Acta Physico-Chimica Sinica  2017, Vol. 33 Issue (3): 500-505   (962 KB)    
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  • Received: October 12, 2016
  • Revised: November 11, 2016
  • Published on Web: November 11, 2016
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    Author
    LIU Ning-Liang
    SHEN Huan
    Multiphoton Dissociation and Ionization Dynamics of Allyl Chloride Using Femtosecond Laser Pulses
    LIU Ning-Liang1,2, SHEN Huan1,2,*   
    1 College of Science, Huazhong Agricultural University, Wuhan 430070, P. R. China;
    2 Institute of Applied Physics, Huazhong Agricultural University, Wuhan 430070, P. R. China
    Abstract: The dissociation and photoionization dynamics of C3H5Cl were studied at 200, 400, and 800 nm with femtosecond laser pulses. The time-of-flight mass spectra, laser power index and photoelectron images were recorded. At short wavelength (200 nm), ionization of the parent molecule was found to be the dominant channel, while other ions were generated by the dissociation of C3H5Cl+. With the shift to long wavelength (e.g., 800 nm), fragment ions became dominant, and were generated through the multiphoton ionization of neutral fragments after the photodissociation of C3H5Cl. These results imply that photodissociation plays a significant role at long wavelength, because neutral fragments are supposed to be generated from the intermediate states reached by 800 nm photons. At 400 nm, the dissociation on the intermediate states is also critical, but is not as high as that at 800 nm. Taken together, our results demonstrate that the dissociation/ionization behaviors of allyl chloride are wavelength-dependent, and reveal the complex dynamics of allyl chloride at 200, 400 and 800 nm.
    Key words: Femtosecond laser pulse     Time-of-flight mass spectrometry     Photoelectron imaging     Allyl chloride     Multiphoton dissociation and ionization    

    1 Introduction

    As a prototype of aliphatic halides, allyl chloride has attracted a great deal of interest due to its complex photodissociation dy-namics1-13. The radiation of a UV photon possibly leads to C-Cl bond breaking and/or HCl elimination. Myers et al.7 measured the photofragment velocity and angular distribution of allyl chloride at 193 nm with a crossed lasermolecular beam apparatus and identified three competing channels occurring upon ππ* excita-tion. Later, Morton et al.8 found that [all C-Cl] : [fast C-Cl] : [slow C-Cl] : [fast HCl] : [slow HCl] : [all HCl] was 1.00 : 0.971 : 0.029 : 0.291 : 0.167 : 0.458, where fast refers to the high recoil kinetic energy channels. Park9 and Liu10 et al. also investigated the C-Cl bond fission dynamics of ally chloride at 235 nm with velocity map imaging technique. Both groups found that at least two channels are involved in the dissociation. As for the ionization dynamics, important constants, such as the accurate ionization potential, were also measured11, 12.

    Since all studies were carried out with nanosecond pulses, Shen et al.13 studied the photodissociation dynamics of allyl chloride in a timeresolved way using femtosecond lasers recently. The lifetimes of the C-Cl bond fission channels as well as the HCl elimination channel were obtained. It was found that C-Cl bond fission channels have two different pathways: one takes (48±1) fs and the other takes (85±40) ps. They are attributed to the predissociation process on the repulsive */πσ* state and dissociation after internal conversion to the vibrationally excited ground state. Two different HCl elimination channels, namely (600±135) and (14±2) ps, were also measured. The (14±2) ps component was due to internal conversion from the ππ*/πσ* state to the ground state and dissociation on the ground state. The (600±135) fs component was proposed to be dissociated on the excited state surface.

    As previously demonstrated by Shen et al.13, time-resolved mass spectra coupled with photoelectron images are a powerful tool to study the time-resolved dynamics of complex molecules. Lifetime constants can be extracted from time-resolved mass spectra while information from the intermediate state can be obtained from photoelectron images. Since two different colors are used in timeresolved experiment, and both of them may generate photoelectrons with different kinetic energies, it is important to clearly identify the photoelectron spectra as well as the intermediate state when each single-color laser is used. In the present work, the multiphoton dissociation and ionization dynamics of allyl chloride were investigated using femtosecond laser pulses at 200, 400 and 800 nm. Mass spectra as well as photoelectron images were recorded at each wavelength. The photodissociation and ionization dynamics were found to be wavelength-dependent. Finally, the implications to the photodissociation dynamics were also discussed.

