High-resolution Rotation Research Articles

Band contour calculations and l-doubling effects in symmetric top vibration-rotation spectra

In this lecture I plan to discuss three aspects of our recent work on the analysis of high resolution rotation—vibration spectra. The first concerns the value of using computer-calculated band contours (or computer-simulated spectra) for the analysis of rotation—vibration structure in the band spectra of heavy molecules, in situations where the structure is only partially resolved. The second concerns the origin and importance of 1-doubling interactions (and xy axis Coriolis interactions) in the observed rotational structure of symmetric top molecules. I shall demonstrate that, at least in certain cases, i-doubling effects produce considerable modifications to the observed band contours of symmetric top molecules. The third subject which I wish to discuss briefly concerns the theoretical treatment of the complete rovibrational Hamiltonian by means of a Van Vleck or contact transformation. This provides the best method of understanding 1-doubling interactions. I shall also present new results, mainly for infrared spectra of cylopropane, illustrating this work. I should like to start by recalling the spectrum of the CH3F molecule in the 1460 cm region, which is illustrated in the lower half of Figure 1. This spectrum was analysed recently by Dr. di Lauro, working in my laboratory'; he demonstrated that the curious appearance of this spectrum is due to the fact that there are two accidentally coincident fundamentals of the molecule within 10 cm1 of each other, which are interacting through an xy axis type of Coriolis perturbation. The fundamentals are an A1 species parallel band, u2 at 14600 cm1, and an E species perpendicular band, 1)5at 14680 cm1. The strong Coriolis interaction between these bands, due to the C2,5(X,Y) Coriolis constant, produces perturbations of the true energy levels and wavefunctions which result in the J structure of the Q branches of the perpendicular band apparently degrading in opposite directions in the high frequency and low frequency wings of the band, and which also result in the very curious appearance of the Q branch of the parallel band close to 1460 cm1. The upper half of Figure 1 shows computer-calculated band contours for this pair of bands, based on a model in which the interaction has been correctly allowed for in its effect on the positions and intensities of all lines in the spectrum. The advantage of this technique is that the computer can be programmed to set up and diagonalize a Hamiltonian matrix numerically, without approximation, despite the fact that it is not possible to obtain analytical expressions for the line positions and intensities. Moreover, the computed

Spectroscopy Research at the McLennan Physical Laboratories of the University of Toronto

Recent research of the Spectroscopy and Molecular Physics Research Group at the Department of Physics, University of Toronto, has been in the general areas: spectroscopy of compressed gases, low temperature spectroscopy, Raman spectra of low pressure gases, optical pumping, applications of lasers to light scattering, and interference spectroscopy in the laboratory and from high altitude balloons. Topics discussed in some detail in this communication are: bound states of hydrogen-foreign gas Van der Waals complexes, intensity anomalies in the Raman effect of solid hydrogen, pressure broadening in the Raman spectra of simple diatomic molecules, a synopsis of work on high resolution rotation and rotation-vibrational Raman spectra of gases, recent results on Brillouin scattering and Raman line widths using lasers, and collision broadening in the spectrum of zinc by optical pumping.