Vibrational Analysis of PH₃ in Normal and Local Mode Models

Three spectral regions of molecule PH₃ (v_s+v_b^*, 2v_s and 3v_s) were studied on a BOMEM DA3.002 Fourier transform spectrometer at a resolution of 0.02cm^-1. A white-type multipass cell was used to increase signal to noise ratio. Three stretching overtone bands|200>,|110>and |300>and several bend-stretch combination bands were found. The anharmonic constants x₁₂, x₂₂, x₂₄, x₄₄ and g₄₄ were derived from the bend-stretch combination band origins using the normal mode model, For the pure streches the Morse frequency ω_m anharmonic constant ωx and inter-bond coupling constant λ were derived by least squares fitting the observed data to hamiltonian matrix in the local mode model. It is also demonstrated that the normal mode model is equivalent to local mode model for stretching vibrations, provided the Darling-Dennison resonance are included and x-Krelations are used between anharmonic constants and Darling-Dennison resonance constants. Further discussion shows that|200> and|300> states of PH₃ are local mode states. The local mode behaviour of PH₃ is owing to its small inter-bond kinetic and potential coupling and their partially cancellation.
* v_s represents the stretching vibrations (v₁ and v₃). v_b represents the bending vibrations (v₂ and v₄)