Molecular-adapted Quantum Defect Orbital Research Articles

Theoretical study of the discrete and continuum spectrum of BeH

The transition intensities supplied in this Letter are directly connected with a description of the discrete and continuum spectrum of BeH. An attempt to meet our goal requires the calculation of the absorption oscillator strengths of several transitions to Rydberg states of BeH, together with differential oscillator strengths which give rise to different dipole-allowed photoionization channels from the molecular ground state. The calculations have been performed with the molecular-adapted quantum defect orbital (MQDO) approach. Predictions of new spectroscopic data on BeH at energies where high Rydberg transitions can take place, including the continuum region of the spectrum have been made for the first time.

Photodissociation of the δ(0,0) and δ(1,0) bands of nitric oxide in the stratosphere and the mesosphere: A molecular‐adapted quantum defect orbital calculation of photolysis rate constants

The availability of data concerning the upper atmosphere is essential for an understanding of both the change in solar activity on the Earth and of the different processes that have direct effects on the biosphere, in particular those with harmful environmental consequences. The main goal of the present work is the theoretical analysis of the photodissociation of nitric oxide (NO), which plays an important role in the chemical and energetic balance of the upper atmosphere. Given that the molecular absorption cross section is directly linked to the photodissociation rate of the molecule, we have first determined photoabsorption cross sections and then used the calculated values to obtain the rate constants for the photodissociation processes. The photodissociation of NO at a given wavelength range is triggered by the predissociation of the δ(0,0) and δ(1,0) bands. Through the analysis of the predissociation rate constants, a study of the dissociative behavior of NO by absorption of light within the VUV spectral region has been performed. The dependence on the altitude arises from the temperature changes to the doppler‐broaded halfwidth, in addition to the state population, and the solar zenith angle of the atmospheric photodissociation rate constants of this molecule have been calculated with the molecular‐adapted quantum defect orbital (MQDO) approach. Altitudes ranging from 20 to 90 km, which comprise the stratosphere and mesosphere, have been considered.

Spectral Properties of Hydrogen Fluoride in the Discrete and Continuum Regions (Rydberg Series in Hydrogen Fluoride)

Hydrogen fluoride has been observed in a number of astrophysical environments, being sizably abundant in some of them, which adds to the interest in the processes in which it participates. The processes that have called for our attention are those that involve the interaction of photons or electrons with HF, as they are largely responsible for the evolution of chemical abundances in such environments. On the other hand, the spectral features of HF are, in a sense, anomalous. This fact has been attributed to the interaction between some of the valence and Rydberg states of the molecule and has motivated us to undertake these calculations. The present work deals with the calculation of absorption oscillator strengths involving Rydberg states of hydrogen fluoride, as well as of differential oscillator strengths for the dipole-allowed photoionization channels of HF, all ending in the ground X 2Π state of HF+. The calculations have been performed with the molecular adapted quantum defect orbital (MQDO) method, which has proved to be reliable in previous studies. It is our belief that the achieved data, reported here for the first time, may be useful in studying the evolution of chemical abundances in important astronomical objects and might aid in future experimental measurements.

Molecular quantum defect orbital (MQDO) calculation of the photoionization of CF3X (X=Br,I): Study of ligand substitution effects

AbstractPhotoionization cross sections of CF3Br and CF3I were calculated with the molecular‐adapted quantum defect orbital (MQDO) approach. The partial cross sections corresponding to the different Rydberg series that constitute the ionization channels of both molecules from their ground states are reported. Good agreement with experiment was found. An analysis of the results reveals that the cross section profiles are largely independent of the nature of the ligands. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Intensities of analogous Rydberg series in CF 3Cl, CF 3Br and in those of their isolated atoms, Cl and Br

In the present Letter, we have studied the Rydberg spectroscopic behaviour of the CF 3Cl and CF 3Br isovalent molecules. Absorption oscillator strengths for both molecules have been calculated with the molecular-adapted quantum defect orbital (MQDO) approach. We have sought and found important similarities between the spectral intensities of analogous transitions in these isovalent molecules. Further similarities with the spectra of their isolated halogen atoms has not only served the purpose of assessing the quality of our calculations, but may also offer some relevant practical use.

