Comparison of Quantum Mechanical and Empirical Potential Energy Surfaces and Computed Rate Coefficients for N2 Dissociation

Physics-based modeling of hypersonic flows is predicated on the availability of chemical reaction rate coefficients and cross sections for the collisional processes. This approach has been built around the use of quantum mechanical calculations to describe the interaction between the colliding particles. In this approach a potential energy surface (PES) is computed by solving the electronic Schrdinger equation and collision cross sections are determined for that PES using classical, semiclassical or quantum mechanical scattering methods. The rate coefficients are computed by integrating the thermally weighted cross sections. State-to-state rate coefficients are determined by only integrating over a thermal distribution of collisional energies. Finally, thermal rate coefficients are determined by summation of the state-to-state rate coefficients for reactions of molecules in all relevant ro-vibrational energy levels. If the flow is in thermal non-equilibrium, the translational, vibrational and rotational energy modes can be represented in different ways: three unique temperatures can be used to describe the distributions, the populations of individual ro-vibrational energy levels can be determined by solving the Master Equation, or through the use of direct simulation in particle-based Monte Carlo sampling. The PES-to-rate coefficient approach had been proposed and attempted in the early days of digital computing, but it is only in the last 15 years that computer hardware and software have been up to the task of calculating accurate interatomic and intermolecular potentials

Dissociation and Internal Excitation of Molecular Nitrogen Due to N + N2 Collisions Using Direct Molecular Simulation

In this work we present a molecular level study of N2+N collisions, focusing on excitation of internal energy modes and non-equilibrium dissociation. The computation technique used here is the direct molecular simulation (DMS) method and the molecular interactions have been modeled using an abinitio potential energy surface (PES) developed at NASA's Ames Research Center. We carried out vibrational excitation calculations between 5000K and 30000K and found that the characteristic vibrational excitation time for the N + N2 process was an order of magnitude lower than that predicted by the Millikan and White correlation. It is observed that during vibrational excitation the high energy tail of the vibrational energy distribution gets over populated first and the lower energy levels get populated as the system evolves. It is found that the non-equilibrium dissociation rate coefficients for the N + N2 process are larger than those for the N2 + N2 process. This is attributed to the non-equilibrium vibrational energy distributions for the N + N2 process being less depleted than that for the N2 +N2 process. For an isothermal simulation we find that the probability of dissociation goes as 1/T(sub tr) for molecules with internal energy (epsilon(sub int)) less than approximately 9.9eV, while for molecules with epsilon (sub int) greater than 9.9eV the dissociation probability was weakly dependent on translational temperature of the system. We compared non-equilibrium dissociation rate coefficients and characteristic vibrational excitation times obtained by using the ab-initio PES developed at NASA's Ames Research Center to those obtained by using an ab-initio PES developed at the University of Minnesota. Good agreement was found between the macroscopic properties and molecular level description of the system obtained by using the two PESs

Evaluation of Orthodontic Treatment Need and Its Correlation with the Perception, Awareness and Satisfaction of Personal Dental Appearance among Dental Students

Introduction: Facial esthetics as well as dental health improvements is the main concern of orthodontic treatment. Dentofacial appearance that deviates from normal may have anegative impact on social, physiological and psychological functions. But individual’s attitude to its malocclusion is an important factor in determining treatment need.Objectives: The study was conducted to assess the perception of malocclusion and need of orthodontic treatment among dental students using index of orthodontic treatment need (IOTN). Also self-awareness and level of satisfaction of personal dental appearance were analyzed. Materials and methods: Sample of 300 dental students was selected, their alginate impressions were poured, and study casts were prepared. IOTN was used to find out the impact of malocclusion on individual’s dental health and psychological well being. A questionnaire was prepared consisting of part 1, self-awareness and part 2, satisfaction and attitudes. Each part consisted of series of questions with alternative answers. Eachselected subject was given this questionnaire for self-evaluation and self-perception of occlusion and facial appearance.  Results: Majority of dental students were observed in grades Iand II of IOTN indicating no or minimal treatment need. The students were aware that malocclusion, orthodontic treatment and facial esthetics are the important factors for self-image and self-esteem

Comparison of Quantum Mechanical and Empirical Potential Energy Surfaces and Computed Rate Coefficients for N2 Dissociation

Comparisons are made between potential energy surfaces (PES) for N2 + N and N2 + N2 collisions and between rate coefficients for N2 dissociation that were computed using the quasiclassical trajectory method (QCT) on these PESs. For N2 + N we compare the Laganas empirical LEPS surface with one from NASA Ames Research Center based on ab initio quantum chemistry calculations. For N2 + N2 we compare two ab initio PESs (from NASA Ames and from the University of Minnesota). These use different methods for computing the ground state electronic energy for N4, but give similar results. Thermal N2 dissociation rate coefficients, for the 10,000K-30,000K temperature range, have been computed using each PES and the results are in excellent agreement. Quasi-stationary state (QSS) rate coefficients using both PESs have been computed at these temperatures using the Direct Molecular Simulation of Schwartentruber and coworkers. The QSS rate coefficients are up to a factor of 5 lower than the thermal ones and the thermal and QSS values bracket the results of shock-tube experiments. We conclude that the combination of ab initio quantum chemistry PESs and QCT calculations provides an attractive approach for the determination of accurate high-temperature rate coefficients for use in aerothermodynamics modeling

