Association reactions in femtosecond laser filaments of hexane studied by time-of-flight mass spectrometry with velocity screening

Abstract

Femtosecond laser filament-induced reactions in gaseous hexane (C₆H₁₄) are studied by time-of-flight ion mass spectrometry. The neutral products are unambiguously distinguished from species produced by ionization for mass analysis, through velocity screening along the flight tube. In addition to hydrogen-capped polyynes, C_nH₂ (n = 4, 6, 8, 10 and 12), the mass spectrometry reveals hydrocarbon molecules in the mass range of m/z 50–146 that have eluded identification in previous studies. The product distributions, together with their dependence on laser pulse energy and repetition rate, provide insight into the association reaction pathways to hydrogen-capped polyynes and other products by laser filaments.

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1 Introduction

Ultrashort strong laser fields (∼10¹³–10¹⁴ W cm⁻²) provide a powerful means to control chemical reactions by driving electrons with their large electric fields.^1–3 Combined with coherent pulse-shaping techniques, the strong-field reaction control demonstrated its efficacy in unimolecular reactions.⁴ Selective bond breaking and rearrangement, and orientation selective ionization were demonstrated with various polyatomic molecules in gas phase such as CO₂,^5,6 OCS,^7,8 H₂O,^9,10 C₂H₂,¹¹ CH₄,¹² CF₄,¹³ methylhalides,^14,15 trifluoro- and trichroloacetone,¹⁶ iodohexane,¹⁷ acetophenone,¹⁸ and orgnometallic molecules.¹⁹ Strong laser pulses have been exploited to manipulate bimolecular reactions between gas-phase atoms²⁰ and molecules²¹ and to intermolecular reactions in clusters.^22,23

The application to many-body reactions has also been demonstrated using femtosecond laser filaments. Laser filament is a needle-like light-emitting body generated by loose focusing of ultrashort laser pulses into a gas or liquid medium. The field intensity of the order of 10¹³ to 10¹⁴ W cm⁻² is maintained over a long distance along the filament as a result of competition between nonlinear focusing and defocusing effects.^24–26 Previous studies identified the generation of nanoparticles and films from gaseous reactants, such as carbon nanospheres from CH₄,²⁷ hydrogenated amorphous carbon nanoparticles and films from C₂H₄,²⁸ as well as metal containing nanoparticles from Al(CH₃)₃ and Al₂Mg(CH₃)₈,²⁹ by means of fluorescence/Raman spectroscopy, electron energy loss spectroscopy (EELS) and transmission electron microscopy (TEM). For gaseous hexane C₆H₁₄, association reactions to hydrogen-capped polyynes C_nH₂ (n = 6, 8, 10 and 12) were observed by UV absorption spectroscopy of the products recovered in cooled solvent.³⁰

In addition to these studies based on the analysis of the recovered products, several in situ studies on laser-filament reactions have been reported. Emission spectroscopy was employed^31,32 to identify the intermediates in laser filaments in a gas mixture of CH₄ and air, where the formation of OH radical is shown to be essential in the chain-branching oxidation reaction in the flame of CH₄ and air mixture.³² Absorption spectroscopy was exploited to investigate reaction intermediates and products, in particular nonfluorescent molecules in their electronic ground state.^33–35 The formation of O₃,³³ nitrogen oxides,^33–35 CO,³⁵ and HCN³⁵ by filamentation in air was identified by UV, visible, and infrared absorption spectroscopy.

In contrast to these optical spectroscopic techniques allowing state-resolved detection of the products, ion-mass spectrometry offers identification of a wide range of products with a high sensitivity. It was first applied to a gaseous sample recovered from the reaction cell after laser irradiation for a few hours, where various carbon hydrates, such as C₂H₂ and C₆H₆ from CH₄,²⁷ and CO₂ and C₂H₂ from a gas mixture of CO and H₂³⁶ were observed. Recently, laser filament reactions have been investigated by direct sampling of the products from a reaction chamber into a mass spectrometer.^37,38 The application to the filament reaction in C₂H₄ successfully identified a variety of association reaction products from C₃H₄ to C₇H₇.³⁸

