Strong-Field Ionization and Coulomb Explosion of Chlorine Weakly

Jul 25, 2012 - Departments of Chemistry and Physics, The Pennsylvania State University, ... The chlorine ion signal intensity for each atomic charge s...
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Strong-Field Ionization and Coulomb Explosion of Chlorine Weakly Bound to Small Water Clusters Matt W. Ross, Cuneyt Berkdemir, and A. W. Castleman, Jr.* Departments of Chemistry and Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: Clusters exhibit an enhancement in ionization rates under intense, ultrafast laser pulses compared to their molecular/atomic counterparts. Studies of ionization enhancement of weakly bound molecules to clusters have not been previously characterized and quantified. We demonstrate that weakly bound ClO to (H2O)n (n = 1−12) clusters and weakly bound HCl to (H2O)n (n = 1−12) clusters produce high atomic charge states of chlorine via Coulomb explosion. Density functional theory (DFT) was used to qualitatively compare the interaction energy of ClO with respect to the number of water molecules as well as HCl with respect to the number of water molecules. The chlorine ion signal intensity for each atomic charge state was observed to be dependent on the molecule− cluster bond strength. The observed ionization enhancement was quantified using semiclassical tunneling theory, and it was found that the Cl3+−5+ and O2+ charge states are enhanced in ionization. Possible mechanisms of ionization enhancement are explored for weakly bound chlorine species.

1. INTRODUCTION Halogen oxides are important in many atmospheric processes and have been shown to play a role in the destruction of ozone.1−4 Chlorine dioxide has been used as an air and water disinfectant due its high reactivity.5,6 Chlorine monoxide radical has been shown to react with ozone to produce oxygen gas and halide radicals.7 It has been previously observed that ClO can form a radical−molecule complex that can enhance formation of ClO dimer.8 Fu et al. used density functional theory to study the formation of the ClO·H2O complex and found that hydrogen bonding occurs between the chlorine atom in the ClO radical and the hydrogen atom in water as well as between a hydrogen atom in the water and the oxygen atom in the ClO radical.9 They found the binding energy of the ClO radical to H2O for the lowest energy structure to be about 0.134 eV. Guo et al. extended this by looking at the microsolvation of ClO·(H2O)n (n = 1−4) clusters using high-level ab initio and density functional theory calculations.10 They found that neutral ClO·(H2O)n (n = 1−4) cluster structures were controlled by water−water interactions with ClO being weakly bound. They concluded that neutral ClO radicals are likely unperturbed by hydrated cluster formation due to their weak interactions. Previous experiments have only investigated ClO− ion using anion photodetachment spectroscopy;11,12 however, experimental characterization of the interaction between ClO and water clusters has not been investigated. The study of strongfield light pulses interacting with matter is useful in determining the electronic and dynamic properties of atoms, molecules, and © 2012 American Chemical Society

clusters. It has been well established that clusters exhibit an enhancement in ionization upon absorbing energy from stronglight pulses.13,14 Small ammonia clusters produced high atomic charge states of nitrogen (up to N4+) at nearly 2 orders of magnitude lower in laser intensity than predicted from tunneling theory using sequential ionization potentials.13 Here, we examine the strong-field ionization of ClO·(H2O)n clusters to gain a better understanding of the water−ClO interaction and compare these to DFT binding energy calculations of ClO2·(H2O)n (n = 1−10) and ClO·(H2O)n (n = 1−10). HCl·(H2O)n (n = 1−10) clusters are investigated for comparison and the enhancement in ionization signal is quantified using semiclassical tunneling theory.

2. EXPERIMENTAL SETUP A detailed description of the experimental setup has been previously described15 and only a brief explanation is given here. ClO2 gas was produced by reacting ∼2 g of sodium chlorite (Alfa Aesar) with high-purity chlorine gas (Matheson) for about 2 h. A small amount of water is also present as a byproduct of the reaction. The resulting gas mixture was seeded in high-purity helium and slowly bled into the vacuum chamber (1 × 10−7 Torr) near the laser interaction region. To produce water clusters with weakly bound ClO, the gas mixture was supersonically expanded by a pulse nozzle into the vacuum Received: April 16, 2012 Revised: July 25, 2012 Published: July 25, 2012 8530