Direct Molecular Simulation of Nitrogen and Oxygen at Hypersonic Conditions

University of Minnesota Ph.D. dissertation. February 2018. Major: Aerospace Engineering and Mechanics. Advisor: Thomas Schwartzentruber. 1 computer file (PDF); xiv, 152 pages.The objective of this thesis is to characterize the gas-phase thermochemical non-equilibrium that occurs during hypersonic flight for nitrogen and oxygen gases. This thesis uses the direct molecular simulation (DMS) method in conjunction with potential energy surfaces (PESs) to provide an in-depth molecular level analysis of internal energy excitation and dissociation of molecular nitrogen due to $N_2+N_2$ and $N_2+N$ interactions. Characteristic vibrational excitation times and non-equilibrium dissociation rate coefficients are calculated using the $ab-initio$ PESs developed at NASA Ames Research Center. Comparison of these rate coefficients and non-equilibrium vibrational energy distributions is carried out against prior work done with nitrogen using an independently developed $ab-initio$ PES at the University of Minnesota. Good agreement was found between properties predicted by the two PESs. Furthermore, comparative studies were carried out for the nitrogen system between the DMS method and the state-to-state method. The results obtained by the two different methods, are found to be in good agreement. The DMS method is used to calculate benchmark data for vibrational energy excitation and non-equilibrium dissociation due to $O_2+O$ interactions. $O_2+O$ interactions are modeled using nine PESs corresponding to. $1^1A'$, $2^1A'$, $1^1A''$, $1^3A'$, $2^3A'$, $1^3A''$ $1^5A'$, $2^5A'$ and $1^5A''$ states, which govern electronically adiabatic (ground-electronic-state) collisions of diatomic oxygen with atomic oxygen. This is the first data set in the aerospace community that incorporates all nine PESs for the $O_2+O$ system and fully describes the dynamics of ground state interactions of diatomic oxygen with atomic oxygen. Characteristic vibrational excitation times are calculated over a temperature range of $T=3000K$ to $T=15000K$. It is observed that the characteristic vibrational excitation time for $O_2+O$ interactions is weakly dependent on temperature and increases slightly with increasing temperature. Vibrational excitation is slowest for interactions in the quintet spin state, with the $1^5A''$ state having the slowest excitation rate, and vibrational excitation is fastest on the $1^1A'$ potential energy surface. Non-equilibrium dissociation rate coefficients are calculated over a temperature range of $T=6000K$ to $T=15000K$ during quasi-steady state (QSS) dissociation, and the results agree well with experimental data. For the $O_2+O_2$ system interactions can occur over singlet, quintet and triplet spin states. An in-depth analysis of excitation and dissociation on the quintet and singlet surfaces is provided and bench-mark data for excitation using all three PESs for $O_2+O_2$ interactions is presented for a temperature range of $T=5000K$ to $T=12000K$ . Finally, this thesis explores internal energy exchange processes in oxygen and nitrogen. Probability distribution functions for vibrational energy change during collisions are presented (due to $N_2+N_2$ non-reactive collisions, $N_2+N_2$ exchange reactions, $N_2+N$ non-reactive collisions, $N_2+N$ exchange reactions, $O_2+O$ non-reactive collisions, and $O_2+O$ exchange reactions). It is shown that non-reactive collisions are less efficient in vibrational energy redistribution when compared to exchange reactions. Furthermore, it is observed that the probability distribution functions for vibrational energy change (for both oxygen and nitrogen) are self-similar and may be modeled by simplified functional forms

Orthodontic Practice in the Times of COVID-19 Pandemic: An Online Survey

Introduction: Coronavirus Disease 2019 (COVID-19) pandemic is not the first one which the globe has faced but never came across a health crisis that moved so quickly across continents. COVID-19 outbreak presently posed a very serious threat to the existence of mankind on earth. The massive impact of COVID-19 pandemic was evident in all aspects of life-personal, social as well as professional. The field of dentistry including orthodontics was no exception to this. Aim: To describe the impact of COVID-19 pandemic on orthodontic practice, exploring the basic sterilisation protocols being followed during COVID-19 pandemic and to predict the future of orthodontics in post-COVID era. Materials and Methods: The Department of Orthodontics and Dentofacial Orthopedics, Faculty of Dental Sciences, SGT University, Gurugram, Haryana, India, conducted this questionnaire based cross-sectional survey in May 2020 over a time span of 20 days. A web-based questionnaire of 15 multiple choice questions in English was created as a Google Form in Google Documents. The link to this form was shared online with the orthodontists and postgraduate students (Orthodontics) of dental colleges situated in the National Capital Region (NCR) through WhatsApp groups and e-mails. A total of 254 responses were received. Results: A 97.6% of respondents agreed that COVID-19 affected their orthodontic practice. The odds ratio between those not likely to resume practice and those likely to resume practice taking educational qualification as the significant predictor was calculated to be 8.976 at 95% confidence intervals. There was wide variation in the selection of the safety protocols by the orthodontists in the present study (p<0.01). Increased digitalisation in orthodontics (45.7%) followed by less demand of orthodontic treatment (18.5%) were opted as the future in post-COVID era (p<0.01). Conclusion: The present survey found that orthodontic community as a whole was affected greatly by COVID-19 pandemic and is quite apprehensive. The future of orthodontics in the post-COVID era is unpredictable presently. Digitalisation in orthodontics is the key option to have minimum physical contact with the patients. The study suggested the need and importance of basic sterilisation protocols and a training program for dental settings during COVID-19 for patient’s as well as clinician’s safety