Hydrogen migration in triply charged acetylene Free

The process of intra-molecular migration of atomic and/or molecular hydrogen is a phenomenon that has been observed in a wide range of molecular species.^1–14 This type of rearrangement plays an important role in several research areas such as chemistry of combustion¹⁵ and interstellar medium (ISM),^16,17 radiation damage to biomolecules,^7,9 and proteomics¹⁸ and is, therefore, of common interest. Predicting the process of hydrogen migration poses a stringent test for theoretical methods, since, now in addition to stable initial and final states of molecular ions, the transient states and migration pathways have to be predicted, which play a significant role in determining the outcome.

The typical time scales for hydrogen migration processes have been determined to be of the order of tens of femto-seconds, using pump–probe experimental schemes along with coulomb explosion imaging (CEI) techniques.^19–21 It is to be noted that molecular dissociation time scales are of the same order. Particle impact and continuous/single pulse experiments with photons were the first to provide experimental evidence for this process^22–24 and, subsequently, theoretical calculations paved the way toward understanding this phenomenon. With the advancement of experimental schemes, time resolved studies of this process became feasible and quantitative evidence made it clear that the process happens on the same time scales as that of molecular fragmentation.

The acetylene molecule is an ideal prototype for investigating hydrogen migration. Several experimental (e.g., Refs. 19–21 and 25–31) and theoretical studies (e.g., Refs. 32–34) have been reported on the hydrogen migration process in this molecule. The present understanding of the process, even for the prototype acetylene, is far from complete, as can be inferred from Refs. 19–21 and 33, which show that the results obtained in different experiments do not always agree among themselves or with the theoretical calculations. Hydrogen migration thus continues to be an area of active research and will continue to be so until a reasonable common ground of understanding is reached.

The experimental investigations on this subject can be divided into two main categories based on the mode of excitation of the neutral. The first category involves excitation by particle impact and single pulse experiments using lasers or synchrotron radiation.^25–31 The second category belongs to experiments employing time-resolved pump–probe schemes.^19–21 In all these studies, the hydrogen migration process is reported to occur on the potential energy landscape of either the monocation $(C2H2+)$²⁰ or the dication $(C2H22+)$^19,21,31,35 of the acetylene molecule.

In the experiments that belong to the first category, the interaction of the neutral molecule with a photon/laser pulse or a particle is responsible for initiation of hydrogen migration. Most of these experiments have used the observation of the two-body breakup channel, $C2H22+→C++CH2+$⁠, as the direct signature of the hydrogen migration process.^26–30 Hishikawa et al.³¹ observed the process by looking at the three-body breakup of acetylene into (H⁺ + C⁺ + CH⁺) initiated by single laser pulses of femto-second duration. This breakup channel was shown to be populated by both concerted and sequential pathways when 35 fs pulses were used. Sequential processes were not observed when the pulse duration was reduced to 9 fs. Thus, it was shown that the laser pulse duration plays a crucial role in deciding the pathways taken toward dissociation in a single pulse experiment.

In the present article, we provide direct experimental evidence of isomerization of $C2H23+$ from acetylene to vinylidene configuration in interaction of the neutral parent with a slow highly charged ion. In our experiment, this was observed by measuring the momenta of all fragments associated with the three-body breakup channel shown in the following equation:

For this particular breakup channel, we report on three different dissociation pathways: (i) concerted breakup in an acetylene configuration (ii) concerted breakup in a vinylidene configuration, and (iii) sequential breakup. Concerted breakup implies the instantaneous dissociation of the trication, while the sequential pathway proceeds via the intermediate formation of $[C2H]2+$ as shown in the following equations:

We also estimate the branching ratio of these three pathways and the kinetic energy release (KER) associated with the unimolecular breakup of $[C2H]2+$ into (C⁺ + CH⁺). The suggestion of possible hydrogen migration in acetylene trication was put forth in the discussion of two earlier studies,^19,31 but no conclusive evidence was given.

