Dynamics of Molecular Negative Ions Unravelled
27 February 2012
Electron collision with atoms and molecules is one of the most efficient ways of transferring kinetic energy into potential energy thereby initiating and enhancing chemical reactions. This is at play in a variety of natural processes and in laboratory as well as industrial applications. While electron impact ionization is an important channel in this energy transfer process, other inelastic processes dominate at low energies where ionization is energetically disallowed. A wide variety of processing plasmas used for various industrial purposes like lighting, semiconductor etching, plasma assisted chemical vapour deposition, gas lasers, pollution control and nanolithography are examples where all these electron induced processes play a decisive role. It is also realized that the electron induced processes are the inevitable links in the creation of molecules, including biological molecules in interstellar medium and radiation damage in biological systems. For example, the initial atomic or molecular process is ionization when highly energetic charge particles or gamma rays interact with a medium. Through the cascade of ionization by the ejected electrons, the number of free low energy electrons increase multifold. These low energy electrons are very efficient in producing negative ions, atoms and radicals, and vibrationally and electronically excited molecules. Being highly reactive, these species readily take part in further chemical reactions. In many processes induced by electrons, they are not just mere carriers of energy but they can induce reactions which cannot be triggered using photons of the same energy as well as those transitions which are spin forbidden. The process that dominates the creation of these reactive species when a low energy electron interacts with a molecule is the formation of electron-molecule resonances, which are excited states of molecular negative ions and are transient in nature.
The decay dynamics of these transient molecular negative ion states determine the nature of the reactive products. This transient ion may decay either through ejection of the extra electron (called autodetachment) or through dissociation. The decay through autodetachment generally leaves the molecule in excited vibrational and/or electronic state. The decay through dissociation produces a stable negative ion and one or more neutral radicals. Thus both the decay processes, starting from electron attachment, produce molecules with excess internal energy or radicals and negative ions all of which are chemically very reactive. Since the nature of the chemical reaction will be dependent on the reactants, the decay dynamics of the transient molecular negative ion play the decisive role in determining the final products. Electron energy loss spectroscopy has been used to determine the vibrational and electronic excitation of the molecules. The resonant attachment followed by dissociation of the molecule which is called dissociative electron attachment (DEA) has been studied using negative ion mass spectrometry. Both these channels have been studied extensively in the last century. The discovery at the turn of the century that low energy electrons play a crucial role in radiation damage of DNA by DEA mechanism renewed the interest in the field of low energy electron interaction with molecules.
At TIFR, we have shown that this competition between the two modes of decay of the transient molecular negative ion provides selectivity in the bond cleavage . Here, the electron energy is used as the parameter that controls the dissociation pattern. It is a well-known fact that the selectivity in the bond cleavage in a molecule can be achieved on the basis of how much energy is provided to it. This is based on the fact that depending on their strength, various bonds in a molecule require different amount of energy to break them. However, the most interesting aspect of our discovery of site selective cleavage of bonds using low energy electrons is the existence of this selectivity at energies considerably higher than that required to break any of the individual bonds. For example, in a molecule like acetic acid (CH3COOH), the minimum energy required to break the O-H bond by electron attachment is 4 eV and that for the C-H bond is 3.5 eV. But the selectivity seen in breaking these bonds is at energies larger than 6 eV (at 6.5 eV only O-H bond breaks and at 9.5eV predominantly C-H bond breaks).
In order to use this selectivity towards controlling any further reaction, it.s important to study the molecular dynamics that causes this effect. It is also important to know the underlying dynamics for fundamental understanding of the process. To that end, we have developed a technique of momentum imaging called Velocity Slice Imaging (for low energy electron interaction) which is an adaptation of the well-known Velocity Map Imaging (VMI) . In the VMI technique , fragment ions formed in the interaction volume (here the region of the electron and molecular beam overlap) are extracted, mass analysed using specially built time of flight mass spectrometer and detected on a two-dimensional position sensitive detector. The mass spectrometer provides the electrostatic fields that forces the ions with a specific set of velocities, hit at a fixed point on the detector irrespective of their point of origin. The velocity (magnitude and direction) distribution (known as the Newton sphere) of the ions appears as a disc on the detector if their time of arrival is not recorded. The original distribution of the Newton sphere of the product ions at the time of their formation is obtained by inverting the patterns obtained from two-dimensional image (the VMI data) using appropriate inversion programs. These programs use the fact that the collision interaction has a cylindrical symmetry about the projectile (here electrons) propagation direction. This technique employs a DC electric field in order to extract the ions from the interaction region and has been in use in photodissociation and photoionization experiments.
In the technique that we developed, the ions are made to fall on the detector in the similar pattern as that in VMI. However, the time of arrival and position of each ion is recorded separately. Since we use low energy electrons as projectiles, the use of DC electric field for extracting the ions from the interaction region is ruled out. This forces us to use the electrons in a narrow but well defined pulse, followed by a pulsed electric field. However, we have converted this handicap to our benefit. In order to record the time of arrival of the ions accurately across the Newton sphere, it needs to be stretched along the flight direction. We achieve this by using a time delay between the electron pulse and the ion extraction field. From the individually recorded position and time of arrival of the ions, the entire Newton sphere is directly read out.
The results of these measurements are the momentum distribution patterns for fragment ions shown in the figure. Here the square of the radial distance of a point in the pattern gives the measure of the initial kinetic energy of the ions. The colour of a pixel indicates the number of fragment ions formed with that specific kinetic energy and ejected in that particular direction. From such images, we determine the initial kinetic energies of the ions formed as well as the direction in which they get ejected with respect to the direction of the incoming electrons. The distributions of these kinetic energy values and the directions of the fragment ejection (also called as angular distribution) are used to understand the dynamics of the dissociation of the parent negative ion by further analysis.
By imaging the momentum distributions of the H- and O- fragments using this technique as shown in the figure, the dynamics of the DEA process in water has been unravelled to a great extent. For more information, please refer to .