With the dramatic development of ultrashort laser technique in the past few decades, the ultrashort intense laser field has brought out some unprecedented physical phenomena, such as multi-photon ionization (MPI),^[1–3] enhanced ionization (EI) or charge-resonance enhanced ionization (CERI),^[4–9] above-threshold ionization (ATI),^[10] dissociative ionization,^[11–14] Coulomb explosion (CE),^[15–23] and so on. As is well known, when the laser intensity is up to 10¹⁴ W/cm², the laser field magnitude is comparable to the Coulomb field generated by an atomic nucleus, the molecular geometry structure can be modified, and thus the molecular chemical bond will be broken by the strong Coulomb repulsive force. Several experimental results indicated that the chemical bond will be stretched from the equilibrium nuclear distance (R_e) in the neutral molecule to a critical distance (R_c), and the CE process occurs at this critical distance.^[6,23–28] Schmidt et al.^[29] explained this phenomenon on the basis of a laser-induced stabilization, and found that the bond elongation is mainly determined by the molecule itself. Based on this viewpoint, Corkum et al.^[4] proposed a theoretical model of laser-induced electron localization enhanced ionization, which concluded that the ionization of the parent ion will be dramatically enhanced around the critical distance R_c since the electron can directly tunnel through the narrow internal barrier to the continuum. Similarly, Bandrauk et al.^[5,6] presented a charge resonance enhanced ionization model, and showed that the enhanced ionization probability at the critical distance R_c is attributed to the transitions between a pair of charge-resonant states that are strongly coupled to the laser field.

As a prototypical linear triatomic molecule, the photoionization and photodissociation of CO₂ and CS₂ molecules under the intense femtosecond laser fields have been extensively studied. As a representative study of the CO₂ molecule, Wu et al.^[30,31] studied the three-body fragmentation dynamics of CO₂ molecule in intense laser fields by using a triple ion coincidence technique, and showed that the geometric structure of before fragmentation is close to that of neutral CO₂, and both the sequential and non-sequential fragment processes can occur in the parent ions while the parent ions can only produce the non-sequential fragment process. In the studies of the CS₂ molecule, Graham et al.^[32] measured the angular distributions of those fragment ions from the CE process of parent ions, and found that the distribution of fragment ions S^m+ is perpendicular to that of fragment ions Cⁿ⁺. Mathur et al.^[33] defined CS₂ molecule as the boundary between a “heavy” molecule (like I₂ or its derivatives) and a “light” molecule (like H₂ or N₂), and showed that the geometric alignment mechanism dominates for the 100-fs laser pulses, while dynamic alignment occurs for the 35-ps laser pulses. Yamanouchi et al.^[18–20] utilized a triple coincidence momentum imaging technique to study both sequential and non-sequential three-body CE processes, i.e., and demonstrated that the significant structural deformation occurs for along both bending and stretching coordinates. In previous studies, this coincidence measurement technique can identify a specific single event of Coulomb explosion from a single parent ion, but it has relatively high requirement for the chamber vacuum pressure and sample vapor temperature. Recently, the DC-sliced ion imaging technique has shown to be a well-established tool to study the molecular photoionization and photodissociation process, where both the speed and angular distributions of the produced ions can be directly measured without any mathematical transformation, and so can provide a more intuitive method. In this paper, we experimentally investigate the CE process of CS₂ molecule under an intense near-infrared (800 nm) femtosecond laser field by the DC-sliced ion imaging technique. The DC-sliced images of four ions S⁺, S²⁺, CS⁺, and CS²⁺ by breaking one C–S bond are measured, and their Coulomb explosion channels are assigned by calculating kinetic energy release (KER) and their angular distributions are extracted from the DC-sliced images. The dissociation dynamics of parent ions is numerically simulated by using a Coulomb energy approximation, and it is shown that the dissociation time decreases when the charge number increases.

