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Femtosecond ionization and Coulomb explosion of small transition metal carbide clusters
Chemical Physics Letters 547 (2012) 13–20
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Femtosecond ionization and Coulomb explosion of small transition metal carbide clusters Matt W. Ross, A.W. Castleman Jr. ⇑ Departments of Chemistry and Physics, The Pennsylvania State University, University Park, PA 16802, United States
a r t i c l e
i n f o
Article history: Received 11 June 2012 In final form 23 July 2012 Available online 1 August 2012
a b s t r a c t Strong-field ionization and subsequent Coulomb explosion of small group 5 metal carbide clusters are explored using ultrashort pulses centered at 624 nm. More efficient Coulomb explosion was observed according to: Ta > Nb > V due to the larger mass of tantalum, the slower cluster expansion times, and lower ionization potentials of large atoms. Minimum laser intensities required for the onset of each atomic charge state of V, Nb, and Ta were found to be nearly identical between metal carbides and previously observed group 5 metal oxide clusters indicating electron delocalization within the cluster. Ionization enhancement is explored by comparison to semi-classical tunneling theory. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Small transition metal clusters have generated considerable interest in the past few years due to their prospect of being used as novel materials [1–5]. Early transition metals are particularly reactive toward many 3p-block elements due to the similar energy of the 3d and 3p orbitals [6]. Studying the interaction of strong light fields with matter can be used to probe the fundamental electronic properties of clusters such as ionization potential and fragmentation energies [7,8]. Identifying mechanisms responsible for strong-field ionization and subsequent Coulomb explosion in various clusters has been an active area of research in the past decade [9–16]. Ultrafast laser pulses can be approximated to strip electrons away from clusters on a faster time scale than nuclear rearrangement [17], resulting in a large buildup of positive charge. These like-charges repel according to Coulomb’s law and result in a Coulomb explosion of the cluster with highly ionized fragments released with large kinetic energies that are dependent on the size of the cluster and composition [17,18]. Coulomb explosion of noble gas clusters [19] has been observed to result in highly-charged species of Xe20+ and Kr18+ at a maximum laser intensity of 1 1015 W/cm2. Molecular dynamics simulations of Xe clusters coupled with z-scan results by Doppner et al. indicated that avalanche cluster inner ionization proceeds before cluster plasmon resonance can occur [20]. The ionization of non-metal clusters has been found to be highly dependent on cluster composition [18,21]. Small ammonia clusters [18] were found to have a significantly different mechanism of ionization than was previously observed in small metal oxide clusters [22]. Previous studies [23–25] on large polyatomic molecules have shown that non-adiabatic ⇑ Corresponding author. Fax: +1 (814) 865 5235. E-mail addresses: [email protected], [email protected] (A.W. Castleman Jr.). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.07.055
multielectron dynamics are significant in systems with many delocalized electrons, which are able to easily react to electromagnetic driving fields. Markevitch et al studied the electron dynamics of anthracene and found that the measured energies of H+ resulting from Coulomb explosion of this molecule was achieved via nonadiabatic electron localization maintained through successive ionizations during H+ expulsion [25]. These results imply that collective electron effects are significant in systems with many loosely-bound electrons. Ionization enhancement in which ion signal is observed at lower laser intensities than predicted using sequential ionization tunneling rates has been observed to occur in cluster systems. We have previously used intensity selective scanning (ISS) to quantify the extent of ionization enhancement in small group 5 metal oxide clusters [22]. Ion signal of the first atomic charge states of vanadium, niobium, and tantalum resulting from Coulomb explosion of oxide clusters was found to be in fair agreement with sequential tunneling rates for each metal. Higher atomic charge states, however, were found to be produced at orders of magnitude lower in laser intensity than predicted from ADK atomic tunneling ionization model using sequential ionization potential values [26]. The gap in laser intensity for the appearance of ion signal for the next charge state was found to be dependent on the type of orbital from which the electron is removed. Of particular interest, the saturation intensity (Isat) of each oxygen charge state appears overlapped with ion signal of the metal having a similar sequential ionization potential [22]. This suggests that electrons from oxygen are exhibiting delocalized behavior within the cluster normally observed in metallic species. Strong-field ionization of small transition metal carbide clusters (similar in size to metal oxide and pure metal clusters) has shown the same maximum charge states between the metal oxide [22], metal carbide [27] and pure metal clusters [28] for a given metal species. This would suggest that the metal species in the cluster is primarily responsible for the enhanced ionization observed;
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however, the extent of this enhancement with respect to cluster composition has not been quantified. In the present Letter, we investigate the ionization enhancement of small group 5 metal carbide clusters with between 2 and 20 atoms utilizing ISS with laser intensities between 5 1012 and 3 1014 W/cm2 and quantify the saturation intensity of each atomic high charge state (metal and nonmetal) observed in metal carbides and metal oxides [22]. The ionization enhancement behavior with respect to cluster composition is also explored based on previous models. 2. Experimental setup A detailed description of the experimental setup has been previously described and only a brief overview is given here [29].
