Spectral line parameters in the (2 ← 0) overtone band and the dipole moment function of the HI molecule
We present here new high-resolution experimental data on the linestrengths and pressure-broadened Lorentzian widths for the P2(13) to R2(12) vibration-rotation lines in the first overtone absorption band of hydrogen iodide. By combining the measured linestrengths with our previous results for the fundamental band [J. Mol. Spectrosc. 218 (2003) 75] and with other published data for the higher-overtone bands of this molecule, an improved dipole moment expansion as a function of the dimensionless reduced nuclear displacement x is obtained: μ( x)=0.4471(5)-0.0772(2) x+0.542(3) x2-1.90(2) x3. This experimental dipole moment function is compared with the results of a few recent non-relativistic and relativistic ab initio calculations. The agreement between theoretical and experimental Herman-Wallis coefficients for the first two vibrational bands of HI is found best as ever reported before.