Multielectron Effects in the Strong Field Sequential Ionization of

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Multi-Electron Effects in the Strong Field Sequential Ionization of Aligned CHI Molecules 3

Sizuo Luo, Wenhui Hu, Jiaqi Yu, Xiaokai Li, Lanhai He, Chuncheng Wang, Fuchun Liu, and Dajun Ding J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05588 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017

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The Journal of Physical Chemistry

Multi-electron Effects in the Strong Field Sequential Ionization of Aligned CH3I Molecules Sizuo Luoa,b , Wenhui Hua,b , Jiaqi Yua,b , Xiaokai Lia,b , Lanhai Hea,b , Chuncheng Wanga,b , Fuchun Liua,b , Dajun Dinga,b a

b

Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, China Jilin Provincial Key Laboratory of Applied Atomic and Molecular Spectroscopy, Jilin University, Changchun 130012, China

Abstract Strong field sequential ionization of symmetric-top CH3 I molecules is studied experimentally by using a combined method of femtosecond laser induced impulsive alignment and timeof-flight mass spectrometry. Both alignment and angular dependent ion yields have been measured and the sequential ionization of multi-electron has been discussed. It’s find that the maximum ionization occurs when the polarization of probe laser is perpendicular to the internuclear axis of molecules, and the signal of fragment ions peaks at the polarization of probe laser is parallel to the internuclear axis of molecules. The angular distribution of ions indicated that ionization of π type orbitals are correspond to the generation of charged parents ions, and ionization of σ type orbitals are correspond to the generation of fragment ions. The sequential release of multi-electron for Coulomb explosion channels are studied by analysis the time evolutions of multi-charged In+ (n=1-4) signals. Keywords: Molecular alignment, Sequential ionization, Multi-electron effects, Angular distribution

1. INTRODUCTION During the strong field laser interaction with molecules, the laser field can exert electric forces on valence electrons exceeding those that bind them, leading to ionization, highharmonic generation (HHG), dissociation and Coulomb explosion (CE) of molecules.1, 2, 3, 4, 5 The dynamics of strong field ionization, dissociation and Coulomb explosion of molecules have been studied by different methods, such as coincidence measurement,6, 7 femtosecond extreme ultraviolet transient absorption spectroscopy8, 9 and measuring the alignment dependent ionization10, 11, 12, 13 and dissociation yields.14, 15 However, in a strong laser field, fragment ions can be produced by tunneling ionization from lower orbitals, postionization excitation of the ionic ground state or sequential release multi electrons.1, 2, 6, 7 The observation of the formation of ionic fragments does not constitute proof that the strong field Email addresses: [email protected] (Sizuo Luo), [email protected] (Dajun Ding) Preprint submitted to J. Phys. Chem. A

