High-temperature Spectral Radiative Properties Of Diatomic Metal Oxides

Thermal radiation is not only a way of energy transmission,but also a carrier of signal transmission.It plays a foundational role in the field of optics and thermal radiation.Accurate and comprehensive spectral radiative properties of diatomic metal oxides are important for the applications in the fields of astronomical observation,target identification and tracking,combustion regulation and diagnosis,thermal protection system design and so on.In the early days,the molecular spectroscopic databases established by European and American countries mainly focused on atmospheric molecules at room temperature.With the development of national defense and aerospace technology,the corresponding spectral radiative properties of high-temperature gas molecules were also established,but metal oxides are still not considered.In experiments,the spectra of metal oxides were only observed at a specific waveband and temperature,which seriously restricts the detection and identification of metal oxides.In addition,a large number of molecules are in high-energy states at high temperatures,which drives molecules to participate in a series of physical and chemical processes such as the dissociation and association.Consequently,molcules radiate stronger spectral lines in some wavebands at high temperatures than those at low temperatures and produce continuous spectra.This significantly affects the calculation of radiation field at high temperatures.Taking common diatomic metal oxides as examples,a calculation method from the diatomic molecular structure to molecular radiative properties is proposed,and a set of spectral data for all bands of MgO and AlO molecules below 15000 K are established.Potential energy curves(PECs)and dipole moments are the basis of spectral radiative properties.The PECs of 14 states of MgO,16 states of AlO and 15 states of TiO were accurately calculated by using the internally contracted multireference configuration interaction method and correlation consistency basis set.The calculated spectroscopic constants and transition dipole moments(TDMs)are consistent with the previous experimental and theoretical data.Based on the PECs,the partition functions of MgO,AlO and TiO molecules can be calculated in the temperature range of 10～15000 K.The results show that MgO molecules are mainly in the ground,first,second and third excited(X~1Σ~+,a~3Π,AlΠ and b~3Σ~+)states.AlO molecules are mainly in the ground state and the first excited state(X~2Σ~+ and A~2Π).TiO molecules are mainly in the ground state(X3Δ).In the medium-high temperature environment,most of the gas molecules are in the bound states and occur the transition between the bound states,resulting in the discrete spectrum.The transition properties of MgO,AlO and TiO molecules can be obtained by the PECs and TDMs,where the calculated radiatve lifetimes of the B~1Σ~+,d3Δ and D1Δ states of MgO,the B~2Σ~+ and C~2Π of AlO and the A3Φ,B~3Π and C3Δ states of TiO are in good agreement with previous experimental and theoretical results.Based on the partition functions and transition properties,the absorption and emission line intensity can be calculated at different temperatures,and thus the main radiative mechanism in different wavebands can be obtained at a specific temperatures.For example,MgO molecules have a strong line intensity in the infrared band at room temperature(300 K),and the transition processes A~1Π?X~1Σ~+,X~1Σ~+?X~1Σ~+ and A~1Π?A~1Π play a key role.The thermal movement of atoms intensifies at high temperatures,which drive two atoms to collide and associate to form a diatomic molecule and emit the continuous spectra.In this paper,we employed the quantum mechanical,semi-classical and resonant methods to calculate the emission cross sections for radiative association processes of MgO and AlO for collision energies in range of 0.0001～10 eV.Then the rate coefficients can be obtained for temperatures in the range of 10～15000 K,which can provide fundamental data for the molecular reaction dynamics.The results show that MgO molecules are formed mainly through the association processes A~1Π→X~1Σ~+ and D1Δ→A~1Π.The AIO molecules are formed mainly by the processes A~2Π→X~2Σ~+,12Δ→A~2Π,1~2Π→X~2Σ~+ and 2~2Σ~-→A~2Π.In the high-temperature radiation field,molecules will absorb a lot of energy to dissociate into two atoms.In this paper,we calculated the continuous absorption cross sections of photodissociation processes of MgO and AlO for the wavelength in range of 50～5000 nm.Then the photodissociation rates for environmental temperatures in the range of 300～15000 K in interstellar,solar and blackbody radiation field can be obtained.The results show that the photodissociation of MgO is dominated by the transition processes X~1Σ~+→F~1Πand a~3Π→e~3Σ~-.The transition process X~1Σ~+→F~1Π mainly occurs in the ultraviolet waveband,and the transition process a~3Π→e~3Σ~-occurs in visible and infrared wavebands.AlO molecules dissociate mainly by the transition processes A~2Π→1"~2Π,A~2Π→2~2Σ~-and X~2Σ~+→C~2Π.The spectral line generated by the transition between two levels is not strictly monochromatic light,namely a spectral line,and it has a certain width and profile.Considering real gas effects,the Voigt line profile is used to describe the effect of temperature on the line broadening,thus the discrete absorption and emission cross sections of MgO and AlO at different temperatures can be calculated.The calculated results are in good agreement with the experimental data,which can further verify the accuracy of the spectral data.The discrete cross sections can be combined with the cross sections of photodissociation and radiative association to obtain the total cross sections.In addition,based on the statistical thermodynamics,the radiative properties in local thermal non-equilibrium is derived by two energy partitioning schemes.The results show that the emission cross sections increase with the increase of vibrational/rotational temperature,while the variation of absorption cross sections is relatively small.The calculated absorption and emission cross sections can provide fundamental data to the calculation of high-temperature gas radiation field.