Research

Current research interests

1. Understanding elementary steps in surface chemistry using quantum state selected molecule-surface scattering experiments
2. Developing tools and diagnostics for high-resolution spectroscopy and quantum state-resolved scattering experiments
3. Developing atom scattering-based microscopy techniques

(1) Understanding elementary steps in surface chemistry

The main objectives here are to look into the energy transfer pathways in molecule surface collisions and understand their role in surface chemistry. Specific questions being studied are listed below.

Understanding the dissociation of CO2 on copper surfaces: The chemical transformation of CO2 to methanol is an important part of the strategy for dealing with the rising CO2 levels leading to the global climate crisis. However, the relatively high thermodynamic and kinetic stability of CO2 means that this process needs careful selection of appropriate catalysts and co-reactants. At present, one of the commonly employed catalysts consists of Cu as the main component (Cu/ZnO/Alumina support) and naturally, CO2 interacting with Cu surfaces is an important model system for understanding the elementary steps involved in this process. Our current work is focused on understanding the nature of the activated dissociation of CO2 on Cu surfaces and its internal energy dependence using molecular beam-surface scattering methods.

Understanding the influence of energy transfer processes in determining the dissociation barrier and probabilities:  For most molecule-surface scattering measurements where the inelastic scattering and the corresponding energy transfer pathways are well understood the reaction probabilities and the corresponding dissociation barrier are not known and vice-versa. In order to provide high-quality benchmarks based on which predictive theoretical models can be tested in a realistic manner, combined information on the energy transfer pathways, their magnitude, and the reaction probabilities as a function of internal energy needs to be known. We are working on developing model systems where both inelastic and reactive scattering will be studied.

(left) Schematic description of some of the elementary steps in surface chemistry. (middle and right) Side and top views of our molecule-surface scattering experimental setup. This setup is designed to study both nonreactive and reactive scattering events. The special features include ion-imaging-based detection and a novel compact He source for monitoring inline surface coverage using specular He reflection.

In order to carry out these measurements we have developed a state-of-the-art molecular beam-surface scattering experimental setup. This setup is designed in such a way that both inelastic energy transfer and reactive scattering events can be measured in the same setup. This setup is equipped with an ion source for surface preparation, an Auger electron spectrometer for surface chemical composition analysis, ion imaging-based detection of scattered particles in a quantum state selected manner (in combination with Resonantly enhanced multiphoton ionization method), and provision for quantum state selected preparation of incident reactants. We also have developed a novel and compact He atom beam source for inline monitoring of surface coverage using specular He reflection, which can be used for measuring reaction probabilities.

(2) Developing tools and diagnostics for high-resolution spectroscopy and quantum state-resolved scattering experiments

(left) Schematic of our homebuilt IROPO/A setup with active feedback for wavelength locking (right) IR-UV double resonance signal showing demonstrating the preparation of CO in v = 0 to 2 state by IR excitation for more than 1000 sec. Up to 44% population transfer (70% of saturation) is achieved.

Preparing and detecting molecules in a quantum state-selected manner is an essential component for understanding energy transfer processes in molecule-surface collisions. We employ molecular beam methods in conjunction with laser-based preparation and detection techniques to achieve this. Collision-free environment, narrow velocity distribution, and low number densities in molecular beams mean that intense, narrow line width and frequency stable lasers are needed for pumping/probing these molecules. We have recently built a widely tunable, intense, single longitudinal mode, frequency stabilized and narrow line width mid-infrared radiation source (~ 1400 to 4700 nm) based on nonlinear optical processes. Our design is based on the ‘Master Oscillator Power Amplifier’ concept which consists of an optical parametric oscillator and amplifier stages. This will enable us to efficiently prepare vibrationally excited molecules for quantum state-resolved collision and reaction dynamics studies.

Along with this, we have also designed and developed a novel cost-effective, high-performance (1 ppm accuracy, 0.05 ppm precision) wavemeter based on the Cross Fringe Fizeau Interferometer. This allows us to lock the IR wavelength to a desired transition. This novel design has been recently filed for a patent and also has been awarded a special grant for developing this idea further.

(3) Using neutral atoms and molecules as a ‘soft’ and ‘universal’ probe for surfaces

Atomic/molecular scattering from surfaces depends on microscopic details of the atom/molecule-surface interactions and hence can be thought of as a sensitive probe for surface structure (physical and chemical). An interesting possibility arising from this is the concept of developing imaging (microscopy) techniques based on atomic/molecular scattering.

(left) Schematic diagram of the apparatus for NAM imaging using pinhole collimation (right) An illustration of the working principle of NAM showing topographical contrast

A unique feature of this approach is that it is a ‘soft’ probing method, since the incident kinetic energy of the particles is of the order of 0.1 eV (~ 10 kJ/mol), much smaller than typical chemical bond energies (no beam-induced damage). Another special feature of this technique is that atomic/molecular scattering is a ‘universal’ phenomenon in the sense that irrespective of the probe/target combination, scattering will almost always occur. This means that a very wide range of samples can be probed. The key challenge in these experiments is to improve the spatial resolution, as unlike their charged particle counterparts, neutral atoms can not be focused in similar ways.

Using the highly sensitive nature of the atom-surface scattering process we have demonstrated that NAM imaging of atomically thin layers of MoS2 films is possible. Moreover, we have also shown that NAM imaging is possible using large atomic clusters, thereby overcoming the problem of diffraction from the collimating pinholes. These results are promising for developing a high lateral resolution microscopy based on atom scattering.

(left) Atomically thin MoS2 layers imaged with He atoms as a probe (right) NAM imaging with large Kr clusters (~10,000 Kr atoms /cluster) as a probe. An unusual inverted contrast is seen in these images when compared with images obtained using monoatomic beams