Dissociative Ionization of Argon Dimer by Intense Femtosecond Laser

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Dissociative Ionization of Argon Dimer by Intense Femtosecond Laser Pulses Qian Cheng, Xiguo Xie, Zongqiang Yuan, Xunqi Zhong, Yunquan Liu, Qihuang Gong, and Chengyin Wu J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 7, 2017

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The Journal of Physical Chemistry

Dissociative Ionization of Argon Dimer by Intense Femtosecond Laser Pulses Qian Cheng,1 Xiguo Xie,1 Zongqiang Yuan,2 Xunqi Zhong,1 Yunquan Liu,1,3 Qihuang Gong,1,3 and Chengyin Wu*,1,3 1

State Key Laboratory for Mesoscopic Physics, Collaborative Innovation Center of Quantum Matter, School of Physics, Peking University, Beijing 100871, China

2

Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China

3

Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China

Manuscript for submission to The Journal of Physical Chemistry A *Email: [email protected]

ACS Paragon Plus Environment

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Abstract

We experimentally and theoretically studied dissociative ionization of argon dimer driven by intense femtosecond laser pulses. In the experiment, we measured the ion yield and the angular distribution of fragmental ions generated from the dissociative ionization channels of (1,1) (Ar22+ → Ar + + Ar + ) and (2,1) (Ar23+ → Ar 2+ + Ar + ) using a cold target recoil ion momentum spectroscopy. The channel ratio of (2,1)/(1,1) is 4.5-7.5 times of the yield ratio of double ionization to single ionization of argon monomer depending on the laser intensity. The measurement verified that the ionization of Ar+ is greatly enhanced if there exists a neighboring Ar+ separated by a critical distance. In addition, the fragmental ions exhibit an anisotropic angular distribution with the peak along the laser polarization direction and the full width of half maximum becomes broader with increasing laser intensity. Using a full three dimensional classical ensemble model, we calculated the angle-dependent multiple ionization probability of argon dimer in intense laser fields. The results show that the experimentally observed anisotropic angular distribution of fragmental ions can be attributed to the angle-dependent enhanced ionization of argon dimer in intense laser fields.


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I. Introduction Tunneling ionization is a fundamental process for molecules in intense femtosecond laser fields. It has triggered many interesting strong-field molecular phenomena, such as above-threshold ionization,1-3 high-harmonic generation,4-6 non-sequential double ionization,7,8 laser-induced electron diffraction9-11 and air lasing in laser filamentation.12-14 When the laser intensity exceeds 1014 W/cm2, more than one electron might be ionized and multiply charged molecular ions are thus formed. Due to the Coulomb repulsion force, the multiply charged molecular ions will quickly dissociate into fragmental ions with high kinetic energy through the so-called Coulomb explosion process. By measuring angular distributions and kinetic energy of fragmental ions, molecular orbital15,16 and geometric structure17-19 are reconstructed for some molecules. However, the laser–molecule interaction is very complicated because of the nuclear motion. The dissociative ionization is far from being completely understood for molecules in an intense laser field. It has been found that, both the angular distributions and the kinetic energy of fragmental ions depend on not only the laser parameters, but also the molecular target. Alnaser et al.15 measured the angular distribution of N+/N+ and O+/O+, which are generated from the dissociative double ionization of N2 and O2, respectively. It was found that the angular distribution reflects the symmetry of the highest occupied molecular orbital (HOMO) of the molecule under investigation when few-cycle laser pulses are applied with low laser intensity. Under this condition, the angular distribution of fragmental ions is mainly determined by the angle-dependent ionization rate, i.e. geometric alignment mechanism. But when the duration of laser pulse becomes longer or the laser intensity becomes higher, the fragmental ions mostly fly along the laser polarization direction.20 Under this condition, dynamic alignment mechanism dominates, in which the laser electric field induces a dipole moment within the molecule. Due to


