Dissociation dynamics of ion-pair states accessed by low-energy electron collisions

Ion-pair (IP) states are the superexcited states of the neutral molecule that dissociate as a pair of positive and negative ions. These states are generally present near the ionization continuum of the molecule. IP states can be accessed by using photoexcitation or by using the electron collision technique. Different photoexcitation techniques are widely used over the years to study the threshold (threshold IP production spectroscopy) and the dynamics (IP imaging spectroscopy) of the IP states. However, the electron collision technique is ignored over the years and only a few studies are available. In this review, we will discuss different experimental techniques to probe IP states by using electron collision and also the dynamics of the IP states that are accessed by the electron collision.


Introduction
Ion-pair (IP) states are the super-excited states of molecules embedded in the ionization continuum. IP production spectroscopy (through photoexcitation or electron collision) is the tool to understand the electronic dynamics (excitation and the corresponding decay processes) of neutral molecules at energies near and above the ionization energy of the molecule (roughly >15 eV) [1]. These states can be accessed directly * Author to whom any correspondence should be addressed.
Original Content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. or through coupling with some intermediate Rydberg states or other super-excited states [2]. The IP formation and its subsequent decay process, by producing a cation and an anion, have been studied quite extensively over the past few decades [2,3]. The photoexcitation spectroscopy was the most popular technique in this subject. Photo-induced IP formation has been studied by using discharge lamps and synchrotronbased vacuum ultraviolet (VUV) light sources [4][5][6][7][8][9][10][11][12][13]. Later the threshold energy of the IP states was measured accurately by employing threshold IP production spectroscopy [14,15]. In recent times, IP imaging spectroscopy provided detailed dynamics of the IP states by using a high-resolution laserbased coherent VUV source [16,17]. Nevertheless, though the employment of photoexcitation techniques has led to a broad understanding of both the IP formation processes and relative decay channels, some limitations in this field must be pointed out. Indeed, photoexcitation studies are typically limited to probing dipole-allowed transitions only, while the probability of quadrupole and higher-order transitions are vanishingly small. As a result, it is very difficult, if not impossible, to access several existing IP states using optical techniques. Also, for molecules where the ionization energy of the parent is lower than the IP threshold, direct photoionization is always the dominant outcome at energies at which IP products may be accessed. As a result, the IP formation is nearly always a minor channel (0.1% or less) [1]. On the other hand, the same IP states of the molecules can be accessed by using electron collision. In the electron-molecule collision studies, the dissociative electron attachment (DEA) and the IP dissociation (IPD) are the only two processes where the fragment negative ions are formed [18][19][20][21][22]. However, there is a basic difference between these two processes; unlike DEA, the anion formation due to IPD does not proceed via resonant electron capture. In fact, in the IPD channel, the incident electron partially/completely transfers its kinetic energy to the molecule and excites it to the IP state. The IP formation is possible as long as the electron energy is equal to or higher than the asymptotic IPD energy. The IP formation by using electron collision has been studied by different groups over the years [20,[23][24][25]. Nevertheless, due to the lack of experimental tools able to probe the dynamics associated with the formation and decay of IP states, such early electron-collision studies mainly focused on determining the energy threshold of the IP process. The very first attempt in this direction was made by Van Brunt and Keiffer in their study on Oxygen molecules [26]. By measuring the angular distribution of the O − ions produced due to the IPD, the authors reported the symmetry of the IP states involved in the process. Their theoretical model established the relation between the symmetry of an IP state with the corresponding angular distribution of the fragment ions [27]. However, the angular distribution measurements of the fragment ions formed due to an electron-molecule collision process were tricky in the earlier days. The turn table setups were used to get the angular distributions of the ions. Although accurate results could have been obtained using this technique, the efficiency was too low. The development of velocity map imaging (VMI) and the velocity slicing (VSI) technique made the situation easy and soon became a well-established benchmark in this field [28]. Using the VSI technique, one can simultaneously measure the kinetic energy and angular distribution of the fragment ions produced from the dissociation process, which eventually helps to understand the dynamics involved in the electron-molecule collision processes. Nowadays, the VSI technique has been employed extensively over the years by several groups to study the dynamics of DEA [29][30][31][32]. However, as the focus of these studies was on the DEA process, the usual approach adopted by the scientific community was that of neglecting the IPD process. To the best of the authors' knowledge, only four reports are available on the dynamics of IPD using the VSI technique to date [22,[33][34][35].
In this review, we will first discuss the widely used experimental techniques to probe IP states by using electron collision. The following sections will briefly elucidate the dynamics of the IP states accessed by the electron collision for three different categories of molecular systems: heteronuclear diatomic (CO), homonuclear diatomic (O 2 ), and polyatomic (CO 2 ) molecules.

