Ion Imaging of MgI+ Photofragment in Ultraviolet Photodissociation of

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry +

Ion Imaging of MgI Photofragment in Ultraviolet Photodissociation of Mass-Selected MgICH Complex +

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Kenichi Okutsu, Kenichiro Yamazaki, Motoyoshi Nakano, Keijiro Ohshimo, and Fuminori Misaizu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01944 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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The Journal of Physical Chemistry

Ion Imaging of MgI+ Photofragment in Ultraviolet Photodissociation of Mass-selected Mg+ICH3 Complex

Kenichi Okutsua, Kenichiro Yamazakia, Motoyoshi Nakanoa,b, Keijiro Ohshimoa, and Fuminori Misaizua*

a

Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aoba,

Aramaki, Aoba-ku, Sendai, 980-8578, Japan b

Institute for Excellence in Higher Education, Tohoku University, 41 Kawauchi, Aoba-ku,

Sendai, 980-8576, Japan

*Corresponding author e-mail: [email protected] Tel: +81-22-795-6577 Fax: +81-22-795-6580 -1-

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract We have observed images of MgI+ fragment ions produced in ultraviolet laser photodissociation of mass-selected Mg+ICH3 ions at 266 nm. Split distribution almost perpendicular to the polarization direction of the photolysis laser was observed in the photofragment image. Potential energy curves of Mg+ICH3 were obtained by theoretical calculations. Among these curves, the excited complex ion dissociated along almost repulsive potentials with several avoided crossings which was connected to MgI+ + CH3. In the ground state of Mg+ICH3, the CH3I was bonded with Mg from the iodine side, and the Mg-I-C bond angle was calculated to be 101.1°. The theoretical results also indicated that the dissociation occurred after the 52A′←12A′ photoexcitation, where the transition dipole moment was almost parallel to the Mg-I bond axis. The MgI+ and CH3 fragments dissociated each other parallel to the direction connecting those center-of-masses which was 67° with respect to the transition dipole moment of 52A′←12A′ photoexcitation. Therefore, the fragment recoil direction was assumed to approach perpendicular tendency against the polarization direction under the fast dissociation process. On the other hand, calculated potential energy curves showed a complicated reaction pathway for MgI+ production including non-adiabatic processes, although the experiment results indicated the fast dissociation reaction.

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1. Introduction An ion imaging technique is now widely applied for various kinds of reaction dynamics experiments.1,2 Photodissociation studies of neutral molecules were performed for the past few decades.3,4 In these studies, fragments were detected as ions after excitation using resonance-enhanced multiphoton ionization. The produced ions were detected by a linear time-of-flight (TOF) mass spectrometer, and both kinetic energy and angular distributions were measured simultaneously by a position-sensitive detector. Recently, ion imaging apparatuses were developed for observing the fragment ions produced from mass selected ions.5-12 In our group, a reflectron TOF mass spectrometer combined with the position-sensitive detector for the photofragment ion imaging was used for the experiment.8,9,11 Photodissociation reactions of metal-ligand complexes were studied in terms of a charge transfer process from metal to ligand molecules.13 In the condensed phase, oxidation and reduction between organohalides (R-X) and organometallic catalysts are key reactions in the catalytic C-C bond formation. To understand catalytic properties of metal ions, the C-X bond dissociation of methyl halides by atomic metal ions was studied in the gas phase.14-17 Photodissociation studies of gas-phase Mg+XCH3 (X = F, Cl, Br, and I) complex ions have been performed using a TOF mass spectrometer for the past twenty years.18-22 It was already reported that a halogen atom of methyl halide was coordinated with Mg in the stable structures of the ground state complexes. At the ground states of Mg+XCH3, the Mg-X-C bond angles were reported to be 180°, 117°, 109°, and 103° for X = F, Cl, Br, and I, respectively.21 The symmetries of the complex structures were C3v for X = F and Cs for others. It was also known from the same paper21 that the positive charge was localized on the Mg atom (0.84-1.01). Observed fragmentation pathways from Mg+XCH3 were also found to depend on the excitation energy and the kind of the halogen atom. In these -3-



