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Emission of Negative Potassium Ions from Single Crystal Potassium Bromide ..... the laser irradiation of KBr was again the negative atomic potassium i...
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J. Phys. Chem. C 2010, 114, 5700–5708

Emission of Negative Potassium Ions from Single Crystal Potassium Bromide during Exposure to 248-nm Excimer Laser Radiation† Kenichi Kimura, S. C. Langford, and J. T. Dickinson* Department of Physics and Astronomy, Washington State UniVersity, Pullman, Washington 99164-2814 ReceiVed: September 30, 2009; ReVised Manuscript ReceiVed: December 2, 2009

Many wide bandgap materials yield charged and neutral emissions when exposed to sub-bandgap laser radiation at power densities below the threshold for optical breakdown and plume formation. In previous work, we reported the observation of negative alkali ions from several alkali halides under comparable conditions. Within our detection sensitivity, we observed no negative halogen ion emission, in spite of the high electron affinities of the halogens. Significantly, the positive and negative alkali ions showed a high degree of spatial and temporal overlap. Our conclusion, based on a detailed study of KCl emissions, was that K- is formed by the sequential attachment of two electrons to K+. Here we provide further evidence for this mechanism based on a study of KBr. 1. Introduction Studies of laser-induced particle emissions from wide bandgap materials, including ionic single crystals such as MgO and several alkali halides, have revealed positive ions, electrons, and neutral atoms and molecules at laser fluences well below the threshold for breakdown and the formation of a visible plume. (Recent work is reviewed in ref 1.) Emission during nanosecond pulses typically requires surface and near surface defects, such as anion vacancies and steps.2–4 In many cases, anion vacancies and steps are created by the laser itself. For instance, pulsed 248-nm radiation can produce pits on NaCl(100) surfaces via vacancy aggregation.3 Step erosion along pit edges and cleavage steps during prolonged irradiation suggests that much of the emitted material ultimately originates from kink sites along these steps. A common but important emission is that of the singly charged cation, such as Mg+ from MgO,5 Na+ from NaNO3,6 and Na+ from NaCl.7 These ions typically have kinetic energies greater than 5 eV; thus, emission stimulated by 248-nm irradiation (photon energy 5 eV) requires the absorption of more than one photon. A sequence of single-photon ionization events involving nearby electron traps can provide repulsive charge centers which electrostatically accelerate the desorbed positive ions to the observed kinetic energies.6,8 Negative ions are routinely produced in matrix assisted laser desorption and ionization (MALDI) of large organic molecules, including biomolecules.9 They are less commonly observed from wide bandgap crystals as one component of the visible plume accompanying optical breakdown.10 Recently, we reported for the first time negative atomic ion emission from an ionic material (an alkali halide: KCl) at laser fluences that were too low to produce optical breakdown.11 In this work, we extend this study to single crystal KBr(100) surfaces exposed to 248 nm excimer laser irradiation, again at intensities well below those required for optical breakdown. We show that, just as we saw in KCl, negative alkali ions (M-) are formed by the sequential attachment of two electrons to the cation (M+); again, we find no †

Part of the “Barbara J. Garrison Festschrift”. * To whom correspondence should be addressed. E-mail: [email protected].

evidence for electron attachment to the observed neutral atomic emission products (M° and X°). 2. Experimental Section KBr samples were cleaved along (100) surface plane of high purity, single crystal blocks obtained from Almaz Optics, West Berlin, New Jersey. Samples were mounted on a translation stage directly in front of and normal to the axis of a UTI 100C quadrupole mass spectrometer (QMS). The instrument detects positive and negative ions with comparable efficiency by appropriately biasing the front cone of the Channeltron electron multiplier (CEM) mounted at the exit of the mass filter. The CEM was operated in the pulse-counting mode. The output of the detector was amplified, discriminated, and pulse counted over 200-ns intervals with an EG&G PARC 914P multichannel scalar. Pulsed laser radiation was provided by a Lambda Physik Lextra 200 KrF excimer laser (248 nm, 5-eV photons; 30-ns pulse width) operated at a pulse repetition rate of 1 Hz. Irradiation was carried out at a pressure of 10-7 Pa. KBr has a 7.4 eV bandgap,12 so that single photon excitations across the bandgap are forbidden. By grounding the metal surfaces surrounding the sample, we ensure that electric fields outside the mass filter are small. Further, the quadrupole electric fields along the axis of the mass filter are perpendicular to the ion velocity. Thus the entire apparatus serves as a time-of-flight (TOF) mass spectrometer at zero average potential, with length equal to the distance from the sample to the exit aperture of the mass filter. Ion kinetic energies were determined by curve fitting the resulting signals to an empirical function corresponding to the flux of ions emitted during the laser pulse with a Gaussian energy distribution. An ensemble of ions with a Gaussian energy distribution, directed normal to a surface mounted a distance d ) 0.28 m from the detector at time t ) 0, produces an ion flux I(t) given by

{

-[E(t) - E0]2 Nmd2 I(t) ) exp 2σ2 √2πσt3

10.1021/jp909417q  2010 American Chemical Society Published on Web 01/07/2010

}

(1)

