Spectral line parameters in the (4 ← 0) overtone band and the dipole moment function of HI

We present new measurements of the line strengths in the third vibrational overtone band in pure HI and its mixtures with Ne, Ar, and Xe, and report results of the Herman-Wallis analysis for this band. Significance of the higher-order terms in the polynomial representation of the dipole moment function is discussed. It is concluded that the spectroscopic data yield the dipole moment function fully described by a cubic polynomial in powers of the reduced displacement from the equilibrium bond length. In the Padé approximant for the dipole moment function, the vicinity of the saddle point near equilibrium also can be accurately fitted with a cubic polynomial. Pressure line broadening and shifting parameters are reported for mixtures of HI with rare gases.