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the anisotropic molecular polarizability, the interaction between the induced molecular dipole moment and the laser electric field delivers a torque to align the molecules along the laser polarization direction. Because dynamic alignment and geometric alignment always coexist, it is very difficult to distinguish the contribution of these two mechanisms to the anisotropic angular distribution of the fragment ions.21 For example, there is yet no consensus for the anisotropic angular distribution of fragmental ions generated by the dissociative ionization of iodine molecules.22 It is also found that the kinetic energy of fragmental ions depends on the laser duration.23,24 If the kinetic energy is assumed to come from the potential energy of the Coulomb repulsion of the charges, the internuclear distance can be deduced at the moment of multiple ionization. The results indicate that multiple ionization is accompanied by the stretching of bond length. Dissociation occurs near a critical internuclear distance for all dissociative multiple ionization pathways when the laser pulse is tens or hundreds of femtoseconds. The critical internuclear distance was found to be approximately two to three times of the equilibrium distance of the neutral molecule. These measurements were explained by charge resonance enhanced ionization or enhanced ionization.25-28 At the critical internuclear distance, the appearance intensity is lowest for multiple ionization and the ionization rate is many orders of magnitude greater than that at either a shorter or longer internuclear distance. In comparison with strongly covalently bound molecules (such as N2, O2, I2), rare gas dimers have a large internuclear distance and the electrons are localized at each atom. The nuclear motion can be neglected during the multiple ionization process because the internuclear distance is already in the critical distance. Thus rare gas dimer is an ideal candidate to investigate enhanced ionization of diatomic molecules. Wu et al. studied the double ionization of ArXe dimer by a single-color elliptically polarized femtosecond laser pulse.29 They found that ArXe


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dimer is similar to stretched diatomic molecules and enhanced ionization dominates. Their results proved the physical picture for the enhanced ionization that the up-field atom got ionized. Dissociative double ionization of argon dimer have been extensively studied.30-37 But most of these studies focus on the ionization mechanism. In this article, we experimentally and theoretically studied dissociative ionization of argon dimer driven by intense femtosecond laser pulses. The results directly verified the predictions of enhanced ionization of molecules. The ionization of Ar+ is greatly enhanced when there exists a neighboring Ar+ separated by a critical distance. The fragmental ions exhibit an anisotropic angular distribution with the peak along the laser polarization direction and the full width of half maximum (FWHM) becomes broader with increasing the laser intensity.

II. Experiment Methods The experiment is carried out on a home-built cold target recoil ion momentum spectroscopy (COLTRIMS).38,39 The argon dimer is generated by supersonic expansion of argon gas through a 30-μm nozzle into vacuum chamber. The argon dimer occupies about 1%~2% of the total gas, and larger clusters also exist with a much small fraction.40,41 The laser pulse centered at 780 nm with a temporal duration of 25 fs is generated from an amplified Ti:Sapphire laser system, and is focused by a spherical mirror with a 75-mm focal length. The intensity is calibrated by measuring the ratio of Ar2+/Ar+.42,43 The supersonic beam then intersects with the laser pulse at the focal spot. The ions and electrons generated in the focal spot are guided by a homogenous electric field and a magnetic field to two time-sensitive and position-sensitive detectors consisting of MCPs and delay-line anodes. The momentum vectors are deduced from the time of flight and the impact position on the detectors. Since most of ionization events come from argon


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monomer, some constraints are applied in the off-line data analysis to select out the dissociative ionization channels of argon dimer. 1) Only two atomic ions are detected within one laser pulse. 2) Their sum-momentum is less than 10 a.u. to account for momentum conservation. 3) The kinetic energy of each atomic ion is restricted to be larger than 0.3 eV to rule out false coincidence of two ionized argon monomers. Here, the following fragmentation channels are selected out to study the dissociative ionization dynamics of argon dimers in intense laser fields. (1,1) Ar22+ → Ar + + Ar + ,

(1)

(2,1) Ar23+ → Ar 2+ + Ar + ,

(2)