pathways, all of the Mg+XCH3 complex ions were dissociated to MgX+ + CH3 by 4.66-eV photoexcitation with characteristic features dependent on the halogen atom: For X = F, the observed anisotropy parameter showed a characteristic of parallel transition, whereas no anisotropy was observed for X = Cl, and perpendicular transition characters were obtained for X = Br and I.20,22 The authors’ group already reported from the ion imaging study that the angular distribution of MgF+ fragment ion in 4.66-eV photodissociation of mass-selected Mg+FCH3 ion indicates a parallel transition character.21 Furuya et al. used the configuration interaction singles (CIS) approach to calculate the excited states of the complex and tried to discuss the Mg+ICH3 → MgI+ + CH3 dissociation process by changing the Mg-I bond distance.20,21 However, they did not obtain calculation results which were fully consistent with the experimental results. Therefore, it was difficult so far to discuss how MgI+ was produced after the excitation of Mg+ICH3. In the present study, the formation mechanism of MgI+ in the 4.66-eV photodissociation of the Mg+ICH3 has been investigated with the apparatus of the reflectron TOF mass spectrometer and the position-sensitive detector.8 We observed an image of the MgI+ fragment ion produced by the photolysis of a mass-selected Mg+ICH3 complex ion. The kinetic energy release and the angular distribution were determined from the observed image. The geometrical structure for the electronic ground state of Mg+ICH3 was calculated by density functional theory (DFT). Instead of the previous CIS theoretical approach,20,21 calculations were performed to reveal the character of the excited states and its details of those transitions by state averaged complete active space self-consistent field (SA-CASSCF). From the results of experiments and calculations, the dissociation mechanism after photoexcitation was discussed.

2. Experimental and theoretical methods -4-


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Details of the experimental setup were already described in the previous papers.8,9,11 The present experimental apparatus consisted of a cluster ion source, a conventional angular-type reflectron TOF mass spectrometer (Jordan Co.) and a position-sensitive detector. The cluster ions consisting of Mg+ and CH3I were produced by a pickup source.23 Binary cluster ions including Mg+ICH3 were generated by irradiation with the second harmonic of a Nd:YAG laser (New Wave, Tempest) to a Mg rod which was mounted 10 mm downstream from a pulsed valve (General Valve, Series 9). Being synchronized with the generation of the Mg+ ions, the CH3I/He mixture gas (~12%) was expanded from the pulsed valve with a stagnation pressure of 4×105 Pa to form cluster ions. The CH3I/He mixture gas was obtained by passing He gas (Japan Fine Product, 99.99995+% pure) through a reservoir containing liquid CH3I (Wako, 99.5+% pure) at room temperature. Cluster ions containing Mg+ and CH3I were collimated with a conical skimmer (2-mm throat diameter). Cluster ions were then introduced into the Wiley-McLaren type acceleration region of the TOF mass spectrometer, by which the ions were accelerated up to typical energy of 1.1 keV. The accelerated ion beam was again collimated by two apertures (2.7 and 1.0 mm diameter) prior to photodissociation. After mass separation of the parent cluster ions in the first drift region (1308 mm), mass-selected Mg+ICH3 ions were irradiated with the fourth harmonic (266 nm, 4.66 eV, 40 mJ/cm2) of Nd:YAG laser (Spectra Physics, GCR-150), which induced electronic excitation of the ions followed by fragmentation. Linearly polarized laser was used for the photolysis, in which the polarization direction was rotated by a λ/2 wave plate (CVI, QWPO) and purified with a Glan-laser polarizer prism (Sigma, GLP). Fragment ions were mass analyzed in the second drift region and finally detected by a dual microchannel plate (MCP) detector (25 mm diameter) with a phosphor screen (Photonis, 3025FM, 60:1, P47). The fluorescence lifetime of the phosphor screen was 55 ns (90 – 10%). A pulsed high voltage was applied to -5-



