G. J. Scherer

The high‐resolution visible overtone spectrum of acetylenea)

Direct, gas‐phase overtone spectra of states corresponding to five and six quanta of C–H stretch have been obtained for four isotopically substituted acetylenes in the region 14 900–18 500 cm−1. Peak positions of individual rotation–vibration lines were determined to a precision of about 0.003 cm−1. A total of 39 bands are analyzed: 16 for 12C2H2; 14 for 13C2H2; five for 12C13CH2; and four for 12C2HD. The rotational structure of the bands is fit to a semirigid rotor Hamiltonian. However, the bands suffer from severe rotational perturbations, with typical matrix elements being on the order of 0.3 cm−1 or less. The error in the determined band origins varies between 0.001 and 0.1 cm−1, depending on the degree of perturbation. Precision rotational constants are also determined for the observed bands. The vibrational term values cannot be adequately explained in terms of the usual slightly anharmonic normal mode expansion. Even though acetylene represents an intermediate case between the local mode and normal...

Classical chaos and quantum simplicity: Highly excited vibrational states of HCN

Direct overtone spectra of H 12C 14N, H 13C 14N, and H 12C 15N have been measured between 15 000 and 18 500 cm−1 with a precision of 0.001 cm−1. These were obtained using intracavity photoacoustic spectroscopy, with a fully automated laser system. The spectra are unperturbed. The transition energies and rotational constants are in good agreement with predictions of first order anharmonic constants. Classical trajectories for HCN have been computed on the best experimentally parameterized potential, and found to be stochastic 12 990 cm−1 above the ground state. Quantal density of states were computed for HCN and show that if extensive vibrational coupling occurs, the observed states would be highly perturbed. The simplicity of the observed states is shown to be expected given a Franck–Condon type limitation on significantly perturbing states. The results show the inapplicability of classical dynamics for predicting the dynamics of molecular vibrations.

The high‐resolution visible overtone spectrum of CH4 and CD3H at 77 K

We have obtained visible overtone spectra with Doppler‐limited resolution of methane and trideuteromethane in the vicinity of six quanta of C–H stretch. At room temperature, the methane spectrum is unresolved. Upon cooling to 77 K in a specially designed photoacoustic cell, methane shows a complicated but rotationally resolved spectrum. The widths of all features in the spectrum are consistent with Doppler broadened linewidths at 77 K. Efforts to assign this spectrum are in progress. The overtone spectrum of CD3H has been recently studied by other workers at a resolution of 0.5 cm−1 [Perry, Moll, Kupperman, and Zewail (preprint)]. The spectrum in this region consists of two parallel bands, one at 16 156 cm−1 and another at 16 230 cm−1. These are assigned as arising from a Fermi resonance between the pure C–H overtone 6ν1 and a combination with the degenerate C–H bend, 5ν1+2ν5. A high resolution spectrum taken at 77 K shows nearly completely resolved K‐subband structure for both bands. The rotational const...

Response to ‘Highly excited states of HCN: The probable applicability of classical dynamics’

The comments of Farrelly and Reinhard on a previous publication1 of the authors are answered. The applicability of classical mechanics to the problem of excited state of HCN is further discussed. (AIP)

Variational calculation of the rotational constants for acetylene and its isotopic derivatives

A method is presented for calculation of rotational constants for a linear molecule by treating the vibrations variationally, but treating the vibration–rotation coupling perturbatively. The method is applied to the potential energy function for acetylene given by L. Halonen, M. S. Child, and S. Carter [Mol. Phys. 47, 1097 (1982)]. It is found that even though the vibrational energy levels calculated from this potential are in excellent agreement with experiment, the calculated rotational constants are poorly predicted. The calculated ΔB’s are systematically inaccurate by about 5% for C2H2 and 8% for C2D2. This indicates that the potential energy function is inaccurate even near equilibrium.

Highly excited HCN: The inapplicability of classical dynamics

A realistic potential surface is used to calculate HCN trajectories. (AIP)A realistic potential surface is used to calculate HCN trajectories. (AIP)

Comment on ‘‘The overtone spectrum of acetylene: A rotational analysis based on a local model description’’a)

Many serious errors in the recent article ‘‘The overtone spectrum of acetylene: A rotational analysis based on a local model description’’ by Bryan R. Henry, M. Ali Mohammadi, and Allan W. Tarr [J. Chem. Phys. 77, 3295 (1982)] are discussed.