Determination of the dipole moment of ¹³C methanol by microwave Stark spectroscopy

Abstract

In this work, precise microwave (MW) frequencies were measured for nine different transitions in the three lowest torsional states of ¹³C-substituted methanol, for a wide range of applied dc electric fields. The Stark shifted frequencies were measured with accuracies of ±10 kHz. The results were analyzed to deduce accurate dipole moment values for the three lowest torsional states. Substantial variation of the dipole moment was observed as a function of the torsional state. This is the first time the dipole moments have been determined for this isotopomer of methanol to such a high degree of precision. The zero-field frequencies were also determined with greater precision than were previously known. Additionally, the transitions in the excited torsional states have not been measured previously. The dipole moment determined for the torsional ground state are μ_a = 0.8947(8) and μ_b = 1.4274(6) D. The dipole moment value increases with torsional excitation. These values will be useful for the calculation of relative strengths of optically pumped far-infrared laser lines and interstellar microwave and millimeter-wave transitions.