Anisotropic Coulomb Explosion of CO Ligands in ... - ACS Publications

Aug 16, 2016 - Hiroki Tanaka, Nobuaki Nakashima, and Tomoyuki Yatsuhashi*. Department of Chemistry, Graduate School of Science, Osaka City University,...
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Anisotropic Coulomb Explosion of CO Ligands in Group 6 Metal Hexacarbonyls: Cr(CO)6, Mo(CO)6, W(CO)6 Hiroki Tanaka, Nobuaki Nakashima, and Tomoyuki Yatsuhashi* Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi, Osaka 558-8585 Japan S Supporting Information *

ABSTRACT: Multiple ionization and subsequent Coulomb explosion have been studied for many organic molecules and their clusters; however, the metal complexes, particularly the large Coulombic interactions expected between a metal and its ligands, have not yet been explored. In this study, the angular distribution of CO+, oxygen, and carbon ions ejected from metal hexacarbonyls (M(CO)6, M: Cr, Mo, W) having Oh symmetry by Coulomb explosion in femtosecond laser fields (>1 × 1014 W cm−2) is investigated. The emissions of oxygen ions are wellexplained in terms of the geometric alignment along a line inclined 45° relative to the CO−M−CO axis in a M(CO)4 plane. Unlike the explosion behavior of the oxygen ions located on the outer part of the molecule, the explosion behavior of the carbon ions was affected by the laser intensity, kinetic energy, and metal. This finding that the emission trends of carbon sandwiched between oxygen and metal atoms were the opposite of those for oxygen was explained by the obstruction by oxygen, the deformation of structure in bending coordinates, and the strong interaction with charged metal. The anisotropic Coulomb explosion of metal complexes reflecting their structural symmetry and central metal charge is a promising candidate for use in the investigation of large Coulombic interactions at the molecular level.

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INTRODUCTION Multiply charged atomic ion emission is an important phenomenon when molecules are exposed to intense laser fields1 or highly charged atoms.2 Generally, geometric alignmentthat is, the stripping of electrons from the largeamplitude lobe of the highest occupied molecular orbitals (HOMOs) along the laser polarization directionoccurs by tunneling in intense laser fields.3 Anisotropic and/or isotropic emission of ions, namely, Coulomb explosion, with respect to the laser polarization direction, is observed as a result of the geometric alignment followed by the ion interactions within the framework of molecule.4 Ohmura et al. have shown that molecules having an asymmetric structure of HOMO can be selectively ionized by using asymmetric electric fields.5 They successfully discriminated the molecule’s head-to-tail orientation by asymmetric Coulomb explosion.6 Recently, control of the electron emission direction by using asymmetric electric fields in the extreme ultraviolet (EUV) region has been achieved.7 However, only the preferable alignment of the molecule with respect to the laser polarization direction is selectively ionized by using symmetric electric fields. For example, nitrogen4 has been examined as a representative diatomic molecule, and acetylene derivatives,8,9 benzene,10,11 and fullerene12 have been studied as representative linear-, planar-, and spherical-shaped molecules, respectively. Molecular structure deformation,13 heavy mass,8,11 and molecular length8 effects have been observed in the Coulomb explosion processes. Moreover, not only the Coulomb © 2016 American Chemical Society

explosion of isolated molecules but also those of molecular complexes14,15 and molecular clusters16 have been examined. Sophisticated methods such as covariance mapping17 and ion momentum imaging18 have been used to investigate the two- and three-body explosions of small molecules such as CS2 19 as well as middle-size molecules such as benzene.20 Recently, even Coulomb explosion in symmetric electric fields has been utilized for the imaging of molecular structure21 and the discrimination of chiral molecules22 and geometric isomers23 because the geometric alignment in the first ionization process regulates the configuration of the multiply charged molecular ions in the laser polarization direction due to the sequential nature of tunnel ionization processes.23 Although many detailed studies on the Coulomb explosion of organic molecules have been reported, elemental metal and metal complexes have not been well-examined. Recently, the multiple ionizations and subsequent Coulomb explosion of various metal clusters were reported by Castelman et al.16,24−26 In addition, we have reported the formation of multiply charged iron up to 6+ from ferrocene (0.8 μm, 40 fs, 3.6 × 1015 W cm−2).27 The existence of Coulomb explosion could not be identified for cyclopentadienyl ligands, probably due to their liberation as neutral molecules and/or neutral radicals.27 Received: May 20, 2016 Revised: August 7, 2016 Published: August 16, 2016 6917

