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Electron–ion coincidence momentum imaging of molecular dissociative ionization in intense laser fields: Application to CS2
Journal of Electron Spectroscopy and Related Phenomena 169 (2009) 97–101
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Electron–ion coincidence momentum imaging of molecular dissociative ionization in intense laser fields: Application to CS2 Akitaka Matsuda a , Mizuho Fushitani a,b , Akiyoshi Hishikawa a,b,c,∗ a b c
Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki, Aichi 444-8585, Japan The Graduate University for Advanced Studies (SOKENDAI), Myodaiji, Okazaki, Aichi 444-8585, Japan PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan
a r t i c l e
i n f o
Article history: Received 5 December 2008 Received in revised form 16 December 2008 Accepted 17 December 2008 Available online 25 December 2008 Keywords: Dissociative ionization Intense laser field CS2 Electron–ion coincidence Momentum imaging
a b s t r a c t An electron–ion coincidence momentum imaging apparatus has been developed and applied to dissociative ionization of CS2 in ultrashort intense laser fields. Photoelectron images of CS2 in 35 fs intense laser fields (2 × 1013 W/cm2 , 800 nm, linearly polarized) recorded in coincidence with the parent ion, CS2 + , show clear concentric ring patterns due to the above-threshold ionization (ATI) process. On the other hand, broad structureless distributions elongated along the direction of the laser polarization are observed in the coincidence electron images for the CS+ and S+ fragment ions. The difference in the electron images indicates that the dissociative ionization does not proceed sequentially by the formation and photodissociation of CS2 + in intense laser fields. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Dissociative ionization is a typical response of molecules in nonresonant intense laser fields [1]. The resultant fragment ions often provide valuable information on the nuclear dynamics occurring in an ultrashort time scale, and thus, a number of experimental studies have been dedicated to the detection of fragment ions, which successfully elucidated characteristic reaction processes reflecting the changes in the nuclear potential [1], such as above-threshold dissociation (ATD), bond softening/hardening [2] and deformation of the geometrical structures [3,4]. These features are associated with the rearrangement of the electron distribution by the strong interaction with the intense laser fields. Molecular ionization is one such processes, which can provide, in principle, more direct information on how the electrons are driven in intense laser fields. Indeed, for atoms in intense laser fields, photoelectron spectroscopy has clearly elucidated the importance of the resonant excitation to ponderamotively shifted Rydberg states (known as “Freeman resonances”) [5]. For molecules, on the other hand, ionization pathways yielding fragment ions are open to compete with the direct formation of parent ions, so that the photoelectron spectra are essentially
composed of contributions from many different ionization pathways. In the present study, we performed electron–ion coincidence measurements on the dissociative ionization of CS2 in intense laser fields to label each photoelectron with the counterpart ion, so that the electron dynamics for different pathways can be discussed separately from the corresponding photoelectron images. Only a few studies [6,7] have been reported on the electron–ion coincidence measurements on molecular processes in intense laser fields until now. The dissociative ionization and the Coulomb explosion of H2 [6] and C2 H5 OH [7] were discussed at relatively high field intensity (∼1014 W/cm2 ) from the coincident photoelectron spectra. Here the experiments are performed at a lower intensity (2.1 × 1013 W/cm2 ) to suppress the “volume effect” originating from the non-uniform distribution of the laser field intensity at the focal spot [8]. In this paper, we first describe the design and performance of our electron–ion coincidence imaging apparatus in detail, and then discuss the possible mechanism of dissociative ionization of CS2 in intense laser fields.
2. Experimental 2.1. Experimental setup
∗ Corresponding author at: Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki, Aichi 444-8585, Japan. Tel.: +81 564 55 7419; fax: +81 564 55 7391. E-mail address: [email protected] (A. Hishikawa). 0368-2048/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2008.12.004
Fig. 1 shows a schematic view of the present electron–ion coincidence experimental setup. The output of a Ti:sapphire laser amplifier system (35 fs, 800 nm, 1 kHz) was introduced into the
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A. Matsuda et al. / Journal of Electron Spectroscopy and Related Phenomena 169 (2009) 97–101
Fig. 1. Schematic of the electron–ion coincidence imaging apparatus. The laser pulses are focused by a concave mirror (f = 75 mm) placed in a ultra high vacuum chamber (<3 × 10−8 Pa) onto target molecules introduced as a skimmed effusive beam. Electrons and ions produced at the interaction region with the laser pulses are guided by four electrodes in opposite directions and detected by position sensitive detectors with delay-line anodes (Roentdek HEX80).