As demonstrated by these studies, the ion mass spectrometry is powerful in investigating the reaction products and intermediates. On the other hand, obtained mass spectra are often contaminated by the fragmentation of the reaction products by ionization,³⁹ preventing a clear understanding of the reaction process from the product distributions. This becomes significant when association reactions to a large molecular species are anticipated, as they are often susceptible to dissociation. Since the discrimination of nacent products from species produced by mass analysis is often challenging, a soft ionization technique suppressing extensive fragmentation upon ionization, such as electrospray ionization (ESI) or matrix-assisted laser desorption ionization (MALDI) is employed when applicable.

Here we introduce an alternative approach to distinguish filament products from ionization-induced fragments. The method is based on the difference in initial velocities between filament products and ionization fragments after ionization, which is applied to identify laser-filament reaction products of hexane (C₆H₁₄). The paper is organized as follows. In Section 2, we describe our experimental setup. The filament products are directly sampled into the time-of-flight mass spectrometer, which allows a clear identification of the products and the intermediates to discuss the reaction processes. The velocity-resolved mass spectroscopy is presented in Section 3. Possible reaction pathways for the formation of hydreogen-capped polyynes and other products are discussed, as well as the effects of the laser repetition rate and laser intensity on the product distributions. The results of the present study are summarized in Section 4.

2 Experiment

The experimental setup is similar to that described previously.³⁸ Briefly, it consists of three sections, a reaction gas cell, a differential pumping stage, and a time-of-flight mass spectrometer (Fig. 1). The output from a Ti:sapphire laser amplifier system (800 nm, 50 fs, 1 kHz) was divided into two by a beamsplitter. The main pulse (98%) was focused into the reaction gas cell by a plano-convex lens (f = 200 mm) to generate a laser filament. The remaining horizontally-polarized pulse (2%) was focused into the mass spectrometer using a plano-convex lens (f = 200 mm), which serves as an ionization probe for the mass analysis of the filament products. Hexane (C₆H₁₄, vapor pressure ∼0.2 atm at room temperature) was continuously supplied into the reaction gas cell using Ar as a carrier gas. The gas flow rate was controlled to keep the pressure inside the reaction gas cell at a constant value of 0.4 atm.

Fig. 1 Schematic of the experimental setup consisting of a reaction gas cell, a differential pumping stage, and a Wiley-McLaren-type time-of-flight (TOF) mass spectrometer. The products of femtosecond laser-filament reactions produced in the reaction gas cell by filament laser (pump) is introduced into the TOF mass spectrometer via an oriffice and a skimmer. The products ionized by ionization laser (probe) are guided by electric fields and detected by a microchannel plate detector. The repetition rate and energy of the filament laser are varied by an optical chopper and neutral density filters, respectively.

After interaction with the laser filaments, the reaction products are introduced into the mass spectrometer as a quasi-continuous molecular beam³⁸ via an orifice (ϕ 200 µm) and a skimmer (ϕ 200 µm) in the differential pumping stage. The filament products ionized by the probe laser pulse are guided by a static electric field to the micro-channel plate detector. Because of the static electric field, ionic species are deflected before entering the flight tube. Therefore the present setup exclusively detects the neutral products from the reaction gas cell.

The field intensity achieved by the probe pulse is estimated to be 8.2 × 10¹³ W cm⁻². The Keldysh parameter , with I_p and U_p being the ionization potential and the poderomotive potential, is less than 1 for I_p < 10 eV, suggesting that tunnel ionization dominates the ionization process in the present study. An optical chopper is inserted to the filament laser beam to study the effect of the repetition rate on the product distributions. The pulse energy is varied by neutral density (ND) filters.

3 Results and discussion

3.1 Time-of-flight ion mass spectrometry with velocity screening

Fig. 2(a) shows a time-of-flight mass spectrum of C₆H₁₄ obtained without the filamentation laser in the reaction chamber. In addition to the peak at t = 6.7 µs corresponding to the parent ion (C₆H₁₄⁺), the spectrum shows many peaks at shorter flight times, some of which have peak intensities even larger than that of the parent ion. A similar mass spectrum is observed by electron impact ionization.⁴⁰ These additional peaks are assigned to fragment ions produced by the probe pulse for the mass analysis.^41,42 The fragmentation was not significant on ethylene.³⁸ The marked contrast to the previous study implies that the product distribution of the filament-induced reaction in hexane requires a secure discrimination between the filament products and the probe products.