III. Results and Discussion Figure 1a shows the time-of-flight mass spectra of supersonic argon gas beam irradiated by 780 nm, 25 fs laser pulses at an intensity of 4×1014 W/cm2. According to the mass to charge ratio, the peaks can be assigned to argon atomic ions (such as Ar+, Ar2+, and Ar3+) and argon cluster ions (such as Ar2+ and Ar3+). In addition to these argon atomic ions and argon cluster ions, there are some background ions (such as H+, H2+, H2O+), which are marked with asterisks. They come from the ionization of residual gas in the chamber. In the case of argon atomic ions and argon cluster ions, the peaks can be divided into two components. One component is very sharp and the other component is relatively wide. The sharp component come from the direct ionization of corresponding neutral target. The wide component come from the dissociative ionization of argon cluster with various sizes. Based on the coincident measurement technique, these two kinds of formation channels can be accurately separated in our experiment. Figure 1b and 1c show the time-of-flight mass spectra from the dissociative ionization channels of argon dimer of


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(1,1) and (2,1), respectively. The widths of the peaks are wide. The wide peaks indicate that the atomic ions generated from the dissociative ionization channels of argon dimers carry large kinetic energy. Because coincidence measurement technology is applied, the three dimensional momentum vectors of correlated fragmental ions generated from the same molecular target are obtained by measuring the time and position that the ions hit the detector. The kinetic energy release (KER) for each dissociation event is calculated through the measured three dimensional momentum vectors of correlated fragmental ions. Through the statistical analysis of all the dissociation events, we obtain the KER distribution for any specific reaction channels. Figure 2a shows the KER distributions from the dissociative ionization channels of argon dimer. The peaks locate at 3.7 eV and 7.3 eV for the channels of (1,1) and (2,1), respectively. When a Coulomb potential is applied to describing the dissociation dynamics of Ar22+ and Ar23+ , the internuclear distance is determined to be of 7.2 atomic unit for both Ar22+ and Ar23+ , as shown in Figure 2b. The internuclear distance of Ar22+ and Ar23+ is close to the equilibrium internuclear distance of argon dimer, which is consistent with previous report.33 These measurements indicate that the internuclear distance is kept unchanged during the multiple ionization process. Figure 3 shows the ion yield ratio of Ar2+/Ar+ and the channel ratio of (2,1)/(1,1) at various laser intensities. In the case of the ion yield ratio of Ar2+/Ar+, the atomic ions Ar2+ and Ar+ are generated through the direct ionization of argon monomer. The ratio of Ar2+/Ar+ reflects the further ionization probability of Ar+ in intense laser fields. In contrast, the channels of (2,1) and (1,1) represent the dissociative ionization channels of argon dimer Ar23+ → Ar 2+ + Ar + and Ar22+ → Ar + + Ar + , respectively. The channel ratio of (2,1)/(1,1) can be regarded as the further ionization probability of one Ar+ with the existence of a neighboring Ar+. We calculate the ratio


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of (2,1)/(1,1) to Ar2+/Ar+ for each laser intensity. It is 7.5 at 2×1014 W/cm2 and 4.5 at 7×1014 W/cm2. This measurement indicates that the ionization of Ar+ is greatly enhanced if there exists a neighboring Ar+. Under the current experimental condition, the ionization is in the sequential tunneling regime, in which the electron is ionized one by one. Tunneling ionization of Ar+ and Ar22+ exhibits different behaviors due to the existence of a neighboring Ar+. As shown in Figure 4a, a potential barrier is formed due to the superposition of the external laser field and the internal Coulomb field when Ar+ is subjected into intense laser fields. The electron can be ionized via tunneling through the barrier. The ionization probability depends on the height and the width of the barrier. In the case of Ar22+, as shown in Figure 4b, an inner barrier is emerged between the two cores when an external electric field is applied. The external laser field will raise the potential energy of the electron in the up-field core and lower that in the down-field core. The electron in the up-field core easily tunnels through the inner barrier between the cores and into the continuum state.29 This is the well-known charge-resonance-enhanced ionization (CREI) and the ionization probability can be dramatically increased at the critical distance.25-28 The critical distance is predicted to be 4/Ip where Ip is the atomic ionization potential in atomic unit.44 In the case of argon dimer, the ionization potential is 15.76 eV. Correspondingly, the critical distance is 6.9 atomic unit, around its equilibrium internuclear distance of 7.2 atomic unit. By using one-dimensional two cores potential, we theoretically calculate the barrier height of Ar+ and Ar22+. In the case of Ar+, the energy of the electron is determined by its ionization potential. The potential can be written by 2