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where N is the total number of ions in the ensemble, E0 and σ are the average and standard deviation of the kinetic energy distribution, and E(t) is the kinetic energy of an ion of mass m arriving at the detection plane at time t after the laser pulse [E(t) ) mV2/2 ) md2/2t2]. The K+ TOF signals in this work typically display two peaks. Although each peak is well described by eq 1, the large number of parameters (two mean energies, two standard deviations, and two amplitudes) result in large uncertainties unless the two peaks are distinct. Unfortunately, the relative intensities of the two peaks vary dramatically with fluence and overlap significantly. The most reliable curve fits are obtained when the two peaks have comparable amplitudes, at fluences near 150 mJ/cm2. Full six-parameter curve fits were performed on this data. To describe the signals at higher and lower fluences, the mean kinetic energies [E0 in eq 1] were fixed at the energies obtained from fits to the 150 mJ/cm2 signals, and only four parameters were adjusted. This procedure allowed for a more consistent description of the evolution of the signal with increasing fluence: the resulting trends in N and σ (the number of particles in each energy distribution, and their standard deviations) were much more clear and consistent. The neutral particles emitted during laser exposure are generally much slower than the corresponding positive ions. Nevertheless, the fastest neutral particles significantly overlap the slower positive ions. To characterize these fast neutral particles, the leading edge of the neutral TOF signals were fit to Maxwell-Boltzmann distributions appropriate for thermal desorption from a surface at temperature T at the time of desorption. For neutral particles emitted at time t ) 0, the signal detected at time t is proportional to the particle density at the quadrupole ionizer at time (t - t’), where t′ is the time required for the ionized neutrals to pass through the quadrupole mass filter. The corresponding signal at the detector, I(t), is given by

I(t) )

(

-mdion2 m 2 exp 2π(t - t)4 kT 2kT(t - t)2 ηNVdion

( )

)

(2)

where the constant η accounts for the ionization and transmission efficiencies, assumed to be uniform over the volume V of the ionizer, N is the total number of particles leaving the surface, dion is the distance from the sample surface to the quadrupole ionizer, m is the particle mass, and k is the Boltzmann constant. The constant η can be determined by measuring the quadrupole output as a function of partial pressure of a gas molecule whose mass is similar to the mass of the studied particle. As noted below, the measured neutral time-of-flight signals show long tails that depart significantly from pure Maxwell-Boltzmann behavior, presumably due to particles emitted well after the laser pulse (t > 0). A number of electrode and detector arrangements were used to probe the charged particles (both positive and negative) produced by laser irradiation. These are described as needed. 3. Results Mass-Selected Ion Time-of-Flight Measurements. Positive atomic ion emission from the alkali halides under 248-nm excimer laser irradiation at fluences below the threshold for plasma formation is dominated by the emission of the singly charged, alkali ion.11 At the fluences used in this study, no positive halogen ions were detected: in particular, Br+ was not detected from KBr. Figure 1 shows mass-selected K+ time-offlight signals acquired at fluences of 100 and 150 mJ/cm2. These

Figure 1. Mass selected time-of-flight signals for K+ from KBr at fluences of (a) 100 and (b) 150 mJ/cm2. The gray curves show the time-of-flight signals for two sets of ions, each with a Gaussian energy distribution. The slow peak is formed by ions with a mean kinetic energy of 4.8 eV and the fast peak by ions with a mean kinetic energy of 7.8 eV. The widths of the energy distributions was chosen to minimize the sum of the squared differences between the model and the data.

signals display two peaks, each of which are well described by the sum of two time-of-flight distributions (corresponding to Gaussian energy distributions) as given in eq 1. As the fluence increases, the heights of both peaks increase dramatically, but the slow peak near 55 µs grows much faster with fluence than the fast peak near 40 µs. Although higher fluences (above 200 mJ/cm2 in this work) are required for the observation of significant negative ion signals, the dominance of the slow peak at these high fluences complicates the characterization of the fast peak by curve fitting techniques. As noted above, the most reliable curve first were obtained using the TOF signal acquired at 150 mJ/cm2 [Figure 1b], where the two TOF peaks are most distinct. These curve fits yield mean kinetic energies of 4.8 and 7.8 eV for the slow and fast peaks, respectively. The curve fit to the TOF signal in Figure 1a employed these values for the two mean kinetic energies, with the other four parameters (N and σ for each of the two peaks) adjusted to minimized the sum of the squared differences between the data and the model. This four-parameter model provided a good description of all the K+ TOF signals described below. Positive ion emission from ionic materials has been attributed to the desorption of ions adsorbed near ionizable surface electron traps, such as anion vacancies (F centers in the alkali halides).6–8,13 Surface electron traps, such as the surface F center, offer favorable sites for the adsorption of positive ions (adions). When the underlying defect is photoionized, the net electrostatic force on the sorbed ion (Adion) becomes strongly repulsive, and the Adion is ejected with considerable kinetic energy. Thus the kinetic energy of the ejected ion reflects the electrostatic environment of the Adion immediately after the underlying

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Figure 3. Fluence dependence of positive and negative potassium ion intensities during 248-nm irradiation on a log10-log10 scale.