Transition metal carbide clusters were produced utilizing a laser vaporization source (LaVa) in which the second harmonic of a Nd:YAG laser is used to ablate a metal rod (V, Nb, and Ta) of 99% purity. A pulsed jet of 40% methane gas seeded in high purity helium is passed over the metal rod while maintaining a pressure of 1 106 torr in the vacuum chamber. A small region of the metal rod is ablated for 2–3 h to remove the outer oxide layer. The resulting plasma of metal, carbon, and helium atoms undergo supersonic expansion leading to the formation of neutral and ionic clusters of up to 25 atoms in size similar to cluster distributions in reference [27]. The clusters then enter an extraction region of a home built Wiley–McLaren time-of-flight mass spectrometer [30]. A previously described [31] colliding pulse ring cavity modelocked dye laser, centered at 624 nm, with average energy of
Figure 1. Ions resulting from Coulomb explosion of (a) small vanadium carbide clusters (b) niobium carbide clusters (c) tantalum carbide clusters along with coproduced pure carbon clusters at a laser intensity of 4 1014 W/cm2.
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1 mJ per pulse and a pulsewidth of 100 femtoseconds (fs), was used to interrogate the clusters perpendicular to the ion flight path. The focused beam reached a peak intensity of 4 1014 W/cm2. The intensity selective scanning method (ISS) [32] was employed to attenuate laser intensity by translating a 60 mm focusing lens. A 2.00 mm slit replaces the last grid of the extraction region to spatially limit the collection of ions to a nearly constant intensity. The positively charged species resulting from the interaction of the ionizing laser with the cluster beam are accelerated toward a meter long field-free region after being steered by an Einzel lens and are detected with a chevron stack of multichannel plate detectors coupled to an oscilloscope and computer for analysis. The operating pressure inside the vacuum chamber was maintained at 1 106 Torr. The saturation intensity (Isat), where the probability of ionization for a particular species becomes unity, is extracted experimentally on a log–log plot at the point where experimental ion signal intersects the laser intensity axis.
Table 1 Measured saturation intensity (Isat) values of ion states for each metal. The saturation intensities are given in units of 1 ± 0.3 1013 W/cm2. Species
V (Isat)
Nb (Isat)
Ta (Isat)
M+ M2+ M3+ M4+ M5+ C+ C2+ C3+
2.42 3.34 3.68 – – 2.85 4.09 4.55
2.11 4.77 5.55 5.73 5.84 2.63 4.77 5.29
1.48 4.68 5.43 6.56 – 3.30 6.32 7.55
mental ion signal [29]. ADK-simulated ion signal using sequential ionization values is compared to experimental signal and used to quantify the observed enhancement in ionization for the cluster species.
3. Tunneling ionization model 4. Results and discussion Above laser intensities of 1 1014 W/cm2 at a wavelength of 624 nm, the Keldysh parameter predicts tunneling to be the predominant mechanism of ionization [33]. The well-known Ammosov–Delone–Krainov (ADK) [26] tunneling rate equations are used to simulate ion signal of high atomic charge states using sequential ionization potential values from literature and this method has been described in great detail elsewhere [34]. The laser intensity axis is calibrated by running ISS scans of background hydrocarbon species resulting from the oil diffusion vacuum pump prior to cluster studies and ADK theory is fit to resulting experi-
4.1. Vanadium atomic charge states from VxCy clusters Figure 1a shows the high atomic charge states produced from Coulomb explosion of small vanadium carbide clusters at an intensity of 4 1014 W/cm2. Figure 2a shows a log–log plot of ion signal vs. laser intensity for the observed high charge states of vanadium along with simulated ion signal from ADK tunneling theory using sequential ionization potential (IP) values [35]. The first charge state shows a slight suppression in ionization from its
Figure 2. ISS curves of high atomic charge states of (a) vanadium and (b) carbon. Experimental data (dots) are compared to ion signal predicted from ADK theory (solid lines) using sequential ionization potential values.
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sequential IP value and ADK theory is found to overestimate tunneling rates. This phenomenon was also observed to a greater extent by Smits et al. with vanadium, niobium, and tantalum upon direct ionization of atoms using an ultrafast infrared laser [36]. They attributed this deviation from ADK theory to be due to multielectron dynamic screening related to the polarizability of the electronic clouds in metals [36]. It is also likely that the different wavelengths used in each experiment affect the observed deviation from theory. V2+ ion signal matches with its sequential IP value in the tunneling region where the multiphoton mechanism of ionization becomes less significant. V3+ ion signal is observed to be enhanced in ionization as it does not match theoretical signal predicted from ADK tunneling ionization rates using the sequential IP value. The gap in laser intensity from the onset of V2+ ion signal to the onset of V3+ signal was found to be nearly identical to that previously observed in small vanadium oxide clusters [22] within experimental error. This indicates a similar ionization enhancement mechanism as was predicted from the observation of the same maximum charge states between metal oxide and metal carbide clusters [27]. Figure 2b shows the high atomic charge states of carbon as a function of laser intensity. C2+ and C3+ ion signal are observed at nearly identical laser intensities, indicating that these are resulting from a mechanism involving the Coulomb explosion of cluster species. Table 1 summarizes the saturation intensity (Isat)
for each of the charge states of vanadium and carbon. The gap in laser intensity required for the onset of ion signal for each charge state of carbon is found to be less than that required for oxygen. This is most likely due to the lower ionization energies of carbon compared to oxygen. 4.2. Niobium atomic charge states from NbxCy clusters Figure 1b shows the high charge states of niobium resulting from Coulomb explosion of small, singly charged niobium carbide clusters. Atomic charge states up to Nb5+ are observed as well as monovalent small pure carbon clusters. Figure 3a shows the high atomic charge states of Nb plotted as a function of laser intensity. Similar to vanadium, the first charge state of niobium is suppressed from that predicted from sequential IP values using ADK theory and Nb2+ ion signal is found to be in good agreement with predicted ion signal from ADK theory using the second sequential IP value of niobium. Enhancement in ionization is observed to begin with Nb3+ ion signal, and occurs over a narrow span of laser intensities. The Isat value of each charge state of niobium and carbon is summarized in Table 1 and similar gaps in laser intensity are observed to those from metal oxide clusters. Figure 3b shows the high charges states of carbon resulting from Coulomb explosion of NbxCy clusters. Interestingly, the gap in laser intensity between
Figure 3. ISS curves of high atomic charge states of (a) niobium and (b) carbon. Experimental data (dots) are compared to ion signal predicted from ADK theory (solid lines) using sequential ionization potential values.