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the head of the detector and was synchronized with the arrival time of MgI+ (m/z = 151). An image of the photon signals was captured by a CCD camera (Hamamatsu, ORCA-ER, C4742-80-12AG-HEAL) placed 300 mm downstream from the detector. Photon signals were accumulated for 50 000 shots of the photolysis laser. The observed image was reconstructed with an inverse-Abel transformation to a sliced image

of the

three-dimensional (3D) velocity distributions of the fragment ions, using pBASEX program.24 As mentioned above and in the previous reports,8,9,11 pulsed high voltage (−2.0 kV, 0.6 µs width) was applied to the surface of the MCP detector. Therefore, an electrostatic field was created between the grounded surrounding flange parts and the MCP surface. Under this condition, the diameter of the ion beam shrunk from that of the original beam. From the simulation of ion trajectories by SIMION software, the diameter of the original MgI+ ion beam was found to shrink to 70.9% of the original beam diameter on the surface of the detector under present experiment condition. The geometrical structure of the Mg+ICH3 ion in the ground state were calculated by the DFT calculations with the M06-2X functional25 with the aug-cc-pVTZ basis set (H, C, and Mg atoms) and aug-cc-pVTZ-pp basis set (I atom) of the GAUSSIAN 09 package.26 The potential energy curves were calculated with SA-CASSCF27,28 calculation and same basis set by changing the I-C bond distance. MOLPRO 2015.1 package29 was used for the SA-CASSCF calculation.

3. Results and discussion 3.1 Ion imaging of MgI+ photofragment In the present study, the Mg+ICH3 ion (m/z = 166) was selected for photodissociation. Mainly three fragment ions (MgI+, ICH3+, and I+) in addition to small amount of CH3+ and -6-


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Mg+ fragment ions were observed in the TOF mass spectrum after the ultraviolet photodissociation of Mg+ICH3 at 4.66 eV, as reported in the previous papers20-22 by the authors. We here focused on the MgI+ fragment ion which showed a completely different dynamics with photofragment MgF+ produced from photodissociation of Mg+FCH3.9 The observed images of MgI+, which were projections of a 3D distribution onto a two-dimensional detector, are shown in Fig. 1a, Fig. 1b. In Fig. 1a, MgI+ fragment ions had a ring distribution with almost constant intensity at every angle under the photolysis condition of E // Z (E, the polarization direction of the photolysis laser; Z, the ion beam direction). On the other hand, split distribution perpendicular to the laser polarization direction was observed for E ⊥ Z condition in Fig. 1b.

Figure 1. Ion images of the MgI+ fragment ions formed by the photodissociation of Mg+ICH3 at 4.66 eV. (a) and (b): observed projection images with E // Z and E ⊥ Z conditions, respectively. (c): reconstructed sliced image of the observed image (b) by the pBASEX program.24 The directions of the E vectors are shown at the upper left sides. These projection images were symmetrized by averaging the quadrants.

The velocity and angular distributions were obtained by reconstructing the observed image (Fig. 1b) with inverse-Abel transformation. Fig. 1c shows a sliced image of the 3D distribution reconstructed from the projection image with E ⊥ Z (Fig. 1b) by the pBASEX -7-



program.24

Recoil velocity distribution of MgI+ fragment ions (Fig. 2) was determined

from the radial distribution of the image in Fig. 1c. Shrinking effect mentioned in the previous papers8,9,11 was considered in the calculation of velocity distribution as noted in Sec. 2. The most probable value of the fragment ion velocity was determined to be v = 466 ± 11 m s−1 by fitting the experimentally obtained distribution with two Gaussian functions. The faster component of the fitted curves with the most probable velocity of ~540 m s−1 corresponds to the edge of the detector, which was analyzed as an artificial peak in the pBASEX program. The peak related to the edge of the detector was checked by analyzing the simulation image. See also Fig. S1 for further information. Therefore, we neglected this artificial peak in the following discussion. In addition, the broad velocity distribution was almost due to the ion beam expansion which was reported in detail in Ref. 8.