Figure 2. Time-of-flight signals for (a) K+ and (b) K- from KBr during 248-nm laser irradiation at a fluence of 330 mJ/cm2. The mean and standard deviations of the kinetic energy distributions derived from curve its of eq 1 to the data are also indicated. Both K+ and K- show a fast component on the leading edge of these TOF curves. The model used to describe the positive ion signal does not provide a good description for the negative ion signal, as explained in the text.

electron trap is photoionized. The observation of two peaks in the TOF signal from KBr suggests that K+ is emitted from two distinct electrostatic environments.6,7,14–17 For K+ from KBr, the simplest defect configuration yielding emission is probably the K+ ion adsorbed on top of a surface F center. We assume that the Adion occupies an idealized lattice site above the surface-one nearest neighbor distance from the center of the electron trap. If work on image charges can be neglected, as in ref 8, the final kinetic energy of the ejected ion, KE, will equal the potential energy of a K+ ion located one nearest neighbor distance from a positively charged lattice site (the electron trap with the electron removed); this energy will be reduced by the electrostatic energy of the original lattice (the binding energy, BE)

KE )

e2 - BE 4πε0r

(3)

where r is the nearest neighbor distance. The predicted kinetic energy for KBr (r ) 0.33 nm, BE ≈ 0.3 eV18) is about 4.1 eV. Considering the approximate nature of our assumptions, this is reasonably consistent with the 4.8 eV kinetic energy derived from curve fits to the signal at 150 mJ/cm2 in Figure 1b. Nearby electron traps photoionized prior to emission would further increase the electrostatic potential energy of the K+ Adion, resulting in yet higher kinetic energies [e.g., the 7.8 eV peak in Figure 1b]. Figure 2a shows a typical positive potassium ion TOF signal from KBr acquired at a fluence of 330 mJ/cm2. At this fluence, the fast peak appears as a shoulder on the much more intense

slow peak. Nevertheless, the fast peak is still much more intense than the fast peaks detected at 100 and 150 mJ/cm2 in Figure 1. The simple four parameter model (adjusting only N and σ for the two energy distributions) provides a good description of the K+ signal at 330 mJ/cm2. In previous work, we observed no Cl- during the 248-nm irradiation of KCl, despite the observation of significant signals due to K-. Again to our surprise, no signals due to the negative anion (Br-) were observed from KBr. Given the high electron affinity of Br (3.55 eV for Br versus 0.50 eV for K)]19 Brshould be much easier to produce than K-, if Br- production is not otherwise hindered. Nevertheless, the dominant negative ion observed during the laser irradiation of KBr was again the negative atomic potassium ion (K-). A mass selected ion TOF signal for K- is shown in Figure 2b, again acquired at a fluence of 330 mJ/cm2. Importantly, the K- TOF distribution is similar, but not identical, to its positive alkali ion counterpart. This includes the shoulder on the leading edge of the main TOF peak. However, the K- signal shows a tail to long times that is not observed in the K+ signal. As discussed below, we attribute K- formation to electron-ion and electron-neutral attachment events that require high particle densities. The observed TOF signals reflect particle fluxes, and the flux is the product of the particle density and velocity. For a given particle flux (signal intensity), the highest particle densities are associated with the slowest particles-that is, those detected at long times after the laser pulse. A similar argument suggests that the slowest particles experience high particle densities for longer times. Negative ion formation should be disproportionately more efficient for particles detected at these longer times. The variation of ion intensity with laser fluence often provides clues to the emission mechanism. Fluences above 400 mJ/cm2 often produce a visible breakdown plasma, so higher fluences were avoided. A log-log plot of the fluence dependence of the total K+ and K- intensities from KBr is shown in Figure 3. The K+ intensity increases roughly with the fourth power of the fluence. The K- intensity varies in a similar fashion, and is typically an order of magnitude less intense. Both intensities show a slight increase in slope as the fluence is raised from 120 to 330 mJ/cm2. The strong coupling between the K+ and K- emissions can be demonstrated in a dramatic fashion by mounting a biased metal grid near the entrance of the quadruple mass filter. Ion time-of-flight signals recorded with grid biases of -20, 0, and +20 V are shown in Figure 4. Importantly, the intensities of both signals drop dramatically as the grid bias is made more positive. With the positive bias voltage, the grid repels the net-

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Figure 4. Mass-selected (a) K+ and (b) K- time-of-flight signals recorded at a fluence of 220 mJ/cm2 per pulse after mounting a biased metal grid at the entrance of the quadrupole mass filter. The intensities of both signals drop dramatically as the bias voltage is made more positive.

positive charge cloud, thus reducing the K+ signal intensity. Since the K- intensity is also reduced, we conclude that the K- ions are also trapped in the positive potential well of the net positive charge cloud. To emphasize the fact that the charge cloud contains both positive and negative charge, we call it a ( charge cloud. Significantly, the highest K- intensities are observed with -20 V applied to the grid. Since most of the ions have less than 5 eV kinetic energy, they would normally be repelled by the applied electric field despite the large number of nearby positive ions. This screening is characteristic of a weak plasma, provided that the grid spacing is significantly larger than the Debye screening length, λD, which is given by

λD )

[ ] ε0kTe nee2

1/2

(4)

where Te is the electron temperature, ne is the electron density, and e is the electron charge. In this experiment, the grid spacing is 500 µm. For negative charge to efficiently pass through the grid, the Debye screening length must be less than about half of the grid spacing, or 250 µm. Assuming an electron temperature Te ≈ 1000 K (0.1 eV), the corresponding electron density at the shield grid would have to be at least 8 × 107 cm-3. This is a substantial electron density-especially considering how much the charge densities must drop as the particles travel from the sample to the grid. We argue below that the electrons in the ( charge cloud play a critical role in negative ion formation. Mass-Selected Neutral Particle Measurements. Electron attachment to neutral K would naturally produce K-. We have shown previously that Na0 and Cl0 are emitted at comparable fluences from NaCl,2 and (as we show below for KBr), electrons

