DIRECT MEASUREMENT OF THE ROTATIONAL G-VALUE IN THE GROUND VIBRATIONAL-STATE OF ACETYLENE BY MAGNETIC VIBRATIONAL CIRCULAR-DICHROISM

The magnetic vibrational circular dichroism (MVCD) spectra of the upsi lon(4) + upsilon(5) combination band of acetylene was measured. From t he result for the P(J('') = 1 --> J' = 0) rotational transition, the g (j)-value in the ground vibrational state for C2H2 was determined to b e + 0.049 (+ 0.005). The sign of the g(j)-value measured by MVCD confi rms that the correct sign is positive as found in a revised molecular beam magnetic resonance experiment and predicted on the basis of molec ular Zeeman studies of other molecules and magnetic effects on heat fl ow in acetylene. MVCD offers an exceptionally simple method of determi ning the sign of molecular g-values for both the ground and excited st ates.