DOI: 10.1021/acs.jpca.6b05113 J. Phys. Chem. A 2016, 120, 6917−6928

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The Journal of Physical Chemistry A Therefore, the role of central metal in the Coulomb explosion of ligands has not been clarified. Although there are a variety of metal complexes, carbonyl complexes may be suitable candidates for exploring the Coulomb explosion dynamics of ligands because of their simple and symmetric structures as well as the straightforward ligand dissociation processes.28 Trushin et al. have ionized three metal carbonyl complexes, Ni(CO)4, Fe(CO)5, and Cr(CO)6, at 1 × 1014 W cm−2 (800 nm, 110 fs; 810 nm, 50 fs; 1350 nm, 30 fs).29 However, the main focus of their studies was not Coulomb explosion but the photodissociation mechanism of singly charged molecular ions, and thus the details of CO+ as well as carbon and oxygen ions were not provided. In this study, we investigate the Coulomb explosion of group 6 metal hexacarbonyls above 1 × 1014 W cm−2. The characteristic Coulomb explosion behavior of atomic ions is explained in terms of the Oh symmetry of complexes.

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EXPERIMENTAL METHODS

Cr(CO)6 (Kanto Chemical, ≥98.0%), Mo(CO)6 (Aldrich, ≥99.9%), and W(CO)6 (Aldrich, 99.99%) were used without further purification. The experimental details have been described elsewhere.8 Briefly, multiple ionization of metal hexacarbonyls was performed with a 40 fs pulse centered at 0.8 μm, and the ions were detected by a linear time-of-flight mass spectrometer. The time-of-flight spectra obtained by averaging 1000 laser shots were used to evaluate kinetic energy as described elsewhere.30 The angular distribution of ions was constructed from the kinetic energy spectra measured at different θ. Here we defined the angle θ that was measured with respect to the polarization plane of the laser fields from the ion flight axis to the detector. When the direction of laser polarization is parallel (θ = 0°, 180°) or perpendicular (θ = 90°, 270°) to the ion flight axis, it is referred to as “parallel” or “orthogonal,” respectively. The kinetic energy spectra were measured for the two quadrants with 4° steps experimentally, and these data were averaged to improve the ratio of signal intensity to noise intensity, since the two quadrants were almost identical. A polar plot was generated from the averaged data to present the information more clearly.

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RESULTS Ionization and Fragmentation of Metal Hexacarbonyls. The time-of-flight spectra of the three metal hexacarbonyls (M(CO)6, M: Cr, Mo, W) measured at 4.9 × 1014 (Figure 1) and 2.5 × 1015 W cm−2 (Figure S1) consisted of the intact molecular ions up to 3+, the CO-loss fragment ions, very small amounts of the CO and O-loss fragment ions such as M(CO)2C2+ and MC+, CO+, oxygen ions, carbon ions, and metal ions. The ion distributions measured above 4.9 × 1014 W cm−2 were almost identical except for actual ion intensity. We will, in this study, focus on the Cv+ (v = 1−3) and Ow+ (w = 1−3) originating from highly charged precursor ions. Those atomic ions appeared as multiple peaks in the time-of-flight spectra because they had certain kinetic energy owing to the Coulomb explosion. The ions with certain kinetic energy are emitted forward and backward against the ion flight axis determined by the applied static electric field and therefore are detected with a certain time delay in the time-of-flight spectrum.30 Crx+ (x = 1−5), Moy+ (y = 1−5), and Wz+ (z = 1−6) also appeared as multiple peaks; however, the abundant ratio of these multiple peaks was identical to the natural abundant ratio of their isotopes. Mo7+ was not observed, but the existence of Mo6+ was not certain because its peaks, if they

Figure 1. Time-of-flight spectra of (a) Cr(CO)6, (b) Mo(CO)6, and (c) W(CO)6. The upper and lower panels show entire and magnified spectra, respectively. Laser intensity was 4.9 × 1014 W cm−2. The sample pressure was 5.0 × 10−5 Pa. The laser polarization was parallel to the ion flight axis. Asterisks show the ions originated from contaminated water and air.

existed, were hindered by the larger peaks of O+. Figure 2 shows the atomic ion yields as a function of laser intensity. The intensity-dependence plots of oxygen and carbon ions were almost identical for the three metal hexacarbonyls. Judging from the trend of multiply charged metal formation, it is safe to say that Cr(CO)6 and W(CO)6 form the lowest and highest charged metal at a fixed laser intensity, respectively. We decided to investigate the oxygen and carbon ion emission behavior in detail at three representative laser intensities: 1.3 × 1014, 4.9 × 1014, and 6918

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Figure 2. Yields of atomic ions ejected from (a) Cr(CO)6, (b) Mo(CO)6, and (c) W(CO)6 as a function of laser intensity: (left) O+ (□), O2+ (○), O3+ (△); (center) C+ (□), C2+ (○), C3+ (△); (right) M+ (□), M2+ (○), M3+ (△), M4+ (▽), M5+ (◇), M6+ (☆). M denotes metal. The sample pressure was 5.0 × 10−5 Pa. The laser polarization was parallel (O+, O2+, O3+, C3+, metal ions) or orthogonal (C+, C2+) to the ion flight axis. Detector sensitivity was not corrected.