ultrahigh vacuum chamber (<3 × 10−8 Pa) along the y-axis through a 3-mm thick synthesized quartz window. The laser polarization was linear along the x-axis. The laser pulses were focused by a concave mirror (f = 75 mm) placed in the vacuum chamber to generate intense laser fields. The sample gas was introduced as a skimmed effusive beam along the x-axis to the interaction region. Electrons and ions produced from the target molecules at the laser focal spot are guided in the opposite directions and detected by fast position sensitive detectors (PSDs) with delay-line readouts (Roentdek HEX80). The flight tubes were magnetically shielded by a mu-metal tube of 1.5-mm thickness (not shown in Figs. 1 and 2). The electron and ion guiding was performed simply by means of four equally spaced parallel-plate electrodes (spacing d = 18 mm). Each electrode had a 40-mm diameter opening to form non-uniform electric fields in Fig. 2. The trajectories show that electrons and ions are focused onto different positions of the detectors depending on the initial velocity, thus realizing the velocity-mapping detection of these charged particles. The applied voltages, VT = 0 V, VM = −510 V, VB = −390 V, Vbase = −900 V, are reversed in order (VT > VB > VM > Vbase ) for the two center electrodes (B and M) to achieve the velocity-mapping condition. The sample number density in the effusive beam was kept low to achieve a low count rate (<0.03 events per laser shot). The amplified and discriminated signals from each end of the delay-line anodes were sent to a time-to-digital converter (Roentdek TDC8, 0.5 ns time resolution) for electron and ion detection, respectively, and stored on an event-to-event basis in hard disks for off-line analysis. The (x, y) position on the PSD and time-of-flight t at the arrival at the detector are determined for single detection events to obtain the momenta of the ions and electrons [9]. 2.2. Determination of the laser field intensity The laser field intensity at the laser focal spot is determined by two different methods. One is to use the ponderomotive shift of
Fig. 2. Non-uniform electrostatic potentials formed by the four electrodes (hole diameter 40 mm) in the present electron–ion coincidence momentum imaging apparatus. The applied voltages, VT = 0 V, VM = −510 V, VB = −390 V, Vbase = −900 V, are reversed in order (VT > VB > VM > Vbase ) for the two center electrodes (B and M) to achieve the velocity-mapping condition. The electric field at the interaction region is 6.3 V/mm. Trajectories show that the electrons and S+ ions ejected from the interaction region of 5 mm are focused to less than 1 mm at the PSD detectors. The dimensions are given in millimeters.
the ionization energy in intense laser fields. The shift is given by the ponderomotive energy Up = e2 I2 /82 n0 me c3 ε0 , where I is the intensity, is the wavelength, and e, n0 , me , c and ε0 are the elementary charge, the refractive index of vacuum, the electron mass, the speed of light in vacuum, and the vacuum permittivity, respectively. Thus the energy E of photoelectrons produced from a molecule having an ionization energy of I0 by n-photon absorption is given as E = nh − (I0 + Up ). Since the poderomotive potential is proportional to I, the field intensity can be directly determined from a measured value of E of a given above-threshold ionization (ATI) photoelectron peak. The other method utilizes the ion recoil momentum spectroscopy in circularly polarized intense laser fields [10,11]. Circularly polarized laser light obtained by an achromatic quarterwave plate was focused on a skimmed molecular beam of D2 . A weak uniform extraction electric field (5.6 V/mm) was applied to measure the momentum of the resultant D2 + ions along the time-of-flight axis (z-axis) at a field intensity below the ionization saturation limit. The obtained momentum spectra along the z-axis were fitted by a model function incorporating the intensity distribution near the focal spot (i.e., “the volume effect”) and the temporal shape of the laser pulses to obtain the peak field intensity. This method is useful especially at relatively large field intensities (>1 × 1014 W/cm2 ) where ATI peaks become barely visible. It
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Fig. 3. Photoelectron images (a) recorded with a gas mixture of CS2 and Xe at a field intensity of 2.1 × 1013 W/cm2 (800 nm, 35 fs) and those recorded in coincidence with CS2 + (b) and Xe+ (c). The total electron–ion coincidence count rate is less than 0.03 per laser shot. The coincidence photoelectron image for Xe+ is in good agreement with the photoelectron image obtained with a pure Xe sample at the same field intensity (d), showing that minor ionization processes can be extracted without degradation.
was found that the results of these two independent measurements were consistent with each other, and the uncertainty of the peak intensity was estimated to be 5% at the field intensity of 2.1 × 1013 W/cm2 . 3. Results and discussion 3.1. Performance test of the electron–ion coincidence imaging apparatus The performance test of the present apparatus for electron–ion coincidence imaging was carried out using a gas mixture of CS2 and Xe. The photoelectron image obtained at a laser peak intensity of 2.1 × 1013 W/cm2 shows concentric ring distributions sharply peaked along the laser polarization vector (ε) as shown in Fig. 3(a). The corresponding time-of-flight mass spectrum in Fig. 4 shows intense peaks due to CS2 + and a smaller amount (∼15% of CS2 + ) of Xe+ . In addition, much weaker peaks due to CS+ and S+ produced by dissociative ionization of CS2 and the doubly charged cation CS2 2+ are observed, indicating that the photoelectron image in Fig. 3(a) contains contributions from various ionization processes. The photoelectron images obtained in coincidence with CS2 + and Xe+ are shown in Fig. 3(b) and (c), respectively. Since CS2 + is the dominant product, the coincidence photoelectron image for CS2 + is similar to the non-coincidence electron image in Fig. 3(a). On the other hand, a different image is obtained when the photoelectrons are detected in coincidence with Xe+ . The coincidence image is in good agreement with the photoelectron image obtained with a pure Xe sample at the same field intensity in Fig. 3(d), which
Fig. 4. Ion time-of-flight spectrum of a CS2 /Xe gas mixture obtained at a field intensity of 2.1 × 1013 W/cm2 (800 nm, 35 fs). The mass resolution M/M is estimated to be ∼1000.
