Stretch-bend Combination Research Articles

Spectroscopic constants for the 2.5 and 3.0 μm bands of acetylene

Fourier-transform interferometer and difference-frequency laser spectra of acetylene have been recorded in the 2.5- and 3.0-μm regions, yielding improved rotation-vibration constants for the ν 3 fundamental and its Fermi-resonant partner, ν 2 + ν 4 + ν 5, for their associated hot bands originating in the ν 4 and ν 5 bends, and for the CH stretch-bend combination bands. An analysis of the Fermi resonance gives more reliable estimates for the deperturbed constants and the anharmonic coupling.

CH local mode excitations and Fermi resonance in trichloroethylene

The CH stretching overtone spectrum of liquid-phase trichloroethylene is studied using the dual beam thermal lens technique (Δ V s = 6) and conventional absorption method (Δ V s = 2-5). The high value of the mechanical frequency is attributed to CH bond strengthening resulting from the electron-withdrawing property of the halogen atoms. The empirical relation between CH bond length and fifth overtone energy predicts that the CH bond in trichloroethylene is 0.002 Å smaller than that in ethylene. The Δ V s = 3 region shows Fermi resonance between pure overtone and stretch—bend combination states. The magnitude of the Fermi resonance matrix element is close to that reported for dichloromethane.

Van der Waals potentials from the infrared spectra of rare gas–HF complexes

High-resolution infrared spectra of the Ar–HF, Kr–HF, and Xe–HF van der Waals molecules have been recorded in the vicinity of the H–F stretching fundamentals, ν1, under thermal equilibrium conditions at T≂211 K with a tunable difference-frequency laser. Rotational structure has been observed up to or approaching rotational predissociation, permitting us to model the effective radial van der Waals potentials for these complexes. These potentials provide good estimates for the binding energies, D0, and the van der Waals stretching frequencies, ν3, in the ground (v1=0) and excited (v1=1) states of the molecules. For v1=0 in Ar–HF, Kr–HF, and Xe–HF, we find D0=102, 133, and 181 cm−1 and ν3=39.2, 41.1, and 43.4 cm−1, respectively. The ν3 modes characterized by the model potentials aid in the assignment of the ν1+ν3−ν3 hot bands observed in our spectra. The band centers for the ν1 fundamentals are all down shifted in frequency from the isolated HF monomer by Δν=−9.654, −17.518, and −29.185 cm−1 for the Ar, Kr, and Xe complexes, respectively, indicating that the van der Waals bonds are some 10% to 15% stronger in the excited vibrational state. This increased vibrational attraction also results in a contraction of the van der Waals radial coordinate manifest in the larger rotational constants observed for ν1; ΔB/B0=+0.35%, +1.00%, and +1.75% for Ar–, Kr–, and Xe–HF. We have also observed the Q branch of the ν1+ν2 stretch–bend combination band in Ar–HF some 70.2 cm−1 above the ν1 fundamental with a large negative ΔB/B0=−2.00% implying a strong anisotropy in the potential.

Vibrational excitations in soft x-ray emission and core ESCA spectra of NH3

A b initio calculations are performed for the energy surfaces of the ground N ls hole and 3a1 hole states of ammonia in order to investigate the vibrational band profiles in its soft x-ray emission and core ESCA spectra. Both the N ls and 3a1 hole states are found to be planar. In comparison with the ground state, the N–H bond length is significantly shortened for the core hole state but relatively unchanged for the 3a1 hole state. The results lead to a core ESCA spectrum dominated by a progression of closely spaced vibrational peaks of the pure bending mode and to a 3a1 x-ray spectrum with excited stretch–bend combination bands.