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the appearance of each ion state of carbon was observed to increase from vanadium to niobium. This would indicate that changing the metal species affects the ionization behavior of carbon in the cluster. This is also consistent with the results observed in group 5 metal oxide studies [22], indicating that changing the metal will change cluster ionization properties.
4.3. Tantalum atomic charge states from TaxCy clusters Figure 1c shows the high charge states of tantalum as well as pure carbon clusters. Figure 4a shows Ta high charge states (with Ta5+ excluded due to low signal intensity) as a function of laser intensity along with ion signal calculated from ADK theroy. The Isat of each charge state of tantalum and carbon are summarized in Table 1. The difference in Isat values (DIsat) between atomic charge states of tantalum again match very well with those found from tantalum oxide clusters [22]. Figure 4b shows the high charge states of carbon as a function of laser intensity along with ion signal calculated using ADK theory. DIsat values between C+, C2+ and C3+ ion signal shown in Table 2 are observed to increase from that observed in niobium; a further indication that a different mechanism of Coulomb explosion is occurring. DIsat is observed to increase with increasing metal radii (V < Nb < Ta). A similar trend was observed in metal oxide clusters and appears to be dependent
Table 2 Difference in saturation intensity values (DIsat) between metal charge states from this Letter with carbides vs. previously reported oxide studies [22]. Differences are reported in units of 1 ± 0.6 1013 W/cm2. Isat difference
V–C
V–O
Nb–C
Nb–O
Ta–C
Ta–O
M2+–M+ M3+–M2+ M4+–M3+ M5+–M4+
0.92 0.34 – –
1.35 0.15 – –
2.66 0.78 0.18 0.11
1.74 0.67 0.42 0.36
3.2 0.75 1.13 –
1.93 0.79 1.37 –
on the atomic size and IP of the metal species. The trends of first IP of the metal species varies according to V < Nb < Ta. DIsat values in carbon are found to change proportionally with those observed in the metal species. This would suggest charge delocalization within the cluster as was proposed for similar size metal oxide clusters [22]. Pure carbon clusters were observed to a larger extent along with tantalum carbide clusters. Figure 5 shows the growth of methane signal with respect to laser intensity along with that predicted from ADK theory using sequential IP values. Due to the spherical shape of the highest occupied molecular orbital (HOMO) of CH4, atomic ADK theory (using an s-like atomic orbital) fits well with the ionization potential from literature [35]. The growth behavior of Cþ 2 was also studied and is shown in Figure 6a. Previous
Figure 4. ISS curves of high atomic charge states of (a) tantalum and (b) carbon. Experimental data (dots) are compared to ion signal predicted from ADK theory (solid lines) using sequential ionization potential values.
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literature measurements of C2+ assign the ionization potential between 11.1–13 eV [37]. Fitting ADK theory to experimental data yields an ionization potential of 11.1 eV (with a p-like atomic orbital). At higher laser intensities, Cþ 2 does not follow volume limited
decay of ion signal in the tantalum system indicating that larger pure carbon and tantalum carbide clusters are fragmenting to produce C2. Cþ 2 produced in the niobium system is shown in Figure 6b. Similar to tantalum, ADK theory matches with an ionization
Figure 5. ISS of CHþ 4 (dots) overlapped with ion signal predicted from ADK theory (solid line) at the literature ionization potential of 12.6 eV.
þ Figure 6. ISS of Cþ 2 from (a) tantalum metal study (b) niobium metal study. Each match their ADK predicted signal at an ionization potential of 11.1 eV, however, C2 from tantalum does not follow volume-limited decay indicating it is produced from fragmentation of larger clusters.
