Abstract
In this study, we analyzed the light absorption by diatomic molecules or colliding atoms in a spectral region dominated by an avoided crossing of adiabatic state levels or crossing of the corresponding diabatic state levels. Our attention was focused on the low-resolution spectrum at a higher gas temperature under local thermodynamic equilibrium conditions. The absorption measurements of mixed vapors of potassium (≈80%) and cesium (≈20%) were made in the temperature range of 542–715 K and the infrared spectral range 900–1250 nm. In this area, the main spectral contributions were the broad A 1 Σ ( u ) + ( 0 ( u ) + ) − X 1 Σ ( g ) + ( 0 ( g ) + ) bands of K2, Cs2, and KCs molecules. There was a crossing of A 1 Σ ( u ) + ( 0 ( u ) + ) and b 3 Π ( u ) ( 0 ( u ) + ) state potential curves and the coupling of this state was due to the matrix element ⟨ A 1 Σ ( u ) + ( 0 ( u ) + ) | H s o | b 3 Π ( u ) ( 0 ( u ) + ) ⟩ of the spin–orbit interaction. Using data for relevant electronic potential curves and transition dipole moments existing in the literature, the spectra of the A 1 Σ ( u ) + ( 0 ( u ) + ) − X 1 Σ ( g ) + ( 0 ( g ) + ) molecular bands of K2, Cs2, and KCs molecules were calculated. Full quantum mechanical and semi-quantum coupled channel calculations were done and compared with their non-coherent adiabatic or diabatic approximations. Through the comparison of our theoretical and experimental spectra, we identified all observed spectral features and determined the atoms’ number density and gas temperature.
Highlights
Several approaches to the theoretical simulation of low-resolution, high-temperature optical spectra of diatomic molecules under local thermodynamic equilibrium conditions have been analyzed in detail by Beuc et al [1]
The broad oscillating structure around nm wasstate related toexcited the maximum of the KCs adiabatic states, the energies and wave functions of the excited states needed to be determined using transition difference potential at 1107 nm, and the peak at 1189 nm was related to the minimum at 1192 coupled channel calculus (QCC)
1205 nm.potential curves’ crossing point, the transition difference potential curves of the excited adiabatic states were monotonic in the first case and had a minimum and a maximum in the second case
Summary
Several approaches to the theoretical simulation of low-resolution, high-temperature optical spectra of diatomic molecules under local thermodynamic equilibrium conditions have been analyzed in detail by Beuc et al [1]. Devdariani et al [2,3,4] and O’Callaghan et al [5] studied this problem within semiclassical theory using a Landau-Zener approximation [6,7]. Their analysis focused on the absorption and emission of light by colliding atoms, their conclusions can be qualitatively applied when interpreting quantum mechanical calculations of bound–bound molecular transitions. On the red side atomic transition doublet, of the first resonant the spectrum is dominated by molecular A1 Σ+
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