    2 Experimental setup

    The experimental setup used here has been described in detail elsewhere14. In brief, ~5% allyl chloride was seeded in He carrier gas. The mixture was expanded through a pulsed valve operating at 10 Hz. After passing a skimmer, which separated the source chamber from the ionization chamber, the molecular beam was interacted with the femtosecond laser pulses at the midway between the repeller and the extractor plates of the electrostatic lens. The generated photoions and photoelectrons were accelerated in the electric field and detected by a two-dimensional position sensitive imaging detector. In order to minimize the effect of the earth′s magnetic field on the photoelectron trajectory, a doublelayer μ-metal shield was installed along the axis of the time-offlight chamber. A photomultiplier tube and a 100 MHz digital oscilloscope (TDS 2012, Tektronix) were used to acquire the timeof-flight mass spectra of the photoions, while a charge coupled device (CCD) camera was used to collect the photoelectron images.

    The laser source employed here was a regenerative amplified Ti:sapphire femtosecond laser system (Coherent, Legend). The Ti: sapphire oscillator was pumped by the second harmonic of a CW Nd: YVO4 laser. A seed beam was generated and then amplified by a Nd:YLF laser pumped regenerative amplifier to generate a~50 fs, 1 mJ pulse centered at 800 nm with a repetition rate of 1 kHz. 400 nm pulse was produced by the second harmonic generation of the fundamental pulse, while 200 nm pulse was generated by the sum frequency of the fundamental (800 nm) and its third harmonic (267 nm). These laser beams were focused using a 25 cm focallength quartz lens. The polarization of the laser beams was set to be vertical to the optical table and parallel to the face of the imaging detector. The typical intensity used was 1.0 × 1012 W∙cm-2 for 200 nm, 1.9 × 1012 W∙cm-2 for 400 nm and 4.5 × 1012 W∙cm-2 for 800 nm at the focusing region for each laser pulse, respectively.

    The photoions and photoelectrons produced were analyzed by an ion time-of-flight mass spectrometer and by a velocity-mapimaging (VMI) device, respectively. The VMI device was also used for mapping the kinetic energy and the angular distributions of the photoelectrons resulting from the photoionization process. Each image was accumulated over 20000 laser shots. The photoelectron kinetic energy was calibrated using (2 + 1) resonanceenhanced multiphoton ionization of iodine atom15.

    3 Results and discussion
    3.1 Ion spectra

    Fig. 1 depicts the ion spectra of allyl chloride with the radiation of 800, 400 or 200 nm femtosecond pulse. As shown in Fig. 1, five peaks can be observed, and based on their respective flight time, they can be assigned to the parent ion C3H5Cl+ and its fragment ions, i.e., C3H5+, C3H4+, C2H3+ and CH2+(as labeled in the Fig. 1). Five kinds of ions are observed at all the three wavelengths. However, their relative intensities are different (Table 1). At 800 nm, the branch ratio of C3H5Cl+ is 17.06%, which is almost three times lower than that of C3H5+(53.26%). At 400 nm, the branch ratio of C3H5Cl+ increases to 30.31%, which is close to that of C3H5+ (37.69%). However, at 200 nm, the branch ratio of C3H5Cl+ in-creases to 55.26%, which is about three times higher than that of C3H5+ (15.97%). Meanwhile, the branch ratios of other ion frag-ments, including CH2+, C2H3+ and C3H4+, show very slight changes at different wavelengths as shown in Table 1. The parent ion shows different branch ratios at these three wavelengths, indicating that different formation mechanisms are dominant at different wavelengths.