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ionization process directly populates different electronically excited ionic states. Various studies show that single active electron (SAE) approximation does not translate well to the multielectron world of polyatomic molecules interaction with strong laser fields.7, 8, 9, 10 The multi-electron effect has been proven to be very important in strong field molecular physics, which the influences have been observed in HHG,5 above threshold ionization (ATI)6, 7 and fragmentation of molecules.14, 15 Furthermore, multi-electron ionization by sequential pathways including nuclear dynamics has been proposed to understanding the kinetic energy release of molecular CE.15, 16, 17, 18 Considering the ionization and fragmentation are the fundament processes that occurs when strong laser field interact with molecules, it’s very important to understanding the contribution of electron dynamics during molecular ionization and fragmentation. The study of multi-electron effect on strong field ionization and fragmentation offer us a method to detangling the generation or proof on the pathways of populates electronically excited states for molecular ions. Ionization and fragmentation of CH3 I molecules in intense femtosecond laser fields have been extensively studied.19, 20, 21, 22, 23, 24, 25 The measurements of high resolution mass spectra and ion velocity imaging give out the kinetic energy release of different fragment ions, which provides the information of molecular potential energy surfaces and dissociation mechanism.19, 20, 21 The sequential ionization mechanism has been proposed to identify the CE channels of fragments. The angular distribution of fragment ions have been measured as a function of the angle between the laser polarization vector and the time of flight (TOF) axis of spectrometer, which the measured angular distributions provide the information to distinguish the contribution of geometric alignment and dynamics alignment on strong field fragmentation of CH3 I.22, 23, 24 These studies provide a lot useful information on strong field ionization and fragmentation of CH3 I. However, the pathways of population of different molecular ion electronic states can’t be obtained from these experimental results. Recently, the formation and ejection of fragments from CH3 I have been studied in asymmetric twocolor femtosecond laser field, and the asymmetric ion distributions of fragments at different phase delay between two lasers have been measured.25 The experimental results show that different fragment ions are generated from multi-orbital ionization and the process of eject multi-electron are discussed. They find that the low kinetic release energy CH+ 3 fragments are generated from ionization of HOMO orbital and both HOMO and HOMO-2 orbitals have contribution on CE of molecules. Because of complex structure and small energy gap between different states of molecules, the molecular ions can be populated at both ground state and excited states after interacted with intense field laser, and the excited ionic states are unstable which can be dissociate to fragments. Strong field ionization induced relaxation and dissociation dynamics in polyatomic molecules are difficult to trace both in theory and in experiment.26, 27, 28 Those experiments are generally performed on samples with an isotropic spatial distribution with alignment and angular averaging, which have limitation on extract the information from experimental data. Molecular alignment method is very useful on studying angular dependence strong field ionization and fragmentation of molecules, which provide us an opportunity to detangle the multi-orbital contribution and to study the dynamics of sequential ionize multielectron from molecules. Furthermore, understanding the dynamics of fragmentation is also very important to control the isomerization and 2


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fragmentation processes of molecules in intense femtosecond laser. Here, we report experiments on strong field laser interaction with aligned CH3 I molecules dedicated to understanding the multi-electron and multi-orbital effects in the strong field sequential ionization of molecules. The alignment and angular dependent of charged parent ions and fragment ions have been measured. The results show that maximum ionization yields appears when the polarization of probe laser is perpendicular to the molecular axis, and the fragment ions signals are maximum when the polarization of probe laser is parallel to the molecular axis. The different angular distribution of parent and fragment ions come from the ionization of multi-orbital electron, the mechanisms of sequential ionization have been discussed by compare the alignment dependent curves of multi-charged iodine ions, Iq+ (q=1-4). 2. EXPERIMENTAL SECTION The experiments utilize a Ti:sapphire oscillator (Coherent, Libra) and a chirped-pulse amplifier (CPA) to produce a 50 f s, 800 nm linearly polarized laser pulses with energies up to 4 mJ at a 1 kHz repetition rate. A Mach-Zehnder interferometer is constructed in which the laser beam is split into pump and probe parts, the laser intensities are varied by using a half-wave plate and polarizer. The pump part is used to trigger the alignment of molecules, and the probe part is sent through a computer-controlled delay stage to ionize the molecules. The spot size of the alignment laser beam is reduced using an aperture, to ensure that the focus was larger than the probe laser and after passing the mirror, the aligning pulses pass through a 70 mm BK7 glass, the GVD in this glass stretches the pulses to ≈ 300 f s (full width at half maximum, FWHM) but maintains their energy. The pump and probe pulses are collinearly and focused by a 250-mm-focal-length lens onto the molecular beam. We adjust the polarization of pump laser by rotate a half-wave plate, and keep the pump laser intensity to the same during the angular dependence measurement experiment. The experiment is performed in the spectrometer with three chambers: the source chamber, the hexapole chamber, and the detection chamber. In the source chamber, the supersonic molecular beam of 2.5% CH3 I seeded in Neon is produced from a pulsed valve at a 4 bar stagnation pressure. The hexapole chamber contains the hexapole rotational state selector, which is composed of six hexagonally placed rods. The radial electric field of the hexapole induces a Stark shift of the molecules with dipole moment, and the rotational states of molecules can be selected out and the trajectories of molecules can be focused to the center axis by positive Stark shift (low-field seeking). The mass spectra of molecules are measured in the detection chamber. The detection chamber is designed base on the electrodes of velocity map imaging experiment setup with repeller, extract and ground electrodes. The ions generated in the interaction region were extracted and accelerated by the electrostatic lens system at the end of time-of-flight tube, and projected onto a 2D detector composed of two microchannel plates (MCP) in Chevron configuration coupled to a phosphor screen. The mass spectra are obtained, accumulated, and averaged by connecting the PMT output with a digital storage oscilloscope and then transferred into a computer. The intensity of femtosecond laser was corrected using the saturate intensity of Xe.29 3



