Spectral Transition Characteristics Of Diatomic Molecules In Atmospheres And Interstellar Space

Molecular spectroscopy plays an extremely important role in thermal radiation of high-temperaure gases.Due to the practical problems faced in the experiments,experimental data are often limited in temperature applicability and spectral range.While theoretical predictions can overcome these limitations.The transition probability is the key intermediate for obtaining the diatomic molecular spectra.The potential energy curves?PECs?and electronic transition dipole moments?TDMs?are the basis for calculating the transition probabilities.At present,avaliable transition probabilites of diatomic molecules in Earth's and Martian atmospheres were obtained based on earlier experimental spectroscopic constants and theoretical TDMs.New experimental spectroscopic constants and theoretical TDMs are reported in the literature in the past decade,so transition probabilities of diatomic molecules in Earth's and Martian atmospheres need to be updated.For the interstellar diatomic molecules,experimental and theoretical spectroscopic data are extremely limited,which has greatly restricted the development of astronomical observation.Elaborate spectral transition data are needed to provide guidelines for astronomical observation.Now the TDMs obtained by quantum chemistry methods generally meet the requirement of high precision spectral modelling.So how to accurately obtain the PECs of electronic states becomes the key to the computation of transition probabilites of diatomic molecules.Spectroscopic constants obtained by experiments can be used to obtain the PEC with higher precision than the quantum chemistry calculations.Therefore,the PECs of the electronic states for diatomic molecules whose experimental spectroscopic constants are relatively complete are generally obtained by the semi-empirical methods.Otherwise,the PECs of the electronic states are computed by the quantum chemistry methods.The experimental spectroscopic constants are relatively complete for diatomic molecules in atmospheres and are relatively scarse for those in the interstellar medium.In this thesis,therefore,the semi-empirical methods are used to calculate the PECs of diatomic molecules in Earth's and Martian atmospheres.The quantum chemistry methods are used to compute the PECs of diatomic molecules in the instestellar medium.The obtained PECs,combined with the TDMs,are used to compute the transition probabilities of the main diatomic components in Earth's and Martian atmospheres,as well as PN,CP,PN+and SiO+in the intestellar space.Based on the above transition parameters,as well as the partition functions,the thermodynamic properties and line intensities of diatomic molecules in Earth's and Martian atmospheres are computed.The details of this study are:Transition probabilities between high vibrational levels are needed to be resolved for radiation modeling of high-temperature gases.Based on the recent experimental spectroscopic constants,the classical Rydberg-Klein-Rees?RKR?method is employed to construct the near-equilibrium part of the PEC,and the theoretical potentials are utilized to extrapolate this PEC up to the dissociation limit.The PECs,along with the recent TDMs in the literature,are used to calculate the transition probabilities of the electronic transition systems for diatomic molecules and then determine the radiative lifetimes of the electronic states.The calculated radiative lifetimes agree well with with the experimental data.Compared with previous theoretical results,radiative lifetimes for most electronic states,such as N₂?B³?g?,O2?C2????,CO?A1??,CO+?B2?+?,have been improved greatly.To efficiently observe the interstellar enviroment,the spectral transition properties of the interstellar medium need to be studied.Diatomic molecules and ions are important components of the interstellar medium.Hence,based on quantum chemistry theory,the spectral transition characteristics of PN,CP,PN+and SiO+are studied in this work,following three steps.First,the valence internally contracted multireference configuration-interaction method with Davidson correction?icMRCI+Q?,including basis-set extrapolation to complete basis set?CBS?,core-valence correction,and scalar relativistic correction,is adopted to compute the PECs of PN,CP,PN+and SiO+,which are introduced to the nuclear Schrodinger equation to obtain the vibrational levels and rotation constants.The vibrational levels and rotation constants are used to fit the spectroscopic constants,which are in good agreement with the experimental data.Then,the TDMs of dipole allowed transitions for PN,CP,PN+and SiO+are determined by icMRCI method with the aug-cc-pV6Z basis set.Finally,the PECs and TDMs are used to predict the transition probabilities of different electronic systems for PN,CP,PN+and SiO+,together with their spectral ranges.It is found that most electronic systems have strong vibrational bands for PN,CP,PN+and SiO+.Transition probabilities of some strong bands are listed in this thesis in order to provide theoretical supports for laboratory measurements and astronomical detection.The specific heats need to be resolved for heat transfer calculations of high-temperature gases.Based on statistical mechanics,the analytical expressions for the partition functions and specific heats of diatomic molecules are derived in thermal equilibrium and non-equilibrium?two-temperature model?.The PECs of diatomic molecules in atmospheres are involved in the rotational dependence of the nuclear Schrodinger equation to obtain the vibrational and rotational levels,which are introduced to the analytical expressions to study the variation of partition functions and specific heats versus the temperature.In equilibrium,the partition functions increase with the increase of temperature.The specific heats increase firstly and decrease later as the temperature increases.The peak values occurs in different temperatures for different diatomic molecues,but mainly lie in the temperature range of 10000-20000K.This is mainly because the electronic,vibrational and rotational levels of diatomic molecules have been all motivated in this temperature range.In nonequilibrium,the large difference between the vibrational and rotational temperatures has great effect on the partition functions and specific heats.The temperature range of specific heats in this thesis is up to 50000K,which can provide data support for heat transfer calculations of high-temperature flow field.Based on vibrational and rotational levels,transition probabilities,and partition functions,the equilibrium radiative source strengths of N₂,N₂⁺,O₂,NO,CN,C₂,CO and CO+ are determined at different temperatures,which agree well with the experimental and theoretical data.At the same time,the non-equilibrium line intensities of N₂ first and second positive systems??v=0?,N₂⁺ first negative system??v=0?,and C₂ swan system??v=0?are predicted,which compare well with the experimental data.Such good agreement further verifies the accuracy of the spectral transition data of diatomic molecules in this thesis.Line intensities of diatomic molecules can provide data support for radiation modeling of high-temperature gases.