𝑉(𝑧) = − |𝑧| − 𝐸 ∙ 𝑧,

(3)


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where z is the coordinate of the electron and E is the laser electric field vector. In the case of Ar22+, enhanced ionization dominates. The energy of the electron in the up-field core is Stackshifted by E·R/2, while the energy of the electron in the down-field core is lowered by E·R/2, where R is the equilibrium internuclear distance. According to enhanced ionization, the electron in the up-field core gets ionized.29 Supposing the electron in the up-field core is on the left and the electron on the right is localized at its parent ion Ar2+, the potential surface of the ionized electron can be written by: 𝑉(𝑧, 𝑅) = −

2 𝑅 |𝑧+ | 2

−

1 𝑅 2

|𝑧− |

− 𝐸 ∙ 𝑧,

(4)

An inner barrier is formed between the two cores. The results demonstrate that the height of the inner barrier is lower than the energy of the electron in the up-field core when the laser polarization is parallel to the molecular axis and the laser intensity is higher than 3×1014 W/cm2. Under this condition the electron in the up-field core can be ionized through over-barrier ionization. In comparison, the laser intensity of 6×1014 W/cm2 is required for over-barrier ionization of Ar+ monomer. As a result, the ionization of Ar+ is greatly enhanced when there exists a neighboring Ar+. This well explained our observation that the ratio of (2,1)/(1,1) is much higher than that of Ar2+/Ar+. Based on the above analysis, both the height of the inner barrier and the energy of the electron strongly depend on the relative angle between the internuclear axis of argon dimer and the laser polarization direction. Therefore, the ionization probability of argon dimer also depends on the relative angle between the internuclear axis of argon dimer and the laser polarization direction. Such dependence can be obtained through measuring the angular distribution of fragmental ions. Figure 5a and 5c show the angular distributions of the ionic fragments generated


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from the channels of (1,1) and (2,1), respectively. It can be seen that the fragmental ions show similar angular distribution for these two channels. The angular distribution is anisotropic relative to the laser polarization direction. Most of the fragmental ions fly along the laser polarization direction. The FWHM becomes broader with increasing the laser intensity and approaches isotropic for the channel of (1,1) when the laser intensity is 7×1014 W/cm2, which is the highest intensity applied in our experiment. Such laser intensity-dependent angular distribution is contrary to that of small diatomic molecules (such as N2 and O2), in which the FWHM becomes narrower with increasing the laser intensity.20 The anisotropic angular distribution has been widely observed for fragmental ions in intense laser fields and has been attributed to dynamic alignment or geometric alignment mechanisms.15,20 In the case of dynamic alignment, due to the anisotropic molecular polarizability, the interaction between the laser electric field and the induced dipole moment aligns the molecule along the laser polarization direction. In the case of geometric alignment, the angular distribution of the fragmental ions is determined by the angle-dependent ionization rate between the laser polarization direction and the molecular axis. When dynamic alignment mechanism dominates, the FWHM becomes narrower with increasing the laser intensity. This is the case of N2 and O2 in intense laser fields with laser intensity entering into the sequential ionization regime.20 Because of the small anisotropic molecular polarizability, the effect of the dynamic alignment is not important for argon dimer. In addition, the FWHM of angular distribution of fragmental ions becomes broader with increasing laser intensity for argon dimer, which is contrary to the prediction of dynamic alignment. This observation further demonstrates that the dynamic alignment is not important in the present measurement.


