Intense-Field Double Detachment of Electrostatically Bound F

Article pubs.acs.org/JPCA

Intense-Field Double Detachment of Electrostatically Bound F−(NF3)n Cluster Anions Y. Albeck,† G. Lerner,† D. M. Kandhasamy, V. Chandrasekaran, and D. Strasser* Institute of Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel ABSTRACT: The interaction of intense laser pulses with size-selected F−(NF3)n clusters is experimentally studied. Intense-field double- and multipledetachment processes in the isolated atomic F− anion and in electrostatically bound F−(NF3) dimer and F−(NF3)2 trimer systems are directly compared. Both dimer and trimer systems are found to exhibit similar enhancement of the highly nonlinear processes, with respect to the atomic system, as reflected in significantly lower saturation intensities. The dependencies of different product channels as a function of laser peak intensity, polarization ellipticity, and laser pulse shape are presented, indicating an efficient nonrescattering mechanism.

1. INTRODUCTION Intense laser pulse interactions with atomic and molecular anions are intrinsically different from interactions with overall neutral or positively charged systems. For neutral systems, tunnel ionization and semiclassical rescattering models provide an intuitive picture for considering intense-field interactions with isolated atoms and molecules,1−4 successfully accounting for complex phenomena such as above-threshold ionization, double ionization, and high-order harmonic generation (HHG).1,4−9 In this picture, an electron is released from a neutral system by tunnel ionization and swept by the strong oscillating electric field of the laser pulse, allowing efficient nonsequential absorption of energy on the order of the ponderomotive potential, corresponding to many quanta of the photon energy. The absorbed energy can manifest itself by emission of high-kinetic-energy electrons, high-energy photons, or double and even multiple ionization of the isolated molecule.5−7 In anionic systems, the lower electron binding energies facilitate the release of electrons from the system at lower field intensities. Furthermore, once an electron is removed from an anion, its motion is not affected by the attractive Coulomb potential present in initially neutral or cationic systems. Anionic systems can therefore be expected to exhibit new mechanisms of intense-field interaction with matter.10−13 However, despite the progress in understanding intense-field interactions with neutral and cationic systems, far less is known about interactions with anionic systems. Nonsequential as well as rescattering processes were considered theoretically to explain the experimentally measured high-energy photoelectron spectra emerging from intense-field interaction with atomic anions.12,14−16 Pedregosa-Gutierrez et al. reported enhanced double detachment of atomic F− by linearly polarized, compared to circularly polarized, 30 fs pulses.11 The observed enhancement was interpreted as first evidence for a nonsequential double-detachment process and © XXXX American Chemical Society

tentatively assigned to the rescattering mechanism, which is acutely suppressed for circularly polarized light. In contrast, Bergues and Kiyan proposed that fast photoelectron spectra can be accounted for by sequential double detachment with longer intense 100 fs pulses.17 A nonsequential mechanism is also supported by pulse shaping studies, showing strong suppression of intense-field double detachment of the molecular SF6− anion by prepulses introduced by negative third-order dispersion (TOD).18,19 However, the weak dependence on polarization ellipticity was found to be inconsistent with semiclassical rescattering dynamics, suggesting that a different nonsequential mechanism dominates intense-field double detachment of molecular SF6−.18,19 In a recent study, intense-field interaction with the atomic F− and molecular SF6− were directly compared, using the “z-scan” method20 to determine the saturation intensities at

(

which the highly nonlinear processes reach 1 −

1 e

) of their

maximal probability.21 The molecular SF6− system showed enhanced double-detachment efficiency compared to the F− anion, reflected in a significantly lower saturation intensity. Nevertheless, both systems were found to exhibit weak polarization ellipticity dependence, indicating a nonrescattering mechanism.21 In the absence of a successful theoretical picture for intensefield interactions with anionic systems, it is important to examine experimentally the origin of the reported enhancement. In particular, it is valuable to determine whether the enhancement is unique to the specific rich electronic structure of the SF6 system or can be expected for any polyatomic anion system. Special Issue: Ronnie Kosloff Festschrift Received: December 2, 2015 Revised: January 14, 2016