shows clear nodal patterns due to the 8-photon resonance with the ac-Stark shifted 4f Rydberg states [12]. A more detailed comparison can be made in the photoelectron spectra in Fig. 5 obtained by angular integration of these photoelectron images. The energy spacing of these peaks is about 1.5 eV corresponding to the energy of a single photon. The 4f Rydberg peaks are shaded towards the higher energy side both in non-coincidence and coincidence spectra due to the 8-photon resonances to the 5g Rydberg states. The laser field intensity estimated from the Xe photolelectron spectra [12] (∼2.0 × 1013 W/cm2 ) is in good agreement with the value (2.1 × 1013 W/cm2 ) obtained by the method described above. These spectra show that the false events due to intense peaks associated with CS2 + are almost negligible at the present count rate, showing that the details of minor ionization processes in intense laser fields, which are often obscured by other dominant ionization pathways, can be discussed from the individual photoelectron images obtained by the present ion–electron coincidence technique. 3.2. Dissociative ionization CS2 in intense laser fields The ion time-of-flight spectrum in Fig. 4 shows two different fragment ions, CS+ and S+ , due to the dissociative ionization: CS2 → CS+ + S + e− ,
(1)
Fig. 5. Photoelectron spectra of Xe in intense laser fields (2.1 × 1013 W/cm2 , 800 nm, 35 fs) obtained for a pure Xe sample (black solid line) and a gas mixture of CS2 and Xe (red solid line) from the coincidence photoelectron images in Fig. 3(d) and (c), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 6. Coincidence photoelectron images for (a) CS+ and (b) S+ produced from CS2 in intense laser fields (2.1 × 1013 W/cm2 , 800 nm, 35 fs). The corresponding ion images are shown for (c) CS+ and (d) S+ .
CS2 → S+ + CS + e− ,
(2)
in addition to the parent ion CS2 + formed by the single ionization process, CS2 → CS2 + + e− . The yields of the CS+
(3) channel in Eq. (1) and the S+
channel in Eq. (2) are 3.8% and 4.3% of the CS2 + yield, respectively. The electron–ion coincidence images of the pathways (1) and (2) are shown in Fig. 6. The observed distributions for the CS+ (Fig. 6(c)) and S+ (Fig. 6(d)) fragment ions are peaked at the center of the ion images, showing that these fragments are mostly formed with small kinetic energies (<1 eV) by the dissociative ionization process, and that the contributions from the Coulomb explosion process [4], CS2 2+ → CS+ + S+ , are virtually negligible (∼0.1). The corresponding coincidence photoelectron images in Fig. 6(a) and (b), show broad structureless features elongated along the laser polarization direction, presenting a sharp contrast with the coincidence electron image for CS2 + exhibiting clear ATI structures (see Fig. 3(b)). The difference between the parent and fragment ions is also seen in the photoelectron energy distribution in Fig. 7. The photoelectron spectrum in coincidence with CS2 + shows several sharp peaks corresponding to the concentric rings observed in Fig. 3(b). On the other hand, the photoelectron spectra in coincidence with CS+ and S+ fragment ions show smooth distributions without distinct peak structures. The dissociative ionization in intense laser fields is often considered to proceed in a stepwise manner, as has been discussed for H2 + [2,13]. The ionization first takes place to populate the parent ions in the stable ground state, which is then followed by further photo-absorption to dissociative excited states leading to fragmentation of the molecule. In this case, the photoelectron spectra for the parent and fragment ions should coincide with each other, since the fragmentation occurs after the electron ejection. Thus, the clear difference observed in the present study between the photoelec-
tron images for the parent (CS2 + ) and the fragment (S+ and CS+ ) ions show that such a sequential photo-absorption process via the ground state CS2 + is not significant in the dissociative ionization of CS2 . Instead, the observed dissociative ionization can be attributed to the direct multiphoton ionization to excited states [14] of CS2 + that can subsequently decay by fragmentation. In this case, the electrons correlated with the CS+ and S+ ions are ejected upon the formation of the excited states, so that the spectra can be different from that for the ground state of CS2 + . Previous studies using a VUV light ˜ 2 g + source show that the first dissociative excited state of CS2 + is C
Fig. 7. Photoelectron energy spectra of CS2 in intense laser fields (2.1 × 1013 W/cm2 , 800 nm, 35 fs) obtained by the coincidence with CS2 + (red solid line), CS+ (blue solid line) and S+ (green solid line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
A. Matsuda et al. / Journal of Electron Spectroscopy and Related Phenomena 169 (2009) 97–101 Table 1 ˜ 2 g + Vertical excitation energies of low lying electronic states of CS2 + from the CS2 X state, and their products. State
Vertical ionization energy [16]
Product ion [15]
˜ 2 g X ˜ 2 u A
10.11 eV
CS2 +
˜ 2 u + B ˜ 2 g + C
12.56 eV
CS2 +
14.49 eV
CS2 +
16.20 eV
CS+ , S+ (CS2 + )