Figure 3. Angular distribution of CO+ ejected from (left) Cr(CO)6 at 4.9 × 1014 W cm−2, (center) Mo(CO)6 at 4.9 × 1014 W cm−2, and (right) W(CO)6 at 4.8 × 1014 W cm−2. The ion intensity in the polar plot is expressed by a linear color code. The radius indicates kinetic energy in electronvolt units. Angle θ in the polar plot indicates the relative angle between the polarization plane of the laser fields and the ion flight axis to the detector. The sample pressure was 5.0 × 10−5 Pa. The kinetic energy spectra were smoothed to improve the signal-to-noise ratio before the construction of angular distribution. The pedestal component originating in 54Cr2+ was superimposed on the angular distribution of CO+ ejected from Cr(CO)6. The ion intensity was normalized at peak kinetic energy.

1.4 × 1015 W cm−2. The singly charged oxygen and carbon ion yields were saturated, and the triply charged ones were negligible at 1.3 × 1014 W cm−2. The triply charged oxygen and carbon ions appeared, and the doubly charged oxygen and carbon ion formation was saturated at 4.9 × 1014 W cm−2. The triply charged oxygen and carbon ion yields were saturated at 1.4 × 1015 W cm−2. Angular Distribution of CO+. We next analyzed the emission behavior of atomic ions; the angular distribution of CO+ was considered here because CO ligands, which are located on the edges of molecules, may provide information about the correlation between the molecular structure and the Coulomb explosion. CO+ showed a triple-peak structure, a sharp peak

accompanying broad side peaks, as shown in Figure 1c. The width of the central peak was very narrow. Thus, the origin of this narrow peak, without noticeable kinetic energy, was the fragmentation of a singly charged molecular ion and perhaps its excited states. In contrast, the width of the side peaks indicates that they are attributed to the Coulomb explosion. The kinetic energy of the side peaks was then calculated as described previously.30 Figure 3 shows the angular distribution of CO+ measured at ∼4.9 × 1014 W cm−2. The shape of the CO+ angular distribution was nearly isotropic but peaked at 0° and 90°. This feature was not observed for atomic ions ejected from linearshape molecules.8 Table 1 shows the kinetic energy of CO+ measured at 4.9 × 1014 W cm−2. Here, we defined the peak 6919

DOI: 10.1021/acs.jpca.6b05113 J. Phys. Chem. A 2016, 120, 6917−6928

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Table 1. Peak and Maximum Kinetic Energies of CO+ and Atomic Ions Ejected from Metal Hexacarbonyls at 4.9 × 1014 W cm−2 kinetic energy, eV Cr(CO)6 ions +c

CO O+ c O2+ c O3+ c C+ e C2+ e C3+ c

peak

a

3.6 19.9 (0.50) 96.2 (0.46) 194 (0.10) 11.9 (1.7) 60.3 (1.6) 141 (0.63)

Mo(CO)6 maximum 17.6 64.4 171 280 40.6 119 228

d

b

peak

a

W(CO)6 maximum

3.4 24.5 (0.46) 94.0 (0.43) 188 (0.12) 14.8 (1.8) 57.5 (1.5) 133 (0.49)

15.8 63.2 166 265 41.9 114 210

b

peak

a

maximumb

3.6 25.4 (0.60) 80.6 (0.43) 181 (0.09) 16.1 (1.9) 56.0 (1.7) 120 (0.76)

13.3 66.1 162 273 44.2 113 199

a

The extinction value, the ratio of the oxygen ion intensity (peak value) measured under an orthogonal condition to that measured under a parallel condition, is shown in parentheses. bDefined as an energy at 10% of the peak ion intensity. cThe laser polarization is parallel to the ion flight axis. d This value is overestimated because of the spectral overlap with Cr2+. eThe laser polarization is orthogonal to the ion flight axis.

Figure 4. Kinetic energy spectra of Ow+ (w = 1−3) ejected from (left) Cr(CO)6, (center) Mo(CO)6, and (right) W(CO)6. The red and black lines indicate the data obtained under parallel (θ = 0°, 180°) and orthogonal (θ = 90°, 270°) conditions, respectively. Laser intensity was 4.9 × 1014 W cm−2. The sample pressure was 5.0 × 10−5 Pa. Ion intensity was normalized to unity at peak kinetic energy.