Fig. 2 (a) Time-of-flight spectrum of hexane recorded without the filamentation laser. The field intensity of the ionization laser is estimated to be 8.2 × 10¹³ W cm⁻². Note that the small peak corresponding to m/z 40 is attributed to C₃H₄⁺ instead of Ar⁺ from the carrier gas, because the tunnel ionization of Ar is less efficient than the hydrocarbon molecules due to the high ionization potential (15.8 eV). (b) Velocity distribution of representative peaks in (a). Side peaks observed in the velocity distributions for C₆H₁₄⁺, C₅H₁₁⁺, and C₄H₉⁺ are due to ions with neighboring mass, which can be assigned to ¹³CC5H14 isotopologue and fragments with different number of hydrogen atoms, such as C₅H₁₂⁺, C₅H₁₀⁺, and C₄H₈⁺, respectively. (c) Full width at the half maximum of the velocity distrubution w_v for the peaks observed in (a). The reference width w⁰_v (solid) and the critical width w^c_v (dashed) are indicated.

Under the space-focusing conditions of a time-of-flight mass spectrometer,⁴³ the spectrum peak width is essentially determined by the distribution of the initial velocity v_z at the time of ionization along the time-of-flight axis (see Fig. 1). The initial velocity of the parent ion (C₆H₁₄⁺) is governed by the transverse velocity of the molecular beam. On the other hand, the fragment ions produced by the ionization can gain additional velocity due to the kinetic energy released by the fragmentation. This results in a broader velocity distribution than that of the parent ion.

Fig. 2(b) shows the velocity distribution along the z-axis for selected ionic species in the time-of-flight spectrum in Fig. 2(a). The v_z component is given as,⁴⁴

Here q and m are the electric charge and the mass of the ion, respectively, and F_acc is the strength of the electric field in the ion extraction region, t is the flight time, and t₀ is that of the ion with v_z = 0. The C₆H₁₄⁺ parent ion exhibits a narrow velocity distribution, while significantly broader distributions are observed for the fragment ions.

Fig. 2(c) shows the full width at half maximum (FWHM) of the velocity distribution, w_v, obtained for each peak from least-squares fitting to a Gaussian function. The width increases for a smaller fragment, which can be interpreted as a result of heavy fragmentation caused by ionization to highly excited or highly charged states. It is worth noting that a significant difference is seen even in the widths of the parent ion and the largest fragment ion, C₅H₁₁⁺, showing that the velocity width can be used to discriminate the nascent filament products in the sample from the probe products. In the following, the widths of C₆H₁₄⁺ and C₅H₁₁⁺ are adopted as the reference width w⁰_v and the critical width w^c_v, respectively, which are used to identify the filament products from hexane.

3.2 Laser-filament product distributions

Fig. 3(a) shows the time-of-flight mass spectrum obtained with a filament laser operated at 480 µJ per pulse and a repetition rate of 500 Hz. The mass spectrum looks similar to that observed without the filament laser. However, the enlarged spectrum in Fig. 3(b) reveals the emergence of new peaks in the spectral range from m/z 10 to 150. The new peaks are more clearly visible in the difference between the spectra recorded with and without the filament (Fig. 3(c)). Hydrogen-capped polyynes C_nH₂⁺ (n = 6, 8, 10, 12) observed in the previous study by UV absorption spectroscopy of recovered samples³⁰ are observed. In addition, the time-of-flight mass spectrometry identifies a number of hydrocarbon molecules as assigned in Fig. 3(c). The broad widths of the peaks around m/z 110 hinder clear assignments, but they could be attributed to C₉H_n⁺ (n = 1, 2).