Experimental techniques
Several experimental techniques have been used over the years by different groups to study electron-molecule collisions. Discussing all those is beyond the scope of the present review hence only a few of them are discussed.

Residual gas analyzer
Residual gas analyzers are mainly designed for mass spectrometric studies and to measure the ion yield of fragment ions generated from different electron-molecule interaction processes. The system is equipped with a small vacuum chamber along with a high-resolution mass spectrometer (e.g. quadrupole mass spectrometer [36]), an electron source, and an effusive molecular beam. Generally, a resistively heated filament with a combination of a few electrodes are used as an electron gun (typical energy resolution is around 0.6 eV). In some studies, the electron gun is replaced by an electron monochromator that guarantees a higher energy resolution [18]. In this system, the electron-molecule collisions occurred in the interaction region and the fragment negative ions are collected by the detector. In some studies, the effusive beam is replaced by supersonic expansion and molecular clusters [18]. This type of system is widely used for mass spectrometric studies, resonant energy, and threshold energy measurements of different molecular fragments.

Turn-table experiment
The ion yield curve, kinetic energy, and angular distribution of the fragment ions can be obtained by adopting a turn-table experimental setup. Figure 1(a) shows the schematic of a turntable experiment. The interaction of a well-collimated electron beam with a molecular beam in a crossed-beam arrangement produces ions in a field-free region. These ions are then drifted with their initial velocity through the spectrometer assembly and collected by the detector. During the angular distribution measurements, either the spectrometer assembly or the electron gun assembly is movable and the other remains fixed. In most of the turn-table experiments, the spectrometer is in a fixed position, while the electron gun is rotating with respect to the spectrometer axis, allowing to collect the ion signal in the laboratory angular ranges (160 • ⩽ θ ⩾ 20 • and 340 • ⩽ θ ⩾ 200 • ). Due to the physical size of the electron gun and detector, complete 0 • -360 • angular measurements are not possible in this system [37]. Although by using a localized magnetic field in the interaction region, this difficulty was overcome, it was not efficient [38,39]. By using an energy analyzer, the spectrometer is tuned to select ions of a particular energy [37]. The ions that enter the spectrometer from the interaction region are accelerated and focused into an electron multiplier. The electron multiplier produces an electron cloud that is collected by the collector plate as a signal. These signals The electron gun assembly consists of an electron gun and a collector plate which is aligned on the same axis. During the angular distribution measurements, the electron gun assembly rotates to different angles with respect to the fixed spectrometer axis. (b) Schematic of a velocity map imaging spectrometer, showing also the electron gun, Faraday cup, and the capillary array that produces the effusive molecular beam (Reprinted from [28], with the permission of AIP Publishing.).
are then amplified and counted with a sealer. By using different types of analyzers, the kinetic energy of the fragment ions is measured. As a major drawback, the poor ion detection efficiency of this technique makes it unproductive for molecules with low IPD cross-section.