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Figure 1. Mass spectrum of ClO2 gas seeded in helium (no clustering). The highest observable atomic charge state from laser field ionization of each atom is Cl2+ and O+ at a laser intensity of 8 × 1014W/cm2.

their sequential ionization potential values from literature. The tunneling rate equations are fit to experimental ion yields to extract effective ionization potentials and can be used to quantify the enhanced ionization observed. 3.2. Density Functional Theory Calculations. Cluster geometries in the electronic ground state were determined using density functional theory (DFT) calculations. Becke’s three-parameter hybrid functional was employed by incorporating the correlation functional of Lee, Yang, and Parr (B3LYP).21,22 In many computational problems related to DFT calculations,23 B3LYP method has been shown to be remarkably accurate and is considered the best compromise between speed and efficiency for relatively large systems. Moreover, the use of B3LYP method has been found well suited to deal with hydrogen-bonded systems.24−26 The basis set of LANL2DZ27 was selected. This uses a D95 V basis set on H and O atoms whereas an effective core potential (ECP) which includes the mass velocity and Darwin relativistic effects plus an ECP-optimized DZ basis set is employed for Cl atom. Frequency calculations were also performed at the B3LYP/ LANL2DZ level of theory. Zero-point vibrational energies (ZPVE) taken from these frequency calculations were assumed to be an upper limit due to the anharmonicity of the potential energy surface. The interaction energy (EInt) with respect to the number of water molecules for a given cluster complex is calculated as the difference between the total energies of the ClO·(H2O)n, ClO2·(H2O)m (n = 1−6, 8, 10, 12; m = 1−6) complexes and the corresponding ClO, ClO2, and H2O molecules as follows:

chamber leading to rapid collisions and cooling. Weakly bound HCl−water clusters were produced by seeding high-purity HCl gas (Matheson) in helium with a small amount of water vapor added. The clusters then enter an extraction region of a homebuilt Wiley−McLaren time-of-flight mass spectrometer.16 A previously described17 colliding pulse ring cavity modelocked dye laser, centered at 624 nm, with average energy of ∼1 mJ per pulse and a pulse width of 100 fs was used to interrogate the clusters perpendicular to the ion flight path. The focused beam reached a peak intensity of ∼1 × 1015 W/cm2. We use the intensity-selective scanning method (ISS) 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 × 10−6 Torr.

3. THEORY 3.1. Tunneling Ionization Model. At laser intensities above 1 × 1013 W/cm2 for a laser with a wavelength centered at 624 nm and a pulse width of ∼100 fs, the Keldysh parameter predicts tunneling to be the predominant mechanism of ionization.18 The well-known Ammosov−Delone−Krainov (ADK) tunneling model is applied to clusters and high charge states of atoms due to its ease of use and success in predicting atomic ionization potentials.19 The method of simulating ion signal using ADK tunneling rate equations has been described in detail elsewhere.15 Briefly, the saturation intensity (Isat) obtained experimentally for each species is mapped to an ionization potential obtained from the simulated ion signal from ADK theory and the volume-limited decay of the pulse.20 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 experimental ion signal. The energy requirements for the high charge states of chlorine and oxygen are compared to

E Int = E[X] + yE[H 2O] − E[X(H 2O)y ]

(1)

where E[X] is the total energy of individual X (X = ClO or ClO 2 ) radicals, yE[H 2 O] is the total energy of the corresponding number of water molecules, and E[X(H2O)y] (y = n for X = ClO or m for X = ClO2) are the total energies of the corresponding complexes. The HCl·(H2O)n (n = 1−6, 8, 10,12) interaction energy is calculated in a similar fashion as E Int = E[Cl−] + E[H3O+] + (n − 1)E[H 2O] − E[HCl ·(H 2O)n ] 8531

(2)

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Figure 2. (a) Pure protonated water cluster distribution with ClO2 gas. (b) Lower region of mass spectrum showing high charge states of chlorine (up to Cl5+) and oxygen (up to O3+) at a laser intensity of 8 × 1014W/cm2.