located about 6 eV above the ground state (see Table 1), which dissociates into CS+ + S and S+ + CS with almost the same branching ratio (0.58 and 0.42) [15], as observed in the present study. The observed kinetic energies of the fragments [15] are also consistent with those observed (<1 eV) here. On the other hand, the He II photoelectron ˜ 2 g + state exhibit peaks as sharp as those of the spectra of the C ground state [16]. The broad structureless photoelectron spectra observed here would, therefore, indicate the contributions from additional processes such as (1) a laser-induced coupling with other excited states of CS2 + and (2) nuclear motions prior to the ionization, possibly induced by the excitations to highly excited states [17–19] in neutral CS2 . It is known that the photoelectrons released by the ionization to the ground state of CS2 + can return to the ion core after the acceleration by the laser field. The “electron recollision” process plays an important role in the high-order harmonics generation and non-sequential double ionization [20] as well as in the molecular excitation [21] in intense laser fields. In the latter case, the returning electron is inelastically scattered by the ion core and loses their energy, so that the final photoelectron distribution associated with the molecular excitation can be different from the photoelectrons escaping directly from the ion core, as observed in the present study. It should be noted, however, that the maximum kinetic energy of the returning electrons (Emax = 3.17Up ) is only 4.0 eV at the present field intensity of 2.1 × 1013 W/cm2 . Since the available collision energy is significantly lower than the excitation energy ˜ 2 g + state from the ground state, the excitation to (∼6 eV) to the C the dissociative state by the electron recollision seems to be pos˜ 2 u ) and the second (B ˜ 2 u + ) excited sible only from the first (A states (see Table 1), which might be populated by the direct ionization discussed above. It is worthy to note the recent discussion [22–24] on the multiple electron collision for the double ionization of atoms at the field intensity below the recollision threshold. If such a mechanism holds in the present case, the accumulated excitation from the ionic ground state would eventually lead to the fragmentation and to the diffuse photoelectron peaks due to the repeated collisions. 4. Summary Electron–ion coincidence imaging apparatus has been developed for molecular dissociative ionization in intense laser fields. The performance of the apparatus was tested by the single ionization of a gas mixture of CS2 and Xe. The photoelectron produced by the minor ionization component Xe+ was securely separated from
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that for the dominant component CS2 + , showing that the details of minor ionization processes in intense laser fields, which are often obscured by other dominant ionization pathways, can be discussed from the individual photoelectron images obtained by the present ion–electron coincidence technique. The electron–ion coincidence imaging technique was applied to CS2 in ultrashort intense laser fields (2.1 × 1013 W/cm2 ). The photoelectron spectra obtained in coincidence with the parent ion CS2 + showed clear ATI features. On the other hand, featureless broad spectra was obtained in coincidence with the fragment ions, CS+ and S+ , indicating that the dissociative ionization does not proceed sequentially by the formation and photodissociation of CS2 + in intense laser fields. The origin of the difference in the spectra was discussed in terms of the direct ionization to the dissociative state and of the (multiple) electron recollision process. Acknowledgements The authors are grateful to Dr. Chien-Ming Tseng for his experimental support. The present study is supported by Grant-in-Aid for Young Scientist (A) (No. 15685001) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References [1] J.H. Posthumus, Rep. Prog. Phys. 67 (2004) 623. [2] P.H. Bucksbaum, A. Zavriyev, H.G. Muller, D.W. Schumacher, Phys. Rev. Lett. 64 (1990) 1883. [3] A. Hishikawa, A. Iwamae, K. Yamanouchi, Phys. Rev. Lett. 83 (1999) 1127. [4] H. Hasegawa, A. Hishikawa, K. Yamanouchi, Chem. Phys. Lett. 349 (2001) 57. [5] R.R. Freeman, P.H. Bucksbaum, H. Milchberg, S. Darack, D. Schumacher, M.E. Geusic, Phys. Rev. Lett. 59 (1987) 1092. [6] H. Rottke, C. Trump, M. Wittmann, G. Korn, W. Sandner, R. Moshammer, A. Dorn, C.D. Schröter, D. Fischer, J.R. Crespo Lopez-Urrutia, J. Ullrich, Phys. Rev. Lett. 89 (2002) 013001. [7] T. Hatamoto, G. Prumper, M. Okunishi, D. Mathur, K. Ueda, Phys. Rev. A 75 (2007) 061402. [8] T. Morishita, Z. Chen, S. Watanabe, C.D. Lin, Phys. Rev. A 75 (2007) 023407. [9] M. Lebech, J.C. Houver, D. Dowek, Rev. Sci. Instrum. 73 (2002) 1866. [10] I.V. Litvinyuk, K.F. Lee, P.W. Dooley, D.M. Rayner, D.M. Villeneuve, P.B. Corkum, Phys. Rev. Lett. 90 (2003) 233003. [11] A.S. Alnaser, X.M. Tong, T. Osipov, S. Voss, C.M. Maharjan, B. Shan, Z. Chang, C.L. Cocke, Phys. Rev. A 70 (2004) 023413. [12] V. Schyja, T. Lang, H. Helm, Phys. Rev. A 57 (1998) 3692. [13] L.J. Frasinski, J.H. Posthumus, J. Plumridge, K. Codling, P.F. Taday, A.J. Langley, Phys. Rev. Lett. 83 (1999) 3265. [14] G.N. Gibson, R.R. Freeman, T.J. McIlrath, Phys. Rev. Lett. 67 (1991) 1230. [15] B. Brehm, J.H.D. Eland, R. Frey, A. Kustler, Int. J. Mass Spectrom. Ion Phys. 12 (1973) 213. [16] P. Baltzer, B. Wannberg, M. Lundqvist, L. Karlsson, D.M.B. Holland, M.A. MacDonald, M.A. Hayes, P. Tomasello, W. von Niessen, Chem. Phys. 202 (1996) 185. [17] I. Kawata, H. Kono, Y. Fujimura, A.D. Bandrauk, Phys. Rev. A 62 (2000) 031401(R). [18] A.N. Markevitch, D.A. Romanov, S.M. Smith, R.J. Levis, Phys. Rev. Lett. 92 (2004) 063001. [19] A. Hishikawa, E.J. Takahashi, A. Matsuda, Phys. Rev. Lett. 97 (2006) 243002. [20] P.B. Corkum, Phys. Rev. Lett. 71 (1993) 1994. [21] H. Niikura, F. Legare, R. Hasbani, A.D. Bandrauk, M.Y. Ivanov, D.M. Villeneuve, P.B. Corkum, Nature 417 (2002) 917. [22] P.J. Ho, R. Panfili, S.L. Haan, J.H. Eberly, Phys. Rev. Lett. 94 (2005) 093002. [23] J.S. Prauzner-Bechcicki, K. Sacha, B. Eckhardt, J. Zakrzewski, Phys. Rev. Lett. 98 (2007) 203002. [24] Y. Liu, S. Tschuch, A. Rudenko, M. Durr, M. Siegel, U. Morgner, R. Moshammer, J. Ullrich, Phys. Rev. Lett. 101 (2008) 053001.