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Figure 1: (Color online) The rotational behavior of nonadiabatic alignment of CH3 I molecules induced by a femtoseocnd laser (300 f s, 5.7 × 1012 W/cm2 ), the alignment evolution of I3+ ion signal as a function of delay between the pump and probe laser is shown in the black circles and the degree of alignment calculated from the time dependent Schr¨ odinger equation (TDSE) is shown in the blue line.

3. RESULTS AND DISCUSSION The evolution of alignment for CH3 I molecules is defined by the measurement of the I signal intensity as a function of the delay time between pump and probe lasers, and the degree of alignment is given by comparing the evolution curve of I3+ signal intensity with the alignment evolution curve from a time dependent Schrödinger equation (TDSE) calculation,30, 31 the results are shown in Fig. 1. The rotational period of CH3 I molecules is τ = 1/2Bc =66.67 ps for a rotational constant B = 0.25 cm−1 .32 The periodically spatial distribution behavior of nonadiabatic alignment of molecules also shown in the figure, the pump laser interacts with random distributed molecules at 0 ps which create a rotational wavepacket and the laser intensity is 5.7 × 1012 W/cm2 according to the best fitting of alignment revival structures. The schematic representation of molecular spatial distribution at alignment and anti-alignment around half and full revivals also present in the figure. During the half revival period, the molecules are prepared parallel to the pump laser polarization at 32.1 ps (alignment), and the molecules are changed to preferentially orthogonal distribution to the pump laser polarization at 34.6 ps (anti-alignment). Around the full revival period, the molecules are anti-aligned at time of 65.2 ps and aligned at 68.2 ps. The delayed intense probe laser is used to ionize and dissociate the rotating molecules. 3+

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Figure 2: (Color online) The time dependent intensity of all ions are shown in (a), and (b) is the obtained typical time of flight mass spectra of CH3 I at the probe laser intensity is 2.0 × 1014 W/cm2 .

The time dependent evolutions of TOF mass spectra of CH3 I molecules obtained from the interaction with a laser pulse at a wavelength of 800 nm, pulse duration of 50 f s and intensity of 2.0 × 1014 W/cm2 are shown in Fig. 2(a). Fig. 2(b) shows a typical TOF mass spectrum obtained from this pump-probe scanning. The dominant peaks in the mass spectrum are + parent ions, CH3 I+ , and fragment ions, Iq+ (q = 1-4), CHq+ n (n=0-3 with q=1, 2), Hn (n=1, 2), and also a small mass spectrum peak that corresponds with a doubly charged parent ion CH3 I2+ also can be clearly resolved in the TOF spectrum. The multiple charged fragment ions are generated from Coulomb explosion (CE) of molecules in an intense laser field.19, 20, 21 The observation of a doubly charged parent ion is indicative for the presence of a potential well in the Potential Energy Surface (PES) of the CH3 I2+ ion along the C-I bond reaction coordinate.21 We also perform the laser intensity dependent measurement on fragment ions signal at different molecular alignment, and no-knee structures have been observed in our measurement, which proves that the sequential ionization is dominate in the strong field multiple ionization of CH3 I, and in the experiment we set the laser intensity at 2.0 × 1014 W/cm2 to make sure that the fragment ions are mainly generated by sequential ionization process. The polarization of the pump and probe lasers is parallel to the axis of the TOF. Its obvious to see that the intensity of ions signal changes as the delay time increases. The time dependent evolutions of different charged parent ions and fragment ions relay on the revival of molecular alignment and the multi-electron ionization mechanism of molecules. Since 5



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Figure 3: (Color online) The alignment dependent evolutions of different ions, (a) and (b) are the evolutions of the parent ion intensity of CH3 I+ and CH3 I2+ , the evolutions of fragments In+ (n=1-4) ion intensity are shown in (c) and the evolutions of CH+ n (n=0-4) are shown in (d).