Figure 5. (a) Time-of-flight signals for K0 (top curve) and Cl° (bottom curve) from KCl at a fluence of 330 mJ/cm2. The K0 signal includes an initial fast peak near 25 µs due to K+, which is observed even with the quadrupole ionizer turned off. The true K0 signal is slower and peaks near 95 µs. Maxwell-Boltzmann distributions were use to characterize the leading edges of the neutral signals. (b) Normalized K0 and K+ densities at time t ) 1 µs after the laser pulse, predicted by transforming the curve fits of eqs 1 and 2 to the data in panel a. The small overlap between these densities would hinder the attachment of electrons confined to the K+ distribution (in the ( charge cloud) with K0 emitted from the surface.

are available for attachment events. Mass-selected TOF signals incorporating K0 and Br0 from KBr exposed to 248-nm radiation at 330 mJ/cm2 are shown in Figure 5. The most intense neutral signal is due to K0 (mass/charge ration 39 amu/e) and peaks near 95 µs. (Because our mass filter does not efficiently reject incoming ions, a peak due to K+ appears near time t ) 25 µs. This K+ peak is faster than the one in Figure 2a because slower K+ ions are rejected more efficiently than fast ones.) The smaller but easily detected signal due to Br0 (mass/charge ratios 79 and 81 amu/e) is also shown. Given the presence of Br0, it is at first glance difficult to account for the absence of Br- (electron affinity 3.55 eV)19 if we attribute K- (electron affinity 0.50 eV)19 to electron attachment on thermal K0. Both the K0 and Br0 signals display tails to long times that are not well described by the Maxwell-Boltzmann distribution of eq 2. This suggests that neutral particles continue to be emitted for several microseconds after the laser pulse. More importantly here, the leading edges of these neutral TOF peaks are well described by the Maxwell-Boltzmann distribution. The corresponding temperatures are considered reasonable measures of the surface temperature at the time of desorption. The solid lines in Figure 5a are least-squares curve fits of eq 2, at a common temperature but different amplitudes, to the leading edges of the K0 and Br0 signals. The resulting effective surface temperature is T ) 2700 K, which corresponds to a translational kinetic energy of ∼0.2 eV. (By way of comparison, effective surface temperatures of 2800 K were observed for Na0 from NaCl during 248-nm irradiation at a somewhat lower fluence.2) Electron attachment to these neutrals, if it occurred, would not

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appreciably alter the kinetic energy of the resulting negative ion. We have previously shown that the ( charge cloud (here delineated by the K+ distribution) contains large numbers of electrons.20 If K- is formed by electron attachment to these thermal neutral particles, we would expect that the K+ and K0 density distributions to overlap significantly. We have transformed the measured Br0, K0, and K+ signals to spatial particle density distributions corresponding to time t ) 100 ns after the laser pulse. The results are plotted in Figure 5b. These spatial distributions are self-similar in time and distance: for particles in free flight, the degree of overlap (or lack thereof) between the particle distributions does not change as the particle distributions evolve after the laser pulse. Thus the poor overlap between the K+ and Br0 distribution in Figure 5b applies to all times after the laser pulse. The poor spatialtemporal overlap between the ( charge cloud and Br0 suggests that Br0 never experiences sufficient electron densities to produce negative ions. This is consistent with the lack of detectable Br-. Since the overlap between the K0 and K- is likewise small, electron attachment to thermal K0 is also unlikely. If K- is formed by electron attachment, we must find another source of K0. Charge Cloud and Electrons. To verify that electrons are emitted directly from the surface, negative charge measurements were made with sufficient time resolution to distinguish the relatively short time-of-flight of electrons from the longer timeof-flight of ions. These measurements employed an electron multiplier mounted in a grounded conical shield that minimizes signals due to scattered laser photons striking the electron multiplier, illustrated in Figure 6a. A grid covering the entrance to the shielded enclosure could be biased to reject or accept particles of a given charge. Figure 6b shows the first microsecond of data acquired with the outer grid biased at +20 V (to attract negative charge outside of the conical shield) at a fluence of 200 mJ/cm2. This signal disappears when the outer grid is biased at -1 V, verifying that these are low energy electrons coming from the sample rather than photoelectrons produced inside the conical shield or from the grid. The arrival time of 230 ns is far too short for negative ions with kinetic energies of a few eV. Thus we conclude that this signal is due to photoelectrons emitted from the KBr surface. These electrons originate from electron traps such as surface F centers and not from the valence band (which would require photon energies of at least 8 eV).21 We have written a simple JAVA program to predict the trajectory of electrons emitted as a function of location on the sample surface and launch angle, and contains all nearby surfaces and their appropriate potentials. The experimental TOF signals in Figure 6b are best matched by these simulations with average electron kinetic energies of 0.3 ( 0.2 eV. This is consistent with our ability to completely eliminate this signal by applying a -1 V potential to the outer grid of the conical shield. We refer to these photoelectrons, which travel directly from the surface to our detector, as “fast” electrons. They do not couple with the slower ( charge cloud. The absorption of single 5 eV photons by electron traps high in the KBr band gap would yield electrons with similar kinetic energies. Single photon excitations from the valence band are not expected to yield photoelectron emission,21 but photoelectron emission from colored KBr (F center density about 3 × 1017 cm-3) has been observed at photon energies as low as 2.1 eV.22 Analysis of the electron kinetic energies as a function of photon energy suggests that the F center ground state lies 2.9 eV below the vacuum level, and thus would yield photoelectrons during

Kimura et al.