Fig. 3 (a) Mass spectra obtained with (red) and without (black) the filament laser pulse (pulse energy: 480 µJ, repetition rate: 500 Hz). Each spectrum is normalized at m/z 29. Fragment ions generated from the parent C₆H₁₄ molecule solely by the probe pulse are mainly observed. (b) Expanded view of the mass spectra shown in (a). (c) Difference spectrum between the spectra obtained with and without the filamentation laser pulse. The hatched areas indicate spectral regions where strong signals of the probe products from hexane hinder clear identification of the filament products.

Fig. 4 plots the full widths at the half maximum w_v of peaks identified in the difference spectrum in Fig. 3(c). The velocity distributions of selected species are displayed as insets in Fig. 4. Some species such as C₄H₃⁺, C₅H₈⁺, C₆H₁₀⁺, C₇H₈⁺, C₈H₈⁺, and C₁₀H₂⁺, have narrow widths comparable with the hexane cation (C₆H₁₄⁺). On the other hand, the peak of C₄H⁺, for example, exhibited a significantly broader width.

Fig. 4 Full-width half maximum of the velocity distrubution of the major peaks observed in the difference spectrum. The velocity distribution of selected peaks are also shown. The horizontal lines represent the reference width w⁰_v and critical width w^c_v, respectively.

Under the present quasi-continuous free-expansion conditions, the fragment products are entrained by the carrier gas to the mass spectrometer. This implies that all the products formed in laser filaments have velocity distributions similar to those of the dominant species of the beam, i.e., C₆H₁₄ and Ar, in the present case. Therefore, the reference and critical widths, w⁰_v and w^c_v, determined in the previous section for C₆H₁₄ can be used to discriminate the filament products from the probe products. Namely, peaks with widths w_v that satisfy w_v < w^c_v are attributed to filament products, whereas those with w_v ≥ w^c_v are classified as probe products.

Fig. 4 shows that mass peak heavier than C₆H₁₄⁺, such as C₈H_n⁺, C₁₀H_n⁺ and C₁₂H_n⁺ are assigned to filament reaction products, while those lighter than C₆H₁₄⁺ are mostly formed by fragmentation, except for C₄H_n⁺ (n = 2 and 3) and C₅H_n⁺ (n = 6 and 8). In principle, each mass peak could contain contributions from both the filament product and the probe product. The resultant peak profile in such a case would consist of narrow and broad components. However, since the velocity distributions are well expressed by single Gaussian profiles as plotted in the insets of Fig. 4, the observed species are attributed to either of the two possible origins.

Fig. 5(a) shows the integrated intensities of the peaks observed in the difference spectrum in Fig. 3. For a more quantitative discussion of the distribution of the filament products, the ionization efficiency of each species should be taken into account.³⁸ As mentioned above, the ionization should be well described by tunnel ionization under the present experimental conditions. The ionization rate can be evaluated by the Ammosov-Delone-Krainov (ADK) theory,^45,46 using the ionization potentials as described in the previous study³⁸ (see also Supplementary Information). Here the structure factor describing the effect of molecular orientation to the laser electric field is not considered for simplicity. Fig. 5(a) also shows the ionization probability P calculated for each species in the spectrum, by time integration of the tunnel rate over the ionization pulse, where a Gaussian intensity profile is assumed for the ionization laser pulse.

Fig. 5 (a) Relative yields of the peaks observed in Fig. 3(c). The number of hydrogen atoms (n) is indicated above each bar. Tunnel ionization probabilities calculated for the probe pulse with a duration of 50 fs and a peak intensity of 8.2 × 10¹³ W cm⁻² are also shown. (b) Relative yields of the products obtained after tunnel ionization efficiency correction. Since the ionization energies of C_nH (n = 4, 6, 8, 10, 12) are unavailable, the ionization potentials are assumed to be the same as that of C_nH₂. The color indicates the ratio r_v = w_v/w⁰_v of each peak with w⁰_v being the reference width determined from the C₆H₁₄⁺ peak. The filament products (r_v < r^c_v) are distiguished from the probe products (r_v ≥ r^c_v).