Figure 2. Velocity distribution of MgI+ formed in the photodissociation of Mg+ICH3 at 4.66 eV. The experimental result (black circle) was fitted with two Gaussian functions (red curves). Blue curve is the sum of the two fitted curves. The faster component of the fitted -8-


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curves was artificial corresponding to the edge of the detector.

3.2 Kinetic energy release and anisotropy parameter Total kinetic energy release in the photolysis, Et, was obtained from the velocity distribution of the fragment ions by the following equation deduced from the energy and linear momentum conservations:

Et =

mp  1 2  mf v  , mn  2 

(1)

in which v is the velocity of the fragment ion, mp, mn, and mf are masses of the parent ion, the neutral counterpart, and the fragment ion, respectively. From the most probable value of the fragment velocity, the total kinetic energy release in the reaction forming MgI+ + CH3 with 4.66-eV photoexcitation was calculated to be Et = 1.89 ± 0.04 eV. In a one-photon photodissociation process with linearly polarized light, angular distribution I(θ) of the photofragment has the form I (θ ) =

1 [1 + βP2 (cos θ )] , 4π

(2)

in which θ is the angle between the recoil direction of the fragment ion and E, β is the anisotropy parameter ranging from −1 to 2, and P2(cosθ) is the second order Legendre polynomial in cosθ.30 In the present study, the β parameter was calculated by averaging the

β value around the most probable velocity (v = 466 ± 11 m s−1) and was obtained to be β = −0.39 ± 0.04. In our previous study of the photodissociation of Mg+ICH3 at 4.66 eV, the energy and angular distributions were determined from the TOF profiles by simulations using forward convolution method.25 Consequently, Et was estimated to be 1.37 eV and β = −0.50 under the assumption of the parent ion temperature T = 100 K. The difference between the previous and the present studies can be attributed partly to the flexibility and uncertainty of -9-



the previous simulation of TOF profiles by multi-parameter fittings. The anisotropy parameter and the kinetic energy release were determined more directly from the present measurements of the photofragment image, and therefore, the reliability of the present results is expected to be higher than that of the previous study.

3.3 Dissociation mechanism of Mg+ICH3 In order to understand the observed kinetic energy and β parameter of MgI+ fragment ions formed by photodissociation of Mg+ICH3 ions, potential energy curves of the ground and excited electronic states for Mg+ICH3 were calculated by single point calculations with SA-CASSCF. Fig. 3 shows the potential energy curves of Mg+ICH3 along the I-C bond distance (RI-C). Other structural parameters were fixed during the calculations. These calculated excitation energies and assignments for 2A′ and 2Aʺ states are summarized in Table 1. The details of the calculation results are also summarized in the supporting information.

Figure 3. Potential energy curves of Mg+ICH3 along the I-C distance coordinate, RI-C, -10-


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calculated by the SA-CASSCF/aug-cc-pVTZ (H, C, and Mg atoms) and aug-cc-pVTZ-pp (I atom) levels. The vertical blue arrow indicates the photoexcitation in the present experiments. The black solid, broken, and dotted lines correspond to 2A′, 2Aʺ, and 4Aʺ states respectively. The red and green lines correspond to 52A′ and 62A′ states respectively.

Table 1. Excitation energies and oscillator strengths of electronic transitions of Mg+ICH3 in the visible to ultraviolet regions calculated at its equilibrium structure of the ground electronic state by the SA-CASSCF/aug-cc-pVTZ (H, C, and Mg atoms) and aug-cc-pVTZ-pp (I atom) levels. Allowed transitions (2A′ and 2Aʺ states) were picked up and the term symbols of the excited states were determined and numbered in ascending order at the ground-state structure of Mg+ICH3. Transition

Electron excitation

Excitation energy [eV]

