Change in the Inversion Spectrum of ND3from Resonant to Nonresonant Absorption

The absorption due to the inversion spectrum of N${\mathrm{D}}_3$ centered near 1600 Mc/sec was measured at pressures from 0.3 to 80 cm Hg in the frequency interval 1100 to 2600 Mc/sec. The inversion frequency was found to decrease with increasing pressure and became substantially zero at about 9 cm Hg. This change occurs at pressures about 15-fold lower than for N${\mathrm{H}}_3$, a result in accord with Margenau's theory. At the higher pressures where the absorption is of the nonresonant or Debye type, the collision diameter is about 6.9A, a value roughly one-half that associated with resonant absorption at low pressures.