    Fig. 1 Time-of-flight mass spectra of allyl chloride with the radiation of (a) 800 nm, (b) 400 nm and (c) 200 nm femtosecond pulses

    Table 1 Branch ratio of each ion after the photoionization of allyl chloride at three wavelengths

    In the present study, the intensities of the femtosecond laser pulses were set to be 1.0 × 1012 W∙cm-2 at 200 nm, 1.9 × 1012 W∙ cm-2 at 400 nm and 4.5 × 1012 W∙cm-2 at 800 nm. At low intensity limit (~1012 W∙cm-2), photoionization products are produced by the multiphoton ionization process normally16. The ionization potential for allyl chloride is 10.20 eV17, suggesting that at least two 200 nm photons (2 × 6.20=12.40 eV), four 400 nm photons (4 × 3.10=12.40 eV) or seven 800 nm photons (7 × 1.55=10.85 eV) are required for the ionization of allyl chloride. In order to further confirm the number of photons absorbed at the three wavelengths, a power dependence measurement of the ion yield was performed (Fig. 2). A slope of 1.94±0.03 was found for C3H5Cl + at 200 nm (Fig. 2(a)), which is consistent with the assumption of a two-photon process. The power indexes for the fragment ions, such as C3H5+ and C3H4+, were found to be 2.13±0.15 and 2.32±0.19, respectively, which are very close to that of C3H5Cl+. These results imply that C3H5+ and C3H4+ are produced from the dissociation of C3H5Cl+, since the dissociation of C3H5Cl+ can be easily saturated due to its limited population. Fig. 2(b) shows the similar measurement at 400 nm. The laser power indexes of C3H5Cl+, C2H3+ and CH2+ were recorded to be 4.41±0.14, 3.71±0.16 and 4.03±0.44, respectively, indicating a four-photon process. However, the laser power indexes for C3H5 and C3H4 are 5.63±0.12 and 4.92±0.15, respectively, which are higher than that of C3H5Cl+. These results suggest that other channels may contribute to these two fragment ions besides the dissociation of C3H5Cl+, which will be discussed in Section 3.3. Fig. 2(c) shows the results at 800 nm. The laser power indexes for C3H5Cl+, C2H3+, and CH2+ were recorded to be 1.44±0.04, 2.14±0.25 and 2.10±0.03, while they were 5.69±0.19 and 5.13±0.31 for C3H5 + and C3H4+, respectively. It seems that none of them shows a seven-photon process at 800 nm. This obvious deviation indicates that other channels may play a significant role in the formation process of these fragment ions besides the dissociation of C3H5Cl+, as will be discussed in Section 3.3. For C3H5+ and C3H4+, similar results were obtained at 400 and 800 nm, and they show a laser power index of ~6 and ~5, respectively. On the other hand, the laser power index of C3H5Cl+ drops to 1.44. Of course, two photons at 800 nm cannot ionize C3H5Cl; instead, at least seven photons are needed. It is likely that at 800 nm, C3H5Cl+ dissociates and other ions are formed as the laser power increases, resulting in a lower laser power index.

    Fig. 2 Laser power index measurement of the generated ions at three wavelengths

    3.2 Photoelectron spectra and ionization dynamics

    The photoelectron images resulted from photoionization at 200, 400 and 800 nm femtosecond laser fields are shown in Fig. 3. The arrow indicates the polarization of the laser. The left part of each image is the two-dimensional raw image. These raw images are the two-dimensional projections of the three-dimensional speed and angular distributions of the photoelectrons. Given that the distributions of the photoelectrons have a cylindrical symmetry around the polarization axis of the photolysis laser, a full threedimensional photoelectron image can be reconstructed using the basis-set expansion method (BASEX)18, as shown in the right half of each image in Fig. 3. Thus, the kinetic energy distributions of the photoelectrons were obtained from the angle integration of the reconstructed image as a function of the radial distance from the center (Fig. 4). Two peaks are observed at 200 nm, and three peaks are observed at 400 nm, while six peaks are observed at 800 nm.

    Fig. 3 Photoelectron images of the resulted electrons in the femtosecond laser field The left part of each image is the two-dimensional raw image, while the right part is the reconstructed three-dimensinal image. The double direction arrow is the polarization of the laser.

    Fig. 4 Kinetic energy distributions of the photoelectrons obtained from the photoelectron images observed peaks are labeled as an, bn and cn (n=1, 2, 3, …, 6) at 200, 400 and 800 nm, respectively.

    The observed peaks can be assigned using the conservation of energy,

    $ {E_{\rm{e}}} = nhv + {E_0}-{E_i} $ (1)

    where n stands for the number of photons involved in the pho-toionization process, hv is the energy for a 200 nm, a 400 nm or an 800 nm photon, E0 is the internal energy of the parent molecule, and it can be neglected under the cold supersonic beam conditions19, Ei is the internal energy of ionic parent at the ith level, and Ee is the kinetic energy of the outgoing electrons.

