Dissociative ionization and Coulomb explosion of ethyl bromide under a near-infrared intense femtosecond laser field

Abstract

The multi-photon dissociation and Coulomb explosion of ethyl bromide C₂H₅Br under near-infrared (800 nm) femtosecond laser field are experimentally investigated by a DC-sliced ion imaging technique. The sliced images of fragment ions C₂H₅⁺, Br⁺, CH₃⁺, CH₂Br⁺, H₂⁺ and C₂H₃Br⁺ are obtained, and their dissociative pathways are assigned by observing their corresponding kinetic energy release (KER) and angular distribution. It is shown that low-KER components of these fragment ions result from multi-photon dissociation of singly charged parent ion C₂H₅Br⁺, while high-KER components come from Coulomb explosion of doubly charged parent ion C₂H₅Br²⁺. It is also shown that the precursor species [C₂H₅⁺…Br⁺] has a longer lifetime than [C₂H₃Br⁺…H₂⁺] and [CH₃…CH₂Br⁺]. In addition, the probable H₂ and H₂⁺ elimination channels are theoretically simulated by Gaussian 09 software packages, and the results show that the former is an asynchronous process while the latter is a synchronous process.

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I. Introduction

When atoms or molecules are exposed to a femtosecond laser field with laser intensities in the range of 10¹³–10¹⁵ W cm⁻², this intense femtosecond laser field will modify the potential energy surface (PES) of the electronic state in the atomic or molecular systems, and thus a variety of novel physical phenomena will occur, such as multi-photon ionization (MPI),^1–4 above threshold ionization (ATI),^5,6 high-order harmonic generation (HHG),^7,8 Coulomb explosion (CE),^9–12 and so forth. On the basis of these related phenomena, one can obtain the atomic or molecular dynamical processes. Alkyl bromide C₂H₅Br was usually taken as study example in the field of molecular dynamics under the intense laser field, and its photodissociation behaviors have been regarded as the reference for understanding photodissociation dynamics of polyatomic molecules.^13–15 In the photodissociation process of alkyl bromide, the dominant dissociation channel is C–Br bond fission to form an alkyl cation and a ground or spin–orbit excited state bromine atom,^16–20 and such observation has an important significance on the photochemical study of atmospheric bromine. Zhang et al. have experimentally studied the photodissociation dynamics of C–Br bond fission in a series of alkyl bromide by ultraviolet (UV) laser excitation,^13,21–23 and shown that fragments Br* (²P_1/2) and Br (²P_3/2) can be attributed to the perpendicular transition to the ¹Q₁ and ³Q₁ states and the parallel transition to the ³Q₀ state. In addition to C–Br bond cleavage, Xu et al. have demonstrated other photodissociation pathways of ethyl bromide cation C₂H₅Br⁺ based on 355 nm laser excitation,²⁴ and concluded that both C₂H₅Br⁺ → C₂H₃⁺ + H₂ + Br and C₂H₅Br⁺ → C₂H₅⁺ + HBr channels are produced through multi-center intermediates according to the anisotropy parameter. Gardiner et al. also confirmed above two dissociation channels by 266 and 355 nm laser excitations,²⁵ and theoretically simulated the lowest thermochemical threshold of accessible photofragmentation pathways from ethyl bromide cation C₂H₅Br⁺, involving C–C, C–Br and C–H bond fissions.

In these previous studies, fragmentation dynamics of ethyl bromide by C–Br, C–C or C–H bond cleavage were usually performed by UV laser excitation, while studying them by near-infrared (IR) laser field was seldom reported. In this work, we present an investigation on the photodissociation process of ethyl bromide C₂H₅Br by 800 nm intense femtosecond laser field based on a DC sliced ion imaging technique. These fragment ions H₂⁺, CH₃⁺, C₂H₅⁺, Br⁺, CH₂Br⁺ and C₂H₃Br⁺ by C–C, C–Br and C–H bond fissions are observed, and low-KER components of these fragment ions are attributed to multi-photon dissociation (MPD) of singly charged parent ion C₂H₅Br⁺ while high-KER components are assigned to Coulomb explosion (CE) of doubly charged parent ion C₂H₅Br²⁺. Furthermore, elimination channels of H₂ and H₂⁺ in MPD and CE processes are also experimentally observed, and the theoretical simulations show that the former is an asynchronous concerned process while the latter is a synchronous concerned process.