the revival of molecular alignment is same for different ions, the multi-electron ionization mechanism can be extracted and studied from the transient variation of ions. The alignment dependent signal intensities of parent ions and fragment ions from strong field laser interact with CH3 I molecules are shown in Fig 3. The alignment dependent single and double charged parent ions (CH3 I+ , CH3 I2+ ) intensities transient curves are shown in Fig 3 (a) and (b), these two transient curves show similar evolutions with maximum signal appears at delay time of maximum anti-alignment (34.6 ps and 66.3 ps) and minimum signal appears at delay time of maximum alignment (32.3 ps and 68.0 ps). This means the charged parent ions prefer to occur at polarization of probe laser is perpendicular to the C-X axis of molecules. However, the modulation depth from alignment to anti-alignment is very different, i.e. the ratio between alignment and anti-alignment ion intensity is 1.2 for CH3 I+ and 2.4 for CH3 I2+ . The modulation depth relays on the anisotropy of ionization processes, stronger modulation comes from ionization process with larger anisotropic. The similar evolutions of these two ions indicated that the ionization orbital have the same shape, so the double charged parent ions CH3 I2+ are generated from first ionize one electron from 6


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molecular HOMO orbital and then ionize the second electron from molecular cation HOMO orbital (π orbital). The alignment dependent fragment ions intensity transient curves are shown in Fig 3 (c) and (d), and the transient curves of fragment iodine ions, Iq+ (q=1-4) are shown in Fig 3(c) which the transient behaviors are complete different from charged parent ions. The transient curves of fragment ions show similar evolutions with maximum signal appears at delay time of maximum alignment (32.3 ps and 68.0 ps) and minimum signal appears at delay time of maximum anti-alignment (34.6 ps and 66.3 ps). This means fragmentation yields are maximum when the polarization of probe laser is parallel to the C-X axis. The different evolutions behavior indicated that the ionization orbital is different, and the fragment iodine ions are produced from ionization of HOMO-1 orbital which have a1 symmetry with electron density maxima localized along the C-X axis (σ orbital). The alignment dependent modulation depth for fragment iodine ions, Iq+ (q=1-4), is increase as the charge state increases (1.2, 1.6, 2.3, 2.8 for Iq+ (q=1-4)). This means more σ type orbitals are involved in strong field induced CE of molecules. The transient curves of fragment ions, CH+ n (n=0-3) are shown in Fig 3 (d), which shown the similar alignment dependent evolution as fragment iodine ions. The modulation becomes larger as more H is lost during the fragmentation process, which suggest more specific and more anisotropy pathways are + involved in fragmentation. Furthermore, other fragment ions such as CH2+ n (n=0-3) and Hn (n=1, 2) also have the similar alignment dependent evolution as fragment iodine ions. The measured evolutions of fragment ions indicate that all of the fragment ions are generated from ionization of σ type orbitals. The dissociation and CE of molecules in intense femtosecond laser is triggered by sequential ionization of electron, which the first electron is release at equilibrium internuclear distance in this case, and the other electrons are ionized by strong filed laser at larger distance because the multi-charged molecules are unstable.16, 17, 33, 34 In our experiment, the increase of alignment dependent modulation can be understand by sequential ionization, which keeps the anisotropic distribution of previous ionization process and the sequential ionization induce larger anisotropic. The distribution of molecular orbital is come from linear combination of atomic orbitals, and the orbital shape is different for molecular ions at different C-I bond length. The length of C-I bond is related to the laser duration and charged state, normally the length is longer for ionization from higher charged molecular states. When the bond of molecules is longer enough, the molecular orbital can be treated as independent atomic orbital.35, 36 The strong anisotropic of highly charged fragment iodine ions proves that the molecular ions orbitals keeps the shape during the process of stretch the C-I bond in this experiment. In order to study the effect of sequential ionization and charge redistribution during multi-electron ionization, the alignment dependent ratios of Im+ /In+ (m-n=1) are given in Fig 4. First, the strong modulations are observed which show the similar behavior with alignment dependent fragment ions signal, this proves the sequential ionization also occurs from σ type orbital. Second, the modulation depth of ratio from alignment to anti-alignment is different, which means the anisotropy of electron distribution is different during sequential ionization from different charged states of molecules. This modulation relays on the shape of ionization orbital and the charge redistribution during fragmentation. The I3+ /I2+ ratio have largest modulation between alignment and anti7