Figure 6. (a) Schematic of charge detection experiment employing a Channeltron electron multiplier mounted in a conical shield to minimize exposure to laser photons. The grid across the aperture of the shield was mounted 6 cm from the sample surface and was grounded for TOF measurements on fast electrons. (b) The TOF signal for negative charge arriving at the detector for the first 800 ns after the laser pulse during 248-nm irradiation at a fluence of 200 mJ/cm2. (c) UV-vis absorption spectra before and after exposure to 5000 pulses of 248 nm irradiation at 400 mJ/cm2.

exposure to 248 nm (5 eV) photons. F centers are also produced in KBr during 248-nm exposure. Figure 6(c) shows UV-vis absorption spectra acquired before and after 248-nm irradiation of the material employed in our fast electron emission measurements. The absorption centered at 620 nm is attributed to the F center and gives the crystal a visible blue color. F centers in the alkali halides are produced when electrons recombine at self-trapped holes; at 248-nm, electron-hole pairs are produced by (often defect mediated) two-photon excitations across the band gap.3 Surface F centers are also generated when KBr is cleaved;23 thus some F centers will be available for electron emission at the onset of 248-nm exposure. Although the behavior of the ( charge cloud under applied electric fields is consistent with high electron densities, we performed pulsed TOF measurements to further verify the presence of electrons traveling with the positive ions. The simple TOF apparatus is illustrated in Figure 7a. The drift tube of length 15.3 cm was biased at +20 V to encourage electrons to travel toward the detector and to positive ions. The front electrode was grounded, and the CEM biased to detect negative charge. As the ( charge cloud entered the region between the front electrode and the extraction plate (with a 1 cm opening), a +20

Emission of Negative Potassium Ions

J. Phys. Chem. C, Vol. 114, No. 12, 2010 5705 the net positive ( charge cloud, and eventually pushes it away from the entrance of the TOF tube-taking most of the electrons with it! This process apparently requires 1-2 µs. For our purposes, the most important observation is the short travel time of the fastest extracted negative charge, confirming the presence of electrons in the ( charge cloud. 4. Discussion

Figure 7. (a) Schematic of pulsed-grid TOF measurements using a simple TOF tube of length 15.3 cm to detect negative charge pulled from the ( charge cloud by a carefully timed +10 V pulse on the extraction plate. A DC voltage of +20 V was applied to the TOF tube. (b) The TOF signal acquired with the CEM front cone biased to detect negative particles 28.2 µs after the 248-nm laser pulse at a fluence of 220 mJ/cm2. This signal is not observed when the pulse to the extraction plate is delivered either before the charge cloud arrives at the extraction plate or after the charge cloud has passed the extraction plate.

V pulse of 6 µs duration was applied to the extraction plate drawing some of the negative charge from the ( charge cloud into the TOF tube. Because the particle densities soon rearrange themselves to screen the electric field of the pulse, the duration of negative charge extraction is limited to a small time interval following the change in potential on the extraction plate in spite of the long pulse duration. The negative charge detected at the CEM is shown in Figure 7b, where time t ) 0 corresponds to the onset of the positive pulse (∼10 ns rise time) on the extraction plate. The leading edge of the resulting TOF signal rises at ∼200 ns after the rise of the positive pulse to the extraction plate. If electrons were to enter the drift tube immediately with the extraction pulse, a 20 eV electron would arrive in ∼60 ns. However, a 5 eV K- ion entering the drift tube at time t ) 0 would arrive at the detector 15 µs later. The short TOF observed is consistent with electrons (and not K-) passing through the 0.15 m length of the drift tube with 20 eV kinetic energy. The extra time to arrive (200 ns vs 60 ns) is due to the extra time required to extract the electrons from the ( charge cloud. We also detect slow negative particles (not shown) arriving at the detector 18-25 µs after the extraction pulse, consistent with the extraction of K- from the charge cloud and subsequent travel acceleration and travel through the drift tube. Neither the fast signal due to electrons nor the slow signal attributed to negative ions are observed when the extraction pulse is applied before the arrival of the ( charge cloud. The decay of the fast TOF peak 1-2 µs after the leading edge shows that electrons continue to be extracted after the onset of the extraction pulse. This extraction voltage pulse also repels