O2+ and C2+. The kinetic energy spectra of oxygen ions constructed from the time-of-flight spectra taken under parallel and orthogonal conditions are shown in Figures 4 (4.9 × 1014 W cm−2) and S2 (1.4 × 1015 W cm−2). The kinetic energies of oxygen ions are summarized in Tables 1 (4.9 × 1014 W cm−2) and S2 (1.4 × 1015 W cm−2). The angular distributions of oxygen ions are shown in Figures 5 (4.9 × 1014 W cm−2) and S3 (1.4 × 1015 W cm−2). The shape of the kinetic energy spectra of oxygen ions emitted from metal hexacarbonyls was almost identical at 4.9 × 1014 W cm−2. It was obvious that a larger amount of oxygen ionsindependent of the kinetic energy, laser intensity, or central metalwas detected in a parallel rather than an orthogonal direction. Further, it is worth noting that the extinction of oxygen ion, that is, the ratio of the oxygen ion intensity (peak value) measured under an orthogonal condition to that measured under a parallel condition, does not depend on the central metal. Thus, the extinctions were ca. 0.5 (O+ and O2+) and ca. 0.1 (O3+) at 4.9 × 1014 W cm−2 (Table 1). The angular distribution showed that the yield of oxygen ions was constant between 45° and 90° on the first quadrant. There were noticeable increases (up to 20%) of both peak and maximum kinetic energies, but other features were almost identical as the laser intensity increased from 4.9 × 1014 to 1.4 × 1015 W cm−2.

kinetic energy as the energy at peak ion intensity. In addition, the maximum kinetic energy was defined as the energy at 10% of the peak ion intensity. To investigate the origin of the precursor ions, we calculated the Coulomb repulsion energy (CE) under the two-body dissociation and point-charge approximation. The CE is calculated on the assumptions that the Coulomb explosion occurs between the M(CO)5u+ (u = 1−4) and CO+, the geometry is identical to the complex at the neutral ground state, and the charge of CO+ is located on the center of mass of CO+. For calculation of the kinetic energy of each ion, the distribution of CE between M(CO)5u+ and CO+ by their masses was taken into account. The results are shown in Table S1. The peak kinetic energy of CO+ (ca. 3.5 eV) indicates that Coulomb explosion of CO+ presumably occurs with M(CO)52+ by taking the reduction of CE due to the CO−M bond elongation into account.31 The maximum kinetic energy (>10 eV), however, indicates that other Coulomb explosion channels with more highly charged central metal exist. Although there may be a possibility that CO+ is liberated from more highly charged precursor ions, the liberation of CO+ by Coulomb explosion is a minor process judging from its ion yields. Kinetic Energy Spectra and Angular Distributions of Oxygen and Carbon Ions above the Saturation Region of 6920

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Figure 5. Angular distribution of Ow+ (w = 1−3) ejected from (left) Cr(CO)6 at 4.9 × 1014 W cm−2, (center) Mo(CO)6 at 4.9 × 1014 W cm−2, and (right) W(CO)6 at 4.8 × 1014 W cm−2. The ion intensity in the polar plot is expressed by a linear color code. The radius indicates kinetic energy in electronvolt units. Angle θ in the polar plot indicates the relative angle between the polarization plane of the laser fields and the ion flight axis to the detector. The sample pressure was 5.0 × 10−5 Pa. Ion intensity was normalized to unity at peak kinetic energy.

Figure 6. Kinetic energy spectra of Cv+ (v = 1−3) ejected from (left) Cr(CO)6, (center) Mo(CO)6, and (right) W(CO)6. The red and black lines indicate the data obtained under parallel (θ = 0°, 180°) and orthogonal (θ = 90°, 270°) conditions, respectively. Laser intensity was 4.9 × 1014 W cm−2. The sample pressure was 5.0 × 10−5 Pa. Ion intensity was normalized to unity at peak kinetic energy.

The small amount of O2+ carrying the small kinetic energy (ca. 10 eV at 1.4 × 1015 W cm−2), which was diminished under an orthogonal condition, may be originated in the Coulomb explosion of liberated CO. The liberated neutral CO from singly charged precursor ions and/or excited ions may be ionized by the subsequent laser pulses (the repetition rate of laser pulses is 1

kHz), because such neutral molecules may remain in the ionization volume for a sufficient duration. We then calculated the kinetic energy of carbon and oxygen ions on the assumption that those ions were formed by the Coulomb explosion of isolated CO (Table S3). By taking the reduction of kinetic energy due to the C−O bond elongation into account,31 the peaks can 6921

DOI: 10.1021/acs.jpca.6b05113 J. Phys. Chem. A 2016, 120, 6917−6928

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The Journal of Physical Chemistry A

Figure 7. Kinetic energy spectra of Cv+ (v = 1−3) ejected from (left) Cr(CO)6, (center) Mo(CO)6, and (right) W(CO)6. The red and black lines indicate the data obtained under parallel (θ = 0°, 180°) and orthogonal (θ = 90°, 270°) conditions, respectively. Laser intensity was 1.4 × 1015 W cm−2. The sample pressure was 5.0 × 10−5 Pa. Ion intensity was normalized to unity at peak kinetic energy.