where E[Cl−] is the total energy of Cl−, E[H3O+] is the total energy of H3O+, (n − 1)E[H2O] is the total energy of the corresponding number of water molecules, and E[HCl·(H2O)y] is the total energy of the complexes. All calculations were carried out using the Gaussian 09 package.28 Different initial geometries were adopted to avoid trapping at local minima on the potential energy surface. The B3LYP hybrid method with large basis sets such as 6311++G(d,p) and aug-cc-pVnZ, (n = T, Q, and 5) are commonly used to treat halogen or hydrogen-bonded complexes29−39 to analyze properties such as electron density, charge transfer, and electron localization. In the present study, however, we focus on calculating molecule−complex interaction energies with respect to the number of water molecules in the complex by determining the appropriate intermolecular and intramolecular bond lengths. For this reason, we selected a small basis set of LANL2DZ, which is the most commonly used relativistic effective core potential with the Gaussian program.28 This basis set offers faster computation times, and results that are reasonably comparable to those obtained experimentally and from higher-level calculations. The bond lengths and bond angles of ClO, ClO2, HCl, and H2O molecules using the B3LYP/LANL2DZ basis set are compared to experimental

results and calculations utilizing larger basis sets in Table T1 (see Supporting Information). The calculated bond lengths of HCl and H2O molecules at the B3LYP/LANL2DZ level of theory in this study are in good agreement with previous theoretical and experimental results.30,31,36,39 Although the calculated bond lengths of ClO in the ClO and ClO2 radicals are higher than those obtained experimentally and theoretically with more accurate computational methods, the intermolecular interaction distances between OCl and OH 2 in the ClO·(H2O)n and ClO2·(H2O)m complexes are in good agreement with previously reported values.29−39

4. RESULTS AND DISCUSSION 4.1. ClO·(H2O)n (n = 1−12) Clusters. Laser field ionization of ClO2 gas produced ClO2+, ClO+, Cl+, and O+ fragments and trace amounts of Cl2+. However, as shown in mass spectra data in Figure 1, no higher atomic charge states of chlorine or oxygen were observed. This result corresponds well with ADK theory using sequential ionization potentials (IP) from the literature40 which predict Cl2+ to be the highest observable charge state with a maximum laser intensity of ∼8 × 1014 W/ cm2. 8532

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values of ClO (1.570 Å)29 and OH (0.957 Å),30 respectively. The ground states predicted to be lowest energy structures are similar to the structures calculated by Fu et al.,9 Galvez et al.,31and Du et al.32 The interaction energy of a water molecule to ClO is calculated to be 0.148 eV including the ZPVE correction (Table 1) which is comparable to previous calculations.29,31,32 After optimizing the geometric structure and calculating the binding energy value of the ClO radical with a water molecule, the number of water molecules was subsequently increased up to 12 using the optimized cluster geometries shown in Figure S1 in the Supporting Information. The ground-state structures were derived using previous calculations from OH·(H2O)n, BrO·(H2O)n, and ClO·(H2O)n geometries33−35 as starting guesses. Recent theoretical papers on ClO·(H2O)n (n = 1)29,31,32 suggest the most energetically stable structure for one ClO radical plus one water molecule is formed by an OCl···OH2 association, shown in Figure S1. This complex shows halogen bonding in which a weak bond between the oxygen atom in water and the chlorine atom in ClO radical forms. The association of OCl···OH2 results in an elongation of the Cl−O bond in the ClO radical by 0.009 Å (Cl−O distance in the ClO radical is calculated as 1.765 Å); however, no changes are observed for the water molecule (O− H distance and HOH angle in the water are calculated as 0.977 Å and 110°, respectively). The ClO bond length elongation results from the increasing electrostatic interaction between Cl in the ClO radical and O in the water and occurs due to large redistribution of electron density affecting the chlorine and oxygen atoms in water, as previously described in detail by Galvez et al.36 The most stable structure for the dihydrated ClO, formed by ClO radical and two water molecules, is shown in Figure S1 and gives relevant bond lengths. For the ClO·(H2O)n (n = 2) complex, the intermolecular interaction distance between ClO and water is actually smaller than that obtained for the ClO·(H2O) complex. This implies that the existence of the second water molecule reinforces the intermolecular interaction between OCl and the water dimer with respect to the corresponding one in the ClO·(H2O) complex. The geometries of the complexes arising due to the ClO−water interaction indicate a cyclic structure when greater than two water molecules are bound; however, for larger clusters the water molecules tend to bond among themselves with ClO outside of the ring (Figure S1). This is in good agreement with the findings of Galvez et al.31 Figure S2 (see Supporting Information) shows the optimized geometries of ClO2·(H2O)m (m = 1−6) complexes along with the ClO bond and intermolecular distances (see Table T1 in the Supporting Information for experimental37 ClO bond length and OClO bond angle in ClO2 radical). The structure of ClO2·H2O has a distorted chainlike structure where H2O and ClO2 are bound by a hydrogen to an oxygen. An oxygen atom in ClO2 is bonded to a hydrogen of H2O, with all the atoms aligned in the same plane. The geometries of H2O and ClO2 change very little when the number of water molecules is increased. The second water molecule adds to the free oxygen of ClO2, further perturbing the geometry of that molecule. The intermolecular bond distances for each water are calculated to be 2.205 Å using the B3LYP/LANL2DZ level of theory, which is slightly shorter than in the ClO2·H2O complex. The results presented here for ClO2·(H2O)m (m = 1,2) are in good agreement with those predicted at B3LYP/6-311++G(3df,3pd) level of theory by Aloisio et al.38 When more than two water molecules add to the ClO2 radical, the water molecules tend to