II. Experimental arrangement

Our experimental setup of DC sliced ion imaging system has been described in our earlier publications,^26,27 and here the simple statement is given. The multi-stage ion lens installed in the main chamber has the similar geometry designed by Suits et al.²⁸ According to ion trajectory simulation by Simon 8.0 software, the voltages applied on these electrodes are set to be U_Repeller = 2000 V, U₁ = 1768 V, U₂ = 1661 V and U₃ = 0 V, respectively. The ethyl bromide vapor seeded in helium gas at a backing pressure of 0.3 atm is expanded into the source chamber through a pulsed valve with a repetition rate of 100 Hz, and the supersonic molecular beam is skimmed before entering the reaction chamber. The femtosecond laser outputted from a regenerative amplification laser system has the center wavelength of 800 nm, pulse width of about 72 fs and repetition rate of 1 kHz, and the linearly polarized femtosecond laser pulse is focused into the reaction area by a lens with the focal length of 400 mm. The ethyl bromide is ionized and dissociated by the femtosecond laser field in the reaction chamber. These fragment ions are accelerated by multi-stage ion lens velocity apparatus and detected by a pair of 77 mm micro-channel plates (MCP) coupled to a P47 phosphor screen, and finally the sliced images of the fragment ions are recorded by an intensified charge-coupled device (ICCD) camera. To identify the fragment ion species, the time-of-flight (TOF) mass spectra are measured by a photomultiplier tube connected to a digital oscilloscope. In this experiment, all the timing sequence is controlled by Stanford Instrument Digital Delay/Pulse Generation (DG535).

III. Results and discussion

Fig. 1 shows three typical TOF mass spectra of ethyl bromide C₂H₅Br with the laser intensities of 1.8 × 10¹³ (a), 4.9 × 10¹³ (b) and 1.5 × 10¹⁴ W cm⁻² (c), here the laser polarization direction is kept to be perpendicular to TOF axis. Since there are two different isotopes of bromine (⁷⁹Br and ⁸¹Br) in nature, those fragment ions that contain bromine atom will exhibit multiple peak structure. The fragment ions C₂H_m⁺ (m = 2–5) and singly charged parent ion C₂H₅Br⁺ are observed with the laser intensity of 1.8 × 10¹³ W cm⁻² (see Fig. 1(a)), which means that C–Br bond fission is the dominant dissociative channel in the lower laser intensities. When the laser intensity is increased to 4.9 × 10¹³ W cm⁻², these fragment ions H⁺, H₂⁺, CH_n⁺ (n = 0–3) and Br⁺ are observed (see Fig. 1(b)), and this illustrates the participation of C–C and C–H bond fissions. Here, the fragment ion H₃⁺ is not observed, which may be due to the more complicated formation process comparing with the fragment ion H₂⁺, which includes the breakage of three C–H bonds and formation of three H–H bonds, and therefore its formation need more demanding experimental condition. When the laser intensity is further increased to 1.5 × 10¹⁴ W cm⁻², highly charged fragment ions C²⁺ and Br²⁺ are observed (see Fig. 1(c)), which shows that the CE process will occur in the higher laser intensities. In this CE process, the ionization probability can be greatly enhanced when the bond length of the molecular ions elongates to a critical distance, which triggers the Coulomb repulsion, and then the Coulomb repulsive energy accelerates the fragmentation process.^29,30 Therefore, the fragment ions produced by the Coulomb explosion process are easy to be observed in the higher laser intensities. In this work, our goal is to explore the photodissociation process via C–Br, C–C and C–H bond fissions.

Fig. 1 TOF mass spectra of ethyl bromide C₂H₅Br under the femtosecond laser field with the laser intensities of 1.8 × 10¹³ (a), 4.9 × 10¹³ (b) and 1.5 × 10¹⁴ W cm⁻² (c).