Figure 8. Angular distribution of Cv+ (v = 1−3) ejected from (left) Cr(CO)6 at 4.9 × 1014 W cm−2, (center) Mo(CO)6 at 4.9 × 1014 W cm−2, and (right) W(CO)6 at 4.8 × 1014 W cm−2. The ion intensity in the polar plot is expressed by a linear color code. The radius indicates kinetic energy in electronvolt units. Angle θ in the polar plot indicates the relative angle between the polarization plane of the laser fields and the ion flight axis to the detector. The sample pressure was 5.0 × 10−5 Pa. Ion intensity was normalized to unity at peak kinetic energy.

be attributed to the results of the Coulomb explosion of CO4+. The kinetic energy of O2+ observed in this experiment was wellcoincident with the kinetic energy (9.5 eV) calculated from the kinetic energy release of CO4+ produced by the ionization of CO at 5.6 × 1014 W cm−2.32 O+ originating from the liberated CO may exist, but a distinct peak is not resolved. The above

calculations underscore that the main fractions of O2+ and O3+ having high kinetic energy are not originated from isolated CO. The origin of their high kinetic energy should be the Coulomb explosion of highly charged metal hexacarbonyls. Thus, it is expected that the terminal oxygen and carbon are in close proximity to the central metal atom to gain large Coulomb 6922

DOI: 10.1021/acs.jpca.6b05113 J. Phys. Chem. A 2016, 120, 6917−6928

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Figure 9. Angular distribution of Cv+ (v = 1−3) ejected from (left) Cr(CO)6, (center) Mo(CO)6, and (right) W(CO)6 at 1.4 × 1015 W cm−2. The ion intensity in the polar plot is expressed by a linear color code. The radius indicates kinetic energy in electronvolt units. Angle θ in the polar plot indicates the relative angle between the polarization plane of the laser fields and the ion flight axis to the detector. The sample pressure was 5.0 × 10−5 Pa. Ion intensity was normalized to unity at peak kinetic energy.

higher value (1.4−1.6 times higher). We did not find any significant variation of the kinetic energy spectra of oxygen ions by central metal. However, we can now state that the kinetic energy distribution of C+ and C2+ ejected from Cr(CO)6 is different in shape, peak and maximum kinetic energies, and extinction value from that ejected from other metal hexacarbonyls at 1.4 × 1015 W cm−2. The kinetic energy spectra of C+ and C2+ ejected from Mo(CO)6 and W(CO)6 became broad due to the increase of the fraction of high (C+, >30 eV; C2+, >75 eV) kinetic energy. The extinctions of carbon ions became slightly larger at 1.4 × 1015 W cm−2: ca. 2.0 (C+), ca. 1.7 (C2+), and ca. 0.63 (C3+) for Mo(CO)6 and W(CO)6. In contrast, the kinetic energy spectrum of C+ ejected from Cr(CO)6 showed a narrower distribution compared with other metal hexacarbonyls. In addition, the extinction of carbon ions ejected from Cr(CO)6 became smaller compared with other metal hexacarbonyls at 1.4 × 1015 W cm−2: the extinctions were 1.2 (C+), 1.1 (C2+), and 0.44 (C3+). These results indicate that carbon ions did not gain high kinetic energy or efficient ejection in the orthogonal direction in the case of Cr(CO)6 at 1.4 × 1015 W cm−2. The angular distributions of carbon ions were measured at 4.9 × 1014 (Figure 8) and 1.4 × 1015 W cm−2 (Figure 9). The angular distributions of carbon ions emitted from hexacarbonyls were almost identical at 4.9 × 1014 W cm−2. However, it should be mentioned that a small feature found in the kinetic energy spectrum of C+ ejected from Cr(CO)6 is reflected in the angular distributions: C+ ejected from Cr(CO)6 has a larger fraction in the parallel direction than C+ ejected from Mo(CO)6 and W(CO)6. This feature was more clearly observed at 1.4 × 1015 W cm−2. On the one hand, the angular distribution of C+ and C2+