    At 200 nm, the molecule would obtain an energy of 12.40 eV after absorbing two photons. The first and second ionization potentials (IP) of allyl chloride are 10.20 and 11.17 eV, respectively17. Thus, the kinetic energy obtained by the photoelectron should be located at 2.20 and 1.23 eV, respectively. This is in accordance with the observed peaks a2 and a1, which center at 2.08 and 1.15 eV (Fig. 4(a)). The discrepancy on the order of ~0.1 eV is probably due to the error in the identification of the peak center since the peaks are pretty broad.

    In our photoelectron spectra, two peaks both result from the ionization of the parent ion at 200 nm. This observation is in line with the mass spectra obtained when 200 nm was used. In Fig. 1 (a), the branch ratio of the parent ion is 55.26%, which is at least three times higher than those of other ions. Nevertheless, the fragment ions, such as C3H5+, C3H4+, C2H3+ and CH2+, are also observed. Normally, the fragment ions are generated from two possible processes20-23. One is the dissociative ionization of the neutral parent molecule, while the other is the ionization of the neutral fragment dissociated from the neutral parent molecule. Due to the different ionization potentials of the parent molecule and its fragments, electrons from each species would show distinct peaks. In our case, we did not observe any peak for the ionization of the neutral fragments, implying that the observed fragment ions, such as C3H5+, C3H4+, C2H3+ and CH2+ at 200 nm, are attributable to the dissociative ionization of the neutral parent molecule.

    At 400 nm, three peaks are observed, which are located at b1 (0.88 eV), b2 (1.32 eV) and b3 (1.70 eV) (Fig. 3(b)). At least an energy of 12.4 eV (four 400 nm photons) is needed to ionize the parent molecule since its IP is 10.20 eV, and two 200 nm photons also result in an energy of 12.40 eV. Thus, we assign the b2 peak to the ionization of the C3H5Cl molecule. However, two other peaks, b1 (0.88 eV) and b3 (1.70 eV) were observed, implying that these peaks might come from other processes, such as ionization of neutral fragments. We assign the b3 peak to the ionization of H2C=CHC:H or the HC∙=CHC∙H2 radical after the absorption of three 400 nm photons as Shen et al.13, who also reported the observation of a 1.67 eV peak. As to the b1 peak, it probably comes from ionization of the C3H5 radical, which has an IP of 8.18 eV (6 × 1.55 8.18=1.12 eV)24, 25. These assignments are in line with the mass spectra observed in Fig. 1(b), where C3H5+, C3H5Cl+ and C3H4+ dominate the mass spectra at 400 nm. The measured photoelectron spectra also indicate that the observed C3H5+ and C3H4+ with 400 nm femtosecond pulse are not derived only from the dissociative ionization of C3H5Cl; and multiphoton ionization of neutral radical also plays a significant role, which is probably the reason why its power index is higher than 4.

    At 800 nm, six peaks, c1 (0.65 eV), c2 (0.99 eV), c3 (1.45 eV), c4 (2.23 eV), c5 (3.01 eV) and c6 (4.58 eV), are observed (Fig. 3(c)). With 800 nm pulse, seven photons could achieve an energy level of 10.85 eV, which is 0.65 eV higher than the IP of the parent molecule. Thus, it is possible that the c1 peak (0.65 eV) is due to the ionization of the C3H5Cl molecule (7 × 1.55 -10.20=0.65 eV). The c2 peak can be assigned to the ionization of C3H5 radical, while the c3 peak can be assigned to the ionization of H2C=CHC: H or the HC*=CHC*H2 radical (6 × 1.55 -7.63=1.67 eV)13. As to other peaks, we notice that c4-c1=1.58 eV, c5-c3=1.56 eV, and c6-c5=1.57 eV. Thus, they are from the same molecule/radical, but absorb one more photon. As suggested previously, ionization of the C3H5Cl molecule is one of the possible pathways to generate c1. Thus, the c4 peak implies that C3H5Cl+ is populated to a highly excited state by absorbing one more photon. Hence, the fragmentation of C3H5Cl+ would lead to a decrease of its branch ratio. On the mass spectra in Fig. 1(c), the branch ratio of C3H5Cl+ is the lowest among these three wavelengths, which is in line with the observations in the photoelectron kinetic energy measurements.

