Knowledge Bank

The technique of Two-Tone Frequency-Modulation Spectroscopy (TTFMS) using a multimode CW dye laser was employed to study high overtone bands of HBr. Individual vibration-rotational line strengths and positions were measured. On the basis of these measurements, rotationless dipole moment matrix elements and Herman-Wallis coefficients were determined for the 6-0, 7-0, and 8-0 overtone bands. Using the rotationless dipole moment matrix elements obtained previously by other workers for the pure rotational, fundamental, and first four overtone bands, together with the present results and numerical vibration-rotational wavefunctions, we have derived a series expansion for the dipole moment function. This function provides an excellent fit to all existing Herman-Wallis coefficients, and thus represents one of the best known dipole moment functions for any diatomic molecule. The accuracy of the present data also confirms the importance and utility of TTFMS when used for performing ultrasensitive molecular spectroscopic measurements of weak overtone bands.