repulsion energy even in the charge states that produce O2+ and O3+. The kinetic energy spectra of carbon ions taken under parallel and orthogonal conditions were measured at 4.9 × 1014 (Figure 6) and 1.4 × 1015 W cm−2 (Figure 7). The peak and maximum kinetic energies and extinction values are listed in Tables 1 (4.9 × 1014 W cm−2) and S2 (1.4 × 1015 W cm−2). The most significant differences in the kinetic energy spectra of carbon ions compared with that of oxygen ions were as follows. (1) The direction of carbon ion emission was dependent on the carbon charge: C+ and C2+ were largely emitted in an orthogonal direction (extinction > 1), whereas C3+ was emitted in a parallel direction (extinction < 1). (2) The degree of the extinction of C+ and C2+ was dependent on the kinetic energy: the extinction became larger at higher kinetic energy, indicating that C+ and C2+ with high kinetic energy were largely emitted in an orthogonal direction. The kinetic energy spectra of C+ and C2+ measured at 4.9 × 1014 W cm−2 were almost identical for the three metal hexacarbonyls. It was noteworthy that we notice that C+ ejected from Cr(CO)6 had a larger fraction below 10 eV compared with other metal hexacarbonyls under a parallel condition. Here, we try to compare the extinction of carbon ions, although the comparison of the extinction at a fixed kinetic energy may not be meaningful, because the extinction is dependent on the kinetic energy. The extinctions determined at peak kinetic energy (orthogonal condition) were ca. 1.8 (C+) and ca. 1.6 (C2+). The extinctions of C3+ were varied (0.5−0.8), probably due to their small ion yields. By increasing the laser intensity from 4.9 × 1014 to 1.4 × 1015 W cm−2, the peak kinetic energy, especially of C+, was shifted to a 6923

DOI: 10.1021/acs.jpca.6b05113 J. Phys. Chem. A 2016, 120, 6917−6928

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The Journal of Physical Chemistry A

Figure 10. Angular distribution and kinetic energy spectra of Ow+ (w = 1−2) and Cv+ (v = 1−2) ejected from Cr(CO)6 at 1.3 × 1014 W cm−2. The ion intensity in the polar plot is expressed by a linear color code. The radius of the polar plot indicates kinetic energy in electronvolt units. Angle θ in the polar plot indicates the relative angle between the polarization plane of the laser fields and the ion flight axis to the detector. The red and black lines in the kinetic energy spectra indicate the data obtained under parallel (θ = 0°, 180°) and orthogonal (θ = 90°, 270°) conditions, respectively. The sample pressure was 7.5 × 10−5 Pa. Ion intensity was normalized to unity at peak kinetic energy.

at the peak kinetic energy became smaller by decreasing the laser intensity. The decrease of the extinction at peak kinetic energy at 1.3 × 1014 W cm−2 would be due to the absence of high kinetic energy components that were dominantly ejected in an orthogonal direction.

ejected from three metal hexacarbonyls had a bimodal shape at 4.9 × 1014 W cm−2. The shape was maintained even at 1.4 × 1015 W cm−2 for Mo(CO)6 and probably W(CO)6. On the other hand, the angular distributions of C+ and C2+ ejected from Cr(CO)6 were elliptical and donutlike at 1.4 × 1015 W cm−2, respectively. These results indicate that the emission of C+ and C2+ from Cr(CO)6 became nearly isotropic at 1.4 × 1015 W cm−2. The small fraction of C2+ carrying the small kinetic energy (ca. 10 eV at 1.4 × 1015 W cm−2), which was diminished under an orthogonal condition, may be originated in the Coulomb explosion of liberated CO as mentioned for O2+. Kinetic Energy Spectra and Angular Distributions of Oxygen and Carbon Ions below the Saturation Region of O2+ and C2+. Figure 10 shows the angular distribution and kinetic energy spectra of singly and doubly charged oxygen and carbon ions ejected from Cr(CO)6 at 1.3 × 1014 W cm−2. The kinetic energy became lower by decreasing the laser intensity from 4.9 × 1014 to 1.3 × 1014 W cm−2. The reductions of the peak kinetic energy of oxygen ions were 90% (O+, 19.9 to 18.0 eV) and 42% (O2+, 96.2 to 40.0 eV) measured under a parallel condition. Those of carbon ions were 69% (C+, 11.9 to 8.2 eV) and 60% (C2+, 60.3 to 36.3 eV) measured under an orthogonal condition. The degree of peak kinetic energy reduction of doubly charged ions was rather large compared with that of singly charged ions. Those of the maximum kinetic energy were 69% (O+, 64.4 to 44.5 eV), 40% (O2+, 171 to 68.6 eV), 77% (C+, 40.6 to 31.1 eV), and 62% (C2+, 119 to 74.4 eV). The finding that the kinetic energy decreased as the laser intensity was reduced indicates the disappearance of the ion ejection channels from more highly charged precursor ions at 1.3 × 1014 W cm−2. The significant changes of the peak and maximum kinetic energy indicate that the production of O2+, C2+, and perhaps C+ was saturated at 4.9 × 1014 but not 1.3 × 1014 W cm−2. The results highlight that the extinction of O+ (0.51) does not change when the laser intensity is decreased from 4.9 × 1014 to 1.3 × 1014 W cm−2, whereas the extinction of O2+ decreases to 0.31. Moreover, the angular distributions of O2+ become narrower. In the case of carbon ions, the extinction estimated