M.W. Ross, A.W. Castleman Jr. / Chemical Physics Letters 547 (2012) 13–20
potential of 11.1 eV, however, Cþ 2 signal follows volume limited decay at higher laser intensities in the niobium system, indicating that larger pure carbon and niobium carbide clusters are not being formed. No Cþ 2 was observed in the vanadium system. The degree of sequential-like ionization observed increases from Ta > Nb > V as tantalum shows better agreement with ADK ion signal (using sequential IP values from literature) than vanadium. 4.4. Cluster ionization behavior Table 2 compares the DIsat between the atomic charge states of vanadium, niobium, and tantalum resulting from oxide vs. carbide clusters. DIsat values are useful to compare differences in ionization behavior between different metal systems. The difference between the M2+ and M+ (M = V, Nb, Ta) state is expected to show large variations since the M+ charge state is produced from field ionization, fragmentation of larger clusters, and Coulomb explosion processes. It is observed that DIsat values of the M3+–M2+, M4+–M3+, and M5+– M4+ only significantly vary with metal composition and are nearly identical between oxide and carbide clusters with the same metal within an experimental error of ±6 1012 W/cm2. This is in agreement with previous studies [27] of metal carbides that found the same maximum charge states as were in the corresponding metal oxides [22]. The maximum observed charge state for each metal was V3+, Nb5+, and Ta5+. If the outer electrons of a cluster are rapidly stripped off in the laser field, then the ionization enhancement mechanism would not be expected to be dependent on cluster size, pulsewidth, and atomic weight since it is driven by the large space charge field within the cluster [38,39]. Our results coupled with those from group 5 metal oxide studies [22] imply that electrons are not localized around the non-metal species within the cluster suggesting metallic-like behavior of the entire system. These similar high charge states imply that a similar mechanism of Coulomb explosion is occurring regardless of the nonmetal species in the cluster. Upon initial field ionization of a cluster, the internal potential energy landscape is altered and the barrier to ionization is lowered. Bandrauk et al. found that the critical internuclear distance for maximum enhanced ionization only applies to bonding electrons found in sigma orbitals based on the Hþ 2 dimer [40]. Electrons in non-localized orbitals were found to experience a steady increase in ionization rate as internuclear distance grows. In a metal system with many delocalized electrons, the ionization rate is expected to experience a more rapid increase. Positively-charged ion cores within the cluster attract neighboring electrons and thus cause a cascading effect of subsequent ionizations until Coulomb explosion occurs. The higher charge states observed in tantalum and niobium versus vanadium are likely due to more efficient Coulomb explosion with higher atomic mass metal species [14]. The heavier metal atoms move slower with respect to electron motions resulting in decreased cluster expansion during the pulse duration and thus a greater number of electrons are removed. These higher charge ion states are observed to deviate from ADK tunneling theory which assumes sequential ionization, whereas ionization enhancement is a non-sequential process. Outer electrons in metal clusters are delocalized and form valance and conduction bands. These electrons have small ionization potentials and upon initial field ionization, many outer electrons are stripped away from the cluster resulting in a large, inhomogeneous electric field within the cluster. This nonadiabatic charge localization has been shown [25] to last several laser periods for polyatomic systems and is sustained through successive ionizations of the cluster species. Ionization enhancement is observed to be more significant in systems containing many delocalized electrons. Similar scale ionization enhancements of the atomic charge states of silicon resulting from Coulomb explosion of small silicon clusters [21] were not ob-
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served. Silicon was found to ionize sequentially matching ADK simulated ion signal. The silicon atoms are covalently bound in silicon clusters and no delocalized electrons are available to cause further ionization enhancement mechanisms. The increasing gap in laser intensity between C+, C2+, and C3+ from V < Nb < Ta suggests that metal atomic size and metal composition drastically affect the ionization behavior of the non-metal species in the cluster. The fact that the metal atomic charge states have nearly the same Isat values as those in metal oxide clusters [22] shows that the delocalized electrons (mostly residing in the d-orbitals of the metal) have a large influence on the observed enhancement in ionization. 5. Conclusion Strong-field ionization of group 5 metal carbide clusters was studied to look for differences in the ionization enhancement mechanism by changing cluster composition. High atomic charge states of each metal were compared to ADK theory using sequential IP values and it was found that the first atomic charge state showed a suppression in ionization due to its coproduction from field ionization and Coulomb explosion of clusters whereas the second charge state fit well with theory. Ionization enhancement in each species was observed to begin with the M3+ charge state (M = V, Nb, Ta) with the laser intensity requirements for the onset of ion signal for each charge state lowered by orders of magnitude from that predicted by ADK theory. Tantalum formed higher atomic charge states compared to vanadium due to its larger atomic size and hence the ease with which outer electrons are removed. DIsat values between the atomic charge states of carbon were observed to increase from V < Nb < Ta. Pure carbon clusters are also coproduced and the ionization potential of Cþ 2 was found to fit an ADK curve with an ionization potential of 11.1 eV, in good agreement with many previous literature values [37]. Comparison of the Isat values for V, Nb, and Ta to their counterpart group 5 metal oxide clusters [22] showed no difference in values within experimental error indicating that the electrons in the clusters are delocalized and contribute greatly to the ionization enhancement behavior. This is in contrast to results found for non-metal clusters in which changes in composition drastically affect the ionization enhancement observed. Our observations suggest that a greater degree of electron delocalization affects the extent of ionization enhancement observed in clusters containing transition metals; in good agreement with previous studies [23– 25] on multielectron nonadiabatic dynamics in polyatomic molecules. The delocalization of electrons within the d-orbitals of the metal causes a large inhomogeneous field within the cluster, lowering the barrier for ionization. Further modeling of the electron dynamics in clusters is necessary to gain a better understanding of the precise mechanisms. Acknowledgments This publication was made possible in part through our AF sponsored grant entitled, Ignition of Propellants through Nanostructured Materials, Grant No. RS120041. We also thank Dr. Scott Sayres for use of his programming code in tunneling theory calculations. References [1] A. Fielicke, P. Gruene, M. Haertelt, D.J. Harding, G. Meijer, J. Phys. Chem. A 114 (2010) 9755. [2] K.S. Molek, T.D. Jaeger, M.A. Duncan, J. Chem. Phys. 123 (2005) 144313. [3] V. Dryza, J.F. Alvino, G.F. Metha, J. Phys. Chem. A 114 (2010) 4080. [4] N. Fukushima, K. Miyajima, F.J. Mafune, Phys. Chem. A. 113 (2009) 2309. [5] K.A. Zemski, D.R. Justes, A.W. Castleman Jr., J. Phys. Chem. 106 (2002) 6136.