A

DOI: 10.1021/acs.jpca.5b11792 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 1. Schematic representation of the photofragment spectrometer setup for characterizations of intense-field interactions with mass-selected anion species. The inset shows a typical cluster mass spectrum.

the z displacement of the laser focal point from the anion beam by taking advantage of two effects. On the one hand, nonlinear processes yield increases with increased peak intensities as the anion beam approaches the focal point. On the other hand, the shrinking interaction volume leads to a decrease of the yield. Consequently, maximal product yield is obtained at a z displacement that balances these opposite trends.20 Albeck et al. showed that the Imax peak intensity at which maximal yield is obtained is directly related to the saturation intensity by a simple geometric factor, G= Imax/Isat.20 Assuming realistic temporal and spatial Gaussian laser beam profiles, this geometric factor was shown to rapidly converge with the increasing nonlinearity of the process to a robust value of G = 3.20 In addition to peak intensity dependence, the response to laser polarization ellipticity is characterized by rotation of a quarter-wave plate, allowing continuous tuning between linear and circular polarization. Using a 128 pixel SLM pulse shaper, positioned between the oscillator and amplifier, the residual spectral phase is compensated to obtain transform-limited (TL) pulses as determined by an iterative multiphoton intrapulse interference phase scan (MIIPS).26 The pulse shape is further controlled by applying an additional spectral phase to induce prepulses or postpulses, as described in the following. As the intense-field interaction products are formed in the moving frame of the anion beam, all photoproducts continue toward the time- and position-sensitive microchannel plate (MCP) detector located downstream of the photofragment spectrometer. Photoproducts are separated according to their charge-over-mass ratio by the second part of the spectrometer potential. Cationic products are accelerated, reaching the detector first, followed by the neutral products of laser−ion interaction, while the parent anion beam is decelerated, arriving last to the detector. In this way, all photoproducts can be efficiently detected and distinguished by their TOF with respect to the laser pulse time. The intrinsically low number densities of the anion beam make it possible to record the correlated photofragments for a single molecule at a time. Reliable comparison of intense-field interactions with different atomic and cluster anions is obtained by continuously scanning the selected ion species, peak intensity, polarization ellipticity, and laser pulse shape within the same computer-controlled experimental sequence.

In this paper, we present experimental characterizations of intense-field interaction with F−-based systems, directly comparing the isolated atomic anion to size-selected cluster anions of the F−(NF3)n form. The low, ∼0.3 eV, dissociation energy of the electrostatically bound complexes offers a unique opportunity to investigate the effect of a neighboring NF3 molecule on the interaction of the anion system with intense laser pulses, with minimal disruption of the F− anion positioned at a ∼2.6 Å distance from the NF3 nitrogen atom.22 The dependence of double- as well as multiple-detachment product yields on the laser peak intensity, polarization ellipticity, and pulse shapes is presented and discussed for the F−(NF3) dimer and F−(NF3)2 trimer systems.