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In order to test whether the anisotropic angular distribution is caused by the geometric alignment and the anisotropic angular distribution reflects the angle-dependent ionization probability, we develop a classical ensemble model to calculate the angle-dependent ionization probability of argon dimer in intense laser fields. Because over-barrier ionization occurs for argon atom when the laser intensity is higher than 2.47×1014 W/cm2, the effect of the neighboring atom is small for the first electron ionization of argon dimer. Under our experimental condition, the laser intensity excesses the threshold for over-barrier ionization of argon atom. It is therefore reasonable in our model to assume that the ionization probability of the first electron of argon dimer is isotropic relative to the alignment angle between the laser polarization and the internuclear axis. In the calculation of the second electron ionization, we start from Ar2+ (i.e. Ar-Ar+) with two active electrons. Each electron is sampled from a microcanonical distribution around its corresponding parent nucleus (Ar+ and Ar2+) with a binding energy equal to its ionization potential. The subsequent evolution of the electrons driven by the intense laser fields is governed by Newton’s equations of motion: 𝑑2 𝑟⃗𝑖 𝑑𝑡 2

= −𝜖⃗(𝑡) − ∇𝑟𝑖 (− ∑2𝑘=1 |𝑅⃗⃗

𝑞𝑘

𝑘 −𝑟⃗𝑖 |

+ |𝑟⃗

1

).

1 −𝑟⃗2 |

(5)

Here, the indexes 𝑖 = 1,2 and 𝑘 = 1,2 denote the two different electrons and the two different nuclei, respectively. 𝑅⃗⃗ and 𝑟⃗ respectively denote the displacement of the corresponding nuclei and electrons. 𝑞𝑘 denotes the charge state of the nuclei. The linearly polarized external field 𝜖⃗(𝑡) has a constant amplitude for the first 10 cycles and is turned off with a 3-cycle ramp.41 In the calculation, the motion of the nuclei is neglected. The ionization events of the electron around the parent ion Ar+ are selected out for statistical analysis to obtain the dependence of the ionization probability of the second electron on the alignment angle between the laser


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polarization and the internuclear axis of argon dimer. It should be emphasized that the thus formed Ar22+ will dissociate into Ar+ and Ar+. The fragmental ions are emitted along the direction of the internuclear axis. In the calculation of the third electron ionization, we start from Ar22+ (i.e. Ar+-Ar+) with two active electrons around their corresponding parent nucleus (Ar2+ and Ar2+). The ionization events of one electron are selected out for statistical analysis to obtain the dependence of the ionization probability of the third electron on the alignment angle. The thus formed Ar23+ will dissociate into Ar2+ and Ar+. Because the ionization is in the sequential regime and the electron is ionized one by one, the angle-dependent multiple ionization is the product of the angle-dependent ionization probability of each electron. Figure 5b and 5d show the theoretical calculated angle-dependent double ionization and triple ionization, respectively. There exist some common features for the theoretical calculated angle-dependent multiple ionization. It is anisotropic relative to the laser polarization direction. The peak is achieved when the laser polarization direction is parallel to the internuclear axis of argon dimer and the FWHM of the angle-dependent multiple ionization becomes broader with increasing the laser intensity. It should be emphasized that, Ar22+ and Ar3+ formed by double ionization and triple ionization will dissociate through the channels of (1,1) and (2,1), respectively. Figure 5a and 5c show the measured angular distributions of these fragmental ions. It can be seen that the angle-dependent ionization agrees with the angular distribution of fragmental ions. The agreement indicates that the dissociation of Ar22+ and Ar23+ is a quick process. The momentum vector of fragmental ions represents the direction of the internuclear axis of argon dimer at the moment of multiple ionization. The anisotropic angular distribution of fragmental ions is caused by the geometric alignment and determined by the angle-dependent ionization probability of argon dimer in intense laser fields.