The relative peak intensity of each compound in Fig. 5(a) is divided by the corresponding ionization efficiency S, which accounts for variations in the ionization probability P near the focal spot.³⁸ The resultant relative yield distribution is shown in Fig. 5(b). Here the relative ratio of the peak width r_v = w_v/w⁰_v is used to distinguish the filament products (r_v < r^c_v) and the probe products (r_v ≥ r^c_v), where r^c_v = w^c_v/w⁰_v is the ratio between the critical and reference widths defined in the previous section. The product distribution spectrum with shows that the hydrogen-capped polyynes, C_nH₂ with n = 4, 6, 8, 10 and 12 are formed and that the smaller polyynes with n = 4, 6, 8, are the major products from the filament. The yields decrease as the number of included carbon atoms increases. A similar spectrum is obtained at a lower ionization pulse intensity of 5.0 × 10¹³ W cm⁻² (see Supplementary Information), showing that it is not sensitive to the ionization pulse intensity under the present experimental conditions.

3.3 Effects of filament laser parameters

3.3.1 Repetition rate. As the repetition rate of the filament laser pulse increases, the time interval of laser irradiation would become shorter or comparable to the timescale of unimolecular decay or diffusion of reaction products. This leads to an accumulation of reaction intermediates subjected to irradiation of subsequent filament laser pulses.^47,48 Indeed, previous studies of laser filamentation in air showed that laser energy absorption by the laser filament at higher repetition rate (1 kHz) becomes considerably larger than at lower repetition rates (1–10 Hz).⁴⁹ This was attributed to the electronically excited molecules formed in the metastable states in laser filaments that undergo ionization by subsequent laser pulses.

Fig. 6 plots the repeptition rate dependence of the yields of the hydrogen-capped polyyne, C_nH₂ (n = 4, 6, 8, 10 and 12). The least-squares fitting to a power function of the repetition rate R^k_r shows that the yields depend linearly (k_r ∼ 1) with the repetition rate R, regardless of the products. This shows that the observed filament reactions are induced by single laser pulses.

Fig. 6 Repetition-rate dependence of the molecular yields per unit time for hydrogen-capped polyynes C_nH₂ (n = 4, 6, 8, 10, 12) with m/z 50, 74, 98, 122, and 146, obtained at the filament laser energy of 300 µJ per pulse. The solid lines show the curve fitting results to a power function R^k_r, where R is the repetition rate. The numbers denote the nonlinear coefficients k_r.

3.3.2 Pulse energy. Laser pulse energy can be another important parameter in laser filament induced reactions. For an isolated molecule, the fragmentation is expected to become significant as the pulse energy increases, as a result of nonlinear excitation and ionization in strong laser fields. In the previous study, it was shown that the product distribution of the laser filament reaction in ethylene is sensitive to the laser pulse energy.³⁸ Fig. 7 shows the yields of the hydrogen-capped polyynes plotted as a function of the pulse energy E of the filamentation laser. The pulse energy dependence is well expressed by E^k_e, with k_e being the nonlinear coefficient. Interestingly, these products have similar values k_e ∼1.7, showing that the product distribution does not vary significantly by pulse energy. This can be interpreted in terms of the intensity clamping effect in a filament. Because of the balance between the nonlinear focusing and defocusing, the laser field intensity is maintained at ∼1 × 10¹⁴ W cm⁻² in a laser filament.^24–26,50 Instead, the increase of input energy results in the increase of filament volume.^51,52 Therefore the product yields increase as the pulse energy, but with no dependence on the product species in this energy range.

Fig. 7 Filamentation laser-energy dependence of the molecular yields for hydrogen-capped polyynes C_nH₂ (n = 4, 6, 8, 10 and 12), obtained at the repetition rate of 500 Hz. The solid line shows the curve fitting results to a power function E^k_e, with E being the pulse energy. The numbers denote the nonlinear coefficients k_e.