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DISCUSSION Anisotropic Emission of CO+. In general, the liberation of CO ligands precedes the dissociation of C−O bond because the dissociation energy of the metal−CO bond (154−192 kJ mol−1)33 is much smaller than that of carbon monoxide (1072 kJ mol−1). Indeed, the liberation of CO+ was observed, but this process was a minor process and mostly occurred at triply charged molecular ions. The formation of such states should be saturated at the laser intensity used in our study, and thus the production of any orientation of the precursor ions of CO+ resulted in a nearly isotropic angular distribution of CO+. However, the angular distribution of CO+ peaked at 0° and 90° may be the evidence of the emission of ligands located perpendicularly to each other. The Anisotropic Emissions of Oxygen Ions and their Saturation. Oxygen atoms are located on the edge of the molecule, and thus the emission of oxygen ions is not obstructed by other atoms at all. Therefore, it is expected that most oxygen ions will be emitted in a parallel direction. However, there was significant emission of oxygen ions in an orthogonal direction in our experiments. The extinction (orthogonal/parallel) is very informative in terms of explaining the characteristic Coulomb explosion behavior of oxygen ions from M(CO)6, because the values are fairly large. For example, the extinction of H+ and carbon ions ejected from acetylene was nearly zero.8 For the case of a large but linear molecule, that is, ethynylbenzene-d, the extinction of D+ (1) found in the extinction of C+ and C2+ as well as the extinction of C3+, which is smaller than that of C+ and C2+ but larger than that of O3+. We suggest taking the molecular deformation and charge of central metal into account to explain the characteristic emission behavior of carbon ions. If the carbon ions are expelled like oxygen ions, the extinctions should be similar to that of oxygen ions. The geometric alignment configurations depicted in Figure 11b cannot explain the large extinction of carbon ions if carbon is ejected in the same direction as oxygen. The large extinctions require the emission of carbon ions in an orthogonal direction not only from CO(B, B′) but also CO(A, A′). To expel carbon in an orthogonal direction, the Coulomb explosion of the carbon should have a certain momentum orthogonal to the polarization plane of laser fields. One important piece of evidence is that the carbon atom is located between the central metal atom and terminal oxygen atom. However, the obstruction of carbon ion emission by a light oxygen atom is not sufficient to explain the results. The deformation of the molecular structure in the out-ofplane bending coordinates before Coulomb explosion would have such a momentum. Figure 11c shows the out-of-plane bending motion of a CO−M−CO axis (in phase and out-ofphase). Carbons of the other CO−M−CO axis in the same M(CO)4 plane deform in the opposite direction to conserve the center of inertia (not shown). Though we do not have information about the potential energy surfaces of metal hexacarbonyls, we could expect the deformation in the stretching and bending coordinates occur simultaneously by coupling with other electronic states in strong laser fields as in the case of carbon dioxide.18 Assuming that carbons are emitted only in the