2. EXPERIMENTAL SETUP The experimental setup for the characterization of intense-field interactions with anionic systems was previously described in detail.18−21 Briefly, anions are produced by a 200 eV pulsed electron gun in a supersonic expansion of argon carrier gas seeded with a ∼2% NF3 precursor gas sample at a total pressure of ∼14 atm. Atomic F− is efficiently produced by dissociative electron attachment to NF3, forming electrostatically bound cluster anions of the F−(NF3)n form.23,24 The cold anions are accelerated up to ∼4.7 keV by a pulsed Wiley−McLaren-type time-of-flight (TOF) mass spectrometer,25 toward a dedicated photofragment spectrometer shown schematically in Figure 1. A typical TOF mass spectrum showing mass peaks corresponding to the F−(NF3)n cluster series with n = 0−4, as well as small peaks assigned to F2−(NF3)n clusters, is presented in Figure 1 inset. A pulsed “mass-gate” deflector at the entrance of the photofragment spectrometer allows selection of a specific cluster species based on their TOF. The selected anions are further accelerated by the Vsp = 4 keV spectrometer potential before reaching the laser−anion interaction region. The peak intensity obtained at the laser−ion interaction region by the ∼3 mJ and 35 fs laser pulse is computercontrolled by displacement of a 250 mm focal length lens reaching up to ∼4 × 1015 W/cm2 at the focal spot, based on an M2 = 1.75 and w0 = 25.5 μm beam waist characterized by beam profile imaging. Due to the finite dimensions of the ion beam target, which are much larger than the laser beam waist, individual anion molecules experience different intensities depending on their radial distance from the center of the Gaussian beam, as well as on their exact position along the laser propagation direction. Nevertheless, reliable saturation intensities can be extracted for different nonlinear processes by zscan analysis20 of their respective product yields as a function of

3. RESULTS AND DISCUSSION Intense laser pulse interaction with atomic F− produces neutral F as well as cationic F+ and F2+ multiple-detachment products. B


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another cation. Also for the trimer system, dicationic atoms are predominantly observed in coincidence with other cations and not in coincidence with neutral products. In contrast to the F−(NF3) dimer, the F−(NF3)2 trimer system clearly exhibits N+ + NFm+ (marked by contour IV) as well as NFm′+ + NFm+ coincidence events indicating simultaneous ionization of both NF3 molecules on the opposite sides of the atomic anion. With the advent of coincidence analysis, it is possible to disentangle different double- and multiple-detachment channels in the cluster systems and compare their intensity dependence to the analogues products of intense-field interaction with the atomic anion. Figure 3a−c presents the z-scan analysis20 for the

The presence of neighboring NF3 molecules results in a much richer fragmentation. Significant ionization and fragmentation also of the NF3 molecules is observed, indicating that the loosely bound NF3 molecules are not merely spectators of the intense-field interaction with the atomic anion. In fact, we do not observe an intact NF4+ cation signal in the product spectra. It is therefore valuable to use the coincidence information to resolve the dominant double- and multiple-detachment channels. Figure 2a shows a typical coincidence map obtained

Figure 2. Coincidence map plots for (a) F−(NF3) and (b) F−(NF3)2, showing two-hit correlations between the dissociative ionization products. The upper and right axes show the respective peak assignments, while contours I−IV highlight specific channels as described in the text. Figure insets show the respective geometries of the dimer and trimer systems.22

for F−(NF3), describing two-hit correlations between different products distinguished by their TOF to the detector, relative to the laser pulse time. Double-detachment products are identified by a cation arriving in coincidence with a neutral product. For example, contour I indicates events corresponding to a NF3+ cation arriving in coincidence with a neutral fragment, tentatively assigned as F, while contour II indicates events corresponding to F+ cations arriving in coincidence with a neutral product. At intensities higher than ∼5 × 1013 W/cm2, detachment of multiple electrons from the F−(NF3) system can produce cation−cation coincidence signals. Contour III indicates F+ cations arriving in coincidence with another cation. It is important to note that under our experimental conditions, random coincidences that can result from fragmentation of two different molecules are negligible, as indicated by the absence of correlated N+ + N+ and N+ + NFm+ coincidences. At peak intensities above ∼2 × 1014 W/cm2, multiple detachment can also produce doubly ionized N2+ and F2+ that can be observed in coincidence with a counterpart atomic cation, indicating the complete disintegration of the system into atomic fragments. Interestingly, the low probability of a dication coincidence with a neutral product suggests that the observed doubly charged atoms are produced from highly charged intermediates, which distribute their charge among the atomic fragments. Addition of a second NF3 molecule to the cluster forms a F−(NF3)2 cluster anion with the two NF3 molecules located on opposite sides of the atomic anion.22 The weak bonding of the central atom to the NF3 molecules is reflected in the lack of intact or partially fragmented clusters in the product spectrum that exhibits the same species as the dimer system. The differences due to the addition of a second NF3 molecule are revealed only in the coincidence map analysis shown in Figure 2b. Similar to the F−(NF3) case, contours I and II mark NF3+ and F+ cations arriving in coincidence with neutral products, while contour III marks coincidences of F+ atomic cations with