    3.3 Implication to the photodissociation dynamics

    Although five different ions are observed at all the three wavelengths, photoelectron spectrum measurements show that they are generated by different schemes. At 200 nm, C3H5Cl+ is predominant and other ions are generated by the dissociative ionization of C3H5Cl. At 400 nm, multiphoton ionization of the neutral fragments plays a significant role. At 800 nm, the branch ratio of C3H5Cl+ is further decreased by dissociation with the absorption of an additional photon. Meanwhile, multiphoton ionization of the neutral fragments is as important as at 400 nm. This wavelength-dependent ionization behavior implies that photodissociation plays a significant role at long wavelength, since neutral fragments are supposed to be generated on the potential energy surface of the intermediate states reached by 400 nm or 800 nm photons.

    The photodissociation dynamics of allyl chloride on the ππ* state and the */πσ* state have been investigated by time-resolved mass spectroscopy coupled with photoelectron spectroscopy13. After absorbing two photons at 400 nm (6.20 eV), the fast predissociation of C-Cl bond on the repulsive */πσ* state takes (48±1) fs, while the dissociation of C-Cl on the vibrationally excited ground state resulting from the internal conversion from the initially prepared ππ* state takes (85±40) ps. The HCl elimination on the excited state takes (600±135) fs and on the ground state resulting from the internal conversion from the ππ* state takes (14±2) ps. With 266 nm light, the molecule is populated to the * state and direct C-Cl dissociation takes~48 fs. In the present study, three intermediate states, ππ*, * and πσ*, are reachable with the absorption of a 200 nm photon. As mentioned in the previous section, only the fast C-Cl bond breaking channel on the */πσ* potential surface is comparable to the pulse duration (50 fs). Thus, ionization or dissociation to C3H5+Cl of C3H5Cl is the dominant channel. Hence, molecules on the ππ* state and the */πσ* state corresponding to the slow dissociation channel will be ionized. Therefore, it is not surprising to find that C3H5Cl+ has a branch ratio as high as 55.26%. As for other ions, they are likely from the dissociation of the parent ion. This is in agreement with our observed photoelectron spectra, where electrons from neutral radicals are not observed. At 400 nm, since no intermediate states can be reached with one 400 nm photon, two 400 nm photons can populate the molecule to the same intermediate states as one 200 nm photon. However, C3H5+ is dominant in the mass spectra, which is different from the results observed at 200 nm. This difference is due to the fact that three 400 nm photons (9.30 eV) populate the molecule to a region which cannot be achieved by 200 nm photons. With this abundant internal energy, more dissociative states can be reached and fragmentation is more likely to occur. Although the pathways for ionization are still dominant after the absorption of two 400 nm photons, they are disturbed by the dissociative states before ionization. At long wavelength (800 nm), more dissociative states are reachable. For example, three 800 nm photons populate the molecule to the * state, where C-Cl bond breaking takes~50 fs. In order to reach the ππ* state populated by two 400 nm photons, one more 800 nm photon is needed. Hence, it can be speculated that less molecules are populated on the ππ* states at 800 nm than at 400 nm. On the mass spectra, the C3H5Cl+ signal is weaker at 800 nm compared with at 400 nm, which is in agreement with the above analysis.

    4 Conclusions

    In the present work, femtosecond laser pulses were used to study the photodissociation and photoionization dynamics of C3H5Cl. The time-of-flight mass spectra as well as photoelectron spectra were obtained. We found that the ionization of C3H5Cl induced by two photons is the primary pathway at 200 nm. Fragment ions are mostly generated through dissociative ionization of the parent molecule. At 400 nm, dissociation of the C3H5Cl molecule plays an important role. The fragment ions show an increasing branch ratio, and they are generated from the dissociation of the C3H5Cl molecule produced by multiphoton ionization. At 800 nm, dissociation on the intermediate states becomes more significant since more intermediate states are involved. The present study provides important information for the ionization and dissociation dynamics of C3H5Cl together with our previous two-color time-resolved study.

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