orthogonal direction, which is indicated by dotted arrows, the extinction of carbon ion is 1.5 ((2 × C(A) + 2 × C(A′) + C(B) + C(B′))/(2 × C(B) + 2× C(B′))) above the saturation regime. However, the extinction is expected to be larger due to the smaller emission probability of C(B, B′) compared with that of C(A, A′), which is located in the M(CO)4 plane. Actually, the extinctions of C+ ejected from Mo(CO)6 and W(CO)6 were more than 1.8 above 4.9 × 1014 W cm−2. According to these hypotheses, the oxygen and metal atoms should stay close to carbon atoms to gain a large Coulomb repulsion. This situation may be fulfilled for the charge states that expel carbon atoms as C+ and C2+. On the basis of the extinction of C3+, the effect would be insignificant for the charge states that expel carbon atoms as C3+. This insignificance may be due to the M−C−O bond elongation in the stretching coordinates. Furthermore, we suggest that the carbon ion emission in an orthogonal direction is strongly affected by the central metal charge: more highly charged metal will have higher Coulomb repulsion energy. The charge number of the bare metal that appeared in the time-of-flight spectra and the actual charge number of metal in the Coulomb explosion process are not directly correlated. However, the charge number of bare metal ion would help to clarify the explosion behavior of carbon ions. The metal ions with the highest charge numbers, which were comparable to C+ in intensity, were as follows: Cr3+, Mo3+, W3+ at 1.3 × 1014 W cm−2; Cr4+, Mo5+, W5+ at 5 × 1014 W cm−2. These results are consistent with the order of the ionization potential of metal ions. The ionization potentials of metal ions in electronvolt units are as follows:37 Cr+ (6.77), Mo+ (7.09), W+ (7.86); Cr2+ (23.3), Mo2+ (23.3), W2+ (24.2); W3+ (50.2), Mo3+ (50.4), Cr3+ (54.2); W4+ (88.4), Mo4+ (90.7), Cr4+ (103); W5+ (140), Mo5+ (145), Cr5+ (173); W6+ (205), Mo6+ (213), Cr6+ (263). The order is reversed above the triply charged state: Cr2+ and more highly charged chromium require more energy to be further ionized than other metals. Judging from the appearance of bare metal ion and the ionization potential of elemental metals, chromium does not readily become highly charged within the complex. Therefore, it is expected that the Coulomb repulsion in Cr(CO)6 would be weaker than that of the other complexes. The difference in the charge number of metal, that is, different Coulombic repulsion, is a key to explaining the kinetic energy spectra of those carbon ions that are strongly affected by the adjacent metal ion. The kinetic energy spectrum is superimposed by the contributions of the different charge states of the precursor ions. The sudden drop of the extinction of C+ and C2+ ejected from Cr(CO)6 at 1.4 × 1015 W cm−2 is due to the increase of the parallel component of low kinetic energy as well as the lack of a high kinetic energy fraction that is observed in Mo(CO)6 and W(CO)6. The increase of a parallel component indicates that the geometric alignment is lost for the precursor ions that emit carbon ions with low kinetic energy. In the cases of Mo(CO)6 and W(CO)6, the geometric alignment configuration is maintained for the more highly charged precursor ions that emit high kinetic energy carbon ions. To prove this hypothesis, we need to investigate the correlation between carbon and metal ions. However, unfortunately, the metal ions are not carrying sufficient kinetic energy to be investigated by covariance mapping17 or momentum imaging18 techniques. Further investigations of metal complexes that contain metals with large variations of ionization potential would be useful. 6926

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CONCLUSION The angular distribution of CO+, oxygen, and carbon ions measured above 1 × 1014 W cm−2 is explained in terms of the Oh symmetry of metal hexacarbonyls. Although the CO ligands are weakly bound to central metal, oxygen and carbon atoms are highly charged in alternative strong electric fields, and the Coulombic repulsion with central metal results in their high kinetic energy. The angular distributions of oxygen ions, emitted mostly in a parallel direction with respect to the laser polarization direction, were insensitive to laser intensity, kinetic energy, and central metal probably because they were located on the outer part of the molecule. In contrast, the emission of carbon atoms that favored the orthogonal direction was dependent on the laser intensity, kinetic energy, and central metal. The opposite emission trends of carbon sandwiched between oxygen and metal atoms are explained by the obstruction by oxygen, the deformation of structure in bending coordinates, and the strong interaction with charged metal. Chromium is less easily ionized to triply and higher charge states than are molybdenum and tungsten. Therefore, the chromium complex could not form carbon ions with high kinetic energy compared with other metal hexacarbonyls. Other metal carbonyl complexes with different symmetry and different central metals such as Fe(CO)5 and Ni(CO)4 as well as heterolytically coordinated metal complexes will shed light on the details of the Coulombic interaction between a highly charged metal and its ligands.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b05113. Rotational periods of metal hexacarbonyls, molecular orbital calculations, time-of-flight spectra measured at 2.5 × 1015 W cm−2, kinetic energy spectra of oxygen ions measured at 1.4 × 1015 W cm−2, angular distributions of oxygen ions measured at 1.4 × 1015 W cm−2, the calculated kinetic energy of M(CO)5u+ and CO+, the peak and maximum kinetic energies of atomic ions ejected from metal hexacarbonyls at 1.4 × 1015 W cm−2, and the calculated Coulomb repulsion energy and kinetic energy of carbon and oxygen ions formed by the Coulomb explosion of isolated CO (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81-6-6605-2554. Fax: +81-6-6605-2552. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported financially in part by JSPS KAKENHI Grant Nos. JP26620014, JP24227002, and JP26107002

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REFERENCES

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