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Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Femtosecond ionization and Coulomb explosion of small transition metal carbide clusters Matt W. Ross, A.W. Castleman Jr. ⇑ Departments of Chemistry and Physics, The Pennsylvania State University, University Park, PA 16802, United States
a r t i c l e
i n f o
Article history: Received 11 June 2012 In final form 23 July 2012 Available online 1 August 2012
a b s t r a c t Strong-field ionization and subsequent Coulomb explosion of small group 5 metal carbide clusters are explored using ultrashort pulses centered at 624 nm. More efficient Coulomb explosion was observed according to: Ta > Nb > V due to the larger mass of tantalum, the slower cluster expansion times, and lower ionization potentials of large atoms. Minimum laser intensities required for the onset of each atomic charge state of V, Nb, and Ta were found to be nearly identical between metal carbides and previously observed group 5 metal oxide clusters indicating electron delocalization within the cluster. Ionization enhancement is explored by comparison to semi-classical tunneling theory. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Small transition metal clusters have generated considerable interest in the past few years due to their prospect of being used as novel materials [1–5]. Early transition metals are particularly reactive toward many 3p-block elements due to the similar energy of the 3d and 3p orbitals [6]. Studying the interaction of strong light fields with matter can be used to probe the fundamental electronic properties of clusters such as ionization potential and fragmentation energies [7,8]. Identifying mechanisms responsible for strong-field ionization and subsequent Coulomb explosion in various clusters has been an active area of research in the past decade [9–16]. Ultrafast laser pulses can be approximated to strip electrons away from clusters on a faster time scale than nuclear rearrangement [17], resulting in a large buildup of positive charge. These like-charges repel according to Coulomb’s law and result in a Coulomb explosion of the cluster with highly ionized fragments released with large kinetic energies that are dependent on the size of the cluster and composition [17,18]. Coulomb explosion of noble gas clusters [19] has been observed to result in highly-charged species of Xe20+ and Kr18+ at a maximum laser intensity of 1 1015 W/cm2. Molecular dynamics simulations of Xe clusters coupled with z-scan results by Doppner et al. indicated that avalanche cluster inner ionization proceeds before cluster plasmon resonance can occur [20]. The ionization of non-metal clusters has been found to be highly dependent on cluster composition [18,21]. Small ammonia clusters [18] were found to have a significantly different mechanism of ionization than was previously observed in small metal oxide clusters [22]. Previous studies [23–25] on large polyatomic molecules have shown that non-adiabatic ⇑ Corresponding author. Fax: +1 (814) 865 5235. E-mail addresses: [email protected], [email protected] (A.W. Castleman Jr.). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.07.055
multielectron dynamics are significant in systems with many delocalized electrons, which are able to easily react to electromagnetic driving fields. Markevitch et al studied the electron dynamics of anthracene and found that the measured energies of H+ resulting from Coulomb explosion of this molecule was achieved via nonadiabatic electron localization maintained through successive ionizations during H+ expulsion [25]. These results imply that collective electron effects are significant in systems with many loosely-bound electrons. Ionization enhancement in which ion signal is observed at lower laser intensities than predicted using sequential ionization tunneling rates has been observed to occur in cluster systems. We have previously used intensity selective scanning (ISS) to quantify the extent of ionization enhancement in small group 5 metal oxide clusters [22]. Ion signal of the first atomic charge states of vanadium, niobium, and tantalum resulting from Coulomb explosion of oxide clusters was found to be in fair agreement with sequential tunneling rates for each metal. Higher atomic charge states, however, were found to be produced at orders of magnitude lower in laser intensity than predicted from ADK atomic tunneling ionization model using sequential ionization potential values [26]. The gap in laser intensity for the appearance of ion signal for the next charge state was found to be dependent on the type of orbital from which the electron is removed. Of particular interest, the saturation intensity (Isat) of each oxygen charge state appears overlapped with ion signal of the metal having a similar sequential ionization potential [22]. This suggests that electrons from oxygen are exhibiting delocalized behavior within the cluster normally observed in metallic species. Strong-field ionization of small transition metal carbide clusters (similar in size to metal oxide and pure metal clusters) has shown the same maximum charge states between the metal oxide [22], metal carbide [27] and pure metal clusters [28] for a given metal species. This would suggest that the metal species in the cluster is primarily responsible for the enhanced ionization observed;
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however, the extent of this enhancement with respect to cluster composition has not been quantified. In the present Letter, we investigate the ionization enhancement of small group 5 metal carbide clusters with between 2 and 20 atoms utilizing ISS with laser intensities between 5 1012 and 3 1014 W/cm2 and quantify the saturation intensity of each atomic high charge state (metal and nonmetal) observed in metal carbides and metal oxides [22]. The ionization enhancement behavior with respect to cluster composition is also explored based on previous models. 2. Experimental setup A detailed description of the experimental setup has been previously described and only a brief overview is given here [29].