MQDO oscillator strengths and emission coefficients for electronic transitions in N 2 and NO

The molecular-adapted quantum defect orbital (MQDO) method has been applied to the calculation of absorption oscillator strengths and Einstein emission coefficients for electronic transitions involving Rydberg states of N 2 and NO. These compounds have been found to play an important role in the evolution of Earth's upper atmosphere. However, the difficulties encountered in both laboratory measurements and theoretical calculations on the photoabsorption of these compounds are responsible for the scarcity of data in the literature. Predictions of unknown transition probabilities for the NO molecule are presently given.

Photodissociation Rate Constants for VUV Processes of CF3Cl and CF2Cl2 in the Upper Atmosphere. A MQDO Study

The availability of data concerning the upper atmosphere is essential for an understanding of both the change in solar activity and the different processes that have direct effects on the biosphere, in particular, those with harmful environmental consequences. The main goal of the present work is the theoretical analysis of the photodissociation of two chlorofluorocarbon (CFC) compounds that play an important role in the chemical and energetic balance of the upper atmosphere, CF3Cl and CF2Cl2. Given that the molecular absorption cross section is directly linked to a molecule's photodissociation rate, we have first calculated cross sections and then have used the achieved values to determine the mechanisms of the photofragmentation processes that CF3Cl and CF2Cl2 undergo at specific vacuum-UV (VUV) wavelengths that are present in the ionosphere. We have focused our study on the calculation of the kinetic rate constants for the processes that can give rise to cations upon photoabsorption, because they are difficult to determine experimentally. Through the analysis of the photodissociation rate constants, we have been able to make a comparative study of the dissociative behavior of CF3Cl and CF2Cl2 when these two CFC's undergo absorption within the VUV spectral region. The atmospheric photodissociation rate constants of CF3Cl and CF2Cl2 have been calculated with the Molecular-adapted Quantum Defect Orbital (MQDO) approach as a function of the altitude and at different solar zenith angles. Altitudes from 60 to 150 km, which fall within the D and E layers of the ionosphere, have been considered. No earlier data of this kind have been found in the literature.

Rydberg Series as Ionization Channels: Photoabsorption and Photoionization of the Atmospherically Relevant Molecule NO

The photoionization of nitric oxide from its ground state has been studied with the molecular-adapted quantum defect orbital (MQDO) method. Partial differential oscillator strengths for the different Rydberg series that constitute the ionization channels of NO from its ground state have been calculated up to a photon energy of 60 eV. The continuity of the calculated differential oscillator strengths across the ionization threshold, that is, in the discrete and continuous regions of the spectrum, has been adopted as a quality criterion, given the scarcity of comparative data.

Intensity calculations of the VUV and UV photoabsorption and photoionisation of CF 3Cl

Absorption oscillator strengths and photoionisation cross-sections for CF 3Cl from its ground state are reported. The molecular-adapted Quantum Defect Orbital (MQDO) method has been employed in the calculations. Partial differential oscillator strengths for the different Rydberg series that constitute the ionisation channels of CF 3Cl, as well as the photoionisation cross-sections, from its ground state are reported. The calculated cross-sections conform fairly well with experimental and theoretical values found in the literature.

Oscillator strengths and ionisation cross sections for the absorption of CF 2Cl 2 in the UV and VUV spectral regions. A MQDO study

Absorption oscillator strengths and photoionisation cross sections of CF 2Cl 2 have been calculated with the molecular-adapted quantum defect orbital (MQDO) approach. The differential oscillator strengths for the different Rydberg series that constitute the ionisation channels of CF 2Cl 2 from its ground state are reported. The MQDO differential oscillator strengths exhibit the expected continuity across the ionisation threshold. In addition, good agreement with experiment is found for the cross sections that correspond to the photoionisation from the (4 b 1+4 b 2) valence MO’s.