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The ionization of molecule in strong laser field is very complicated. The enhanced ionization occurring near the critical internuclear distance emphasizes the effect of nuclear motion on the ionization. When the internuclear distance is around the critical distance, the ionization rate is greatly enhanced through the inner barrier if the molecular axis is parallel to the electric laser field. However, if the molecular axis is perpendicular to the laser field, the effect of neighboring nuclear can be neglected and the ionization of molecule is similar to that of atom (e.g. atomic-like ionization). Therefore, the enhanced ionization probability depends on the angle between the molecular axis and the laser polarization, which leads to the anisotropic angular distribution of fragmental ions with peak along the laser polarization direction. As laser intensity increases, the enhanced ionization along laser field will penetrate into the saturation regime. Nevertheless, the ionization probability perpendicular to the laser electric field continues to increase. As a result, the FWHM becomes broader with increasing laser intensity. Another interesting phenomenon one may note is that the angular distributions in Figure 5c are systematically wider than those in Figure 5d. This phenomenon can be attributed to the atomiclike ionization, which is underestimated in our theoretical simulation because tunneling ionization is not taken into account in our classical model.

IV. Conclusions In conclusion, we have studied dissociative ionization of argon dimers in intense femtosecond laser pulses. Argon dimer can be regarded as two isolated argon atoms separated by 7.2 atomic unit, a critical distance required by enhanced ionization. Because of the large internuclear distance, the electron cannot freely move between the two cores and localizes around each of the cores. As a result, charge resonance enhanced ionization dominates for argon


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dimers in intense laser fields. In the experiment, we measured the channel yield and angular distribution of fragmental ions from the channels of (1,1) (Ar22+ → Ar + + Ar + )

and (2,1)

(Ar23+ → Ar 2+ + Ar + ). Depending on the laser intensity we have applied, the channel ratio of (2,1)/(1,1) is 4.5-7.5 times of the yield ratio of Ar2+/Ar+ with Ar2+ and Ar+ being generated through direct ionization of argon monomer. The measurement directly verified the prediction of enhanced ionization, the ionization of Ar+ is greatly enhanced if there exists a neighboring Ar+ separated by a critical distance. In addition, the fragmental ions generated through the channels of (2,1) and (1,1) exhibit an anisotropic angular distribution relative to the laser polarization direction. The peak is achieved when the laser polarization direction is parallel to the internuclear axis of argon dimer and the FWHM becomes broader with increasing laser intensity. Using a full three dimensional classical ensemble model, we calculated the angle-dependent multiple ionization probability of argon dimer in intense laser fields. These calculations agree with our experimental observations. The agreement indicates that experimentally observed anisotropic angular distribution of fragmental ions origins from the angle-dependent ionization of argon dimer in intense laser fields. Our results directly confirmed the predictions of enhanced ionization of molecules induced by intense laser fields.

Acknowledgement This work is supported by the National Natural Science Foundation of China under Grants No. 11625414, No. 11604312, No. 21673006, No. 11474009, and No. 11434002.