DC sliced images can provide the related information of fragment ions, involving the KER and angular distributions. To further investigate the fragmentation dynamics of ethyl bromide under the intense femtosecond laser field, DC sliced images of fragment ions C₂H₅⁺, Br⁺, CH₃⁺, CH₂Br⁺, H₂⁺ and C₂H₃Br⁺ via C–Br, C–C and C–H bond fissions are obtained with the laser intensity of 1.1 × 10¹⁴ W cm⁻², and their corresponding velocity distributions are calculated, as shown in Fig. 2. Here, all velocity distributions are fitted with Gaussian function, and the peak velocities and kinetic energy releases are also labeled. It is noted that the fragment ion C₂H₃Br⁺ cannot be observed in the mass spectrum because of its weak signal intensity, but it can be identified by our dc-sliced ion imaging system with high detection efficiency and sensitivity. In general, the fragment ions with low KER come from the MPD process of parent ions, while those with high KER come from the CE process.^26,31 Thus, these fragment ions with low KER in Fig. 2 should result from the MPD process of singly parent ion C₂H₅Br⁺, and these corresponding MPD channels can be assigned as follows

Fig. 2 DC sliced images of fragment ions C₂H₅⁺ (a), Br⁺ (b), CH₃⁺ (c), CH₂Br⁺ (d), H₂⁺ (e) and C₂H₃Br⁺ (f) with the laser intensity of 1.1 × 10¹⁴ W cm⁻², together with their corresponding KER distributions ((g)–(l)). The open circles () denote the experiment data, the solid lines () are the fitting Gaussian peaks, and the solid lines () are the simulated results.

C–Br bond cleavage:

C₂H₅Br → Br + C₂H₅⁺ + e⁻ (1)

C₂H₅Br → Br⁺ + C₂H₅ + e⁻ (2)

C–C bond cleavage:

C₂H₅Br → CH₃ + CH₂Br⁺ + e⁻ (3)

C₂H₅Br → CH₃⁺ + CH₂Br + e⁻ (4)

C–H bond cleavage:

C₂H₅Br → H₂ + C₂H₃Br⁺ + e⁻ (5)

C₂H₅Br → H₂⁺ + C₂H₃Br + e⁻ (6)

Under the intense femtosecond laser field, the ethyl bromide can also be ionized to the multi-charged parent ions, and then dissociates into fragment ions with high KER by Coulomb repulsive force. As well known, in the two-body CE model, the two fragment ions produced by the two-body CE process should have a relationship as follows^26,27,31,32

where X and Y denote the two fragment ions, M is mass of fragment ions, and p and q are charge numbers of the ions. According to eqn (7), the two fragment ions Br⁺ (1.29 eV) and C₂H₅⁺ (3.55 eV) can be attributed to the CE channel as follows

C₂H₅Br → Br⁺ + C₂H₅⁺ + 2e⁻ (8)

Utilizing the same method, the two fragment ions CH₃⁺ (3.85 eV) and CH₂Br⁺ (0.59 eV) by C–C bond cleavage are also attributed to the following CE channel

C₂H₅Br → CH₃⁺ + CH₂Br⁺ + 2e⁻ (9)

Similarly, the two fragment ions H₂⁺ (8.79 eV) and C₂H₃Br⁺ (0.16 eV) by C–H bond cleavage can be assigned to such a CE channel

C₂H₅Br → H₂⁺ + C₂H₃Br⁺ + 2e⁻ (10)

To further investigate the CE process of doubly charged parent ion C₂H₅Br²⁺, the angular distributions of these fragment ions from DC sliced images in Fig. 2 are extracted, and the calculated results are shown in Fig. 3. These experimental data are theoretically fitted by Legendre polynomial function,³³ and such a function is given as follows

where θ is the ejection angle of fragment ions measured from the laser polarization direction, which represents the instantaneous spatial distribution at the moment of CE process, a_L is the Legendre expansion coefficient, and the expectation value 〈cos² θ〉 is the characterization of fragmentation anisotropy. The values of a_L and 〈cos² θ〉 are summarized in Table 1. It is noted that the two fragment ions that come from the same CE channel show the similar expectation value 〈cos² θ〉 (i.e., fragmentation anisotropy), which can further confirm above photodissociation channel assignments in the CE process.

Fig. 3 Angular distributions of fragment ions C₂H₅⁺ (a), Br⁺ (b), CH₃⁺ (c) and CH₂Br⁺ (d). The open circles () are experimental data and the solid lines () are the simulated results by Legendre polynomial function.

