Dissociative Ionization and Coulomb Explosion of CHBrCl₂ in Intense Near-Infrared Femtosecond Laser Fields

Abstract

We experimentally demonstrate the dissociative photoionization of CHBrCl₂ molecules in a femtosecond laser field by time-of-flight mass spectrum and dc-slice imaging technology. The results suggest that the low kinetic energy components are from the dissociative ionization process of single-charged molecular ions. The angular distribution of fragment Cl⁺ ions can be attributed to the features of dissociative state and molecular configuration, and that of Br⁺ ions results from the electronic wave-packet evolution and combination of the multi-dissociation processes. The high kinetic energy components are from the Coulomb explosion of multi-charged molecular ions, and the error of the C-Br distance involved in the Coulomb explosion can be explained by the movement of the effective charge center of the polyatomic molecule.

1. Introduction

The molecular dynamic process in the intense femtosecond laser has attracted extensive attention for wide applications in ultrafast laser techniques [1,2,3,4,5,6]. Keldysh et al. put forward and developed theories to judge the electron(s) behavior under different laser intensities [7,8,9]. When femtosecond laser intensity is low, the molecular internal barrier is hardly suppressed. Therefore, molecules have to absorb multi-photons to reach an ionic state, then parent cations dissociate into fragment ion(s) and neutral parts with relatively low kinetic energies. By analyzing the kinetic energy release distributions and angular distribution, the excited states of singly charged molecular ions and nuclei motion dynamics can be revealed [10,11]. As laser intensity still increases, the molecular internal barrier would be strongly suppressed, even below the valence electron energy. Hence, the electrons are mostly stripped away by a higher intensity laser field, and the multi-charged parent ions would rapidly dissociate into fragments by internal Coulomb repulsive force, which is named as Coulomb explosion (CE). With the help of Coulomb explosion imaging, the abundant dynamic processes in intense laser fields, such as the evolution of excited state wave-packets and the related molecular structure deformation, have been discovered [12,13].

It has attracted considerable interest in the photodissociation of halogenated hydrocarbons (e.g., CH₂XY, where X, Y = F, Cl, and Br) for the possibility of selective control on the dissociation. In recent years, many studies have focused on the carbon halogen bond cleavage by single-photon excitation using an ultraviolet laser field. W. B. Tzeng et al. studied the photodissociation of CH₂BrCl at 193 nm, the branching ratio of C-Cl versus C-Br bond rupture was 4.5, and the major reaction pathway originated from the C-Cl bond fission rather than the weaker C-Br bond [14]. M. D. Person et al. have studied the photodissociation of BrCH₂COCl at a 248 nm laser field. Compared with a predicted statistical branching ratio of 1:30, the branching ratio was 1.0:1.1 for the fission of the C-Cl and C-Br bonds, which resulted from the relative strengths of the electronic coupling between the initially excited nπ*-bound configuration and the two nσ* states, and the weaker coupling inhibited the adiabatic crossing over the barrier to C-Br bond fission [15]. Che. D. C et al. studied the photolysis of ClBrCHCF₃ at 234 nm by a hexapolar electrostatic field and velocity-map imaging detection system. They found that the relative yield ratio of Cl to Br was found to be approximately 2, and the results were not only relevant for a detailed understanding of adiabatic versus diabatic coupling mechanisms, but also pointed out the possibility of selectively inducing specific dissociation pathways [16].

The selective photodissociation dynamic process of CHBrCl₂ molecules is also a problem worthy of attention. Unlike UV radiation, the near-infrared femtosecond laser field provides a good insight into the complex electron dynamics and nuclei behavior in an ultrashort timescale by investigating these phenomena and exploring the potential physical mechanisms. However, there are only a few previous studies of CHBrCl₂ in an intense femtosecond laser field. In this paper, we experimentally demonstrate the dissociative ionization of CHBrCl₂ molecules in an intense 800 nm femtosecond laser field. It is shown that the CHBrCl₂ molecules can be singly ionized and dissociate into low kinetic energy fragment ions, and such a process is further explained by the appearance energy of the fragment ions based on the ab initio calculations. However, the CHBrCl₂ molecules can also be ionized to multi-charged molecular ions and dissociate into high kinetic energy fragment ions by Coulomb repulsion. The effective charge center of the doubly charged molecular ions can be used to explain the error of the C-Br distance involved in the Coulomb explosion. In addition, it is also shown that the dissociation paths can be controlled by laser intensity.