Figure 3. The z-scan analysis showing the intensity dependence of selected final channels for the isolated F− (a), F− (NF3) dimer (b), and F− (NF3)2 trimer (c) systems. The F+ yield for the atomic system and analogous F+ + neutral channels in the cluster species are represented by empty circles. Squares represent multiple detachment channels, that is F2+ products of the atomic anion and F+ + X+ for the cluster anions. The intensity dependence for the molecular NF3+ cation in coincidence with a neutral product are presented by triangles.

atomic and cluster systems, which extracts saturation intensities for different double- and multiple-detachment channels from their respective yield dependence on laser peak intensity. As the laser peak intensity is varied by systematic displacement of the laser focal point from the anion beam that changes also the interaction volume, processes with different saturation intensities exhibit their maximal yields at z displacements corresponding to different Imax peak intensities. The full lines in Figure 3a−c indicate fitting with an approximate analytic function of the form n

Y (z) = ηA(z)(1 − e−σI(z) )

(1)

The function reflects the linear dependence of the yield on the interaction volume A(z) and nonlinear dependence on the C


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The Journal of Physical Chemistry A inversely proportional peak intensity I(z). The σ and n parameters represent a power law model approximation for the nonlinear cross section dependence on peak intensity, while η is an overall normalization factor. Due to the finite width of the ion beam as well as the Rayleigh length, A(z) dependence on displacement is proportional to [(z − z0)2 + zR2]. The optimization of z0 and zR fit parameters that are sensitive to the precise position and width of the ion beam is performed consistently for all product channels, while η, σ, and n fitting parameters are fitted separately for each process. Although the analytic form successfully reproduces the measured yields, as can be seen in Figure 3, the simplified function does not include the variation of peak intensity across a realistic Gaussian beam profile. Therefore, we do not assign a physical meaning to the fitting parameters, using the fit only to guide the eye and extract the displacement at maximal yield. The corresponding Imax values, which are determined based on separately performed measurements of the laser beam profile as a function of z, are then divided by the geometric factor G = 3 to obtain saturation intensities.20 Figure 3a shows the yields of double- and triple-detachment products of an intense-field interaction with F− as a function of peak intensity. The intense-field double detachment of the atomic system to produce F+, which is represented by the open circles in Figure 3a, saturates at 26 ± 4 × 1013 W/cm2 peak intensity. In contrast, triple detachment, which requires an additional ∼36 eV to produce F2+, is represented by the open squares and reaches its maximal yield at z = 0 displacement. Thus, only a lower limit of Isat > 1.37 PW/cm2 can be determined for the corresponding triple detachment saturation intensity. Figure 3b shows the z-scan analysis of the F−(NF3) dimer. Intense-field double-detachment yields of the NF3+ + F final product channel are presented by open triangles, reaching maximal yield at z = 10.5 mm displacement, which is significantly higher than the z = 5.5 mm required for maximal double-detachment yield from the atomic system. Consequently, the double-detachment saturation intensity for the F−(NF3) dimer is 8 ± 1 × 1013 W/cm2, more than 2 times lower than the corresponding Isat for the atomic anion. As the ionization potential of neutral NF3 is 12.9 eV,27 significantly lower than the 17.4 eV ionization potential28 of the F atom, we consider also the intensity dependence of F+ in coincidence with a neutral fragment that is also found to saturate at a relatively low peak intensity, as shown by the open circles in Figure 3b. Multiple-detachment channels, represented by open squares, producing correlated singly charged cations, reach their maximal yield at z = 7.5 mm. The corresponding Isat = 15 ± 3 × 1013 W/cm2 is more than 8 times lower than the lower limit determined for the analogues triple detachment of the atomic anion. Thus, the lower saturation intensities clearly indicate the enhancement of both double and multiple detachment by the presence of a loosely bound neighboring molecule. Figure 3c presents the effect of adding a second NF3 molecule on the intense-field interaction with the anionic complex. The double-detachment channels leading to NF3+ and F+ in coincidence with neutral fragments, as well as triple detachment leading to two correlated cations, exhibit their maximal yields at peak intensities comparable to the analogues channels of the dimer complex. Table 1 summarizes the saturation intensities derived for different final channels for the F−(NF3)n atomic and cluster systems studied in the present work as well as previously