Transition metal carbide clusters were produced utilizing a laser vaporization source (LaVa) in which the second harmonic of a Nd:YAG laser is used to ablate a metal rod (V, Nb, and Ta) of 99% purity. A pulsed jet of 40% methane gas seeded in high purity helium is passed over the metal rod while maintaining a pressure of 1 106 torr in the vacuum chamber. A small region of the metal rod is ablated for 2–3 h to remove the outer oxide layer. The resulting plasma of metal, carbon, and helium atoms undergo supersonic expansion leading to the formation of neutral and ionic clusters of up to 25 atoms in size similar to cluster distributions in reference [27]. The clusters then enter an extraction region of a home built Wiley–McLaren time-of-flight mass spectrometer [30]. A previously described [31] colliding pulse ring cavity modelocked dye laser, centered at 624 nm, with average energy of
Figure 1. Ions resulting from Coulomb explosion of (a) small vanadium carbide clusters (b) niobium carbide clusters (c) tantalum carbide clusters along with coproduced pure carbon clusters at a laser intensity of 4 1014 W/cm2.
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1 mJ per pulse and a pulsewidth of 100 femtoseconds (fs), was used to interrogate the clusters perpendicular to the ion flight path. The focused beam reached a peak intensity of 4 1014 W/cm2. The intensity selective scanning method (ISS) [32] was employed to attenuate laser intensity by translating a 60 mm focusing lens. A 2.00 mm slit replaces the last grid of the extraction region to spatially limit the collection of ions to a nearly constant intensity. The positively charged species resulting from the interaction of the ionizing laser with the cluster beam are accelerated toward a meter long field-free region after being steered by an Einzel lens and are detected with a chevron stack of multichannel plate detectors coupled to an oscilloscope and computer for analysis. The operating pressure inside the vacuum chamber was maintained at 1 106 Torr. The saturation intensity (Isat), where the probability of ionization for a particular species becomes unity, is extracted experimentally on a log–log plot at the point where experimental ion signal intersects the laser intensity axis.
Table 1 Measured saturation intensity (Isat) values of ion states for each metal. The saturation intensities are given in units of 1 ± 0.3 1013 W/cm2. Species
V (Isat)
Nb (Isat)
Ta (Isat)
M+ M2+ M3+ M4+ M5+ C+ C2+ C3+
2.42 3.34 3.68 – – 2.85 4.09 4.55
2.11 4.77 5.55 5.73 5.84 2.63 4.77 5.29
1.48 4.68 5.43 6.56 – 3.30 6.32 7.55
mental ion signal [29]. ADK-simulated ion signal using sequential ionization values is compared to experimental signal and used to quantify the observed enhancement in ionization for the cluster species.
3. Tunneling ionization model 4. Results and discussion Above laser intensities of 1 1014 W/cm2 at a wavelength of 624 nm, the Keldysh parameter predicts tunneling to be the predominant mechanism of ionization [33]. The well-known Ammosov–Delone–Krainov (ADK) [26] tunneling rate equations are used to simulate ion signal of high atomic charge states using sequential ionization potential values from literature and this method has been described in great detail elsewhere [34]. The laser intensity axis is calibrated by running ISS scans of background hydrocarbon species resulting from the oil diffusion vacuum pump prior to cluster studies and ADK theory is fit to resulting experi-
4.1. Vanadium atomic charge states from VxCy clusters Figure 1a shows the high atomic charge states produced from Coulomb explosion of small vanadium carbide clusters at an intensity of 4 1014 W/cm2. Figure 2a shows a log–log plot of ion signal vs. laser intensity for the observed high charge states of vanadium along with simulated ion signal from ADK tunneling theory using sequential ionization potential (IP) values [35]. The first charge state shows a slight suppression in ionization from its
Figure 2. ISS curves of high atomic charge states of (a) vanadium and (b) carbon. Experimental data (dots) are compared to ion signal predicted from ADK theory (solid lines) using sequential ionization potential values.
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sequential IP value and ADK theory is found to overestimate tunneling rates. This phenomenon was also observed to a greater extent by Smits et al. with vanadium, niobium, and tantalum upon direct ionization of atoms using an ultrafast infrared laser [36]. They attributed this deviation from ADK theory to be due to multielectron dynamic screening related to the polarizability of the electronic clouds in metals [36]. It is also likely that the different wavelengths used in each experiment affect the observed deviation from theory. V2+ ion signal matches with its sequential IP value in the tunneling region where the multiphoton mechanism of ionization becomes less significant. V3+ ion signal is observed to be enhanced in ionization as it does not match theoretical signal predicted from ADK tunneling ionization rates using the sequential IP value. The gap in laser intensity from the onset of V2+ ion signal to the onset of V3+ signal was found to be nearly identical to that previously observed in small vanadium oxide clusters [22] within experimental error. This indicates a similar ionization enhancement mechanism as was predicted from the observation of the same maximum charge states between metal oxide and metal carbide clusters [27]. Figure 2b shows the high atomic charge states of carbon as a function of laser intensity. C2+ and C3+ ion signal are observed at nearly identical laser intensities, indicating that these are resulting from a mechanism involving the Coulomb explosion of cluster species. Table 1 summarizes the saturation intensity (Isat)
for each of the charge states of vanadium and carbon. The gap in laser intensity required for the onset of ion signal for each charge state of carbon is found to be less than that required for oxygen. This is most likely due to the lower ionization energies of carbon compared to oxygen. 4.2. Niobium atomic charge states from NbxCy clusters Figure 1b shows the high charge states of niobium resulting from Coulomb explosion of small, singly charged niobium carbide clusters. Atomic charge states up to Nb5+ are observed as well as monovalent small pure carbon clusters. Figure 3a shows the high atomic charge states of Nb plotted as a function of laser intensity. Similar to vanadium, the first charge state of niobium is suppressed from that predicted from sequential IP values using ADK theory and Nb2+ ion signal is found to be in good agreement with predicted ion signal from ADK theory using the second sequential IP value of niobium. Enhancement in ionization is observed to begin with Nb3+ ion signal, and occurs over a narrow span of laser intensities. The Isat value of each charge state of niobium and carbon is summarized in Table 1 and similar gaps in laser intensity are observed to those from metal oxide clusters. Figure 3b shows the high charges states of carbon resulting from Coulomb explosion of NbxCy clusters. Interestingly, the gap in laser intensity between
Figure 3. ISS curves of high atomic charge states of (a) niobium and (b) carbon. Experimental data (dots) are compared to ion signal predicted from ADK theory (solid lines) using sequential ionization potential values.
