The rotational spectrum and molecular parameters of ClO in the v=0 and v=1 states

The $\text{J =}\text{9}\text{2}\text{←}\text{7}\text{2}$, ${}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and ${}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, ground vibrational state transitions of ³⁵ClO and ³⁷ClO and the ${}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, excited vibrational state transitions of ³⁵ClO have been observed in the 164–167 GHz region. Additional measurements have also been made on the $\text{J =}\text{3}\text{2}\text{←}\text{1}\text{2}$ and $\text{J =}\text{5}\text{2}\text{←}\text{3}\text{2}$ transitions of both the ground and excited vibrational states. All measurements were made using millimeter oscillators which were phase locked to harmonics of a Hewlett-Packard microwave spectrometer's X-band source. Λ-doubling splitting of a few ${}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ transitions was resolved.When magnetic and nuclear quadrupole hyperfine terms off-diagonal in J and $\text{Ω}$ in the Hund's case (a) representation were included in addition to the usual diagonal terms, an excellent fit to all of our observed transitions resulted. The most significant change from previously determined parameters is the centrifugal distortion constant D for which the values, D₀ = 0.03972(26) MHz for ³⁵ClO, D₀ = 0.03888(32) MHz for ³⁷ClO and D₁ = 0.0395(21) MHz for ³⁵ClO are obtained. Values of 1.56959(1) Å for the equilibrium bond length and 854(7) cm^?1 for the equilibrium vibrational frequency are derived from the measured spectra. In addition, values for the Λ-doubling constant β_p and the quadrupole coupling constant eQq₂ were derived from the measured spectra for the first time.