Table 1. Summary of the Main Experimental Results of Atomic, Molecular, and Cluster Anion Multiple-Detachment Processesa channel SF6

−b

F− F−(NF3)

F−(NF3)2

SF5+ +

+F F +X F+ + X + F+ F2+ NF3+ + F F+ + X F+ + X + F2+ + X+ NF3+ + F F+ + X F+ + X + N+ + NFm+ F2+ + X+

β

Isat (1013 W/cm2) 9 12 18 26 > 137 8 7 15 43 9 8 13 9 38

R

± ± ± ±

2 4 6 5

6 5 10 2

± ± ± ±

4 3 4 1

0.45 0.6 0.75 0.15

± ± ± ±

0.1 0.05 0.05 0.1

± ± ± ± ± ± ± ± ±

1 2 3 15 2 1 3 2 12

6 5 4 3 3 4 5

± ± ± ± ± ± ±

2 2 2 2 2 3 3

0.5 0.45 0.3 0.2 0.3 0.3 0.2

± ± ± ± ± ± ±

0.05 0.05 0.05 0.1 0.15 0.15 0.1

Channel-specific saturation intensities, as well as β and R parameterization of the ellipticity dependence slope, and circular to linear yield ratios, respectively. bSF6− results were obtained from previously published results.19 a

reported work for double and multiple detachment of the molecular SF6− anion.19 The saturation intensities determined for both cluster species are comparable to the analogues intensities reported for the SF6− molecular anion and are enhanced compared to the atomic anion case. Strikingly, also multiple detachment involving much higher ionization levels of the molecular and cluster systems saturate at peak intensities comparable to or lower than double detachment of atomic F−. In contrast, triple detachment of the atomic anion does not show saturation even at an order of magnitude higher peak intensities. Intense-field interaction with neutral clusters was extensively studied, showing enhancement of multiple ionization and HHG that increase with increasing cluster size.29−32 In contrast to neutral cluster studies, mass-selected anions allow addition of one cluster molecule at a time, indicating that the enhancement observed here for the intense-field multipledetachment mechanism does not simply scale with the increase of the system’s geometric cross section or overall number of atoms. The enhanced response of the molecular and cluster systems compared to that of the atomic anion must therefore be due to a fundamental difference between atomic and molecular systems. In our previous works, intense-field double detachment of both atomic F− and molecular SF6− systems was already shown to exhibit a nonrescattering mechanism.18,19,21 The dotted line in Figure 4 shows the ellipticity dependence trend of the intense-field double-detachment process in the atomic F− system, recorded at ∼2 × 1014 W/cm2 peak intensities to avoid a saturation effect.21 The empty circles show the doubledetachment yield sensitivity following the addition of a neighboring NF3 molecule, measured at lower peak intensity to avoid significant saturation approaching Imax. For comparison, the dashed line in Figure 4 shows a typical response of a 2 rescattering mechanism, characterized by an exponential e−βε 33−35 dependence on the polarization ellipticity ε, with β > 30. Clearly, neither system exhibits the acute sensitivity to ellipticity, a distinct fingerprint of rescattering dynamics. The measured ellipticity dependence can be parametrized by a D