Oscillator strength

2

2

Mg 3px←Mg 3s

3.01

2.827

2

2

Mg 3py←Mg 3s

3.43

3.083

2

2

Mg 3s←I 5py

3.96

0.966

2

2

Mg 3s←I 5px

4.47

0.130

2

2

I 6px←Mg 3s

4.83

1.416

2

2

I 6px←I 5py

4.85

0.122

2

2

4 Aʺ←1 A′

I 6px←I 5py

5.12

0.401

52A′←12A′

2 A′←1 A′ 1 Aʺ←1 A′ 2 Aʺ←1 A′ 3 A′←1 A′ 4 A′←1 A′ 3 Aʺ←1 A′

Mg 3pz←Mg 3s

5.46

4.956

2

2

I 6px←I 5pz

5.66

0.100

2

2

Mg 3py←I 5py

6.28

2.770

2

2

8 A′←1 A′

I 6px←I 5px

6.45

2.856

52Aʺ←12A′

6 A′←1 A′ 7 A′←1 A′

Mg 3px←I 5py

6.52

0.107

2

2

Mg 3s←I 5px

6.84

1.746

2

2

Mg 3pz←I 5py

7.46

1.439

9 A′←1 A′ 6 Aʺ←1 A′

As shown in Table 1, 52A′←12A′ was the transition with the largest oscillator strength. After the photoexcitation to 52A′ states of the parent Mg+ICH3 ion, the MgI+ ion was not expected to be formed directly by fragmentation from these states, because the excited -11-



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potential energy curves do not correlate to the dissociation pair of MgI+ + CH3. However, after the excitation to 52A′ state, avoided crossing with 62A′ state was found at very near (RI-C ~2.3 Å) from the equilibrium distance of the ground state (RI-C = 2.16 Å). As shown in Fig. 3, the 62A′ state has a repulsive character in the RI-C coordinate and finally reaches the dissociation pair. The direct excitation to other states around 52A′ state excitation was excluded from the calculation results (Table 1); the oscillator strength was too small compared with excitation to the 52A′ state. Therefore, after the excitation to 52A′, the Mg+ICH3 ion was dissociated to MgI+ + CH3 via 62A′ repulsive potential. As several A′ symmetry potentials were crossed with the 62A′ potential as shown in Fig. 3, the dissociation occurred via non-adiabatic processes at the avoided crossings. For a polyatomic molecular system, the anisotropy parameter could be express as the following equation,31

β = 2 P2 (cos χ ) P2 (cos α ) ,

(3)

where χ is the angle between the transition dipole moment and the dissociation axis, α is the molecular rotation angle during photodissociation. Assuming the axial recoil approximation in which the fragments move outward along the straight line defined by the internuclear axis of the molecule just after the excitation, the rotation angle α will be α = 0, and thus

P2(cosα) = 1. The anisotropy parameter for 52A′←12A′ (χ = 67.0°) was estimated to be 2P2(cosχ) = −0.54, from eq. 3 (see Fig. 4 for angle χ). These values are consistent with the experimental result of β = −0.39 ± 0.04, in which the angular distribution of the MgI+ fragment had a nature of perpendicular transition. The direction of the transition dipole moment of 52A′←12A′ was nearly parallel to the Mg-I bond axis, as shown in Fig. 4. However, the angles between these dipole moments and the dissociation axis (Fig. 4), which connect the center-of-masses of two fragments (MgI+ and CH3), were calculated to be around 65°. Therefore, when the time scale of the dissociation was much faster than the -12-


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rotational periods of the parent ion, the recoil direction of the fragment ion can be assumed to approach perpendicular tendency against E. From the differences between the experimentally obtained β value (−0.39 ± 0.04) and the theoretical result (−0.54), the rotation angle α could be estimated from eq. (3) as α = 24°.The potential energy curves (Fig. 3) suggest that the dissociation occurred via complicated pathway (avoided crossings) unlike dissociation via simple single repulsive potential. However, the anisotropy parameter obtained in the present study agreed well with the estimated β parameter assuming short dissociation lifetime. These results indicate that the dissociation reaction occurred rapidly compared with the rotation period of the parent ion.

Figure

4. Geometrical structure of the parent Mg+ICH3 ion optimized with

M06-2X/aug-cc-pVTZ (for H, C, Mg atoms) and aug-cc-pVTZ-pp (for I atom) levels. Dissociation axis corresponds to the line connecting the center-of-masses (red dots) of the two fragments, MgI and CH3. Transition dipole moment vector calculated by SA-CASSCF which correspond to 52A′←12A′ (Blue arrow), and angle between dissociation axis and the dipole moment is also shown.

