Infrared Line and Band Strengths and Dipole Moment Function in HCl and DCl

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Abstract
Strengths of the individual isotopic lines have been measured in the 2–0, 3–0, 2–1, and 3–2 bands of HCl35 and HCl37 and in the 1–0, 2–0, and 3–0 bands of DCl35 and DCl37. The relative line strengths for the 1–0 band of DCl are shown to agree with that expected theoretically including the effect of the interaction of vibration and rotation, but the line strengths in the 2–0 bands agree about equally well with the rigid rotor and with the theoretical expressions including the vibration‐rotation interaction. Experimental band strengths and vibrational matrix elements are derived from the measured line strengths for each band. These data are combined with previous results on the HCl 1–0 band to obtain power series expansions for the electric dipole moment function of the hydrogen chloride molecule. It is found that the effective charge is of the same sign as the permanent moment and that the second derivative of the dipole moment with respect to internuclear distance at re is probably of opposite sign. In general, for the same number of terms in the dipole moment expansion it is found that Morse vibrational wave functions give considerably better agreement between observed and calculated matrix elements than do anharmonic oscillator functions retaining terms in the potential energy through the fourth power in the internuclear separation.