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Figure 4. Circles show the integrated signal of all cation doubledetachment products for the F−(NF3) dimer as a function of ellipticity for peak intensities of ∼1014 W/cm2, normalized to the yield for

Figure 5. Double detachment of the dimer response to the laser pulse shape as a function of the corresponding peak intensity relative to the TL pulse. Circles indicate the response to spectral chirp (which was found not to depend on chirp sign), and the triangles and inverted triangles mark the respective positive and negative TOD responses. The inset shows examples for shaped pulses with 60% TL peak intensity obtained with 103 fs2 chirp and ±12 × 104 fs3 TOD.

2

linearly polarized light. The data are fitted with a Y(ε) = A · e−βε + B ellipticity response function shown by the full line and characterized by a β = 5 ± 2 and a circular to linear ratio R of 0.5 ± 0.05. The dotted line is the F− double-detachment ellipticity response (β = 2, R = 0.15),21 while the dashed line represents the typical acute suppression of rescattering mechanisms by polarization ellipticity, with β = 30 and a vanishing yield for circular polarization.

in Figure 5, the pulse shape dependencies reflect mainly the response to the changing peak intensity and not to the presence of prepulses or postpulses. In order to disentangle the asymmetry of the pulse shapes induced by spectral phase manipulation from the peak intensity, we performed additional experiments with a different type of asymmetric pulse shape. We divided the spectral bandwidth of the ultrafast pulse into two spectrally resolved pulses that are separated in time by applying a φ(ω) = Δt|ω − ω0| spectral phase. In addition, the blue part of the spectrum was chirped to obtain asymmetric pulse shapes with a chirped pulse arriving before or after the main pulse. The degree of chirp was selected such that the chirped prepulse could efficiently detach a single electron, while the peak intensity of the main pulse that is more than 2 times higher was sufficient to perform double detachment. The resulting asymmetric pulse shapes are shown in Figure 6a insets for Δt = ±100 fs. As we did not observe any significant product yield dependence on the time delay beyond the peak intensity change effect for overlapping pulses at |Δt| < 100 fs, Figure 6a and b compares the average responses of intense-field double detachment to a chirped prepulse and postpulse. As shown in Figure 6a, the F+ yield from the atomic system does not exhibit any significant dependence on pulse symmetry. In order to avoid saturation effects, the measurement was performed at a ∼0.2 PW/cm2 peak intensity that does not lead to F2+ production. The lack of a distinct enhancement or suppression due to the induced chirped prepulses does not allow conclusions about the sequential versus nonsequential nature of the intense-field mechanism in the atomic F− anion. In contrast, Figure 6b shows that chirped prepulses induce significant suppression of the entire cation product spectrum for the F−(NF3) complex, suggesting a nonsequential intense-field double-detachment mechanism. If we consider a sequential detachment mechanism, the first detachment is likely to lead to dissociation of the electrostatically bound complex and form neutral F atoms. The subsequent ionization of the neutral F atoms should then exhibit a similar saturation intensity as the same sequential