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Electron–ion coincidence momentum imaging of molecular dissociative ionization in intense laser fields: Application to CS2 Akitaka Matsuda a , Mizuho Fushitani a,b , Akiyoshi Hishikawa a,b,c,∗ a b c
Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki, Aichi 444-8585, Japan The Graduate University for Advanced Studies (SOKENDAI), Myodaiji, Okazaki, Aichi 444-8585, Japan PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan
a r t i c l e
i n f o
Article history: Received 5 December 2008 Received in revised form 16 December 2008 Accepted 17 December 2008 Available online 25 December 2008 Keywords: Dissociative ionization Intense laser field CS2 Electron–ion coincidence Momentum imaging
a b s t r a c t An electron–ion coincidence momentum imaging apparatus has been developed and applied to dissociative ionization of CS2 in ultrashort intense laser fields. Photoelectron images of CS2 in 35 fs intense laser fields (2 × 1013 W/cm2 , 800 nm, linearly polarized) recorded in coincidence with the parent ion, CS2 + , show clear concentric ring patterns due to the above-threshold ionization (ATI) process. On the other hand, broad structureless distributions elongated along the direction of the laser polarization are observed in the coincidence electron images for the CS+ and S+ fragment ions. The difference in the electron images indicates that the dissociative ionization does not proceed sequentially by the formation and photodissociation of CS2 + in intense laser fields. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Dissociative ionization is a typical response of molecules in nonresonant intense laser fields [1]. The resultant fragment ions often provide valuable information on the nuclear dynamics occurring in an ultrashort time scale, and thus, a number of experimental studies have been dedicated to the detection of fragment ions, which successfully elucidated characteristic reaction processes reflecting the changes in the nuclear potential [1], such as above-threshold dissociation (ATD), bond softening/hardening [2] and deformation of the geometrical structures [3,4]. These features are associated with the rearrangement of the electron distribution by the strong interaction with the intense laser fields. Molecular ionization is one such processes, which can provide, in principle, more direct information on how the electrons are driven in intense laser fields. Indeed, for atoms in intense laser fields, photoelectron spectroscopy has clearly elucidated the importance of the resonant excitation to ponderamotively shifted Rydberg states (known as “Freeman resonances”) [5]. For molecules, on the other hand, ionization pathways yielding fragment ions are open to compete with the direct formation of parent ions, so that the photoelectron spectra are essentially
composed of contributions from many different ionization pathways. In the present study, we performed electron–ion coincidence measurements on the dissociative ionization of CS2 in intense laser fields to label each photoelectron with the counterpart ion, so that the electron dynamics for different pathways can be discussed separately from the corresponding photoelectron images. Only a few studies [6,7] have been reported on the electron–ion coincidence measurements on molecular processes in intense laser fields until now. The dissociative ionization and the Coulomb explosion of H2 [6] and C2 H5 OH [7] were discussed at relatively high field intensity (∼1014 W/cm2 ) from the coincident photoelectron spectra. Here the experiments are performed at a lower intensity (2.1 × 1013 W/cm2 ) to suppress the “volume effect” originating from the non-uniform distribution of the laser field intensity at the focal spot [8]. In this paper, we first describe the design and performance of our electron–ion coincidence imaging apparatus in detail, and then discuss the possible mechanism of dissociative ionization of CS2 in intense laser fields.
2. Experimental 2.1. Experimental setup
∗ Corresponding author at: Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki, Aichi 444-8585, Japan. Tel.: +81 564 55 7419; fax: +81 564 55 7391. E-mail address: [email protected] (A. Hishikawa). 0368-2048/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2008.12.004
Fig. 1 shows a schematic view of the present electron–ion coincidence experimental setup. The output of a Ti:sapphire laser amplifier system (35 fs, 800 nm, 1 kHz) was introduced into the
98
A. Matsuda et al. / Journal of Electron Spectroscopy and Related Phenomena 169 (2009) 97–101
Fig. 1. Schematic of the electron–ion coincidence imaging apparatus. The laser pulses are focused by a concave mirror (f = 75 mm) placed in a ultra high vacuum chamber (<3 × 10−8 Pa) onto target molecules introduced as a skimmed effusive beam. Electrons and ions produced at the interaction region with the laser pulses are guided by four electrodes in opposite directions and detected by position sensitive detectors with delay-line anodes (Roentdek HEX80).
