The internal coordinate Morse oscillator as a means to simplify the anharmonicity problem in the thermodynamics of systems of polyatomic molecules

Two postulates simplify the evaluation and algebraic complexity of the expression for the anharmonic correction to the ground rotational–vibrational energy of a polyatomic molecule. The two postulates are: (1) anharmonicity of vibration is associated with bond stretching coordinates only; all other internal coordinates can be represented by harmonic potentials; (2) bonds vibrate according to a Morse potential. The result is the replacement of tedious computer calculations by a 'back of an envelope' type of computation. Relationships between anharmonic corrections to the ground state vibrational energies of isotopically different molecules are derived. Good agreement with exact results is obtained. Quantitative and qualitative applications are given.

Beiträge zum Stark-Effekt der Moleküle 205Tl19F und 39K19F / Contributions to the Stark-Effect of the Molecules 205Tl19F and 39K19F

The dominant contribution to the Stark-effect energy of polar 1Σ diatomic molecules can be calculated from the model of the rigid rotator. Additional terms arise from the anharmonicity of vibration, the centrifugal distortion, the vibration-rotation interaction and the electronic polarizability.These contributions to the Stark-effect have been investigated for the molecules 205TlF and 39KF with a molecular beam electric resonance apparatus suitable to detect rotational transitions. Measure-ments have been performed at values of electrical field corresponding a) to a minimum in the frequency for the transition (J, mJ) = (1, 0) → (2,0) for vibrational states v = 0, 1, 2 and b) cor-responding to the electrical field where the transitions (J, mJ) = (1, 0) (0, 0) and (1, 0) → (2, 0) for v = 0 occur at the same frequency.Interpretation of our data requires more precise values of the Dunham coefficients than have been published to date. These coefficients therefore have been recalculated from rotational transitions measured at zero electrical field.