Intensities Of Overtone Bands Research Articles

Dwarf Carbon Star Model Atmospheres And Synthetic Spectra

Dwarf carbon (dC) stars are believed to be main-sequence stars which have received carbon-rich material from a former AGB carbon star companion. We have computed model atmospheres, synthetic spectra, and colours for dC stars and compared them with observations. Our models are computed with an updated version of the MARCS code (Jørgensen et al. 1992, A&A 261, 263) and include atomic and molecular line blanketing, grain formation, and collision-induced absorption (CIA) processes (Borysow et al. 1997, A&A 324, 185). The models assume hydrostatic equilibrium, which for giant stars (i.e., high luminosity and low gravity) would be inconsistent with the inclusion of dust. Typical gravities of dC stars are, however, considerably larger than the acceleration due to radiation pressure even on dust grains with a relatively high absorption coefficient.For the cooler solar metallicity models, grain formation can be substantial, but it depends strongly on the assumed type of dust. In a test model of Teff = 2800 K, C/O = 1.17, and log(g) = 5, 1% of the free carbon (i.e., that not bound in CO molecules) condensed when the dust was assumed to be amorphous carbon, whereas as much as 30% condensed when nano-diamonds (of the extra-solar form which are abundant in carbonaceous chondrite meteorites and are believed to have formed in stellar outflows) were allowedfor. This difference in degree of condensation is due to the very small diamond opacity compared to that of amorphous carbon. In both cases, however, the main effect of the dust was a 400 K heating of the surface layers. The dust itself is not clearly visible in the synthetic spectrum, but the heating and correspondingly reduced molecular partial pressures cause a weakeningof spectral features in the 3μm region (due mainly to reduced C2H2 abundance in the upper layers) and reduced CO fundamental and first overtone band intensities.

Calculation of vibrational fundamental and overtone band intensities of H2O

Vibrational intensities are calculated for the fundamental and overtone transitions of H2O up to approximately 18 000 cm−1. The intensities are determined from a dipole moment function expanded in the three internal bond coordinates. The expansion coefficients are computed ab initio at the second-order Mo/ller–Plesset level of theory with a 6-311G** basis set. Vibrational wave functions are calculated either from a three-dimensional harmonically coupled anharmonic oscillator (HCAO) model which uses Morse oscillators to represent both the stretches and the bend of H2O, or from a variational calculation employing the best available potential energy surface and an exact kinetic energy operator. To obtain the most meaningful vibrational intensities we define dipole moment components using the Eckart embedding. Both the HCAO and the variational intensities agree quite well with the experimental results, which span eight orders of magnitude. From the calculations we predict that it may be possible to detect as yet unobserved vibrational transitions of H2O.

Identification of combination and overtone bands in the matrix infrared spectrum of NC3NC

Ten overtone and combination bands of iso-dicyanoacetylene (NC3NC), produced by in situ pulsed ultraviolet laser photoisomerization of dicyanoacetylene in an argon matrix, have been assigned in our Fourier-transform absorption spectra. Experimental band frequencies, relative band intensities, and derived anharmonicity constants are compared in detail to CEPA-1 theoretical calculations. The agreement here, even for overtone band intensities, is excellent.

Mechanical and electrical anharmonicities of the linear hydrogen cyanide H-bonded trimer

The anharmonicity of the hydrogen-bonded (N … H) and proton-donor (C—H) stretching modes of the linear (HCN)3 species are calculated at the SCF level with the 4-31G and 6-31G** basis sets. Third and fourth energy derivatives and second and third dipole moment derivatives, which are needed to calculate the anharmonic terms, were evaluated by means of the one-dimensional Chebyshev polynomial fitting to the calculated ab initio points. Frequencies and intensities of overtone bands are reported and the effect of geometry relaxation on the polynomial fitted derivatives as well as mechanical anharmonicity on the vibrational intensities discussed. Key words: H-bonding, anharmonicity, vibrational frequencies, intensities.

Determination of a new molecular constant from overtone vibrational spectra

The quasiclassical Landau-Lifshitz formula is applied to higher overtone vibrational transitions. The transition probabilities are expressed in terms of the repulsive branch of the molecular potential, which can be characterized by a constant β describing its rapid rise in the region of strong interatomic repulsion. Values of β are calculated from the higher overtone band intensities for both diatomic molecules and the local vibrations in polyatomic molecules.

Intensity distributions in molecular overtone vibrational spectra

It is shown that new information about the repulsive branch of the molecular potential can be obtained from higher overtone band intensities. An expression is given for the intensity distribution and a molecular constant, introduced to describe the rapid potential rise in the region of strong interatomic repulsion, is calculated using experimental data for a number of diatomic systems. Anomalous low-intensity bands are predicted and identified in absorption spectra of diatomic molecules.

Theory of Overtone Vibrational Transitions with Application to the HF, DF and OH Emission Spectra

Abstract The higher overtone band intensities are expressed in terms of the repulsive branch of the molecular potential, which can be characterized by a new molecular spectroscopic constant β. The β values are calculated from experimental band intensities.