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Patterns in Ionization of O2 and N2 by Short Laser Pulses. Phys. Rev. Lett. 2004, 93, 113003. 16. Alnaser, A. S.; Maharjan, C. M.; Tong, X. -M.; Ulrich, B.; Ranitovic, P.; Shan, B.; Chang, Z.; Lin, C. D.; Cocke, C. L.; Litvinyuk, I. V. Effects of Orbital Symmetries in Dissociative Ionization of Molecules by Few-Cycle Laser Pulses. Phys. Rev. A 2005, 71, 031403(R). 17. Wu, C.; Wu, C.; Song, D.; Su, H.; Yang, Y.; Wu, Z.; Liu, X.; Liu, H.; Li, M.; Deng, Y.; et al. Nonsequential and Sequential Fragmentation of CO23+ in Intense Laser Fields. Phys. Rev. Lett. 2013, 110, 103601. 18. Pitzer, M.; Kunitski, M.; Johnson, A. S.; Jahnke, T.; Sann, H.; Sturm, F.; Schmidt, L. Ph. H.; Schmidt-Böcking, H.; Dörner, R.; Stohner, J.; et al. Direct Determination of Absolute Molecular Stereochemistry in Gas Phase by Coulomb Explosion Imaging. Science 2013, 341, 1096-1100. 19. Kunitski, M.; Zeller, S.; Voigtsberger, J.; Kalinin, A.; Schmidt, L. Ph. H.; Schöffler, M.; Czasch, A.; Schöllkopf, W.; Grisenti, R. E.; Jahnke, T.; et al. Observation of the Efimov State of the Helium Trimer. Science 2015, 348, 551-555. 20. Voss, S.; Alnaser, A. S.; Tong, X. -M.; Maharjan, C. M.; Ranitovic, P.; Ulrich, B.; Shan, B.; Chang, Z.; Lin, C. D.; Cocke, C. L. High Resolution Kinetic Energy Release Spectra and Angular Distributions from Double Ionization of Nitrogen and Oxygen by Short Laser Pulses. J. Phy. B: At. Mol. Opt. Phys. 2004, 37, 4239-4257. 21. Posthumus, J. H.; Plumridge, J.; Thomas, M. K.; Codling, K.; Frasinski, L. J.; Langley, A. J.; Taday, R. F. Dynamic and Geometric Laser-Induced Alignment of Molecules in Intense Laser Fields. J. Phy. B: At. Mol. Opt. Phys. 1998, 31, L553-L562. 22. Ellert, Ch.; Corkum, P. B. Disentangling Molecular Alignment and Enhanced Ionization in


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31. Ulrich, B.; Vredenborg, A.; Malakzadeh, A.; Meckel, M.; Cole, K.; Smolarski, M.; Chang, Z.; Jahnke, T.; Dörner, R. Double-Ionization Mechanisms of the Argon Dimer in Intense Laser Fields. Phys. Rev. A 2010, 82, 013412. 32. Manschwetus, B.; Rottke, H.; Steinmeyer, G.; Foucar, L.; Czasch, A.; Schmidt-Böcking, H.; Sandner, W. Mechanisms underlying Strong-field Double Ionization of Argon Dimers. Phys. Rev. A 2010, 82, 013413. 33. Ulrich, B.; Vredenborg, A.; Malakzadeh, A.; Schmidt, L. Ph. H.; Havermeier, T.; Meckel, M.; Cole, K.; Smolarski, M.; Chang, Z.; Jahnke, T.; Dörner, R. Imaging of the Structure of the Argon and Neon Dimer, Trimer, and Tetramer. J. Phys. Chem. A 2011, 115, 6936-6941. 34. Wu, J.; Vredenborg, A.; Ulrich, B.; Schmidt, L. Ph. H.; Meckel, M.; Voss, S.; Sann, H.; Kim, H.; Jahnke, T.; Dörner, R. Multiple Recapture of Electrons in Multiple Ionization of the Argon Dimer by a Strong Laser Field. Phys. Rev. Lett. 2011, 107, 043003. 35. Kim, H.-K.; Gassert, H.; Schöffler, M. S.; Titze, J. N.; Waitz, M.; Voigtsberger, J.; Trinter, F.; Becht, J.; Kalinin, A.; Neumann, N.; et al. Ion-Impact-Induced Interatomic Coulombic Decay in Neon and Argon Dimers. Phys. Rev. A 2013, 88, 042707. 36. Miteva, T.; Chiang, Y.-C.; Kolorenč, P.; Kuleff, A. I.; Gokhberg, K.; Cederbaum, L. S. Interatomic Coulombic Decay following Resonant Core Excitation of Ar in Argon Dimer. J. Chem. Phys. 2014, 141, 064307. 37. Ren, X.; Jabbour Al Maalouf, E.; Dorn, A.; Denifl, S. Direct Evidence of two Interatomic Relaxation Mechanisms in Argon Dimers Ionized by Electron Impact. Nat. Commun. 2016, 7, 11093.