2. Experiment and Theory Calculation

The experimental device has been described in our previous work [17,18], and here, only a brief description is provided. A mode-locked Ti:sapphire laser was used as an excitation source, with a central wavelength of 800 nm, a pulse duration of 70 fs (FWHM), and a repetition rate of 1 kHz. The output femtosecond laser pulse was focused into the gaseous sample by a lens with a 400 mm focus length, and the laser intensity in the focal volume was estimated between 5.0 × 10¹³ and 1.5 × 10¹⁴ W/cm², which was calibrated by the Ar²⁺/Ar⁺ yield ratio [19]. The gaseous molecules were seeded into the chamber by Argon with 0.2 atm and then ejected into the ionization region by a pulse valve (Parker General, the frequency of 100 Hz, and the duration time of 150 µs). Typical pressure in the source chamber was about 1.0 × 10⁻⁷ mbar. The supersonic molecular beam passed into the reaction chamber (~4.0 × 10⁻⁹ mbar) through a 0.2 mm skimmer, and the laser pulses intersected it at right angles in the region between the repeller and the extractor, where the photo-fragments were guided to fly to the detector. The ion cloud was stretched to meet the slice condition, and detected by a pair of microchannel plates (MCPs) coupled with a P47 phosphor screen. The two-dimensional momentum images of each fragment were the central slice (around P_z = 0) of the corresponding three-dimensional momentum distribution, and were obtained by a charge-coupled device camera (PI-MAXII, Princeton Instrument) with 5 ns time resolution. In the whole experimental process, all the timing sequence control was implemented by a DG645 (Stanford Instrument Digital Delay/Pulse Generator).

To better illustrate the ionization and dissociation processes of the CHBrCl₂ molecule, the related theoretical calculations were performed with GAUSSIAN 16 [20], and the relaxed force constant was obtained with the COMPLIANCE software package [21]. The geometry optimization of the reactants, transition states, and fragments was carried out at the B3LYP level with a 6-31G* basis set [22]. Vibrational frequencies and relaxed force constants of each optimized structure were computed to ensure that the energies of stable structures and transition states were the local minimum and maximum along the reaction coordinate, respectively, and the unscaled harmonic frequencies were used to obtain the zero-point energy at the G4 level [23]. The potential energy surfaces of the excited state were carried out at the B3LYP level with the cc-pVTZ basis set [24]. The ionization potential was 10.94 eV for the CHBrCl₂ molecule, which was close to the 10.96 eV [25]. The good agreement between our calculations and previous experiment results can demonstrate the feasibility of the theoretical methodology used in this paper.

3. Results and Discussion

3.1. Time-of-Flight Mass Spectrum and DC-Sliced Images

Figure 1 shows the time-of-flight (TOF) mass spectrum of CHBrCl₂ molecules irradiated by the 800 nm femtosecond laser pulses with the laser intensity of 6.0 × 10¹³ (a) and 1.5 × 10¹⁴ W/cm² (b), respectively. The background ions H⁺, H₂O⁺, N₂⁺, and Ar⁺ (seeding air) were ignored, and the ions C⁺, Cl⁺, CHCl⁺, Br⁺, CHCl₂⁺, CHBr⁺, and CHBrCl⁺ and molecular ion CHBrCl₂⁺ have been observed. Due to the isotopes of ³⁵Cl/³⁷Cl and ⁷⁹Br/⁸¹Br, all the fragment ions containing Cl and Br atoms exhibited multi-peak structures. Considering the laser pulse duration (FWHM = 70 fs) in our experiment, these fragmentations are mainly attributed to the dissociation of parent ions. The weak signal of CHBrCl₂⁺ demonstrates that its chemical property is unstable. Thus, it is prone to be photolyzed in the intense laser field. We focused on the rupture of the C-Br and C-Cl bonds. Hence, the relationship between ions’ yields and laser power was analyzed, which is shown in Figure 2. The yield ratio of Cl⁺/CHBrCl⁺ increased from 0.03 to 2, but that of Br⁺/CHCl₂⁺ increased slowly and remained constant at 1. This indicates that laser intensity influences the above dissociation pathways. The related discussion is carried out in the next section.