VMI
The schematic of a simple VMI spectrometer is shown in figure 1(b). This is one of the most advanced systems for electron-molecule collision studies and is widely used by several groups [28,31,40]. The basic procedure in the experiment entails the interaction of an electron beam and a molecular beam, where one of them is in pulsed mode. A molecule in the molecular beam can be oriented in any arbitrary direction in free space. During the cross-beam electron-molecule interaction, fragment ions produced by the DEA/IPDs are contained within an expanding sphere, referred to as a 'Newton Sphere'. By applying a suitable extraction pulse into the pusher plate, the total Newton Sphere is extracted from the interaction region and projected into the position-sensitive detector (PSD). The spectrometer is designed to maintain the VMI condition, i.e. all the ions with a given velocity vector are mapped onto a single point on the detector regardless of their origin. The VMI technique allows to establish a direct relationship between the image radius (r) and the fragment ions' kinetic energy (kinetic energy ∝ r 2 ). It is an adaptation of time of flight mass spectrometry, and as such also has the ability to separate, and selectively detect ions by mass. This ion imaging technique was first invented by Chandler and Houston where the authors used a 2d PSD to map the kinetic energy of the fragment ions [41]. In contrast to the conventional time of flight (TOF) method (where the ion kinetic energy is measured in a temporal structure), this ion imaging technique is capable of simultaneously measuring the kinetic energy of the fragment ions along with its 360 • spatial angular distribution. A decade later, Eppink and Parker introduced an updated VMI technique [42]. The authors removed the grid electrodes, replacing them with circular ring electrodes which act as electrostatic lenses. The advantage of this setup over the previous design by Chandler and Houston is the removal of the distortions and transmission losses arising from the grid electrodes. The simplest VMI design requires three electrodes and the resulting electric field structure is inhomogeneous which allows mapping of the same velocity ions in the same position on the detector, irrespective of their origin in the interaction region. Better performances can be achieved with more sophisticated ion-optic designs containing more electrostatic lenses [43]. Abel inversion or back projection method is used to reconstruct the complete three-dimensional information of the Newton Spheres from the 2D image [44]. However, due to the Abel inversion, the kinetic energy resolution is ultimately limited by the quality of spatial mapping guaranteed by the detection system. Furthermore, the reconstructed images contain some noise along the symmetry axis. To overcome this problem, a time-slicing method has been proposed by Gebhardt et al [45] In this technique, only the central time slice of the Newton sphere is collected by applying a suitable time gate into the detector. The initial kinetic energy and angular distribution of the fragment ions can be extracted from this central slice, which is parallel to the detector plane and perpendicular to the applied extraction pulse. Later Townsend et al [46] introduced DC slicing, where all the spectrometer voltages, including the first MCP voltages, were kept constant during the ion extraction. This slicing technique is then adapted by Nandi et al for the first time to study the DEA dynamics and later IPD by electron impact [28]. However, time slicing has a few limitations. It is inefficient when the ion count rate is low and when the ions have a tiny time spread. In that case, the Abel inversion is a better option and is widely used [30]. (b) Energy diagram representing the potential energy curves to understand the mechanism involved in the IP dissociation of CO [51] (Reproduced from [51]. © IOP Publishing Ltd. CC BY 3.0.).

Reported IPD dynamics of different molecules
Several IPD studies through electron collision have been reported over the years by several groups [20,[23][24][25]47], although most of them are focused on determining the IP threshold only. Recently, Szymanska et al studied the fragment anion formation in the electron collision with ethylene [25]. They investigated the negative ion formation for an electron energy range of 0-90 eV and found the DEA process for electron energy between 5 and 17 eV, while the IPD occurs beyond 13 eV. The authors used two different experimental setups for their experimental investigation, (i) TOF mass spectrometer operating with the VSI technique, and (ii) a twosector field mass spectrometer. The authors found the pres- anionic fragments in the IPD region. In order to determine the experimental threshold of the IPD process, they fit the ion yield curve with equation (1b) and compared it with the thermodynamically calculated appearance energies. The following section describes the dynamics of the IP states of three different molecules.