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2+ alignment, which means the ionization orbital shape of CHm+ have strongest anisotropy. 3 I The measurement of alignment dependent multi charged fragment ions with an improved theoretical calculation including the treatment of sequential ionization, nuclear dynamics and charge redistribution offers the possibility to study the multi-electron and multi-orbital dynamics of molecules in intense femtosecond laser.15, 24

Figure 4: (Color online) The alignment dependent ratio of Im+ /In+ (m-n=1) in intense femtosecond laser.

The molecules is aligned along the polarization of alignment laser when the delay time is 68.2 ps at the full revival and the degree of alignment is ⟨cos2 θ⟩=0.7. In order to measure the angular dependence of the ionization and fragmentation yields, we setting the probe laser at the delay time of 68.2 ps. The angular distributions are measured by rotate the polarization of alignment laser and adjusting the intensity of laser to the same, as shown in Fig 5(f). Fig. 5 displays the ionization and fragmentation yields of molecules as a function of the angle between the polarization of alignment laser and the polarization of ionization laser. The angular dependent of parent ion (Fig 5(a)) showing a clear minimum of the signal when θ = 0◦ , and a maximum yield at θ = 90◦ . This angular distribution is come from tunneling ionization of HOMO orbital of CH3 I molecules. The angular distributions of different fragments are shown in Fig. 5 (b)-(e) (i.e. I+ , I2+ , I3+ and CH+ 3 ), and which shown a maximum when the ionization laser polarization is parallel to the molecular axis and a minimum when the polarization of ionization laser is perpendicular to the molecular axis. These distributions agree with the angular distribution of tunneling ionization of σ type orbitals. The anisotropy of fragments Iq+ (q=1-3) is increase as the charged states increase, which 8


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is agreed with the observation in alignment dependent measurements. It’s well known that the fragmentation of molecules is a very complex process, the sequential ionization, nuclear dynamics, charge redistribution and electron recollision are involved during the fragmentation.14, 15, 16, 17 The sequential release of multi-electron from multi-orbital of molecular ions is correspond to the angular distribution of fragment ions, and the charge redistribution of molecular ion orbitals with nuclear dynamics may do not have important contribution during multi electron ionization of CH3 I molecules in our current laser conditions.

Figure 5: (Color online) The angular distribution of ionization and fragmentation of CH3 I molecule, the experimental measurement (black circles) of the angular distribution of CH3 I+ , I+ , I2+ , I3+ and CH3 + are shown in (a)-(e). The blue lines are from the best fitting of Legendre polynomial. The method of measuring the ionization and fragmentation yield of aligned CH3 I molecules as a function of the angle between the polarization axes of the alignment and probe pulses is shown in (f).

During the strong field laser interact with molecules, the angular distributions of the various fragment ions are affected by dynamic and geometric alignments from intense probe laser. For heavy molecules like CH3 I, the dynamics alignment from 50 f s probe laser pulse is not important.21, 22, 23 With the nonresonant laser align the molecules, the angular distribution of dissociative ionization and Coulomb explosion only affect by the selection of orientation due to the angular dependence of the ionization rate. Tanaka et al.37 perform an experiment to study strong field ionization of CH3 X (X=F, Cl, Br, I), and they find that the strong field ionization yields of CH3 X can be treated as atomic like ionization due to electron localization on the halogen atom, which means HOMO orbital is dominate in ionization. Walt et al.24 find that the multi-electron effects is important during strong field fragmentation of 9