Details of our home-built DC-sliced ion imaging system have been described in our earlier publication,^[27] and only a brief description is given here. The CS₂ sample (99.9% purity) carried by 1 atm helium gas is expanded adiabatically into the source chamber through a pulsed valve with the repetition rate of 100 Hz, and then skimmed into the high-vacuum main chamber with a base pressure of ∼ 10⁻⁸ mbar. The multi-stage ion lens in the main chamber has a similar configuration described by Suits et al.,^[34] and the optimized voltages applied on the electrodes are U_Repeller = 2000 V, U₁ = 1760 V, U₂ = 1660 V, and U₃ = 0 V. The super-sonic molecular beam interacts with the intense femtosecond laser field in the main chamber, and the fragment ions are first accelerated by a multi-lens velocity mapping apparatus and then fly freely to a dual micro-channel plate (MCP) coupled with a P47 phosphor screen. The sliced images of the fragment ions are obtained with an intensified charge-coupled device (ICCD) camera, and the time-of-flight (TOF) mass spectra are achieved by a photomultiplier tube (PMT) connected to a digital oscilloscope. All the timing sequence control is implemented by a digital delay pulse generator (DG535 Stanford Research System).

Figure 1 shows two typical TOF mass spectra of CS₂ molecule irradiated by the 800-nm femtosecond laser field with a laser central wavelength of 800 nm and the pulse duration of 70 fs for the laser intensity of 4.9 × 10¹³ W/cm² (see Fig. 1(a)) and 1.3 × 10¹⁴ W/cm² (see Fig. 1(b)). In our experiment, the laser polarization direction is always perpendicular to the TOF axis. When the laser intensity is set at 4.9 × 10¹³ W/cm², as shown in Fig. 1(a), these fragment ions C⁺, S^m+ (m = 1–3), CS⁺, and can be observed, which indicates that the two C–S bonds can be broken in this lower laser intensity. When the laser intensity is increased to 1.3 × 10¹⁴ W/cm², as shown in Fig. 1(b), some highly charged fragment ions C²⁺, C³⁺ and S⁴⁺ appear, which shows that the appearance potential of fragment ion C^m+ is higher than that of fragment ion S^m+ with the same charge number. Generally, the generation of highly charged ions (such as C²⁺, C³⁺, S²⁺, S³⁺, S⁴⁺) indicates the participation of the CE processes in our experiment. In addition to the two parent ions and no other multiply charged parent ions are observed at the full range of our laser intensities, which may be due to the instability of the highly charged parent ions. In this work, we aim to explore the formation mechanism of fragment ions S⁺, S²⁺, CS⁺, and CS²⁺ by breaking one C–S bond.

Fig. 1.

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	Fig. 1. The TOF mass spectra of CS2 molecule irradiated by 800 nm femtosecond laser pulses with a laser intensity of 4.9 × 1013 (a) and 1.3 × 1014 W/cm2 (b).

When the polyatomic molecules are subjected to an intense femtosecond laser field, the fragment ions can be produced from different photoionization and photodissociation channels. With the help of DC-sliced image technique, we can clearly discriminate these different dissociative ionization channels of the fragment ions with same mass-to-charge ratio. Figure 2 presents the pseudo-color DC-sliced images of fragment ions S⁺, S²⁺, CS⁺, and CS²⁺. One can see that each fragment ion involves more than one dissociative ionization channel. Usually, the fragment ions with high KER should result from the CE process, while those with low KER should result from the multi-photon dissociative ionization process. Here we focus on the CE process of the four fragment ions. As is well known, in the two-body CE model, the two partner ions should meet with the momentum conservation condition. In other words, the KERs of the two fragments should satisfy the following relationship:^[27,35,36]

Table 1 lists the mass ratio M(CS^q+)/M(S^p+), KER ratio (S^p+)/KER(CS^q+), and relative experimental error Δ. Considering the experimental condition and data processing, if the experimental error Δ is less than 5%, the above listed channel assignments are considered to be correct. As can be seen, both the channels (1,2) and (2,1) result from the CE process of parent ion but their total KERs are different, which can come from the different precursor states. In addition, there is a spot in the central position for the fragment ions S⁺ and CS⁺, which can be assigned to the (1,0) and (0,1) dissociative ionization channels of parent ion Here, we do not discuss this dissociation process since our focus is the CE process.