In our measurement, Xe⁹⁺ ions with velocity ≈0.5 a.u. were made to interact with an effusive jet of neutral acetylene molecules under single collision conditions. The ions and electrons generated from this interaction were separated using an electric field of 40 V/mm and were detected by a position sensitive microchannel plate (MCP) and a channel electron multiplier (CEM) detector, respectively. The position sensitive MCP is located at the end of a time-of-flight mass spectrometer enabling the measurement of both the time-of-flight and position of recoil ions. The start trigger for the data acquisition (DAQ) was taken from detection of electrons by the CEM detector. The DAQ setup is equipped with a multi-hit time to digital convertor (TDC), which enables the measurement of time and position information for each ion of every event in coincidence. From the measured time-of-flight and position information, the momentum of each fragment ion was determined. More details on the experimental setup may be found in Ref. 36.

The possible contamination of the (H⁺, C⁺, CH⁺) breakup channel [see Eq. (1)] from coincident measurement of (H⁺, C⁺, ¹³C⁺) is eliminated by applying the condition of momentum conservation during data analysis. The coincident detection of (H⁺, C⁺, ¹³C⁺) is likely to come only from four body breakup of [H¹³CCH]⁴⁺ when one of the high energy H⁺ fragment is lost due to limited collection efficiency of our experimental setup. The requirement of momentum conservation also eliminates the contamination because of incorrect assignment of fragment masses, which may arise due to possible overlap of time-of-flights of C⁺ and CH⁺ fragments.

The three-body coincident data on the dissociation channel shown in Eq. (1) is analyzed using the method of native frames³⁷ to check for the possibility of sequential breakup via the two steps shown in Eqs. (2) and (3), respectively. In this method, the first step of the breakup is analyzed in the center-of-mass frame of $C2H23+$ and the second step is analyzed in the center-of-mass frame of C₂H²⁺. The KER distribution for the first step and the second step is determined independently and the angular correlation between the fragments generated in the two steps is established. The result of this analysis is shown in Fig. 1, which is a distribution of KER associated with the second step of the sequential breakup, referred to as KER₂, and the angle (θ) between the direction of the first two-body breakup and second breakup in their respective center-of-mass frames. For an intermediate molecular ion formed in the first step, $[C2H]2+$ in the present case, if the lifetime of the populated electronic state is much larger than its rotational period, the angular correlation between the momenta of the fragments formed in the two steps is lost. Thus, we expect to get a uniform distribution parallel to the θ-axis. As shown in Fig. 1, the uniform vertical distribution is the evidence of such a long lived state of $[C2H]2+$⁠. Concerted breakup processes appear within a limited range of angles.

The figure prominently shows two additional features located around θ = 20° and θ = 160°. To gain more clarity, we divided the distribution shown in Fig. 1 in two regions A and B based on the value of KER₂ and made two separate Newton diagrams for events contributing to the two regions. A Newton diagram provides a visual representation of the angular correlation between the momentum vectors of all three fragment ions. In our case, the momentum of the H⁺ fragment is taken as the reference and the relative momenta of C⁺ and CH⁺ are shown in the upper half and lower half of the figure, respectively. These Newton diagrams are shown in Figs. 2(A) and 2(B). The appearance of a semicircular arc like feature, as shown in Fig. 2(A), is a signature of the sequential mode of breakup and the localized high intensity areas observed in such diagrams are related to the concerted pathways.³⁸ 

From the Newton diagram shown in Fig. 2(B), we note that the angle between the momentum vector of the H⁺ fragment and C⁺ fragment takes two very distinct values, one centered around 25° and the other centered around 165°. It has been shown in earlier studies^19,31 that a smaller value of this angle is associated with the acetylene configuration and a larger value is associated with the vinylidene configuration. Thus, for the concerted breakup of $C2H23+$⁠, contributions from both acetylene and vinylidene configurations are clearly observed.

To get the branching ratio for the three breakup pathways, we have exploited the fact that the distribution from the sequential pathway is expected to be a uniform band parallel to the y-axis of Fig. 1. By taking the events within the θ range from 100° to 120°, which do not have any contribution from the concerted pathways, we can generate the complete set of events for the whole θ range. Then, by subtracting this complete distribution of sequential events from the distribution of all events shown in Fig. 1, we obtain the branching ratio for the three pathways. The branching ratio is measured to be of the order of 91%, 0.6%, and 8.4% for concerted breakup in the acetylene configuration, concerted breakup in the vinylidene configuration, and the sequential breakup pathways, respectively. Our experimental setup does not have complete 4π collection efficiency for energetic protons, and thus, these values for the branching ratio are only approximate.