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Figure captions Figure 1. (a) Time-of-flight mass spectra of supersonic argon gas beam irradiated by 780 nm, 25 fs laser pulses at an intensity of 4×1014 W/cm2. The peaks marked with asterisks come from the ionization of residual gas in the chamber. (b), (c) Time-of-flight mass spectra of Ar2 from the channels of (1,1) and (2,1), respectively.

Figure 2. (a) Kinetic energy release for channels of (1,1) and (2,1). (b) The internuclear distance distributions for Ar22+ and Ar23+ .

Figure 3. Ion yield ratio of Ar2+/Ar+ and channel ratio of (2,1)/(1,1) at various laser intensities.

Figure 4. Schematic diagram of tunneling ionization of (a) Ar+ and (b) Ar22+.

Figure 5. (a), (c) Experimentally measured angular distributions of Ar+/Ar+ generated from the (1,1) channel and Ar2+/Ar+ generated from the (2,1) channel, respectively. (b), (d) Theoretically calculated angle-dependent double ionization probability and triple ionization probability of Ar2 in intense laser fields, respectively. (I0: 1014W/cm2).


21


8

107 106 105 104 103 102 101 10

3+

**

Ar Ar2+

+

Ar

(a)

+

*

Ar2

+

Ar3

0

Counts

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20

40

60

80

100

120

(b)

3

10

Ar2

2+

+

Ar + Ar

+

2

10

1

10

0

20

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60

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3

(c)

10

Ar2

2

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2+

Ar + Ar

+

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10

0

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60

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Mass-to-charge ratio Figure 1.


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1.2 1.0

(1,1) (2,1)

(a)

0.8 0.6

Normalized counts

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0.4 0.2 0.0

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2

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6

8

10

Kinetic energy release (eV) 1.2 1.0

(b)

0.8 0.6 0.4 0.2 0.0

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6

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Internuclear distance (a.u.)

10

Figure 2.


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0.1

Ratio

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0.01

(2,1)/(1,1) 2+ + Ar /Ar 2

3

4

5

6 14

7 2

Laser intensity (10 W/cm )

Figure 3.


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Figure 4.


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(a)

(b)

90

1.0 135 0.8 0.6 0.4 0.2 0.0 180 0.2 0.4 0.6 0.8 225 1.0

3 I0

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45

5 I0 7 I0 0

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315

0

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270

(d)

90

1.0 135 0.8 0.6 0.4 0.2 0.0 180 0.2 0.4 0.6 0.8 225 1.0

315

3 I0

45

5 I0

90

135

45

7 I0 0

180

315

0

225

270

315 270

Figure 5.


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TOC graphic


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Figure 1. (a) Time-of-flight mass spectra of supersonic argon gas beam irradiated by 780 nm, 25 fs laser pulses at an intensity of 4×1014 W/cm2. The peaks marked with asterisks come from the ionization of residual gas in the chamber. (b), (c) Time-of-flight mass spectra of Ar2 from the channels of (1,1) and (2,1), respectively. 139x140mm (300 x 300 DPI)


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Figure 2. (a) Kinetic energy release for channels of (1,1) and (2,1). (b) The internuclear distance distributions for Ar22+ and Ar23+. 81x90mm (300 x 300 DPI)



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Figure 3. Ion yield ratio of Ar2+/Ar+ and channel ratio of (2,1)/(1,1) at various laser intensities. 81x80mm (300 x 300 DPI)


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Figure 4. Schematic diagram of tunneling ionization of (a) Ar+ and (b) Ar22+. 118x172mm (300 x 300 DPI)



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Figure 5. (a), (c) Experimentally measured angular distributions of Ar+/Ar+ generated from the (1,1) channel and Ar2+/Ar+ generated from the (2,1) channel, respectively. (b), (d) Theoretically calculated angledependent double ionization probability and triple ionization probability of Ar2 in intense laser fields, respectively. (I0: 1014W/cm2). 159x141mm (300 x 300 DPI)


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Dissociative Ionization of Argon Dimer by Intense Femtosecond Laser

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