Positive ion emission from ionic materials during 248-nm laser irradiation has been attributed to an electrostatic mechanism,6 where ions adsorbed at or near surface electron traps are ejected when the underlying electron trap is photoionized by the laser. This emission is strongly enhanced by treatments that increase the number of surface defects.24 Ion kinetic energies of 4-10 eV are typical.8 In many cases, the mean ion energy is greater than the photon energy. As required by conservation of energy, the fluence dependence of positive ion emission is nonlinear. We have proposed that emission requires a sequence of defect-mediated, single photon absorption events that empty nearby electron traps; when a suitable nearby trap is emptied, it can serve as a final state for photoemission from the trap beneath the adsorbed positive ion, which is then emitted.6 Positive Ion and Electron Emission. In recent work,2 we have shown that neutral emission intensity from NaCl during 248-nm irradiation increases with bulk sample temperature in an Arrhenius fashion. The activation energy corresponds to the thermal activation energy for thermally assisted F center formation due to bandgap excitations (requiring the absorption of two-photons, which we verified). F centers formed in the bulk diffuse to the surface, where they contribute to the formation of single atomic layer pits (observed by AFM)25–27 and step erosion.2,28 This process would provide high densities of surface electron traps, suitable for alkali ion adsorption and subsequent positive emission. Adsorbed positive ions are presumably provided by diffusion from kink sites along surface steps;29 ions diffusing across the surface are readily trapped at F centers to form the Adion-on-electron trap defect configuration for emission. F centers are formed by a similar process in KBr during 248-nm irradiation, and are expected to yield intense ion emissions by a similar, two photon driven process. Surface and near surface F centers are expected to be important sources of photoelectrons. As described above, we have estimated the kinetic energy of the photoelectrons shown in Figure 6b to be 0.3 ( 0.2 eV. We have not found theoretical calculations of the position of the surface F center ground state relative to the vacuum level for KBr (for NaCl, the ground state of the surface F center lies about 4.3 eV below the vacuum level30); thus, 248 nm photons (5 eV energy) would yield photoelectron kinetic energies of about 0.7 eV-close to what we observe. Early in the laser pulse, some of these electrons will escape and charge the surface positive. Subsequent bombardment by laser photons and previously emitted electrons would provide high densities of low energy electrons near the sample surface. Significant numbers of these electrons become trapped in the potential well of the emitted positive ions. We have previously described this state as a ( charge cloud.4,11,31–33 Negative Ion Formation. The observation of both positive and negative ions from single crystals during irradiation at fluences below the threshold for optical breakdown is unexpected. The positive surface charge generated during irradiation would hinder escape of negative ions emitted directly from the surface. Furthermore, the poor spatial-temporal overlap between the ( charge cloud and thermal neutral atoms (Figure 5) and the absence of detectable Br- argue strongly against electron

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attachment to neutral atoms emitted from the surface. We are forced to consider the formation of K- via the attachment of two electrons to K+. Although double electron attachment at first glance seems unlikely, this mechanism is consistent with the strong overlap between the M+ and M- TOF distributions. High electron densities are required if electron attachment is to produce significant K0 and K- densities as the ( charge cloud travels from the surface to our detectors. Given the especially high electron densities during and immediately after the laser pulse, most of the electron attachment events are expected while the ( charge cloud is still close to the sample surface, and we suggest that significant particle densities and attachment probabilities are required on time scales on the order of a microsecond or less. In addition, the electron kinetic energy cannot be too high, or the newly formed neutrals and negative ions will be reionized before they reach the detector. In summary, high neutral densities and overlap of low energy electrons and positive ions are required for double electron attachment. To estimate the electron densities in the ( charge cloud, we take advantage of its (approximate) electrical neutrality. Small differences in the electron and positive ion densities would generate strong electric fields oriented to attract or repel electrons as necessary to ensure approximate charge neutrality (typically to a few percent).31,32 Further, the light mass of the electron assures that the required electron density will change quickly, on time scales of nanoseconds. Therefore we equate the electron density with the expected ion density just after the laser pulse. In the case of KCl, we have made careful positive charge measurements and K+ intensities as a function of fluence. At fluences where the K+ intensity from KCl is similar to the K+ intensities observed from KBr, we find that the ( charge cloud carries at least 1010 electrons at the highest fluences employed for KBr.11 To estimate the corresponding charge density, we assume that the initial volume of the charge cloud is equal to the product of the laser spot area (0.21 cm2) and the distance traveled by 4.7 eV K+ during the most intense part of the laser pulse (about 0.005 cm in 10 ns). Assuming that emission is uniform throughout area of the laser spot and constant over 10 ns, the initial volume of the ( charge cloud is about 10-3 cm3. The corresponding electron () ion) density is 1013 cm-3. Because the ions display a range of speeds and directions, the charge density will gradually drop. For the purposes of estimating the number of recombination events, we assume that this density is maintained for 1 µs. Electron attachment to positive ions can occur via both (twobody) radiative attachment and (three body) collisional attachment

radiative attachment

collisional attachment

K++e- f K0 + hν

K+ + 2e- f K0 + e-

(5a)

(5b)

In the absence of more detailed data on K+ neutralization, we employ an approximate expression for attachment rates developed by Stevefelt et al. that applies to many positive ions.34

P)

1.6 × 10-10 6.0 × 10-9 - 1.37 [e ] + [e ] + T0.63 T2.18 e e 3.8 × 10-9 - 2 [e ] T4.5 e