Figure 2a shows pure protonated water clusters with ClO2 gas. DFT calculations using B3LYP/LANL2DZ level of theory predict weak interaction energies of ClO with respect to (H2O)n (n = 1−6, 8, 10, 12) clusters and ClO2 with respect to the (H2O)m (m = 1−6) clusters, indicating that these molecules fragment from the water clusters subsequent to Coulomb explosion, in good agreement with the experimental ion signal observed in Figure 2a. The binding energies between ClO and (H2O)n clusters (n = 1−6, 8, 10, 12), and ClO2 and (H2O)m clusters (m = 1−6) are compared in Figure 3 and summarized

Figure 3. Comparison of the interaction energies of ClO2, ClO, and HCl species with respect to the number of water molecules in each complex at the B3LYP/LANL2DZ level of theory.

Table 1. Interaction Energy (EInt) with Respect to the Number of Water Molecules in the HCl·(H2O)n, ClO·(H2O)n, and ClO2·(H2O)m Complexesa ClO2 no. of H2O 1 2 3 4 5 6 8 10 12

EInt

EInt with ZPVE

0.199 0.538 1.229 2.339 3.130 3.889 − − −

0.130 0.408 0.956 1.899 2.595 3.227 − − −

ClO

HCl

EInt

EInt with ZPVE

EInt

EInt with ZPVE

0.188 0.712 1.542 2.608 3.454 4.266 5.691 7.346 8.952

0.148 0.535 1.204 2.144 2.894 3.584 4.880 6.290 7.632

− 8.160 9.392 10.329 11.365 12.192 13.797 15.283 17.121

− 8.094 9.211 10.045 10.856 11.578 12.961 14.360 15.747

a

Calculations are performed using the B3LYP/LANL2DZ level of theory and EInt with ZPVE indicates that zero-point vibrational energy corrections are included in the interaction energy with respect to the number of water molecules. All units are in eV.

in Table 1 in which the interaction energies with respect to the number of water molecules are given with/without ZPVE correction. The optimized structures for these clusters are shown in Figures S1 and S2, respectively, in the Supporting Information. At the B3LYP/LANL2DZ level of theory, the computed bond lengths for ClO and OH in the ClO·(H2O)n (n = 1) complex are in the comparable range with the experimental 8533

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Figure 4. High charge states of chlorine (up to Cl5+) and oxygen (up to O3+) produced from pure protonated water clusters with HCl gas at a laser intensity of 8 × 1014W/cm2.

Figure 5. HCl+ ion signal plotted as a function of laser intensity. The dotted line represents a fit using multiphoton ionization model. The solid green line is the best fit ion signal using ADK theory and the solid blue line is the ADK theory fit using the HCl+ sequential literature IP of 12.75 eV.40

water interaction is likely to be the more significant species participating in ionization enhancement. Figure 2b enlarges the lower mass region of the mass spectrum to highlight the high charge states of chlorine and oxygen produced at a laser intensity of ∼8 × 1014 W/cm2. Cl5+ is clearly observed, indicating that all valence electrons in the porbital of chlorine were removed. O3+ is also observed produced from the Coulomb explosion of pure protonated water clusters. O3+ has a similar ionization potential (IP) (54.9 eV) to Cl5+ (67.8 eV) and are observed to be the maximum high charge states of each atomic species. Ultrafast ionization processes can be approximated to occur faster than fragmentation, and therefore, it is likely that weakly bound chlorine participates in the Coulomb explosion of water clusters to produce high charge states of both chlorine and oxygen. For comparison, HCl·(H2O)n (n = 1−10) clusters were studied to see if the same maximum charge state of chlorine would be observed.