To further investigate the related dissociation processes, the dc-sliced imaging technology was utilized to measure the kinetic energy releases (KERs) and angular distributions of fragment ions. Figure 3 presents the pseudo-color sliced images of the observed ions: (a) Cl⁺, (b) Br⁺, (c) CHBrCl⁺, and (d) CHCl₂⁺, with the laser intensity of 1.5 × 10¹⁴ W/cm². The corresponding velocity distributions were extracted, where the experimental data were fitted with the multi-peak Gaussian function, and the related KER values were labeled in eV. In Figure 3a, the kinetic energy distribution of Cl⁺ ions exhibited two peaks, which were located at 0.25 and 2.7 eV, respectively. In Figure 3b, the fragment Br⁺ ion exhibited two peaks, with the corresponding values at 0.18 and 1.45 eV. However, the high kinetic energy peak of CHBrCl⁺ ions almost vanished, and there was one peak at 0.13 eV in Figure 3c. In Figure 3d, the fragment CHCl₂⁺ ion had two peaks at 0.17 and 1.39 eV, respectively.

3.2. Dissociative Ionization of CHBrCl₂ Molecules in 800 nm Femtosecond Laser Fields

The laser pulse duration in our experiment was 70 fs (FWHM), and it is reasonable that most single ionizations will take place in the first few cycles, then the dissociations will occur. Hence, the lower-velocity components of Cl⁺, Br⁺, CHBrCl⁺, and CHCl₂⁺ ions should belong to the dissociative ionization of the single-charged molecular ion CHBrCl₂⁺. The estimated reaction pathways are listed as follows.

• Channel (1)  CHBrCl₂ → CHBrCl₂⁺ + e⁻ → Cl⁺ + CHBrCl + e⁻
• Channel (2)  CHBrCl₂ → CHBrCl₂⁺ + e⁻ → Br⁺ + CHCl₂ + e⁻

To confirm these dissociation processes, the appearance energies (E_AP(1) = 17.57 and E_AP(2) = 15.65 eV) of the abovementioned channels were calculated, respectively. Appearance energies expressed in Equation (1) represent the required energy to switch on the channel, where E_i and E_j are the total energy of reaction products and reactants, respectively.

E_AP = ΣE_i − ΣE_j,

(1)

E_AV = nhν − E_AP,

(2)

E_T = (m_aV_a²/2)(m_a + m_b)/m_b,

(3)

After absorbing enough photons and overcoming the ionization potential barrier, the redundant energy, which is named available energy (E_AV) expressed in Equation (2), released into the internal energy of fragments, and the translational energy (E_T) of the reaction channel. E_T can be obtained by Equation (3), where V_a denotes the velocity of fragment ions, and m_a and m_b represent the mass of fragment ion a and neutral fragment b, respectively. As listed in

Table 1. The related results of the above-estimated reaction channels.

Channel	EAP (eV)	KERs (eV)	ET (eV)	EAV (eV)
(1)	17.57	0.25	0.32	1.03
(2)	15.65	0.18	0.35	1.40

, the experimental results are in good agreement with the calculated results, which demonstrates that the above-estimated channels originate from the dissociative ionization of single-charged molecular ion CHBrCl₂⁺.

Angular distribution of fragment ions also reflects the molecular dynamics process in an intense field. By integrating the reconstructed 2D velocity over a proper range of speed at each angle, the angular distributions of Cl⁺ and Br⁺ ions are shown in Figure 4. It was found that the angular distribution of Br⁺ ion is isotropic, but that of Cl⁺ ion is an anisotropic distribution parallel to the laser polarization direction. To explore the reasons for the above differences, we calculated the potential energy curves (PECs) of the excited states of the parent ion CHBrCl₂⁺ along the C-Cl and C-Br bonds. As shown in Figure 5, the equilibrium nuclear distance (R_e) of the C-Cl and C-Br bonds of molecule CHBrCl₂ was 1.78 and 1.95 Å, respectively. Following the Franck–Condon principle, the electron wave-packets of molecular ions CHBrCl₂⁺ was excited to the high-lying states at this equilibrium nuclear distance. Then, the electron wave-packets will evolve along the potential energy curve of the excited states Here, the minimum energy of the ground state was set to zero for the sake of convenience.

