Previously, an experiment was conducted to study the light-matter interaction of methane when introduced to a laser field. The researchers used ionized deuterated methane (CD₄⁺ instead of CH₄) to facilitate the study. Ionizing the molecule allowed it to be sent through an ion beam, and the heavier version of the molecule was chosen to slow the particles. This made it easier to detect when different parts of the molecule hit the detectors in case of fragmentation due to the laser field. This experiment led to branching ratios of CD₄⁺ breaking into CD₂⁺ and D₂ or 2 separate Deuteriums that we do not currently understand.

My goal this summer was to gain deeper insight into why this molecule interacts the way it does by employing molecular dynamic (MD) simulations to study the behavior of CD₄⁺ . The aim was to correlate the simulated data with the experimental results. MD simulations are computational methods used to study the physical movements of atoms and molecules over time. These simulations output data regarding the positions and velocities of all particles in the system relative to time, with the totality of this data forming the trajectory of the molecule.

The trajectories that I worked with tracks how the molecule CD₄ changes into CD₄⁺. This is crucial as the ionization process prior to being sent through the ion beam may have altered the results of the experiment. To achieve this, a trajectory was set up with a geometrically optimized molecule, and frequencies related to that geometry were obtained. Multiple possible starting conditions for CD₄ were initiated before adjusting the calculations to that of its ionized state. This is important as the ionized state of this molecule does not share the same preferred geometry or vibrational modes. The trajectory will show if and how the molecule fragments during its evolution to the ionized state.

Fig. 1. A heat map showing the relationship between a geometry of CD₄⁺ optimized around the bond length of two Deuteriums and the bond angle between them, and the potential energy of the system above its minimum in eV’s. Geometries with lower related potential energies are preferred, thus is two Deuteriums are ejected, it seems they would prefer to leave close together, likely paired.

Fig. 2. A motion plot of the Deuterium-Carbon bond lengths over time for the Deuteriums that ended up the furthest away from the carbon atom at the end of its trajectory for 3394 trajectories. All trajectories that end above 3 Angstroms are bonds that are considered broken.

Fig. 3. A motion plot of the Deuterium-Carbon bond lengths over time for the Deuteriums that ended up the second furthest away from the carbon atom at the end of its trajectory for 3394 trajectories. None go over 3 Angstroms; thus all of these bonds are intact.

Fig. 4. A histogram on a logarithmic scale showing the same information shown in Fig. 2, to show the difference in how many bonds were still intact vs how many were broken.

Initially, Fig. 2 seems to show a noticeable number of trajectories with anharmonic behavior, with the deuteriums breaking off when they shouldn’t be. When the data is mapped out onto a histogram as on fig. 4, we can see that the trajectories hardly ever dissociate. Of the largest bond lengths found throughout the trajectories, only 273 exceeded 3 Angstroms, which is a distance at which they are likely no longer bonded, meaning only ~8% fragmented. None of the second largest bond lengths exceed 3 Angstroms, thus only one Deuterium will be ejected in any of these trajectories.

The set up of my trajectories should mimic what happens when CD₄⁺ is ionized, which is important to know as this should simulate the gas being ionized just prior to being sent through an ion beam, so if the molecules are dissociating at this point, it will affect everything following. My trajectories only give an insight into the very beginning of what occurred during the experiment. With what I have gathered, future groups can pick up this project to do further analysis and simulations to further our understanding of what occurred

Acknowledgments

Thank you to Dr. Loren Greenman for directing my research and providing excellent guidance this summer, as well as for running the REU program. Thanks also to Kim Coy and Bret Flanders for organizing and running the program. I would also like to thank Dr. Greenman’s research group for their support and for making me feel welcomed. Additionally, my thanks to Kansas State University for hosting the REU program and to the National Science Foundation for funding it. This material is based upon work supported by the National Science Foundation under Grant No. #2244539. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.

 

Final Presentation