bond among themselves in contrast to the situation found for ClO·(H2O)n (n = 4−6) complexes. The accuracy of this prediction depends not only on high-level ab initio or DFT calculations but also on using appropriate basis sets which are a robust analytical functional form to simulate the interaction between the ClO/ClO2 radical and water. These calculations performed at the B3LYP/LANL2DZ level of theory are useful in predicting the optimized structures and binding energies of ClO·(H2O)n and ClO2·(H2O)m complexes. Since the B3LYP method has emerged as a good compromise between computational cost, coverage, and accuracy of results, the LANL2DZ basis set has been employed to calculate the equilibrium geometries and spectroscopic properties of small molecules or clusters.34 As shown in Figure 3 and Table 1, the ClO2·(H2O)m complexes have a weaker interaction energy with respect to the number of water molecules than the ClO·(H2O)n complex. The ClO radical has consistently stronger interaction energies to water clusters than does ClO2; therefore, the ClO− 8534

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Figure 6. Log−log plot of ion signal vs laser intensity for chlorine atomic charge states. Solid lines represent ADK simulated signal using literature sequential IP values. Signal to noise ratio decreases dramatically below 3 × 1013 W/cm2 where multiphoton ionization is expected to have a larger contribution to the overall ionization mechanism.

4.2. HCl·(H2O)n (n = 1−10) Clusters and ISS Measurements. Figure 4 shows the high atomic charge states produced from Coulomb explosion of weakly bound HCl to protonated water clusters (with a water cluster distribution shown in Figure 2a). Similar to ClO−water clusters, Cl5+ and O3+ are observed as the maximum atomic charge states. H−37Cl2+ is also observed in the mass spectrum indicating a metastable multicharged diatomic species that has previously been observed in metal oxide clusters with strong binding energies.14 The ion signal intensity ratio obtained for high atomic charge states of chlorine in HCl−water clusters is greater than that obtained from ClO−water clusters under identical laser intensity/grid voltage conditions, indicating that the stronger HCl−water interaction energy is significant in the production of high atomic charge states of chlorine. The interaction energies of HCl with respect to the number of water molecules in the HCl·(H2O)n (n = 1−6, 8, 10, 12) complexes are calculated in Table 1, and the results are shown in Figure 3 by incorporating ZPVE corrections. The geometric structures and relevant bond lengths of HCl·(H2O)n (n = 1−6, 8, 10, 12) complexes are shown in Figure S3 (see Supporting Information). The H−Cl bond length of the HCl radical is calculated to be 0.041 Å longer than the corresponding experimental value.30 Based on the geometric structures that were previously predicted using DFT calculations,39 optimization of only one of the many possible geometries was performed for n = 1−6, 8, 10, 12 clusters. The stable structure of HCl·(H2O)n (n = 1) results in an elongation of the H−Cl bond length (0.057 Å) as shown in Figure S3 and leads to the binding energy per water molecule of 0.402 eV, including ZPVE corrections. This energy value is relatively higher than that of ClO·(H2O) and ClO2·(H2O). As indicated using B3LYP/LANL2DZ+ECP level of theory, the position of the proton of HCl in the HCl·(H2O)n clusters depends on the cluster size. In the case of n ≥ 2 water molecules, calculations show that the HCl bond length elongates as the number of