For Cl⁺ ion, molecular ions CHBrCl₂⁺ are firstly excited to state S₁₀, which is determined by the E_AP of the channel (1), then quickly dissociated into fragments. Thus, the influence of molecular rotation on the ejection of fragment ions was very little. Angular distribution of fragment Cl⁺ ion should be highly anisotropic. However, that assumption is contrary to the experimental result. We guess that the angular distribution of fragment Cl⁺ ion is not only related to the excited state but also related to the molecular structure. For dissociation along with the C-Cl bond, the possibility of any breaking of the C-Cl bonds is almost equal. Therefore, the position of the Cl atom in the molecular structure establishes the initial angular distribution of Cl⁺ ions, which weakens the anisotropy of the angular distribution of fragment Cl⁺ ion.

According to the appearance energy of channel (2), the molecular ions will be populated to the excited state S₆. Then, there are three possible dissociation pathways to produce Br⁺ ion. For the dissociation pathway (1), electronic wave packets still evolve along the potential energy curve. Then, the molecular ions quickly dissociate along with the C-Br bond. For the dissociation pathway (2), the evolution of electronic wave-packets firstly follows the same route as pathway (1). Due to the strong coupling between states S₆ and S₅ in the range of 2.5~3.0 Å, some of these wave-packets can transfer from the upper state down to the lower state, and then the dissociation processes occur along the potential energy curve of state S₅. For the dissociation pathway (3), the prepared wave-packets in the coupling area will oscillate along the potential well of bound state S₅ periodically. Some of the wave-packets’ energy exceeds the energy barrier of the S₅ state, which can directly cross the energy barrier. Finally, some of the molecular ions dissociate into fragment Br⁺ ion. Due to the existence of multiple dissociation pathways, the dissociation period actually increases, which strengthens the influence of molecular rotation on the angular distribution of ions. Therefore, the angular distribution of Br⁺ ions is almost isotropic.

3.3. Coulomb Explosion of the Double-Charged Molecular Ion CHBrCl₂²⁺

With the increase of laser intensity, the CHBrCl₂ molecules can be ionized to the multi-charged molecular ions, then dissociate into fragment ions with higher KERs by Coulomb repulsive force. In the two-body Coulomb explosion process, the kinetic energy of a pair of fragment ions should satisfy Equation (4), where X and Y denote a pair of fragment ions, M is the mass of fragment ions, and p and q are the charge numbers of the ions:

KERs(X_p+)/KERs(Y_q+) = M(Y_q+)/M(X_p+),

(4)

Total KERs = 14.4pq/r,

(5)

According to Equation (5), the Br⁺(1.45 eV) and CHCl₂⁺(1.39 eV) ions can be attributed to the CE channel as follows:

• Channel (3)  CHBrCl₂ → CHBrCl₂²⁺ + 2e⁻→Br⁺ + CHCl₂⁺ + 2e⁻

The angular distributions of fragment ions further confirm the CE process. The angular distributions of ions Br⁺ and CHCl₂⁺ are shown in Figure 6. They have the same angular distribution, which validates that the above dissociation channel (3) belongs to the CE process, along with the C-Br bond.

In the classic CE model, the total KERs of one CE channel are expressed as Equation (5), where r indicates the internuclear distance in Å. Once the total KERs are measured, the critical distance (R_c) of the CE channel can be determined. According to our experimental results, the CE of CHBrCl₂²⁺ started at R_c = 5.07 Å. The R_c is far longer than the equilibrium internuclear distance (R_e = 1.95 Å), which is caused by the internal Coulomb force stretching the nuclear distance. For polyatomic molecular ion CHBrCl₂²⁺ involved in CE processes, is it appropriate to use Coulomb repulsion forces between two atoms? Can the real potential energy relationship of molecules changing along the chemical bond determine the release of kinetic energy? To answer these questions, we calculated the potential energy curves of the multi-charged molecular ion CHBrCl₂²⁺ along with the C-Br bond at the B3LYP/6-31G* level. The Coulomb repulsive curve also has been shown in Figure 7. Here, the energy in 10 Å on the potential energy curve (PEC) was equal to that of the Coulomb repulsive curve (CRC) for convenience [26].