The formation of the intermediate molecular ion $[C2H]2+$ has been seen earlier in ion-impact induced three body sequential breakup of acetylene trication, but its fragmentation was shown to yield the ion pair $(H++C2+)$⁠, which is different from what we see in our experiment. This fragmentation channel may be expressed as

This sequential mode of breakup [Eq. (4)] is seen only with Ne⁸⁺ ions having velocity ≈1.42 a.u.³⁹ and not when Ar⁸⁺ ions having velocity ≈0.35 a.u.⁴⁰ are used. In our experiment with Xe⁹⁺ ions having velocity ≈0.5 a.u., we do not observe this sequential pathway though we do see concerted three body breakup into $(H++H++C2+)$ fragments. In terms of ion–molecule interaction processes, the dominant mode of electron removal from the molecular target is ionization, for excitation with Ne⁸⁺ ions and electron capture for excitation with Ar⁸⁺ and Xe⁹⁺ ions.⁴¹ Thus, even within the same electron capture regime, the dissociation pathways are very sensitive to the projectile velocity as well as its charge state.

An ion–molecule collision experiment is a case of impulsive excitation. In our experiment, the projectile Xe⁹⁺ is moving at a velocity of ≈11 Å/fs and hence the distance covered by the projectile in each femto-second is much larger than the dimension of the molecule under study. To estimate the bond length doubling time of acetylene trication using the Coulomb explosion model, we performed a simple calculation assuming the initial geometry of the trication to be that of the neutral parent. The bond length doubling time for the breakup of $[C2H2]3+$ into (H⁺, C⁺, CH⁺) comes out to be ≈4.3 fs for the H–C bond and ≈14 fs for the C–CH bond. These time scales are much smaller than the characteristic time scales of H-migration reported in the literature.^19–21,33

As the charge state of a molecular ion increases, the probability of prompt Coulomb explosion gains priority over sequential breakup and possibly over hydrogen migration as well. The maximum charge state that can be achieved in acetylene prior to opening up a complete four-body Coulomb explosion channel is 3+. When a small molecule loses three or more electrons, it often undergoes prompt dissociation due to Coulomb repulsion and releases the excess energy in the form of kinetic energy of its fragments. Though several stable molecular dications are known (such as $N22+$⁠, CO²⁺, and $C2H22+$⁠), the same cannot be said for molecular ions having charge 3 or above. Hydrogen migration on a triply charged molecular ion is thus totally unexpected as in this process, the H-atom has to move from one molecular site to another and this takes a finite time. The triply charged molecular ion will have to retain its molecular composition until migration is complete for its signature to appear in an experimental measurement. In the present article, we have provided direct experimental evidence of hydrogen migration in acetylene trication. It is quite probable that the process of hydrogen migration is active even in higher charge states for larger molecules, but at present, this is an unexplored domain.

In conclusion, we have provided the first clear experimental evidence of hydrogen migration in triply charged acetylene formed in the interaction of the neutral parent with a slow highly charged ion. Our experimental results show that the three body dissociation of $C2H23+$ into (H⁺ + C⁺ + CH⁺) occurs via three pathways: (i) concerted breakup in an acetylene configuration, (ii) concerted breakup in a vinylidene configuration, and (iii) sequential breakup via formation of a C₂H²⁺ intermediate. The branching ratio for these three pathways and the existence of a metastable state for the $[C2H]2+$ dication are reported. The importance of specific characteristics of the excitation source, the pulse duration for laser pulses, and the velocity and charge state for ion-impact, in determining the dissociation pathways, is highlighted. Hydrogen migration is, thus, not limited to singly or doubly charged molecular ions of acetylene but can happen even for triply charged acetylene, the highest charge state that can be reached before complete four-body Coulomb explosion becomes possible.

The authors are thankful to the staff of the Inter University Accelerator Centre and the Low Energy Ion Beam Facility (LEIBF) group for providing all the necessary facilities. J.Y. thanks the Council for Scientific and Industrial Research (CSIR), India, for providing financial support. J.R. acknowledges the Department of Science and Technology, Government of India, for financial assistance (Grant No. CRG/2018/001165).