water molecules increase and the geometric structures are proton-transferred forms. As shown in Figure 3, the HCl radical has stronger interaction energies with respect to the number of water molecules than the ClO and ClO2 radicals and this is reflected in the experimental intensity ratio of atomic charge states of chlorine observed. Figure 5 is a plot of HCl+ ion signal as a function of laser intensity. The dotted line represents the multiphoton ionization fit at lower laser intensities where the Keldysh parameter18 predicts this to be the dominant mechanism of ionization. It was found that ∼5 photons are necessary for the appearance of HCl+; noninteger values can result from more complex dissociation processes and tunneling ionization contributions. This gives an effective ionization potential of ∼10 eV, which corresponds well with the best fit ADK ion signal simulation (11 eV) at laser intensities ∼1 × 1013 W/cm2 where tunneling ionization has a significant contribution to the ionization mechanism. Interestingly, these values are similar to the ionization potentials of small water clusters calculated by Novakovskaya et al. (∼9−12 eV).40 At higher laser intensities where tunneling ionization plays a more significant role, the literature ionization potential of 12.75 eV41 is shown to more closely match HCl+ ion signal. Both ADK curves were fit using atomic tunneling theory with a p-like orbital representing the highest occupied molecular orbital (HOMO) of HCl+. This assumption was previously shown to be valid by Akagi et al. studying the laser tunneling ionization from HCl molecule since the HOMO of HCl has mostly p-like character.42 At lower laser intensities, it is likely that HCl+ ion signal is formed by dissociation from water clusters. Therefore, the effective appearance potential of HCl+ weakly bound to water clusters is between 10 and 11 eV. Atomic chlorine shows high charge states up to Cl5+ in Figure 4, indicating the removal of all p-electrons from its valence orbital. Figure 6 shows a log−log plot of ion signal versus laser intensity for the high atomic charge states of 8535

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correspond well to the ADK model using literature IP values. The Cl3+−Cl5+ signal is observed to overlap with Cl2+ ion signal, indicating enhanced ionization in which the valence electrons are ionized at the same laser intensity. This is a clear deviation from ADK theory which assumes a single-active electron (SAE) approximation in which only the outermost electron interacts with the laser field.19 This indicates that the laser intensity required for ionization of the Cl3+−Cl5+ charge states is lowered by nearly an order of magnitude, indicative of an “enhancement” in ionization. Figure 7 shows experimental ion signal of O+ and O2+ as well as that predicted from ADK theory using sequential IPs from literature (O3+ is observed in the mass spectrum, but has too low of a signal intensity to perform ISS measurements). Table 2 summarizes the charge states of oxygen fit using ADK theory versus literature IP values. At higher laser intensities, O+ ADKsimulated signal matches experimentally observed ion signal. O2+ ion signal has a clear gap in laser intensity from O+ ion signal; however, ionization is somewhat enhanced as the experimentally observed ion signal does not closely match the sequential IP value. The ionization potential of O2+ (35.1 eV) is very close to that of Cl3+ (39.9 eV) where enhanced ionization was observed to begin. This is also in agreement with Figure 1 in which no O2+ was observed upon ionization of pure ClO2 gas with no clusters present. 4.3. Ionization Enhancement Mechanism. For small, nonmetal cluster systems in which delocalized electrons contribute less significantly, the ionization ignition model (IIM) has been shown to be consistent with the observed experimental enhancement results for several cluster systems.43 IIM assumes that, after an initial ionization field event occurs from the laser (via tunneling), the detached electron leaves the cluster unscreened and results in an internal inhomogeneous electric field. This will lower the Coulomb potential barriers of nearby valence electrons in the cluster and effectively lower the required energy for tunneling ionization to occur. This barrier

chlorine resulting from Coulomb explosion as well as the ion signal calculated using ADK theory with sequential ionization values. At higher laser intensities where tunneling ionization is expected to contribute more to the ionization mechanism, Cl+ and Cl2+ ion signals are shown to match with ADK-predicted signal using literature41 sequential IP values of 13.01 and 23.8 eV, respectively. Below laser intensities of ∼5 × 1013 W/cm2, multiphoton ionization has a much greater contribution and the experimental ion signal will not closely match tunneling models. Due to the significant drop in ion signal as laser intensity decreases, it is difficult to accurately measure the number of photons required using multiphoton ionization models since significant fluctuations in ion signal are observed at laser intensities below 2 × 1013 W/cm2. Fragmentation of HCl may also contribute to Cl+ ion signal at lower laser intensities. Table 2 lists the effective ionization potential of each Table 2. Ion Appearance Potential (Iapp), Saturation Intensity (Isat), and Effective Ionization Potential (IP) Values Measured from Experimental Data versus Sequential Ionization Potential Literature Values41a Iapp

ion HCl Cl+ Cl2+ Cl3+ Cl4+ Cl5+ O+ O2+ a

+

6 6 7 1.5 1.5 1.5 6 1.5

× × × × × × × ×

Isat 12

10 1012 1012 1013 1013 1013 1012 1013

3 7.2 1.5 1.7 1.7 1.7 7.2 8.3

× × × × × × × ×

13

10 1013 1014 1014 1014 1014 1013 1013

IP (exptl ADK fit)