As shown in Figure 7, when the total KERs of the channel (3) was 2.84 eV, the calculated C-Br distance in CRC and PEC was 4.33 and 5.07 Å, respectively. Why is the effective distance of C and Br atoms longer in Coulomb explosion? We believe that the reason is the shift of the effective charge centers in the polyatomic molecule systems. Next, we calculated the effective centers of double-charged molecular ion CHBrCl₂²⁺. Firstly, the space configuration and charge distribution of the parent ions were calculated, and then the charge center coordinates on the quasi-fragment ion were obtained according to Equation (6). After the charge center coordinates were obtained, the critical value of the effective Coulomb explosion of polyatomic molecular ions was obtained by using the Euclidean distance. The results are in good agreement with the calculation results, indicating that it is different from the traditional classical point charge model. Hence, the movement of the effective charge center caused by molecular configuration needs to be considered in the process of the Coulomb explosion of polyatomic molecules.

(x, y, z) = Σq_i(x_i, y_i, z_i)/Σq_i,

(6)

For fragment Cl⁺ and CHClBr⁺ ions, they did not satisfy the Equation (4). We believe that there is a multi-body Coulomb explosion process, as shown in channels (4) and (5). It also explains the increase in the Cl⁺/CHBrCl⁺ ion yield ratio, as shown in Figure 2, and the relaxed force constant of the C-Cl and C-Br bonds in doubly charged molecular ion CHBrCl₂²⁺ was 2.75 and 1.57 mdyn/Å, respectively. The larger the relaxed force constant of the chemical bond, the more energy is needed for the corresponding chemical bond fracture [21]. Thus, it was confirmed that the C-Br bond was broken before the C-Cl bond broke in the CE process of parent ion CHBrCl₂²⁺. This shows the feasibility of using a femtosecond laser field to control the dissociation processes.

• Channel(4)  CHBrCl₂ → CHBrCl₂²⁺ + 2e⁻ → Br⁺ + CHCl₂⁺ + 2e⁻
• Channel(5)  CHBrCl₂ → CHBrCl₂²⁺ + 2e⁻ → Br⁺ + CHCl₂⁺ + 2e⁻

4. Conclusions

In summary, the dissociative ionization of CHBrCl₂ molecules has been investigated in the near-infrared (800 nm) intense femtosecond laser field by the dc-sliced imaging technology. The Cl⁺, Br⁺, CHCl2⁺, and CHBrCl⁺ ions were extracted, and their corresponding kinetic energies and angular distributions were analyzed. The experimental results indicated that the low kinetic energy components of these fragment ions were produced via the dissociative-ionization process, where the neutral CHBrCl₂ molecules were ionized to singly charged molecular ions by the multi-photon ionization process, and then dissociated into fragments. The different angular distributions of fragment ions was attributed to the electronic wave-packets’ evolution on the excited states and molecular configuration. At a high laser intensity, the molecules were firstly ionized to doubly charged molecular ions and then dissociated by the driving force of the Coulomb repulsion, and generated high kinetic energy fragment ions. The error of the C–Br distance involved in the Coulomb explosion can be explained by the movement of the effective charge center of the polyatomic molecule. The increase of the yield ratio of Cl⁺/CHBrCl⁺ in Figure 2 resulted from the multi-body Coulomb explosion process. Additionally, our results provide guidance on how to manipulate the electronic dynamics and dissociation process in femtosecond laser fields by accurately selecting laser parameters. Future works will include ionizing polyatomic molecules in few-cycle laser fields and observing the multi-body Coulomb explosion based on a cold target recoil coincidence measurement. These efforts are currently underway.