(6)

where P is the attachment probability in s-1 and [e-] is the electron density in cm-3. The first term on the left corresponds to radiative attachment, and the last term on the right corresponds to collisional attachment. The middle term results from the interplay of collisional and radiative processes. The first (radiative) term may be compared with Wane and Aymer’s calculations based on photoionization cross sections, which yield a attachment probability of about 1 × 10-12 s-1 per cm3 at electron temperatures near 1000 K (0.1 eV);35 the value obtained from eq 6 is about twice this. The other terms presumably display similar accuracies. For electron temperatures of 1000 K and electron densities of 1013 cm3, eq 6 predicts a collisional attachment probability of 1 × 104 s-1 or 1 × 10-2 µs-1, 4 orders of magnitude higher than the radiative attachment probability (about 1 s-1 or 1 × 10-6 µs-1). Thus we expect that collisional attachment dominates under the conditions of this work. Collisional attachment would provide up to 108 reneutralized K+ ions in a microsecond. Unfortunately, reliable electron attachment probabilities for low energy electrons to neutral potassium atoms are not available. An approximate rate can be obtained using the simple picture of radiative attachment described by Massey (1972), where the electron polarizes the neutral atom and experiences an attractive force that allows for an unstable orbit. Assuming a point-like dipole center of attraction, the resulting (Langevin) potential is give by

V(r) ) -

Re2 8πε0r4

(7)

where R is the atomic polarizability and r the distance from the atomic nucleus to the electron (impact parameter). Massey showed that there is an orbiting cross-section (where the electron spirals in while radiating classically due to the acceleration) given by

σattach )

( ) Rπe2 2ε0E

1/2

(8)

where E is the electron kinetic energy and R ) 43 × 10-30 m3 for atomic potassium. Quantum mechanical calculations of the capture cross section oscillate about this result, with a maximum deviation below 10%.36,37 These estimates neglect scattering from valence electrons, suggesting that eq 8 is an upper bound. For 0.1 eV electrons, σattach ≈ 10-14 cm2. The attachment rate is given by the product of σattach and the electron flux. At a density of 1013 cm-3, the flux of 0.1 eV electrons (2 × 107 cm/ s) is about 2 × 1020 cm-2 s-1. The resulting attachment rate is about 2 × 106 s-1, which is more than sufficient to account for the observed negative ions. Significantly, high-lying Rydberg states can be extremely polarizable.38 If electron attachment to K+ forms neutral Rydberg atoms, negative ion formation by electron attachment to these could be especially efficient.

Emission of Negative Potassium Ions We have observed similar patterns of negative ion formation in NaCl, KCl, and LiF.11 In each case, we attribute negative ion formation to double electron attachment to positive ions: positive alkali ions are converted to their negative ion counterparts. In other materials, other negative ion formation mechanisms can dominate. We have attributed negative ion emission from fused silica during 157-nm irradiation to collisional electron attachment to neutral atoms and molecules emitted directly from the surface. These negative ions appear in the region where the emitted neutral atoms and molecules overlap the ( charge cloud.33 In the case of fused silica, the neutral densities in the overlap region are much higher than we observe in the alkali halides; the especially high neutral density in the overlap region favors negative ion production there rather than in the middle of the ( charge cloud. Further, the low atomic polarizabilities of the relevant neutral particles (0.80 Å3 for O and 5.4 Å3 for Si, versus 43.4 Å3 for K)39 hinder radiative attachment; electron attachment to neutral O in particular appears to be collisional, rather than radiative.33 The observation of negative ions under conditions where the initial state carries a positive charge is remarkable. Even under this relatively gentle stimulation by the laser, the initial electron and positive ion densities are sufficiently high for double electron attachment to K+. At somewhat higher laser fluences, we observe optical breakdown, where neutral K is ionized by electron impact. Electron impact ionization is the inverse of collisional electron-ion attachment. Recombination dominates at low electron temperatures (below breakdown), while ionization dominates at high electron temperatures (above breakdown). Although nanosecond infrared laser pulses can significantly heat the electrons in the ( charge cloud,40 ultraviolet absorption is weak at these charge densities.31,32 We attribute electron heating to the acceleration of electrons in the electrostatic potential well of the net positive ( charge cloud. Although dynamic arguments show how electrons can become trapped in the net positive ( charge cloud,31 it is not clear to us what determines how much negative charge is trapped. Thus we cannot yet predict the potential well depth, the rate of ( charge cloud expansion, or the degree of coupling between positive and negative charges as a function of time. An understanding of these processes is required if we are to fully understand the process of UV laserinduced breakdown in ionic solids. 5. Conclusions We have observed negative alkali ions from single crystal KBr exposed to 248-nm excimer laser radiation at fluences below the threshold for optical breakdown. The negative ion time-of-flight signals show strong spatial-temporal overlap with the positive ion densities (K+), but virtually no overlap with the neutral particle densities (neither K0 nor Br0). Thus, we conclude that the negative alkali ion (e.g., K-) is not produced by electron attachment to the thermally emitted alkali (e.g., K0). Since the great majority of the negative charge is electrostatically coupled to the positive ions, the lack of positive ion/neutral bromine overlap accounts for the lack of detected negative bromine (Br-). The electron densities immediately after the laser pulse appear to be sufficient to produce negative alkali ions by double electron attachment to positive alkali ions. The formation of negative ions by electron-ion and electronneutral attachment is confined to the interior of an electrostatically coupled cloud of positive ions and electrons. These clouds are formed at low fluences and grow increasingly dense as the fluence as raised. The initial state of this ( charge cloud and its evolution after the laser pulse plays an important role in the