IP (lit.)41

11.2 13.0 23.9 24.0 24.0 24.0 13.6 24.3

12.75 13.01 23.8 39.9 53.5 67.8 13.6 35.1

All intensities are given in W/cm2 and energies in eV.

high charge state of chlorine fit using ADK model versus the sequential IP value from the literature. A clear gap in laser intensity for the onset of ion signal is observed between Cl+ and Cl2+, indicative of sequential ionization and is observed to

Figure 7. Log−log plot of ion signal vs laser intensity for oxygen atomic charge states. Solid lines represent ADK simulated signal using literature sequential IP values. 8536

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lowering effect is increased as the charge state of the ion in the cluster is increased, consistent with the experimental findings of Cl3+−Cl5+ having large enhancements in ionization while the ionization rate of O2+ is only slightly enhanced. Deviation of ion signal of high atomic charge states from sequential IP values is indicative of ionization enhancement in which collective electron effects are significant. IIM predicts enhancement through the internal lowering of the Coulomb potential for the outer valence electrons. Due to the large number of fragmented clusters in the system, electron impacts would also be expected to further enhance ionization of clusters in addition to the IIM mechanism. Weakly bound chlorine species are subjected to the inhomogeneous field created by the water clusters subsequent to field ionization and participate in Coulomb explosion of the cluster. The strength of the bond to the cluster affects the extent of enhanced ionization observed as ClO·(H2O)n (n = 1− 12) has a lower signal intensity ratio of high charge states than HCl·(H2O)n (n = 1−10) at identical laser intensity and grid voltage conditions. Therefore, weakly bound species are able to participate in enhanced ionization via the IIM mechanism43 with small contributions from electron impact processes. The internal lowering of the Coulomb potentials of neighboring species to highly charged ions via the IIM mechanism43 will affect weakly interacting species in close proximity to the cluster. Pulse-width measurements would be useful in future studies to better understand the role of collective electron effects versus intracluster space-charge ionization enhancement via IIM mechanism which is pulse-width independent.43 As the number of water molecules within the cluster is increased, enhancements in the ionization rate are expected to become larger as more electrons are able to participate in the overall mechanism.

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ASSOCIATED CONTENT

S Supporting Information *

Figures of DFT structures of each cluster system described in the text and validation of the B3LYP/LANL2DZ methodology. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (814) 865-7242. Fax: (814) 8655235. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Science Foundation (NSF)., grant no. ATM-0715014, for financial support as well as Dr. Scott Sayres for use of his programming code in tunneling theory calculations.

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REFERENCES

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5. CONCLUSION We have shown that weakly bound ClO and weakly bound HCl to protonated water clusters can participate in ionization enhancement processes and contribute to subsequent Coulomb explosion upon exposure to ultrafast pulses. DFT calculations suggest that the ClO bond strength to water clusters will be less than that of HCl to water clusters corresponding to the observed increase in ion signal intensity of the high atomic charge states of chlorine from the ClO system to the HCl system. HCl+ was shown to have an effective appearance potential ∼10−11 eV from multiphoton and ADK tunneling theory analysis, indicating that ion signal is likely originating from dissociation from water clusters. Enhanced ionization is observed to occur past the Cl3+ state and a slight enhancement is observed in O2+, both with ionization potentials around 35− 40 eV. Cl3+−Cl5+ ion signal is observed to overlap in the ISS curves, indicating ionization enhancement and a breakdown of the SAE approximation assumed in ADK tunneling theory with sequential IP values. This enhancement in ionization was found to be consistent with IIM as the inhomogeneous electric field in the cluster would allow for enhanced ionization to occur in weakly bound species before fragmentation can occur. Electron impact ionization is also expected to have a significant contribution to the ionization enhancement mechanism due to heavy cluster fragmentation in the system. 8537

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