2

Y(ε) = A · e−βε + B fit function, shown by the solid line. As summarized in Table 1, double detachment of the cluster systems shows a low β, indicating weak ellipticity dependence and a finite circular to linear yield ratio (R), which are both inconsistent with a rescattering-based mechanism. The fact that similar ellipticity dependence is observed for the atomic, cluster, and molecular anion systems with very different electronic structures suggests that the ellipticity dependence reflects the underlying nonlinear dependence on the laser electric field strength, which decreases with ellipticity. In our previous work,21 we observed very different responses of intense-field double detachment in the atomic F− and molecular SF6− systems to the laser pulse shape. Negative or positive TOD of the laser pulse induces prepulses or postpulses, respectively, arriving before or after the main intense pulse. Double detachment of the molecular SF6− anion was suppressed by the presence of prepulses, compared to postpulses, consistent with a nonsequential double-detachment mechanism that is quenched by early detachment. In contrast, double detachment of atomic F− was found to exhibit strong suppression by pulse shapes induced by both negative and positive TOD, reflecting a strong dependence on peak intensity. Thus, no distinct suppression nor enhancement, which could be interpreted as indication for a nonsequential or sequential mechanism, respectively, could be attributed to TOD-induced prepulses.21 To directly compare the response to different types of pulse shapes, Figure 5 shows the response of the F−(NF3) dimer double-detachment yield to the laser pulse shape as a function of the peak intensity fraction relative to the peak intensity of the TL pulse. The triangles and inverted triangles represent response to positive and negative TOD. Furthermore, to separately estimate the effect of the asymmetric pulse profiles and nonlinear peak intensity dependence, the circles show the double-detachment yield response to spectral chirp that stretches the pulse symmetrically without forming prepulses or postpulses. Similar to the previously studied atomic and molecular systems, we did not observe a significant effect of the chirp sign.18 As clearly shown E


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vibrational manifold and dissociation continuum states. In particular, the higher density of states in molecular and cluster systems can lead to enhancement of shakeup-like processes that promote efficient multiple ionization without rescattering that can be excluded based on the weak polarization ellipticity dependence.36−38 Furthermore, the subtle differences observed in the response to pulse shaping manipulations can arise from double-detachment competition with molecular dissociation. We therefore propose that theoretical investigation, needed to confirm the nature of the efficient intense-field doubledetachment mechanism, can be performed on simplified model systems in which an atomic anion is electrostatically bound to a simple molecule.

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AUTHOR INFORMATION

Author Contributions †

Y.A. and G.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge funding from the European Community’s seventh famework [FP7/2007-2013] under Grant Agreement #247471, as well as from the Legacy Heritage fund (Israel Science Foundation). We also acknowledge the assistance of Prof. Ori Cheshnovsky of Tel-Aviv University who provided us with the cold ion source system as well as valuable knowhow.

Figure 6. (a) Photofragment spectra of F− interacting with a chirped prepulse-shaped laser pulse (solid line) or postpulse (dotted line). Pulse shapes are generated by applying spectral phase (see the text for details) and are illustrated in the inset with respective lines. (b) photofragment spectra of F−(NF3) resulting from similar pulse shape measurements, in this case showing clear suppression of doubledetachment yield for the prepulse case.

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process starting from the atomic anion. Thus, a nonsequential mechanism is also supported by the significant enhancement observed for the cluster species. The nonsequential nature is further supported by the equivalent saturation intensities for NF3+ and F+ production from the cluster anions, despite the very different ionization potentials of the corresponding neutral species.27,28

REFERENCES

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4. CONCLUSIONS Intense-field interaction with electrostatically bound F−(NF3)n clusters are studied for n = 1,2 and compared with the interaction of an isolated atomic F− anion. Coincidence map analysis of the interaction products shows that the loosely bound NF3 molecules are not spectators and produce rich dissociation channels. Laser peak intensity dependence is characterized by the z-scan method,20 and saturation intensities are obtained for different double- and multiple-detachment channels. The loosely bound cluster species exhibit enhancement of intense-field double- and multiple-detachment mechanisms compared to the atomic anion, resulting in saturation intensities that are comparable to the previously studied SF6− molecular anion.18,19 The enhancement of multiple detachment in these loosely bound systems indicates that the underlying mechanism is not unique to the previously studied SF6− system. In contrast to atomic F−, clear suppression of double detachment by chirped prepulses is observed for the electrostatically bound complexes. Thus, although double detachment of the clusters shows different TOD dependence than the molecular SF6− system, suppression by chirped prepulses suggests a similarly nonsequential mechanism. We conclude that the apparent enhancement must result from a fundamental difference between the atomic and molecular systems, such as the coupling of electronic states to the F


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Intense-Field Double Detachment of Electrostatically Bound F

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