Investigation of the ν1−2ν2 fermi diad of OF2 by means of IR-MW double resonance
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Diad
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The vibration-rotation structure of the infrared absorption bands due to ν21+ν3, 2ν20+ν3, and ν1+ν3 have been observed and measured for Br79CN and Br81CN. The average value of α3 for these two isotopic molecules was found to be +0.000677 cm—1. The Fermi resonance constant for the resonance diad was calculated to be W12=35.41 cm—1 for Br79CN and W12=35.48 cm—1 for Br81CN. The positions of the Q branches of a number of other bands were also measured.
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Diad
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Acetylene
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The theory of the effect of resonance, in particular Fermi resonance and l-type resonance, on the effective centrifugal distortion coefficient of a molecule for a given vibration state is developed. The theory is applied to certain cases where l-type resonance occurs and to certain cases where both l-type resonance and Fermi resonance occur. No instances appear to have been investigated where Fermi resonance occurs without l-type resonance being present. The agreement between the theory and the experimental results appears to be satisfactory to the approximation of the theory.
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Polyatomic ion
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The infrared spectra of the ν5 and ν3 + ν6 bands of CH3I have been measured with a resolution of about 0.03 cm−1. The excited states of these bands form a Fermi diad and, from the analysis of about 600 lines in the spectra, the matrix element of the Fermi-resonance operator was determined. From this the cubic normal-coordinate force constant k356 was found to be 29.40 cm−1. In addition to the Fermi resonance, a further perturbation due to an x-y-type Coriolis resonance between the states | υ5 = 1 〉 and | υ3 = 1, υ6 = 1 〉 was discovered and analyzed.
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Near infrared absorption of pure CO 2 in the v 1 + v 3 , 2v 2 0 + v 3 Fermi diad region around 3650 cm -1 and in the 2v 1 + v 3 , v 1 + 2v 2 0 + v 3 , 4v 2 0 + v 3 Fermi triad region around 5000 cm -1 was measured at temperatures between 298 K and 500 K to a maximum pressure of 3100 bar. Density tuning of Fermi resonance is studied and compared with literature data on gaseous, liquid, and solid CO 2. Band maximum positions determined within an extended temperature and density range indicate appreciable rotational freedom up to about the critical density (0.45 g · cm -3 ) and repulsive interactions clearly coming into play above 0.9 g · cm -3 . The vibrational intensity of the Fermi triad, up to about 1.0 g · cm -3 is found to be independent of temperature and pressure within ± 2.5%, wich enables (pure) CO 2 concentrations in a large region of states to be precisely and directly measured via quantitative high-pressure high-temperature near infrared spectroscopy.
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Abstract For a molecule affected by Fermi resonance, we establish, up to third order of approximation, the general formulae relating the spectroscopic constants to the mean value of the vibrational energy (in any Fermi polyad) and to the spacings of vibrational energy levels (in a Fermi diad or a Fermi triad).
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Triatomic molecule
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Absorption bands of carbon dioxide involving the Fermi diads 10°0, 02°0 and 10°1, 02°1 have been investigated and molecular constants obtained. The variation in the centrifugal stretching constants which has been observed in the component levels is such that the average centrifugal stretching constant of the diad is very nearly the same as that of the vibrationless ground state of CO2.
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Vibronic spectroscopy
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