Table 1 Legendre expansion coefficients a_L (L = 2, 4, 6) and expectation value 〈cos² θ〉 of fragment ions C₂H₅⁺, Br⁺, CH₃⁺, CH₂Br⁺, H₂⁺ and C₂H₃Br⁺ by Coulomb explosion

Channels	Ions	a2	a4	a6	〈cos^2 θ〉
(7)	C2H5^+	0.46	0.15	0.07	0.35
	Br^+	0.46	0.15	0.06	0.35
(8)	CH3^+	0.59	0.10	0.03	0.41
	CH2Br^+	0.61	0.04	−0.05	0.40
(9)	H2^+	0.59	−0.01	0.05	0.39
	C2H3Br^+	0.61	0.04	−0.02	0.39

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As known to all, when the short-lived precursor specie rotates as a whole in the space, its corresponding fragment ions will have stronger angular anisotropy. In Table 1, the leading expansion coefficient a₂ is an important anisotropy parameter, which characterizes the lifetime of the precursor specie state, and the precursor specie with the longer lifetime will induce the smaller a₂ value of the fragment ions.^33–35 In our experiment, the a₂ value by C–Br bond fission is determined to be a₂ = 0.46, which is smaller than that by C–C bond fission (a₂ = 0.61) as well as C–H bond fission (a₂ = 0.61). That is to say, the precursor specie [C₂H₅⁺…Br⁺] has the longer lifetime than [C₂H₃Br⁺…H₂⁺] and [CH₃…CH₂Br⁺]. This means that the precursor specie [C₂H₅⁺…Br⁺] is prepared in a deeper well of potential energy surface, and dissociates into the fragment ions C₂H₅⁺ and Br⁺ by Coulomb repulsive force following by the C–Br bond stretch.

As shown in eqn (5), (6) and (10), the elimination of H₂ or H₂⁺ is the most complicated dissociative process that includes two C–H bond fissions and H–H bond formation. To understand how the fragment H₂ or H₂⁺ is produced from the parent ion C₂H₅Br⁺ or C₂H₅Br²⁺, the probable photodissociation pathways are theoretically simulated by Gaussian 09 software packages.³⁶ The geometry of parent ions, transition states (TS) and dissociative products are optimized at the MP2 level with a 6-311++G(d,p) basis set. Meanwhile, the energies of related fragment ions are further refined by CCSD/6-311++G (d,p) with MP2 level zero-point energy corrections (ZPE). Fig. 4 shows the structure evolution during the formation of H₂ from the singly charged parent ion C₂H₅Br⁺. The dissociation pathways of H₂ elimination from the same C atom and two C atoms are both calculated at the same theoretical level, and it is found that the energy barrier of H₂ elimination from two C atoms is much higher than from the same C atom, and so here only the H₂ elimination from the same C atom is presented. As shown in Fig. 4, the optimized geometry of parent ion C₂H₅Br⁺ is with C–H bond lengths of 1.10 Å, ∠H–C–H of 109° and H–H distance of 1.78 Å. The elimination of H₂ starts with the C–H bond stretch to form TS. In the TS structure, two C–H bond lengths are elongated asymmetrically to 1.78 Å and 1.55 Å, ∠H–C–H is quickly decreased to 26°, and the distance between two H atoms is reduced to 0.79 Å, which is much shorter than that of parent ion C₂H₅Br⁺. After surpassing TS with energy of 2.30 eV, the singly charged parent ion C₂H₅Br⁺ dissociates into two parts C₂H₃Br⁺ and H₂. The values of the three main parameters ∠H–C–H, two C–H bond lengths and H–H distance along reaction coordinates are listed in Table 2. As can be seen, ∠H–C–H is decreased, H–H distance is shortened, and the two C–H bond lengths are asymmetrically elongated due to the uneven distribution of surface charges in the two H atoms, which indicate that the C₂H₅Br → H₂ + C₂H₃Br⁺ elimination channel is an asynchronous concerned dissociation.

Fig. 4 The dissociation pathway for H₂ elimination from the singly charged parent ion C₂H₅Br⁺, and here the theoretical simulation is performed at MP2/6-311++G (d,p) level.