Heteronuclear diatomic molecule: CO
The IP states of CO were accessed by electron collisions quite long ago by Vaughan [48] and Lozier [49]. The authors reported the excitation function of the O − ions, which was well enough to locate the threshold energy to access the IP states in the Franck-Condon transition window. Lozier [49] compared the intensity of C + and O − ions in the IPD energy range and found the IP threshold at 20.9 ± 0.1 eV. However, the dissociation mechanism of the IP states of CO molecules has been reported only recently [33]. By using the VMI technique, the authors measured the kinetic energy and angular distribution of the O − ions over the IPD energy range. Through the analysis of the kinetic energy distribution, they proposed the excitation and dissociation mechanism involved in the process, whereas, studying the angular distribution, they provided the symmetry of the IP states. Figure 2 represents the kinetic energy distribution of the O − fragments formed due to the IPD process at different electron energies. Two different kinetic energy bands are observed for all the incident electron energies. One strong peak near 0 eV, followed by another broad band between 0.7 and 2.0 eV are observed. With increasing electron energy, the lower kinetic energy band remains unchanged whereas, for the higher kinetic energy band, an initial increase is observed. The authors described this behavior with the involvement of two different excitation mechanisms, direct excitation and/or indirect excitation to the IP state. By using the potential energy curves, the authors attributed the low kinetic energy ions to the indirect excitation to the IP states and the higher kinetic energy ions to the direct excitation to the IP states. By calculating the total kinetic energy released by the dissociation process, the authors reported the appearance energy for the direct excitation to the IP state. From the measured kinetic energy of the O − ions (1.5 eV kinetic energy peak from figure 2) and by using the conservation of linear momentum, they identified that in the FC transition window, the IP state lies 24.4 eV above the ground state of the CO molecule. Due to the partial energy transfer from the incident electron to the CO molecule, this IP state can be accessed directly, as long as the energy of the incident electron beam is more/equal to 24.4 eV. However, the reported excitation function of the O − ions indicates that IPD starts near 20 eV electron energy, which is almost 4.4 eV below the identified location of the IP state in the FC transition window [20]. This suggests the involvement of an indirect excitation mechanism where the IP state is accessed through the coupling of the excited parent Rydberg state of the CO molecule. The same mechanism is observed for molecules in the photoexcitation studies, where the parent molecule is excited to the Rydberg states that converge to the ground or electronically excited states of the ion.
These may be directly or indirectly coupled to the IP states in the IPD limit. Excitations below the IP threshold may give rise to long-lived pseudoatomic states where the anion acts as a heavy electron and the system resembles a Rydberg hydrogen atom [50]. For both photoexcitation and electron collision, a similar mechanism is observed. The O − counts below 24.4 eV electron energy in the excitation function, and the unchanged nature of the near zero O − kinetic energy band with increasing electron energy, strongly suggests the involvement of indirect excitation to the dissociative IP state. Thus, the molecule is first excited into a Rydberg state through a complete or partial energy transfer that crosses the IP state at the IPD limit.
Here, at the crossing point, the covalent bond of the associated Rydberg state transits into an ionic bond via electronic charge transfer from the carbon to the oxygen atom at a large interatomic distance. This results in IP formation through the predissociation process of the molecular Rydberg state, while the dynamics of the IPD are restricted by the degree of coupling between the initially excited Rydberg state and the IP state. According to the selection rules, the IP state and the predissociated Rydberg state should contain the same symmetry [52].
In figure 2, the presence of low kinetic energy ions for all electron energies indicates that the predissociation process occurs throughout the entire electron energy range. The higher kinetic energy band of the O − ions has a different nature compared to the lower kinetic energy band. In figure 2, for 25 eV incident electron energy the higher kinetic energy peak is observed at 1.2 eV. With increasing incident electron energy, the kinetic energy peak shifted to 1.5 eV before it attains a locked behavior. The initial increase of the ion kinetic energy with increasing electron energy is due to the access to different repulsive parts of the IP state. The locked kinetic energy nature is explained in two possible ways by the authors, (i) with increasing incident electron energy, the IP state being out of the FC region, and due to partial energy transfer from the incident electron to the molecule, the ground state molecule excites to the same IP state. (ii) It could be due to excitation to a higherlying Rydberg state by complete or partial energy transfer that crosses the IP state and triggers the indirect excitation process. The authors summarized the results as, near the threshold, the IP formation process occurs mainly via the indirect excitation process, whereas the direct excitation starts near 25 eV incident electron energy. Another indirect process may get involved beyond 25 eV. Both the indirect and direct excitation mechanisms dominate over the complete IP energy range through the partial energy transfer from the incident electron to the molecule. Later, the authors extract the angular distribution of the O − ions for both the kinetic energy bands and fit it with equation (2). The fitted angular distribution indicates that the symmetry of the IP states involved in the process is Σ and Π.