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CH3 X (X=F, Cl, Br, I) molecules in two-color laser, the fragment ions are generated from ionization of HOMO and HOMO-2 orbitals. In our experiment, multi-orbitals contribution is important for fragmentation at high laser intensity, and ionization of σ type orbitals is dominated in the process of fragmentation. Furthermore, the highly charged fragment ions are generated after sequential release multi-electron from multi-orbital of molecular ions. Of course, we cannot exclude the contribution of other orbital during fragmentation, and it is very difficult to fully distinguish the contribution of different orbitals during strong field laser CE of molecules in our current measurement. By using the method of channel- and angle resolved measurement in the molecular frame,7, 15 the relative contributions of multiorbitals may be able to be clarified more accurate and the charge redistribution of orbital during sequential ionization can be studied in the future. 4. SUMMARY We have investigated the angular sensitive photoionization and fragmentation process of CH3 I molecules in intense femtosecond laser (2.0 × 1014 W/cm2 ) by using well aligned molecules. Very different alignment and angular dependence have been observed for parent ions and fragment ions. Our results shown the ionization is prefer to occurs at the polarization of probe laser is perpendicular to the molecular axis and the fragmentation is prefer to occurs at the polarization of probe laser is parallel to the molecular axis of CH3 I. From the analysis of angular distribution of double charged parent ions, the first electron is released by ionization of molecular HOMO orbital and the second electron is ionized from molecular + cation HOMO orbital. The fragment ions, Iq+ (q = 1-4), CHq+ n (n=0-3 with q=1, 2), Hn (n=1, 2) are generated from multi electron ionization of σ type orbitals. The sequential release of multi-electron for Coulomb explosion channels are traced step by step by analysis the time evolutions of multi-charged In+ (n=1-4) signal. The measurement of alignment dependent multi-charged fragment ions offers us the possibility to understanding the influence of sequential ionization and charged redistribution during CE of molecules in intense femtosecond laser. 5. ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973Program) under Grant nos: 2013CB922200 and the National Natural Science Foundation of China under Grant nos: 11627807, 11534004, 11127403. 6. REFERENCES [1] Yamanouchi, K. The next frontier. Science 2002, 295, 1659-1660. [2] Posthumus, J, H. The dynamics of small molecules in intense laser fields. Rep. Prog. Phys. 2004, 67, 623-665. [3] Wang, Q.; Wu, D.; Jin, M.; Liu, F.; Hu, F.; Cheng, X.; Liu, H.; Hu, Z.; Ding, D.; Mineo, H.; et al. Experimental and theoretical investigations of ionization/dissociation of cyclopentanone molecule in a femtosecond laser field. J. Chem. Phys. 2008, 129, 204302.