The pulse energy dependence observed in the present study shows a marked contrast to the filament reactions in ethylene. In the latter case, the yields of filament products such as C₅H_n substantially increase with an increase of pulse energy (155 µJ per pulse) by a factor of two, resulting in a substantial difference in the product distribution.³⁸ This may be attributed to the difference in the ionization efficiency between these species. Indeed, previous studies of strong field ionization of ethylene⁵³ show that the ionization yield becomes saturated at a higher field intensity (∼1 × 10¹⁴ W cm⁻²) than that of hexane (∼6 × 10¹³ W cm⁻²),⁴¹ though their ionization potentials are similar (10.5 eV for ethylene and 10.1 eV for hexane⁵⁴). This implies that the filament clamping intensity for ethylene can be higher than that for hexane, which would explain the difference in the pulse energy dependence of the product distribution between hexane and ethylene.

3.4 Association reactions in femtosecond laser filaments of hexane

As discussed in the previous subsection, the filament reactions in the present study are induced by a single laser pulse. The products with masses smaller than hexane (m/z 86) could be formed by the fragmentation of the parent molecule in the laser filament. Indeed, the mass spectrum of hexane presented in Fig. 3(a) shows that various fragment ions formed at a field intensity (8.2 × 10¹³ W cm⁻²) close to the clamp intensity of a filament (∼1 × 10¹⁴ W cm⁻²). This in turn suggests that neutral fragments observed in the present study could be produced in the filaments as their counterparts. For example, neutral products C₄H_14−n could be produced as the counterpart of C₂H_n⁺ (n = 1–6) observed in Fig. 3, if they are produced by two-body fragmentation of the parent ion, C₆H₁₄⁺. The absence of these neutral species, C₄H₈, C₄H₉, C₄H₁₀, C₄H₁₁, C₄H₁₂ and C₄H₁₃, in Fig. 5 suggests that they are formed either by more substantial fragmentation of the parent ion through, e.g., a three-body dissociation or possibly by association reactions of smaller fragments. This applies to other small neutral species observed in the present study.

The neutral products larger than hexane could be formed by association reactions between the filament reaction products. Reactions of small hydrogen-deficient hydrocarbon radicals with other hydrocarbons have been extensively studied because of their importance in the formation of large carbonaceous molecules such as polycyclic aromatic hydrocarbons, fullerenes, and soot.^55–63

The butadiynyl radical (C₄H), which has attracted attention due to its abundance in interstellar molecular clouds and comets, has been proposed to undergo hydrogen abstraction reactions with saturated hydrocarbons, C₄H + C_nH_2n+2 (n = 1–4) → C₄H₂ + C_nH_2n+1, to form CH₃, C₂H₅, C₃H₇, and C₄H₇.⁵⁷ The reactions with unsaturated hydrocarbon molecules, e.g., C₄H + C₄H₆ → C₈H₆ + H, were also proposed. As for the polyynes, recent experiments^56,59–62 show that the chain length of polyynes can be extended by reactions with C_nH (n = 1–8). The C₄H radical can contribute to the formation of larger polyynes, such as C₈H₂, C₁₀H₂, and C₁₂H₂, through the reactions C₄H + C₄H₂ → C₈H₂ + H,⁶¹ C₄H + C₆H₂ → C₁₀H₂ + H,⁶² and C₄H + C₈H₂ → C₁₂H₂ + H,⁵⁵ respectively.

However, the velocity-screened product spectrum in Fig. 5(b) shows that C₄H is not produced as a filament product, which would otherwise appear as a narrow peak at m/z 49 in the mass spectra.⁶⁴ This suggests that the pathways involving C₄H have minor contributions to the association reactions in laser filaments. Instead, the C₆H radical observed in the product spectrum implies other pathways leading to C₁₀H₂ by C₆H + C₄H₂ → C₁₀H₂ + H and to C₁₂H₂ by C₆H + C₆H₂ → C₁₂H₂ + H⁵⁵ as both C₄H₂ and C₆H₂ are observed in the spectrum. Further investigation of these reaction pathways is needed, but previous studies of closely related systems involving C₄H radicals reacting with small polyynes indicate that radical-addition to C₄H₂ and C₆H₂ would be essentially barrier-less reactions readily leading to to chain growth of polyynes. Another pathway was suggested to form a smaller polyyne C₈H₂ via C₆H + C₂H₂ → C₈H₂ + H,⁵⁹ though C₂H₂ is not confirmed as the product by the present study due to the overlap with the probe product C₂H₂⁺ from hexane (see Fig. 3). The previous crossed-beam studies also show that hydrogen-capped polyynes containing an odd number of carbon atoms can be formed, for example, by C₃H + C₆H₂ → C₉H₂ + H.⁶² However, the product spectrum (Fig. 5) obtained shows that C₉H₂ is not produced in the present study. This is consistent with the absence of C₃H in the product spectrum after the velocity discrimination.