Homonuclear diatomic molecule: O 2
Most of the IPD dynamics of the O 2 molecule have been studied mainly around the IP threshold region using lasers [53][54][55]. These studies indicate that the conical intersection between a Rydberg state and a repulsive IP state is responsible for the IPD around the threshold. However, there was no information available for the IP states that are accessed by electron collisions. In a theoretical paper, Van Brunt presented a model that establishes the connection between the symmetry of the IP states and the angular distribution of the fragment ions [27]. This pioneering work provided a crucial tool for understanding the dynamics and symmetry of the IP states accessed by electron collision. The dissociation dynamics of O 2 IP states accessed by electron collisions were first reported by the same group [26]. By using a turn-table experimental setup (as described in section 2.2) the authors measured the kinetic energy and angular distribution of the O − ions. The authors found three kinetic energy bands peaking at 0, 1.9, and 3.36 eV, respectively. Later, they measured the angular distribution of the O − ions for the 1.9 and 3.36 eV kinetic energy bands. They fitted the experimentally measured angular distribution with their theoretical model and found that the Σ u and Π u symmetry IP states are responsible for the 1.9 and 3.36 eV ions, respectively. The first attempt to study the electron impact IPD using the VSI technique was made by Nandi et al [57]. However, the authors did not discuss the dynamics involved in the process. Later, Nag and Nandi [22,56] studied the IPD dynamics of O 2 molecule. The authors reported the ion yield of the O − ions produced due to low-energy electron collisions. Figure 3(a) shows the ion yield curve of O − ions from electron collision with gas phase O 2 molecule. A resonant peak due to the DEA at 6.5 eV, and almost continuous ion formation due to IPD after nearly 15.3 eV, are observed. To determine the threshold of the IPD process, they fitted the rising part of the ion yield curve ( figure 3(b)) by the equation given by Fiegele et al [58]: Here, the parameter b represents a constant for the background correction, while a is the scaling factor that is set to zero beneath the threshold. E th stands for the threshold energy of the IPD process and n is the exponent of the fitted curve. The authors found the parameter values as a = 44.27, b = 42.91, and n = 1.435 for the best-fitted curve. From this fitted value, the authors reported the experimental threshold (E th ) of the IPD process at 15.3 eV. However, the thermochemically obtained threshold for the IPD process is 17.25 eV. According to the authors, this discrepancy is due to the energy resolution of the electron beam used in the measurement, which is 0.6 eV.
In this study, the kinetic energy and angular distribution of the O − ions over the IPD energy range were also investigated. Figure 4(b) represents the kinetic energy distribution of the O − fragments for different electron energies. Analogously to the one found in the previous case for the CO molecule (3.1), two different kinetic energy bands were found for all the incident electron energies. One strong peak near 0 eV, followed by  another broad band peaking at the higher energy, are found. With increasing electron energy, a shift in the higher kinetic energy peak is observed as well, before it attains a locked kinetic energy behavior. However, the near-zero kinetic energy peak is present for the whole IPD energy region. This finding is similar to what was observed previously in the IPD of CO and the authors explained the dynamics of O 2 in a similar way. According to the authors, an additional kinetic energy peak near 2.5 eV starts appearing above the 35 eV incident electron energy. This may be the same third band found also by Van-Brunt [26]. The extremely low intensity of this third peak could be due to the employment of the flat-slicing technique which gives more weightage to the low kinetic energy ions. In comparison with the CO molecule, the authors explained this behavior with the involvement of two different excitation mechanisms, direct excitation and/or indirect excitation to the IP state. The low-kinetic energy ions are due to the indirect excitation to the IP states (through predissociation of an initially accessed Rydberg state) and the higher kinetic energy ions are due to the direct excitation to the IP states. From the kinetic energy distribution, the authors reported the location of the IP state around 19 eV above the ground state in the Franck-Condon transition region. It has been also stated that the O − ions which are created with higher kinetic energy might also be generated via direct excitation to an IP state.
In order to determine the symmetry of the IP states, the authors measured the angular distribution of the O − ions and fit it with the expression provided by Van Brunt [27] (identical to the expression derived by Zare [59]) and used by Chakraborty et al [33] in the CO study.
Here, K is the momentum transfer vector between the incident and scattered electron. κ denotes the product of the momentum transfer vector, K, and the distance of the closest approach between the impinging electron and the center of mass of the molecule. The j l (κ)s are spherical Bessel functions and Y µ l (θ, ϕ) are spherical harmonics. If Λ i and Λ f are the projection of electronic axial orbital angular momentum on the molecular axis for the initial and the final states, respectively, then, µ = |Λ f − Λ i |. Dealing with homonuclear molecules, the angular momentum quantum number l ⩾ |µ| is restricted to have only even or odd values depending on whether the initial and final molecular states have the same or opposite parity. From the fitted angular distribution data, the authors found that IP states with symmetry Σ u and a Π u are present in the entire energy range. This observation is in agreement with the measurement of Van Brunt where they found that an IP state with symmetry Σ u is responsible for the formation of O − ions with kinetic energy 1.9 eV. The authors observed the involvement of a Π g state with increasing incident electron energies. In a recent report Larsen et al [60] discussed the photodissociation dynamics of IP formation in O 2 by using the twophoton absorption technique. By measuring the angular distribution of the O + ions, the authors confirmed the presence of IP states with symmetries 3 Σ − g and 3 Π g in the IP region. The IP states reported by Larsen et al [60] are not observed by Nag and Nandi [22,56] or by Van Brunt and Kieffer [26]. This is not surprising due to the fact that the photoexcitation and the electron collision processes are restricted by different selection rules and so are the IP states accessed by them. It's not possible to access all the IP states of a molecule by using a single excitation technique. This warrants the importance of the electron collision technique further. In the most recent study on the IPD of O 2 by Kundu et al [35], the authors used a wedge-sliced technique [31,61]. They found the involvement of quintet Rydberg states with the dissociative pathways of the superexcited states. Predissociations of different Rydberg states, along with direct dissociation, have also been confirmed.