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[4] Wang, Q.; Wu, D.; Zhang, D.; Jin, M.; Liu, F.; Liu, H.; Hu, Z.; Ding, D.; Mineo, H.; Dyakov, Y. A.; et al. Ionization and dissociation processes of pyrrolidine in intense femtosecond laser field. J. Phys. Chem. C 2009, 113, 11805-11815. [5] Smirnova, O.; Mairesse, Y.; Patchkovskii, S.; Dudovich, N.; Villeneuve, D.; Corkum, P.; Ivanov, M. Y. High harmonic interferometry of multi-electron dynamics in molecules. Nature 2009, 460, 972-977. [6] Boguslavskiy, A. E.; Mikosch, J.; Gijsbertsen, A.; Spanner, M.; Patchkovskii, S.; Gador, N.; Vrakking, M. J. J.; Stolow, A. The multielectron ionization dynamics underlying attosecond strong-field spectroscopies. Science 2012, 335, 1336-1340. [7] Mikosch, J.; Boguslavskiy, A. E.; Wilkinson, I.; Spanner, M.; Patchkovskii, S.; Stolow, A. Channel- and angle-resolved above threshold ionization in the molecular frame. Phys. Rev. Lett. 2013, 110, 023004. [8] Loh, Z. H.; Leone, S. R. Ultrafast strong-field dissociative ionization dynamics of CH2 Br2 probed by femtosecond soft x-ray transient absorption spectroscopy. J. Chem. Phys. 2008, 128, 204302. [9] Lin, M. F.; Neumark D. M.; Gessner, O.; Leone, S. R. Ionization and dissociation dynamics of vinyl bromide probed by femtosecond extreme ultraviolet transient absorption spectroscopy. J. Chem. Phys. 2014, 140, 064311. [10] Litvinyuk, I. V.; Lee, K. F.; Dooley, P. W.; Rayner, D. M.; Villeneuve, D. M.; Corkum, P. B. Alignmentdependent strong field ionization of molecules. Phys. Rev. Lett. 2003, 90, 233003. [11] Paviˇcić, D.; Lee, K. F.; Rayner, D. M.; Corkum, P. B.; Villeneuve, D. M. Direct measurement of the angular dependence of ionization for N2 , O2 , and CO2 in intense laser fields. Phys. Rev. Lett. 2007, 98, 243001. [12] Kumarappan, V.; Holmegaard, L.; Martiny, C.; Madsen, C. B.; Kjeldsen, T. K.; Viftrup, S. S.; Madsen, L. B.; Stapelfeldt, H. Multiphoton electron angular distributions from laser-aligned CS2 molecules. Phys. Rev. Lett. 2008, 100, 093006. [13] Hansen, J. L.; Holmegaard, L.; Nielsen, J. H.; Stapelfeldt, H.; Dimitrovski, D.; Madsen, L. B. Orientation-dependent ionization yields from strong-field ionization of fixed-in-space linear and asymmetric top molecules. J. Phys. B 2012, 45, 015101. ´ Ivanov, [14] Oppermann, M.; Weber, S. J.; Morales, F.; Richter, M.; Patchkovskii, S.; Csehi, A.; Vibók, A. M.; Smirnova, O.; Marangos, J. P. Control and identification of strong field dissociative channels in CO+ 2 via molecular alignment. J. Phys. B 2014, 47, 124025. [15] Xie, X.; Doblhoff-Dier, K.; Xu, H.; Roither, S.; Schöffler, M. S.; Kartashov, D.; Erattupuzha, S.; Rathje, T.; Paulus, G. G.; Yamanouchi, K.; et al. Selective control over fragmentation reactions in polyatomic molecules using impulsive laser alignment. Phys. Rev. Lett. 2014, 112, 163003. [16] Codling, K.; Frasinski, L. J. Dissociative ionization of small molecules in intense laser fields. J. Phys. B 1993, 26, 783-809. [17] Cornaggia, C.; Lavancier, J.; Normand, D.; Morellec, J.; Agostini, P.; Chambaret, J. P.; Antonetti, A. Multielectron dissociative ionization of diatomic molecules in an intense femtosecond laser field. Phys. Rev. A 1991, 44, 4499-4505. [18] Erattupuzha, S.; Covington, C. L.; Russakoff, A.; Lötstedt, E.; Larimian, S.; Hanus, V.; Bubin, S.; Koch, M.; Gräfe, S.; Baltuˇska, A.; et al. Enhanced ionisation of polyatomic molecules in intense laser pulses is due to energy upshift and field coupling of multiple orbitals. J. Phys. B 2017, 50, 125601. [19] Liu, H.; Yang, Z.; Gao, Z.; Tang, Z. J. Ionization and dissociation of CH3 I in intense laser field. J. Chem. Phys. 2007, 126, 044316 [20] Wang, Y.; Zhang, S.; Wei, Z.; Zhang, B. Velocity map imaging of dissociative ionization and coulomb explosion of CH3 I induced by a femtosecond laser. J. Phys. Chem. A 2008, 112, 3846-3851. [21] Corrales, M. E.; Gitzinger, G.; Vazquez, J. G.; Loriot, V.; Nalda, R.; Banares, L. Velocity map imaging and theoretical study of the coulomb explosion of CH3 I under intense femtosecond IR pulses. J. Phys. Chem. A 2012, 116, 2669-2677. [22] Graham, P.; Ledingham, K.; Singhai, R.; Hankin, S.; McCanny, T.; Fang, X.; Kosmidis, C.; Tzallas, P.; Taday, P.; Langley, A. J. On the fragment ion angular distributions arising from the tetrahedral molecule CH3 I. J. Phys. B 2001, 34, 4015-4026. [23] Ma, R.; Wu, C.; Xu, N.; Huang, J.; Yang, H.; Gong, Q. Geometric alignment of CH3 I in an intense