Spectral intensities of Rydberg transitions in carbonyl compounds. Formaldehyde, carbonyl fluoride and phosgene

Absorption oscillator strengths for electronic transitions involving Rydberg series (including the continuum) of formaldehyde, carbonyl fluoride and phosgene have been calculated with the molecular-adapted quantum defect orbital (MQDO) procedure. These compounds are known to play an important role in the evolution of the interstellar medium and the Earth’s upper atmosphere. The results have been analysed on the grounds of the scarce comparative data found in the literature and by compliance with continuity across the ionisation threshold. The similarities observed between the calculated intensities of analogous transitions in the isovalent molecules F 2CO and Cl 2CO have served the purpose of assessing the quality of our calculations. New data, which may aid in future experimental measurements, are supplied.

Intensities of Vibronic Transitions for the Main Bands Observed in the Electronic Spectrum of Atmospherically Relevant Nitric Oxide

Absorption oscillator strengths for the γ(A2Σ+−X2Π), δ(C2Π−X2Π), and ε(D2Σ+−X2Π) bands of NO, which are relevant to the study of atmospheric processes are reported. We have calculated both electronic and vibronic transition probabilities with the molecular-adapted quantum defect orbital (MQDO) method, which has proved, in previous applications, to be an adequate theoretical tool in the context of Rydberg spectra. The results appear to be in good accord with the available experimental values and, with some exceptions, conform fairly well with theoretical data derived from rather more sophisticated methods.

Study of Rydberg transitions in NaHe

Einstein coefficients and oscillator strengths for electronic transitions in the Rydberg radical NaHe have been calculated with the molecular-adapted quantum defect orbital (MQDO) method. The correctness of our estimations has been established on the basis of their good agreement with the results of a reliable complex calculation available in the literature. Transition probability data involving Rydberg states lying higher in energy than those so far reported are supplied.

Photoabsorption of Nitrous Oxide through Rydberg States in the Bound and Continuum Spectral Regions: Main Ionization Channels

The oscillator strength spectral density for the partial (through different individual Rydberg series) and total photoabsorption processes which determine the photoionization of N2O from its ground state has been studied. Despite its relevance for atmospheric chemistry and astrochemistry, only a few data seem to be available in the literature. The molecular-adapted quantum defect orbital (MQDO) method has been employed in the calculations. For the different ionization channels, continuity between the bound and continuum spectral regions has been achieved in the corresponding partial differential oscillator strengths. Good agreement with experiment has been found for the total photoionization cross section.

Theoretical study of the Rydberg spectra of chemically relevant isovalent radicals

AbstractIn the present work we have studied the Rydberg spectroscopic behavior of the CH3 and SiH3 isovalent radicals. The molecular‐adapted quantum defect orbital (MQDO) method has been employed in our calculations, which have been extended to the radicals' isolated central atoms, C and Si. The similarities observed between the intensities of analogous transitions in the atoms and the corresponding radicals have not only served the purpose of assessing the quality of our calculations but also offer some potential practical usefulness. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Excitation Energies and Molecular Quantum Defect Orbital Transition Intensities for Rydberg States of ArH

The size-consistent self-consistent complete active space singles and doubles configuration interaction ((SC)2-CAS-SDCI) procedure has been applied to the calculation of vertical excitation energies, as well as the ionization energy, of the Rydberg radical ArH. Absorption oscillator strengths and Einstein emission coefficients for a number of electronic transitions have been obtained with the molecular adapted quantum defect orbital (MQDO) method. The adequacy of the two theoretical procedures employed in the present work is discussed.

Photoabsorption processes in nitrous oxide and formaldehyde

AbstractAbsorption oscillator strengths and photoionization cross sections for electronic transitions involving Rydberg states that are relevant to the photochemistry of N2O and H2CO are reported. These compounds have been found to play an important role in the evolution of Earth's upper atmosphere. However, the difficulties encoutered in both laboratory measurements and theoretical calculations on the photoabsorption of these compounds are responsible for the scarcity of data in the literature. The present calculations have been performed with the molecular‐adapted quantum defect orbital (MQDO) method, of which the adequacy for this type of studies has recently been assessed. A comparative analysis of the photoabsorption intensities in the molecules and their constituting atoms has enabled us to predict the variation of the extent of atomic character of the molecular Rydberg orbitals with the degree of excitation. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001