Triatomic molecule: CO 2
There is only one study present to date on the electron impact IPD of CO 2 molecule. The authors measured the ion yield curve of O − ions produced from IPD of CO 2 molecule by the process [34] In this study, the authors used equation (1b) and fit it with the rising part of the measured ion yield curve to determine the threshold of the IPD process (similar to what was done in the O 2 molecule [22,56]). The reported fitting parameters for the best-fitted curve are, a = 29.557 15, b = 26.069 94, and n = 1.549 with the threshold of the IPD process, E Th = 18.1 eV. The authors measured the kinetic energy and angular distribution of the O − ions over the IPD energy range. Figure 5(b) represents the kinetic energy distribution of the O − fragments for different electron energies. Unlike the diatomic molecules, the authors could not observe two different kinetic energy bands here. One possible explanation could be that the excess energy is distributed as the internal excitation (rovibrational excitation) of the neutral conjugate (CO), which was not possible for the dissociation of a diatomic molecule like CO and O 2 . This type of excess energy redistribution was also found in low-energy DEA to triatomic and other polyatomic molecules [31,62]. From the variation of most probable kinetic energy with the incident electron energy, the authors found that the ion kinetic energy first increases with the increasing electron energy and then gets locked. By comparing this finding with the IPD dynamics of the CO molecule [33], they described this locked kinetic energy behavior as discussed in section 3.1. By using quantum chemical calculations, they found that a significant amount of the IPD results from a direct transition to the IP state, along with the indirect transition. The authors extracted the angular distributions of the O − ions from the sliced images for different incident electron energies. They found that the number of scattered O − ions in the forward direction is slightly greater than in the backward direction. Since the CO 2 molecule is linear and thus possesses inversion symmetry, this type of forward-backward asymmetry is unexpected. The authors argued that this asymmetry is due to the linear momentum transfer effect consideration during the laboratory frame angular distribution [63]. The authors also observed a decrease in the angular anisotropy with the increasing incident electron energy. In the study of the O 2 molecule, Nag and Nandi [22] also found the gradual loss of anisotropy in the angular distribution of the anions with increasing incident electron energy. Here the authors argued that the anisotropy is due to the K dependence of the angular distribution as predicted by Zare [59]. The authors measured the angular distribution of the low kinetic energy O − ions for different incident electron energies and fitted them with the expression derived by Van Brunt (equation (2)). From the fitted angular distribution, the authors reported that the possible symmetry of the IP states near the IP threshold region is Σ and Π.

Conclusions
The electron impact IPD technique is a novel method that paves the way to study the electronic dynamics of neutral molecules at energies near and above the ionization energy of the molecule. There are several IP states with different symmetries are present near the ionization energy of the parent, and those states may be coupled to each other or with some Rydberg states that are present in the same energy range. Transition to different IP states by the photons is constrained by certain selection rules, while this is not the case for electron collision. Hence, both electron collision and photoexcitation techniques are equally important for IPD studies. This review covers the recent experimental developments to study the IP states and the reported dissociation dynamics of different molecules. The review also shows that the excitation and dissociation mechanisms of the IP states are indifferent to both electron collision and photoexcitation techniques up to some extent. Near the IP threshold, the IP states are generally accessed through the predissociation of a molecular Rydberg state. However, it is also possible to access the IP states directly from the ground state of the parent and this direct excitation mechanism has a higher possibility for diatomic molecules compared to the polyatomic. The kinetic energy and angular distribution of the negative/positive ions formed due to the IPD provide information on both the location and symmetry of the IP states. Through the electron-collision technique, one can efficiently access the IP states that lie above the ionization energy of the parent molecule in the Franck-Condon transition window.

Data availability statement
No new data were created or analyzed in this study.