Fig. 2.

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	Fig. 2. The pseudo-color DC sliced images of fragment ions S+ (a), S2+ (b), CS+ (c), and CS2+ (d) with the laser intensity of 1.5 × 1014 W/cm2.

Fig. 3.

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	Fig. 3. The velocity distributions of fragment ions S+ (a), S2+ (b), CS+ (c), and CS2+ (d). The red circles are the experimental data, and the blue solid lines are the simulated results by the multiple Gaussian functions.

Table 1.

Table 1.

Table 1. Mass ratio (Ratio1), KER ratio (Ratio2), and relative error for the four CE channels. . Channels Etotal/eV Rc/Å Ratio 1 Ratio 2 Δ (1, 1) 3.84 3.75 1.38 1.37 0.7% (1, 2) 7.44 3.87 1.38 1.44 4.3% (2, 1) 7.86 3.66 1.38 1.41 2.2% (2, 2) 13.75 4.19 1.38 1.33 3.6%

Table 1. Mass ratio (Ratio1), KER ratio (Ratio2), and relative error for the four CE channels. .

In addition to the KER distribution, the angular distribution of fragment ions also plays an important role in assigning the dissociation channels. The two fragment ions from the same dissociation channel should have the similar angular distribution. Figure 4 presents the angular distributions of four pairs of fragment ions mentioned above. As expected, each pair of fragment ions has a similar angular distribution, which can further verify the above CE channel assignments. Furthermore, one can see that the fragment ions that result from the dissociation process of the higher parent ions will show a narrower angular distribution.

Fig. 4.

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	Fig. 4. The angular distributions of the two fragment ions that correspond to the four CE channels (1, 1) (a), (1,2) (b), (2,1) (c), and (2,2) (d).

It has been proved that the total KER E_total from the CE process and the critical distance R_c should satisfy the following relationship:^[23,26]

To better understand this energy conversion process, we use a Coulomb potential approximation to theoretically simulate the dissociation process of parent ions, and here the doubly charged parent ion is used as an example for detailed illustration. Assuming that the initial momentum of fragment ions S⁺ and CS⁺ are both zero, the critical distance R_c of parent ion is known as 3.75 Å, and the two fragment ions S⁺ and CS⁺ move along the Coulomb potential. Here, we introduce the Hamiltonian in the center-of-mass coordinate system as follows:

Fig. 5.

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	Fig. 5. The time evolution of Coulomb energy (red dotted lines) and KER (black solid lines) for the four CE channels (a), (b), (c), and (d).

Table 2.

Table 2.

Table 2. Time requirement for different percentages of Coulomb energy to KERs in the four CE channels. . Channels 10% 30% 50% 70% 90% (1,1) 41 84 136 237 666 (1,2) 30 62 100 176 498 (2,1) 27 56 94 162 456 (2,2) 24 49 91 140 396

Table 2. Time requirement for different percentages of Coulomb energy to KERs in the four CE channels. .

In summary, the CE process of CS₂ molecule under the near-infrared (800 nm) intense femtosecond laser field has been experimentally investigated with a DC-sliced ion imaging technique. The CE channels of fragment ions S⁺, S²⁺, CS⁺, and CS²⁺ by breaking one C–S bond were confirmed by calculating their corresponding velocity distribution and angular distribution. The Coulomb explosion process of highly charged parent ions was theoretically simulated by using a Coulomb potential approximation. It was shown that the chemical bond break occurred at a critical distance, and the dissociation time of parent ion decreased with the increase of the charge number due to the stronger Coulomb repulsive force, and the theoretical result gave a good explanation as to why the measured KER in the experiment was much smaller than the theoretical calculation and the fragment ions from the higher parent ions showed a higher KER and narrower angular distribution. We believe that these experimental and theoretical results will be very useful for further understanding the CE process of polyatomic molecule under the intense femtosecond laser field.