Table 2 Molecular structure evolution from the singly charged parent ion C₂H₅Br⁺ characterized by the three main parameters ∠(H–C–H), two C–H bond lengths and H–H distance along the reaction coordinates calculated at MP2/6-311++G(d,p) level

∠H–C–H (°)	C–H bond lengths (Å)	H–H distance (Å)
109	1.10, 1.10	1.78
77	1.12, 1.16	1.42
46	1.20, 1.32	0.99
36	1.30, 1.48	0.88
26	1.55, 1.78	0.79
25	1.60, 1.83	0.78
21	1.91, 2.12	0.75
16	2.46, 2.61	0.74

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The energy path of H₂⁺ elimination channel from doubly charged parent ion C₂H₅Br²⁺ is shown in Fig. 5. It is noted that the optimized geometry of C₂H₅Br²⁺ is quite different from the structure of C₂H₅Br⁺. The two C–H bonds are stretched to 1.37 Å, which are increased by 0.27 Å comparing with C₂H₅Br⁺. Meanwhile, the distance between two H atoms is much closer, only 0.87 Å, which is almost half of the stable structure of C₂H₅Br⁺. Such an observation indicates that Coulomb repulsive force will greatly change the molecular structure when the two electrons are stripped off under the intense femtosecond laser field. The elimination of H₂⁺ also starts with the C–H bond stretch. In the TS structure, ∠H–C–H is decreased from 37° to 26°, the two C–H bond lengths are symmetrically elongated to 1.61 Å, and H–H distance is reduced to 0.81 Å. After surpassing TS, the doubly charged parent ion C₂H₅Br²⁺ dissociates into the fragment ions H₂⁺ and C₂H₃Br⁺. It is noticeable that the energy of TS is lower (0.03 eV) than that of the doubly charged parent ion C₂H₅Br²⁺, and this phenomenon has been demonstrated in previous studies.^37,38 That is to say, the doubly charged parent ion C₂H₅Br²⁺ is trapped in a very shallow quasi-bound well of the potential energy surface, and can produce a barrierless reaction process. This conclusion can also be confirmed by the experimental observation that no doubly charged parent ion C₂H₅Br²⁺ is observed in the full range of laser intensity. The values of the three main parameters ∠H–C–H, two C–H bond lengths and H–H distance are listed in Table 3. It is noted that the two C–H bonds are simultaneously broken and H–H bond is simultaneously formed, which shows that H₂⁺ elimination from the doubly charged parent ion C₂H₅Br²⁺ is a synchronous concerned process.

Fig. 5 The dissociation pathway for H₂⁺ elimination from the doubly charged parent ion C₂H₅Br²⁺, and here the calculation level is MP2/6-311++G (d,p).

Table 3 Molecular structure evolution from the doubly charged parent ion C₂H₅Br²⁺ characterized by the three main parameters ∠(H–C–H), two C–H bond lengths and H–H distance) along the reaction coordinates calculated at MP2/6-311++G(d,p) level

∠H–C–H (°)	C–H bond lengths (Å)	H–H distance (Å)
37	1.37, 1.37	0.87
36	1.40, 1.40	0.86
33	1.48, 1.48	0.84
31	1.52, 1.52	0.83
26	1.61, 1.61	0.81
25	1.80, 1.80	0.78
21	2.07, 2.07	0.76
17	2.50, 2.50	0.75

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IV. Conclusions

In summary, we have experimentally and theoretically studied the photodissociation process of ethyl bromide C₂H₅Br under the near-IR (800 nm) intense femtosecond laser field by the DC-slice imaging technique. These fragment ions C₂H₅⁺, Br⁺, CH₃⁺, CH₂Br⁺, H₂⁺ and C₂H₃Br⁺ by C–C, C–Br and C–H bond fissions were measured and their corresponding KER and angular distributions were calculated. We confirmed that these fragment ions with low KER are attributed to the MPD process of singly charged parent ion C₂H₅Br⁺, while those components with high KER result from doubly charged parent ion C₂H₅Br²⁺ via the CE process. The angular anisotropy of the fragment ions by the CE process suggested that the precursor specie [C₂H₅⁺…Br⁺] has a longer lifetime than [C₂H₃Br⁺…H₂⁺] and [CH₃…CH₂Br⁺]. Moreover, the elimination channels of H₂ and H₂⁺ were observed, and ab initio calculations showed that H₂ elimination is an asynchronous concerned reaction while H₂⁺ elimination is a synchronous concerned reaction.

Acknowledgements

This work was partly supported by National Natural Science Fund (nos 51132004 and 11474096), and Shanghai Municipal Science and Technology Commission (no. 14JC1401500).

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