Association reaction with metastable C₄H₂* can be another reaction routes to polyynes as discussed in a gas flow reaction study.⁶⁵ For example, C₈H₂ was identified as a reaction product between C₄H₂* and C₄H₂.⁶⁵ Since the peak with m/z 50 identified in Fig. 5 can be assigned to both C₄H₂ and C₄H₂*, such reaction may contribute in forming C₈H₂. The formation of C₇H₆ was also observed in the reaction between C₄H₂* with C₃H₆, which may also contribute in forming products other than hydrogen capped polyynes in the present study. Unfortunately, C₃H₆ is not confirmed as a filament product in the present study, as the mass peak overlaps with C₃H₆⁺ formed by the dissociative ionization of hexane by the ionization laser pulse.

Anionic intermediates, which evade observation by the present experimental setup, may also contribute to the product distribution. It was reported⁶⁶ that negative ions are formed in aerial laser filament. In the present case, C₆H⁻ might be formed in the filament because of the large electron affinity (3.8 eV).⁶⁷ The previous crossed beam experiments have shown that C₆H⁻ would react with C₂H₂ to form C₈H⁻, C₆H⁻ + C₂H₂ → C₈H⁻ + H₂,⁶⁸ which may undergo subsequent reactions such as ion–ion recombination or ion-neutral associative detachement⁶⁹ to form neutralized C₈H₂. The contributions from such anionic carbon chain growth requires further studies to understand the filament induced reactions.

4 Summary and outlook

The present study demonstrated the time-of-flight mass spectrometry of the products from femtosecond filaments in hexane, where the velocity screening proved to be a powerful approach to securely distinguish the filament neutral products from the probe products. Direct sampling from the reaction cell allows for identification of various intermediates and products, including hydrogen-capped polyynes, C_nH₂ (n = 4, 6, 8, 10 and 12) as well as other hydrocarbon molecules that escaped from identification in the previous studies.

Possible routes to the formation of the hydrogen-capped polyynes in laser filaments are discussed, including collisional reaction among the fragments. The synthesis of hydrogen-capped polyynes C_nH₂ has attracted wide interests in both materials science^70,71 and astrochemistry,⁷² where chain growth reactions C_2nH + C_2mH₂ → C_2m+2nH₂ + H are proposed to be responsible for the synthesis of polyynes. Present study suggests that C₆H and C₈H radicals or possibly metastable/anionic species play more important roles than C₄H in the filament-induced reactions to large hydrogen-capped polyynes (n = 8, 10 and 12). Further investigation on the reaction of different molecular species would elucidate the characteristic features of laser filament reactions in more depth.

Strong laser fields offer a unique means of controlling chemical reactions through their electric-field waveforms as discussed in Introduction. In particular, closed-loop optimization employing mass spectrometry provides a powerful approach to tailoring the waveform, with reaction dynamics at each step monitored by product distribution, as previously demonstrated for unimolecular reactions.^18,19 The present velocity-resolved ion mass spectrometry constitutes an ideal tool for optimizing the yields of neutral products, thereby providing a pathway to efficiently control both unimolecular and many-body chemical reactions via laser waveform shaping.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this article are not publicly available upon publication because it is not technically feasible and/or the cost of preparing, depositing, and hosting the data would be prohibitive within the terms of this research project. The data are available from the authors upon reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5cp03867g.

Acknowledgements

We thank Misato Sawaki and Chiaki Kubo for their contribution in the early stage of the experiments and Makoto Yamada for valuable discussion on the velocity-screening mass spectrometry. The present study was supported by JSPS KAKENHI grant numbers JP26620004, JP17K05094, and JP21K03521.

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