mg+ emission a
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