ultrahigh vacuum chamber (<3 × 10−8 Pa) along the y-axis through a 3-mm thick synthesized quartz window. The laser polarization was linear along the x-axis. The laser pulses were focused by a concave mirror (f = 75 mm) placed in the vacuum chamber to generate intense laser fields. The sample gas was introduced as a skimmed effusive beam along the x-axis to the interaction region. Electrons and ions produced from the target molecules at the laser focal spot are guided in the opposite directions and detected by fast position sensitive detectors (PSDs) with delay-line readouts (Roentdek HEX80). The flight tubes were magnetically shielded by a mu-metal tube of 1.5-mm thickness (not shown in Figs. 1 and 2). The electron and ion guiding was performed simply by means of four equally spaced parallel-plate electrodes (spacing d = 18 mm). Each electrode had a 40-mm diameter opening to form non-uniform electric fields in Fig. 2. The trajectories show that electrons and ions are focused onto different positions of the detectors depending on the initial velocity, thus realizing the velocity-mapping detection of these charged particles. The applied voltages, VT = 0 V, VM = −510 V, VB = −390 V, Vbase = −900 V, are reversed in order (VT > VB > VM > Vbase ) for the two center electrodes (B and M) to achieve the velocity-mapping condition. The sample number density in the effusive beam was kept low to achieve a low count rate (<0.03 events per laser shot). The amplified and discriminated signals from each end of the delay-line anodes were sent to a time-to-digital converter (Roentdek TDC8, 0.5 ns time resolution) for electron and ion detection, respectively, and stored on an event-to-event basis in hard disks for off-line analysis. The (x, y) position on the PSD and time-of-flight t at the arrival at the detector are determined for single detection events to obtain the momenta of the ions and electrons [9]. 2.2. Determination of the laser field intensity The laser field intensity at the laser focal spot is determined by two different methods. One is to use the ponderomotive shift of
Fig. 2. Non-uniform electrostatic potentials formed by the four electrodes (hole diameter 40 mm) in the present electron–ion coincidence momentum imaging apparatus. The applied voltages, VT = 0 V, VM = −510 V, VB = −390 V, Vbase = −900 V, are reversed in order (VT > VB > VM > Vbase ) for the two center electrodes (B and M) to achieve the velocity-mapping condition. The electric field at the interaction region is 6.3 V/mm. Trajectories show that the electrons and S+ ions ejected from the interaction region of 5 mm are focused to less than 1 mm at the PSD detectors. The dimensions are given in millimeters.
the ionization energy in intense laser fields. The shift is given by the ponderomotive energy Up = e2 I2 /82 n0 me c3 ε0 , where I is the intensity, is the wavelength, and e, n0 , me , c and ε0 are the elementary charge, the refractive index of vacuum, the electron mass, the speed of light in vacuum, and the vacuum permittivity, respectively. Thus the energy E of photoelectrons produced from a molecule having an ionization energy of I0 by n-photon absorption is given as E = nh − (I0 + Up ). Since the poderomotive potential is proportional to I, the field intensity can be directly determined from a measured value of E of a given above-threshold ionization (ATI) photoelectron peak. The other method utilizes the ion recoil momentum spectroscopy in circularly polarized intense laser fields [10,11]. Circularly polarized laser light obtained by an achromatic quarterwave plate was focused on a skimmed molecular beam of D2 . A weak uniform extraction electric field (5.6 V/mm) was applied to measure the momentum of the resultant D2 + ions along the time-of-flight axis (z-axis) at a field intensity below the ionization saturation limit. The obtained momentum spectra along the z-axis were fitted by a model function incorporating the intensity distribution near the focal spot (i.e., “the volume effect”) and the temporal shape of the laser pulses to obtain the peak field intensity. This method is useful especially at relatively large field intensities (>1 × 1014 W/cm2 ) where ATI peaks become barely visible. It
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Fig. 3. Photoelectron images (a) recorded with a gas mixture of CS2 and Xe at a field intensity of 2.1 × 1013 W/cm2 (800 nm, 35 fs) and those recorded in coincidence with CS2 + (b) and Xe+ (c). The total electron–ion coincidence count rate is less than 0.03 per laser shot. The coincidence photoelectron image for Xe+ is in good agreement with the photoelectron image obtained with a pure Xe sample at the same field intensity (d), showing that minor ionization processes can be extracted without degradation.
was found that the results of these two independent measurements were consistent with each other, and the uncertainty of the peak intensity was estimated to be 5% at the field intensity of 2.1 × 1013 W/cm2 . 3. Results and discussion 3.1. Performance test of the electron–ion coincidence imaging apparatus The performance test of the present apparatus for electron–ion coincidence imaging was carried out using a gas mixture of CS2 and Xe. The photoelectron image obtained at a laser peak intensity of 2.1 × 1013 W/cm2 shows concentric ring distributions sharply peaked along the laser polarization vector (ε) as shown in Fig. 3(a). The corresponding time-of-flight mass spectrum in Fig. 4 shows intense peaks due to CS2 + and a smaller amount (∼15% of CS2 + ) of Xe+ . In addition, much weaker peaks due to CS+ and S+ produced by dissociative ionization of CS2 and the doubly charged cation CS2 2+ are observed, indicating that the photoelectron image in Fig. 3(a) contains contributions from various ionization processes. The photoelectron images obtained in coincidence with CS2 + and Xe+ are shown in Fig. 3(b) and (c), respectively. Since CS2 + is the dominant product, the coincidence photoelectron image for CS2 + is similar to the non-coincidence electron image in Fig. 3(a). On the other hand, a different image is obtained when the photoelectrons are detected in coincidence with Xe+ . The coincidence image is in good agreement with the photoelectron image obtained with a pure Xe sample at the same field intensity in Fig. 3(d), which
Fig. 4. Ion time-of-flight spectrum of a CS2 /Xe gas mixture obtained at a field intensity of 2.1 × 1013 W/cm2 (800 nm, 35 fs). The mass resolution M/M is estimated to be ∼1000.