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femtosecond laser field. Chem. Phys. Lett. 2005, 415, 58-63. [24] Song, Y. D.; Chen, Z.; Sun, C. K.; Hu, Z. Angular distributions of CH3 I fragment ions under the irradiation of a single pulse and trains of ultrashort laser pulses. Chin. Phys. B 2013, 22, 013302. [25] Walt, S. G.; Ram, N. B.; Conta, A.; Tolstikhin, O. I.; Madsen, L. B.; Jensen, F.; Wörner, H. J. Role of multi-electron effects in the asymmetry of strong-field ionization and fragmentation of polar molecules: The methyl halide series. J. Phys. Chem. A 2015, 119, 11772-11782. [26] Madsen, C. B.; Anis, F.; Madsen, L. B.; Esry, B. D. Multiphoton above threshold effects in strong-field fragmentation, Phys. Rev. Lett. 2012, 109, 163003. [27] Lu, P. F.; Zhang, W. B.; Gong, X. C.; Song, Q. Y.; Lin, k.; Ji, Q. Y.; Ma, J. Y.; He, F.; Zeng, H. P.; Wu, J. Electron-nuclear correlation in above-threshold double ionization of molecules. Phys. Rev. A 2017, 95, 033404. [28] Ferré, A.; Boguslavskiy, A. E.; Dagan, M.; Blanchet, V.; Bruner, B. D.; Burgy, F.; Camper, A.; Descamps, D.; Fabre, B.; Fedorov, N.; Gaudin, J.; et al. Multi-channel electronic and vibrational dynamics in polyatomic resonant high-order harmonic generation. Nat. Commun. 2015, 6, 5952. [29] Wallace, W. C.; Ghafur, O.; Khurmi, C.; Sainadh, S.; Calvert, J. E.; Laban, D. E.; Pullen, M. G.; Bartschat, K.; Grum-Grzhimailo, A. N.; Wells, D.; et al. Precise and accurate measurements of strongfield photoionization and a transferable laser intensity calibration standard. Phys. Rev. Lett. 2016, 117, 053001. [30] Luo, S. Z.; Zhu, R. H.; He, L. H.; Hu, W. H.; Li, X. K.; Ma, P.; Wang, C. C.; Liu, F. C.; Roeterdink, W. G.; Stolte, S.; et al. Nonadiabatic laser-induced orientation and alignment of rotational-state-selected CH3 Br molecules. Phys. Rev. A 2015, 91, 053408 . [31] Luo, S. Z.; Hu, W. H.; Yu, J. Q.; Zhu, R. H.; He, L. H.; Li, X. K.; Ma, P.; Wang, C. C.; Liu, F. C.; Roeterdink, W. G.; et al. Rotational dynamics of quantum state-selected symmetric-top molecules in nonresonant femtosecond laser fields. J. Phys. Chem. A 2017, 121, 777-783. [32] Mallinson, P. D. The microwave spectrum of CH2 DI. J. Mol. Spectrosc. 1975, 55, 94-107. [33] Normand, D.; Cornaggia, C.; Lavancier, J.; Morellec, J.; Liu, H. X. Multielectron dissociative ionization of O2 in an intense picosecond laser field. Phys. Rev. A 1991, 44, 475. [34] Wu, C. Y.; Yang, Y. D.; Wu, Z. F.; Chen, B. Z.; Dong, H.; Liu, X. R.; Deng, Y. K.; Liu, H.; Liu, Y. Q.; Gong, Q. H. Coulomb explosion of nitrogen and oxygen molecules through non-Coulombic states. Phys. Chem. Chem. Phys. 2011, 13, 18398-18408. [35] Wernet, P.; Odelius, M.; Godehusen, K.; Gaudin, J.; Schwarzkopf, O.; Eberhardt, W. Real-time evolution of the valence electronic structure in a dissociating molecule. Phys. Rev. Lett. 2009, 103, 013001. [36] Li, W.; Agnieszka, A.; Becker, J.; Hogle, C. W.; Sharma, V.; Zhou, X.; Becker, A.; Kapteyn, H. C.; Murnan, M. M. Visualizing electron rearrangement in space and time during the transition from a molecule to atoms. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 20219-20222. [37] Tanaka, M.; Murakami, M.; Yatsuhashi, T.; Nakashima, N. Atomiclike ionization and fragmentation of a series of CH3 -X (X: H, F, Cl, Br, I, and CN) by an intense femtosecond laser. J. Chem. Phys. 2007, 127, 104314.

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Multielectron Effects in the Strong Field Sequential Ionization of

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