J. Phys. Chem. C, Vol. 114, No. 12, 2010 5707 onset of optical breakdown and the subsequent formation of a visible plume at higher fluences. An improved understanding of ( charge cloud behavior would contribute to applications of laser ablation that exploit particle emissions for chemical analysis and thin film deposition. In particular, the ability to control the electron densities and net charge of the ( charge cloud may improve the reproducibility and quality of the end results. Acknowledgment. This work was supported by the U.S. Department of Energy under Grant DE-FG02-04ER-15618 and the Research Institute, National Printing Bureau of Japan. References and Notes (1) Dickinson, J. T. Physical and chemical aspects of laser materials interactions. Proceedings of the NATO AdVanced Study Institute on Photonbased Nanoscience and Technology: from Atomic LeVel Manipulation to Materials Synthesis and Nano-BiodeVice Manufacturing, 2005, Sherbrooke, Quebec, Canada. (2) Nwe, K. H.; Langford, S. C.; Dickinson, J. T. J. Appl. Phys. 2005, 98, 013506. (3) Nwe, K. H.; Langford, S. C.; Dickinson, J. T. J. Appl. Phys. 2005, 97, 043501. (4) Dickinson, J. T.; Langford, S. C.; Bandis, C.; Dawes, M. L.; Kawaguchi, Y. Appl. Surf. Sci. 2000, 154-155, 291. (5) Dickinson, J. T.; Langford, S. C.; Shin, J. J.; Doering, D. L. Phys. ReV. Lett. 1994, 73, 2630. (6) Ermer, D. R.; Shin, J.-J.; Langford, S. C.; Hipps, K. W.; Dickinson, J. T. J. Appl. Phys. 1996, 80, 6452. (7) Nwe, K. H.; Langford, S. C.; Dickinson, J. T. Appl. Surf. Sci. 2002, 197-198, 83. (8) Bandis, C.; Langford, S. C.; Dickinson, J. T. Appl. Phys. Lett. 2000, 76, 421. (9) Karas, M.; Hillenkamp, F. Anal. Chem. 1988, 60, 2299. (10) Langford, S. C.; Jensen, L. C.; Dickinson, J. T.; Pederson, L. R. J. Appl. Phys. 1990, 68, 4253. (11) Kimura, K.; Langford, S. C.; Dickinson, J. T. J. Appl. Phys. 2007, 102, 114904. (12) Henke, B. L.; Liesegang, J.; Smith, S. D. Phys. ReV. B 1979, 19, 300. (13) Kano, S.; Langford, S. C.; Dickinson, J. T. J. Appl. Phys. 2001, 89, 2950. (14) Dawes, M.; Langford, S. C.; Dickinson, J. T. Appl. Surf. Sci. 1998, 127-129, 81. (15) Kawaguchi, Y.; Dawes, M. L.; Langford, S. C.; Dickinson, J. T. Appl. Phys. A: Mater. Sci. Process. 1999, 69, S621. (16) John, S. R.; Leraas, J. A.; Langford, S. C.; Dickinson, J. T. Appl. Surf. Sci. 2007, 253, 6283. (17) George, S. R.; Leraas, J. A.; Langford, S. C.; Dickinson, J. T. J. Appl. Phys. 2009, submitted. (18) Magill, J.; Bloem, J.; Ohse, R. W. J. Chem. Phys. 1982, 76, 6227. (19) Miller, T. M. Electron affinities. In Handbook of Chemistry and Physics, 71st ed.; Linde, D. R., Ed.; CRC Press: Boca Raton, FL, 1990; p 10. (20) Ermer, D. R.; Langford, S. C.; Dickinson, J. T. Appl. Surf. Sci. 1998, 127-129, 977. (21) Ejiri, A.; Hatano, A.; Nakagawa, K. J. Phys. Soc. Jpn. 1994, 63, 314. (22) Kashkai, A. D.; Berezin, A. A.; Arseneva-Geil, A. N.; Matveev, M. S. Phys. Status Solidi B 1972, 54, 113–119. (23) Fredericks, W. J.; Cook, C. J. Phys. ReV. 1961, 121, 1693. (24) Dickinson, J. T.; Shin, J. J.; Langford, S. C. Appl. Surf. Sci. 1996, 96-98, 316. (25) Wilson, R. M.; Pendleton, W. E.; Williams, R. T. Radiat. Eff. Defects Solids 1994, 128, 79. (26) Such, B.; Czuba, P.; Piatkowski, P.; Szymonski, M. Surf. Sci. 2000, 451, 203. (27) Szymonski, M.; Kolodziej, J.; Such, B.; Piatkowski, P.; Struski, P.; Czuba, P.; Krok, F. Prog. Surf. Sci. 2001, 67, 123. (28) Ho¨che, H.; Toennies, J. P.; Vollmer, R. Phys. ReV. B 1994, 50, 679. (29) Nwe, K. H.; Langford, S. C.; Dickinson, J. T. J. Appl. Phys. 2005, 97, 043502. (30) Ha¨kkinen, H.; Manninen, M. J. Chem. Phys. 1996, 105, 10565. (31) Ermer, D. R.; Langford, S. C.; Dickinson, J. T. J. Appl. Phys. 1997, 81, 1495. (32) Shin, J. J.; Ermer, D. R.; Langford, S. C.; Dickinson, J. T. Applied Physics A: Mater. Sci. Process. 1997, 64, 7.

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