shows clear nodal patterns due to the 8-photon resonance with the ac-Stark shifted 4f Rydberg states [12]. A more detailed comparison can be made in the photoelectron spectra in Fig. 5 obtained by angular integration of these photoelectron images. The energy spacing of these peaks is about 1.5 eV corresponding to the energy of a single photon. The 4f Rydberg peaks are shaded towards the higher energy side both in non-coincidence and coincidence spectra due to the 8-photon resonances to the 5g Rydberg states. The laser field intensity estimated from the Xe photolelectron spectra [12] (∼2.0 × 1013 W/cm2 ) is in good agreement with the value (2.1 × 1013 W/cm2 ) obtained by the method described above. These spectra show that the false events due to intense peaks associated with CS2 + are almost negligible at the present count rate, showing that the details of minor ionization processes in intense laser fields, which are often obscured by other dominant ionization pathways, can be discussed from the individual photoelectron images obtained by the present ion–electron coincidence technique. 3.2. Dissociative ionization CS2 in intense laser fields The ion time-of-flight spectrum in Fig. 4 shows two different fragment ions, CS+ and S+ , due to the dissociative ionization: CS2 → CS+ + S + e− ,
(1)
Fig. 5. Photoelectron spectra of Xe in intense laser fields (2.1 × 1013 W/cm2 , 800 nm, 35 fs) obtained for a pure Xe sample (black solid line) and a gas mixture of CS2 and Xe (red solid line) from the coincidence photoelectron images in Fig. 3(d) and (c), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 6. Coincidence photoelectron images for (a) CS+ and (b) S+ produced from CS2 in intense laser fields (2.1 × 1013 W/cm2 , 800 nm, 35 fs). The corresponding ion images are shown for (c) CS+ and (d) S+ .
CS2 → S+ + CS + e− ,
(2)
in addition to the parent ion CS2 + formed by the single ionization process, CS2 → CS2 + + e− . The yields of the CS+
(3) channel in Eq. (1) and the S+
channel in Eq. (2) are 3.8% and 4.3% of the CS2 + yield, respectively. The electron–ion coincidence images of the pathways (1) and (2) are shown in Fig. 6. The observed distributions for the CS+ (Fig. 6(c)) and S+ (Fig. 6(d)) fragment ions are peaked at the center of the ion images, showing that these fragments are mostly formed with small kinetic energies (<1 eV) by the dissociative ionization process, and that the contributions from the Coulomb explosion process [4], CS2 2+ → CS+ + S+ , are virtually negligible (∼0.1). The corresponding coincidence photoelectron images in Fig. 6(a) and (b), show broad structureless features elongated along the laser polarization direction, presenting a sharp contrast with the coincidence electron image for CS2 + exhibiting clear ATI structures (see Fig. 3(b)). The difference between the parent and fragment ions is also seen in the photoelectron energy distribution in Fig. 7. The photoelectron spectrum in coincidence with CS2 + shows several sharp peaks corresponding to the concentric rings observed in Fig. 3(b). On the other hand, the photoelectron spectra in coincidence with CS+ and S+ fragment ions show smooth distributions without distinct peak structures. The dissociative ionization in intense laser fields is often considered to proceed in a stepwise manner, as has been discussed for H2 + [2,13]. The ionization first takes place to populate the parent ions in the stable ground state, which is then followed by further photo-absorption to dissociative excited states leading to fragmentation of the molecule. In this case, the photoelectron spectra for the parent and fragment ions should coincide with each other, since the fragmentation occurs after the electron ejection. Thus, the clear difference observed in the present study between the photoelec-
tron images for the parent (CS2 + ) and the fragment (S+ and CS+ ) ions show that such a sequential photo-absorption process via the ground state CS2 + is not significant in the dissociative ionization of CS2 . Instead, the observed dissociative ionization can be attributed to the direct multiphoton ionization to excited states [14] of CS2 + that can subsequently decay by fragmentation. In this case, the electrons correlated with the CS+ and S+ ions are ejected upon the formation of the excited states, so that the spectra can be different from that for the ground state of CS2 + . Previous studies using a VUV light ˜ 2 g + source show that the first dissociative excited state of CS2 + is C
Fig. 7. Photoelectron energy spectra of CS2 in intense laser fields (2.1 × 1013 W/cm2 , 800 nm, 35 fs) obtained by the coincidence with CS2 + (red solid line), CS+ (blue solid line) and S+ (green solid line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
A. Matsuda et al. / Journal of Electron Spectroscopy and Related Phenomena 169 (2009) 97–101 Table 1 ˜ 2 g + Vertical excitation energies of low lying electronic states of CS2 + from the CS2 X state, and their products. State
Vertical ionization energy [16]
Product ion [15]
˜ 2 g X ˜ 2 u A
10.11 eV
CS2 +
˜ 2 u + B ˜ 2 g + C
12.56 eV
CS2 +
14.49 eV
CS2 +
16.20 eV
CS+ , S+ (CS2 + )