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the appearance of each ion state of carbon was observed to increase from vanadium to niobium. This would indicate that changing the metal species affects the ionization behavior of carbon in the cluster. This is also consistent with the results observed in group 5 metal oxide studies [22], indicating that changing the metal will change cluster ionization properties.
4.3. Tantalum atomic charge states from TaxCy clusters Figure 1c shows the high charge states of tantalum as well as pure carbon clusters. Figure 4a shows Ta high charge states (with Ta5+ excluded due to low signal intensity) as a function of laser intensity along with ion signal calculated from ADK theroy. The Isat of each charge state of tantalum and carbon are summarized in Table 1. The difference in Isat values (DIsat) between atomic charge states of tantalum again match very well with those found from tantalum oxide clusters [22]. Figure 4b shows the high charge states of carbon as a function of laser intensity along with ion signal calculated using ADK theory. DIsat values between C+, C2+ and C3+ ion signal shown in Table 2 are observed to increase from that observed in niobium; a further indication that a different mechanism of Coulomb explosion is occurring. DIsat is observed to increase with increasing metal radii (V < Nb < Ta). A similar trend was observed in metal oxide clusters and appears to be dependent
Table 2 Difference in saturation intensity values (DIsat) between metal charge states from this Letter with carbides vs. previously reported oxide studies [22]. Differences are reported in units of 1 ± 0.6 1013 W/cm2. Isat difference
V–C
V–O
Nb–C
Nb–O
Ta–C
Ta–O
M2+–M+ M3+–M2+ M4+–M3+ M5+–M4+
0.92 0.34 – –
1.35 0.15 – –
2.66 0.78 0.18 0.11
1.74 0.67 0.42 0.36
3.2 0.75 1.13 –
1.93 0.79 1.37 –
on the atomic size and IP of the metal species. The trends of first IP of the metal species varies according to V < Nb < Ta. DIsat values in carbon are found to change proportionally with those observed in the metal species. This would suggest charge delocalization within the cluster as was proposed for similar size metal oxide clusters [22]. Pure carbon clusters were observed to a larger extent along with tantalum carbide clusters. Figure 5 shows the growth of methane signal with respect to laser intensity along with that predicted from ADK theory using sequential IP values. Due to the spherical shape of the highest occupied molecular orbital (HOMO) of CH4, atomic ADK theory (using an s-like atomic orbital) fits well with the ionization potential from literature [35]. The growth behavior of Cþ 2 was also studied and is shown in Figure 6a. Previous
Figure 4. ISS curves of high atomic charge states of (a) tantalum and (b) carbon. Experimental data (dots) are compared to ion signal predicted from ADK theory (solid lines) using sequential ionization potential values.
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literature measurements of C2+ assign the ionization potential between 11.1–13 eV [37]. Fitting ADK theory to experimental data yields an ionization potential of 11.1 eV (with a p-like atomic orbital). At higher laser intensities, Cþ 2 does not follow volume limited
decay of ion signal in the tantalum system indicating that larger pure carbon and tantalum carbide clusters are fragmenting to produce C2. Cþ 2 produced in the niobium system is shown in Figure 6b. Similar to tantalum, ADK theory matches with an ionization
Figure 5. ISS of CHþ 4 (dots) overlapped with ion signal predicted from ADK theory (solid line) at the literature ionization potential of 12.6 eV.
þ Figure 6. ISS of Cþ 2 from (a) tantalum metal study (b) niobium metal study. Each match their ADK predicted signal at an ionization potential of 11.1 eV, however, C2 from tantalum does not follow volume-limited decay indicating it is produced from fragmentation of larger clusters.