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Comparing with the previous photodissociation imaging experiment of Mg+FCH3,9 we have found that angular distributions of the photofragment recoil directions are dependent upon the geometric structures of the parent complex ions in the electronic ground states. As reported in Ref. 9, the Mg+FCH3 ion was dissociated to MgF+ and CH3 after 22A1 ← 12A1 photoexcitation at 4.66 eV, which mainly corresponds to Mg 3pz ← Mg 3s electron excitation. The ground state structure of Mg+FCH3 has C3v symmetry with Mg-F-C bond angle of 180°, and the transition dipole moment of 22A1 ← 12A1 excitation is parallel with the Mg-F-C bond axis. Because the line connecting the center-of-masses of the fragments is also parallel with the Mg-F-C bond axis, the recoil direction of the MgF+ fragment was expected to be parallel with the transition dipole moment. The observed anisotropy parameter for the MgF+ fragment was β = 1.09, which was consistent with the prediction that the dissociation had a parallel transition character. Although the photoexcitation of Mg+ICH3 contained a similar character of Mg 3pz ← Mg 3s (52A′ ← 12A′) excitation in the present study with that of Mg+FCH3 at the same photon energy (4.66 eV), the MgI+ fragment was ejected to the direction almost perpendicular to the transition dipole moment. This marked difference of the recoil angular distribution between MgI+ and MgF+ fragments is due to the structure difference between the Mg+ICH3 and Mg+FCH3 parent ions.

4. Conclusion In the present study, an Mg+ICH3 complex ion was dissociated in the ultraviolet region (266 nm, 4.66 eV), and a product ion, MgI+, was detected by the ion imaging experiment. The image of the MgI+ fragment ion was found to have a split distribution perpendicular to the polarization direction of the photolysis laser. Kinetic energy release and angular distribution were analyzed from the sliced image reconstructed by the pBASEX program: Total kinetic energy release was determined to be Et = 1.89 ± 0.04 eV, and anisotropy -14-


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parameter was determined to be β = −0.39 ± 0.04. Unlike the previous analysis using the TOF profiles, the total kinetic energy release and the anisotropy parameter were directly determined from the observed image of fragment ions. Potential energy curves for the electronic ground and excited states were obtained by calculations using the SA-CASSCF method. From the potential energy curves, the dissociation was expected to occur by the photoexcitation corresponding to 52A′ ←12A′ transition, followed by avoided crossings to the 62A′ state. As a result, the Mg+ICH3 ion was dissociated to MgI+ + CH3 via repulsive 62A′ potential and the following non-adiabatic processes. The obtained anisotropy parameter was consistent with the fast process of MgI+ formation through the repulsive potential curve with several non-adiabatic processes and had a good agreement with the value which was estimated from the optimized structure of the Mg+ICH3 ground state and the calculated transition dipole moments.

Associated content Supporting information Simulation image and its analyzed result, Cartesian coordinates of optimized Mg+ICH3 in electronic ground state, and details of the calculation results.

Acknowledgements The authors are grateful to Professor Yoshihiro Yamakita, Professor Kiichiro Koyasu, Dr. Masataka Saito and Hiroshi Hoshino for their helpful assistances of the experiment in the early stage of this study. They also thank Yuji Nakashima and Keita Fujimoto for their helpful advices of the data analysis. The authors also thank Dr. Takuya Horio for providing his computer program for the images based on the pBASEX method. This work was supported by a Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for -15-



Challenging Exploratory Research, Grant No. 25620007. Calculations were partly performed using the Research Center for Computational Science, Okazaki, Japan.

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Figure 1 312x100mm (300 x 300 DPI)



Figure 2 176x111mm (300 x 300 DPI)


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Figure 3 177x112mm (300 x 300 DPI)



Figure 4 124x79mm (300 x 300 DPI)


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Ion Imaging of MgI+ Photofragment in Ultraviolet Photodissociation of

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