located about 6 eV above the ground state (see Table 1), which dissociates into CS+ + S and S+ + CS with almost the same branching ratio (0.58 and 0.42) [15], as observed in the present study. The observed kinetic energies of the fragments [15] are also consistent with those observed (<1 eV) here. On the other hand, the He II photoelectron ˜ 2 g + state exhibit peaks as sharp as those of the spectra of the C ground state [16]. The broad structureless photoelectron spectra observed here would, therefore, indicate the contributions from additional processes such as (1) a laser-induced coupling with other excited states of CS2 + and (2) nuclear motions prior to the ionization, possibly induced by the excitations to highly excited states [17–19] in neutral CS2 . It is known that the photoelectrons released by the ionization to the ground state of CS2 + can return to the ion core after the acceleration by the laser field. The “electron recollision” process plays an important role in the high-order harmonics generation and non-sequential double ionization [20] as well as in the molecular excitation [21] in intense laser fields. In the latter case, the returning electron is inelastically scattered by the ion core and loses their energy, so that the final photoelectron distribution associated with the molecular excitation can be different from the photoelectrons escaping directly from the ion core, as observed in the present study. It should be noted, however, that the maximum kinetic energy of the returning electrons (Emax = 3.17Up ) is only 4.0 eV at the present field intensity of 2.1 × 1013 W/cm2 . Since the available collision energy is significantly lower than the excitation energy ˜ 2 g + state from the ground state, the excitation to (∼6 eV) to the C the dissociative state by the electron recollision seems to be pos˜ 2 u ) and the second (B ˜ 2 u + ) excited sible only from the first (A states (see Table 1), which might be populated by the direct ionization discussed above. It is worthy to note the recent discussion [22–24] on the multiple electron collision for the double ionization of atoms at the field intensity below the recollision threshold. If such a mechanism holds in the present case, the accumulated excitation from the ionic ground state would eventually lead to the fragmentation and to the diffuse photoelectron peaks due to the repeated collisions. 4. Summary Electron–ion coincidence imaging apparatus has been developed for molecular dissociative ionization in intense laser fields. The performance of the apparatus was tested by the single ionization of a gas mixture of CS2 and Xe. The photoelectron produced by the minor ionization component Xe+ was securely separated from
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that for the dominant component CS2 + , showing that the details of minor ionization processes in intense laser fields, which are often obscured by other dominant ionization pathways, can be discussed from the individual photoelectron images obtained by the present ion–electron coincidence technique. The electron–ion coincidence imaging technique was applied to CS2 in ultrashort intense laser fields (2.1 × 1013 W/cm2 ). The photoelectron spectra obtained in coincidence with the parent ion CS2 + showed clear ATI features. On the other hand, featureless broad spectra was obtained in coincidence with the fragment ions, CS+ and S+ , indicating that the dissociative ionization does not proceed sequentially by the formation and photodissociation of CS2 + in intense laser fields. The origin of the difference in the spectra was discussed in terms of the direct ionization to the dissociative state and of the (multiple) electron recollision process. Acknowledgements The authors are grateful to Dr. Chien-Ming Tseng for his experimental support. The present study is supported by Grant-in-Aid for Young Scientist (A) (No. 15685001) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References [1] J.H. Posthumus, Rep. Prog. Phys. 67 (2004) 623. [2] P.H. Bucksbaum, A. Zavriyev, H.G. Muller, D.W. Schumacher, Phys. Rev. Lett. 64 (1990) 1883. [3] A. Hishikawa, A. Iwamae, K. Yamanouchi, Phys. Rev. Lett. 83 (1999) 1127. [4] H. Hasegawa, A. Hishikawa, K. Yamanouchi, Chem. Phys. Lett. 349 (2001) 57. [5] R.R. Freeman, P.H. Bucksbaum, H. Milchberg, S. Darack, D. Schumacher, M.E. Geusic, Phys. Rev. Lett. 59 (1987) 1092. [6] H. Rottke, C. Trump, M. Wittmann, G. Korn, W. Sandner, R. Moshammer, A. Dorn, C.D. Schröter, D. Fischer, J.R. Crespo Lopez-Urrutia, J. Ullrich, Phys. Rev. Lett. 89 (2002) 013001. [7] T. Hatamoto, G. Prumper, M. Okunishi, D. Mathur, K. Ueda, Phys. Rev. A 75 (2007) 061402. [8] T. Morishita, Z. Chen, S. Watanabe, C.D. Lin, Phys. Rev. A 75 (2007) 023407. [9] M. Lebech, J.C. Houver, D. Dowek, Rev. Sci. Instrum. 73 (2002) 1866. [10] I.V. Litvinyuk, K.F. Lee, P.W. Dooley, D.M. Rayner, D.M. Villeneuve, P.B. Corkum, Phys. Rev. Lett. 90 (2003) 233003. [11] A.S. Alnaser, X.M. Tong, T. Osipov, S. Voss, C.M. Maharjan, B. Shan, Z. Chang, C.L. Cocke, Phys. Rev. A 70 (2004) 023413. [12] V. Schyja, T. Lang, H. Helm, Phys. Rev. A 57 (1998) 3692. [13] L.J. Frasinski, J.H. Posthumus, J. Plumridge, K. Codling, P.F. Taday, A.J. Langley, Phys. Rev. Lett. 83 (1999) 3265. [14] G.N. Gibson, R.R. Freeman, T.J. McIlrath, Phys. Rev. Lett. 67 (1991) 1230. [15] B. Brehm, J.H.D. Eland, R. Frey, A. Kustler, Int. J. Mass Spectrom. Ion Phys. 12 (1973) 213. [16] P. Baltzer, B. Wannberg, M. Lundqvist, L. Karlsson, D.M.B. Holland, M.A. MacDonald, M.A. Hayes, P. Tomasello, W. von Niessen, Chem. Phys. 202 (1996) 185. [17] I. Kawata, H. Kono, Y. Fujimura, A.D. Bandrauk, Phys. Rev. A 62 (2000) 031401(R). [18] A.N. Markevitch, D.A. Romanov, S.M. Smith, R.J. Levis, Phys. Rev. Lett. 92 (2004) 063001. [19] A. Hishikawa, E.J. Takahashi, A. Matsuda, Phys. Rev. Lett. 97 (2006) 243002. [20] P.B. Corkum, Phys. Rev. Lett. 71 (1993) 1994. [21] H. Niikura, F. Legare, R. Hasbani, A.D. Bandrauk, M.Y. Ivanov, D.M. Villeneuve, P.B. Corkum, Nature 417 (2002) 917. [22] P.J. Ho, R. Panfili, S.L. Haan, J.H. Eberly, Phys. Rev. Lett. 94 (2005) 093002. [23] J.S. Prauzner-Bechcicki, K. Sacha, B. Eckhardt, J. Zakrzewski, Phys. Rev. Lett. 98 (2007) 203002. [24] Y. Liu, S. Tschuch, A. Rudenko, M. Durr, M. Siegel, U. Morgner, R. Moshammer, J. Ullrich, Phys. Rev. Lett. 101 (2008) 053001.