M.W. Ross, A.W. Castleman Jr. / Chemical Physics Letters 547 (2012) 13–20
potential of 11.1 eV, however, Cþ 2 signal follows volume limited decay at higher laser intensities in the niobium system, indicating that larger pure carbon and niobium carbide clusters are not being formed. No Cþ 2 was observed in the vanadium system. The degree of sequential-like ionization observed increases from Ta > Nb > V as tantalum shows better agreement with ADK ion signal (using sequential IP values from literature) than vanadium. 4.4. Cluster ionization behavior Table 2 compares the DIsat between the atomic charge states of vanadium, niobium, and tantalum resulting from oxide vs. carbide clusters. DIsat values are useful to compare differences in ionization behavior between different metal systems. The difference between the M2+ and M+ (M = V, Nb, Ta) state is expected to show large variations since the M+ charge state is produced from field ionization, fragmentation of larger clusters, and Coulomb explosion processes. It is observed that DIsat values of the M3+–M2+, M4+–M3+, and M5+– M4+ only significantly vary with metal composition and are nearly identical between oxide and carbide clusters with the same metal within an experimental error of ±6 1012 W/cm2. This is in agreement with previous studies [27] of metal carbides that found the same maximum charge states as were in the corresponding metal oxides [22]. The maximum observed charge state for each metal was V3+, Nb5+, and Ta5+. If the outer electrons of a cluster are rapidly stripped off in the laser field, then the ionization enhancement mechanism would not be expected to be dependent on cluster size, pulsewidth, and atomic weight since it is driven by the large space charge field within the cluster [38,39]. Our results coupled with those from group 5 metal oxide studies [22] imply that electrons are not localized around the non-metal species within the cluster suggesting metallic-like behavior of the entire system. These similar high charge states imply that a similar mechanism of Coulomb explosion is occurring regardless of the nonmetal species in the cluster. Upon initial field ionization of a cluster, the internal potential energy landscape is altered and the barrier to ionization is lowered. Bandrauk et al. found that the critical internuclear distance for maximum enhanced ionization only applies to bonding electrons found in sigma orbitals based on the Hþ 2 dimer [40]. Electrons in non-localized orbitals were found to experience a steady increase in ionization rate as internuclear distance grows. In a metal system with many delocalized electrons, the ionization rate is expected to experience a more rapid increase. Positively-charged ion cores within the cluster attract neighboring electrons and thus cause a cascading effect of subsequent ionizations until Coulomb explosion occurs. The higher charge states observed in tantalum and niobium versus vanadium are likely due to more efficient Coulomb explosion with higher atomic mass metal species [14]. The heavier metal atoms move slower with respect to electron motions resulting in decreased cluster expansion during the pulse duration and thus a greater number of electrons are removed. These higher charge ion states are observed to deviate from ADK tunneling theory which assumes sequential ionization, whereas ionization enhancement is a non-sequential process. Outer electrons in metal clusters are delocalized and form valance and conduction bands. These electrons have small ionization potentials and upon initial field ionization, many outer electrons are stripped away from the cluster resulting in a large, inhomogeneous electric field within the cluster. This nonadiabatic charge localization has been shown [25] to last several laser periods for polyatomic systems and is sustained through successive ionizations of the cluster species. Ionization enhancement is observed to be more significant in systems containing many delocalized electrons. Similar scale ionization enhancements of the atomic charge states of silicon resulting from Coulomb explosion of small silicon clusters [21] were not ob-
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served. Silicon was found to ionize sequentially matching ADK simulated ion signal. The silicon atoms are covalently bound in silicon clusters and no delocalized electrons are available to cause further ionization enhancement mechanisms. The increasing gap in laser intensity between C+, C2+, and C3+ from V < Nb < Ta suggests that metal atomic size and metal composition drastically affect the ionization behavior of the non-metal species in the cluster. The fact that the metal atomic charge states have nearly the same Isat values as those in metal oxide clusters [22] shows that the delocalized electrons (mostly residing in the d-orbitals of the metal) have a large influence on the observed enhancement in ionization. 5. Conclusion Strong-field ionization of group 5 metal carbide clusters was studied to look for differences in the ionization enhancement mechanism by changing cluster composition. High atomic charge states of each metal were compared to ADK theory using sequential IP values and it was found that the first atomic charge state showed a suppression in ionization due to its coproduction from field ionization and Coulomb explosion of clusters whereas the second charge state fit well with theory. Ionization enhancement in each species was observed to begin with the M3+ charge state (M = V, Nb, Ta) with the laser intensity requirements for the onset of ion signal for each charge state lowered by orders of magnitude from that predicted by ADK theory. Tantalum formed higher atomic charge states compared to vanadium due to its larger atomic size and hence the ease with which outer electrons are removed. DIsat values between the atomic charge states of carbon were observed to increase from V < Nb < Ta. Pure carbon clusters are also coproduced and the ionization potential of Cþ 2 was found to fit an ADK curve with an ionization potential of 11.1 eV, in good agreement with many previous literature values [37]. Comparison of the Isat values for V, Nb, and Ta to their counterpart group 5 metal oxide clusters [22] showed no difference in values within experimental error indicating that the electrons in the clusters are delocalized and contribute greatly to the ionization enhancement behavior. This is in contrast to results found for non-metal clusters in which changes in composition drastically affect the ionization enhancement observed. Our observations suggest that a greater degree of electron delocalization affects the extent of ionization enhancement observed in clusters containing transition metals; in good agreement with previous studies [23– 25] on multielectron nonadiabatic dynamics in polyatomic molecules. The delocalization of electrons within the d-orbitals of the metal causes a large inhomogeneous field within the cluster, lowering the barrier for ionization. Further modeling of the electron dynamics in clusters is necessary to gain a better understanding of the precise mechanisms. Acknowledgments This publication was made possible in part through our AF sponsored grant entitled, Ignition of Propellants through Nanostructured Materials, Grant No. RS120041. We also thank Dr. Scott Sayres for use of his programming code in tunneling theory calculations. References [1] A. Fielicke, P. Gruene, M. Haertelt, D.J. Harding, G. Meijer, J. Phys. Chem. A 114 (2010) 9755. [2] K.S. Molek, T.D. Jaeger, M.A. Duncan, J. Chem. Phys. 123 (2005) 144313. [3] V. Dryza, J.F. Alvino, G.F. Metha, J. Phys. Chem. A 114 (2010) 4080. [4] N. Fukushima, K. Miyajima, F.J. Mafune, Phys. Chem. A. 113 (2009) 2309. [5] K.A. Zemski, D.R. Justes, A.W. Castleman Jr., J. Phys. Chem. 106 (2002) 6136.
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