Infrared-microwave double-resonance spectroscopy of the ClO2 radical: A textbook example

The capability of the ir-mw double-resonance technique to produce precise spectroscopic data has been thoroughly tested in a study of the ClO₂ free radical. Although the experiments are based on fixed-frequency gas lasers, the results of the work show that rotational and vibrational information can be obtained from practically all infrared absorption bands of ClO₂ within the operating frequency range of the lasers. A wide range of microwave frequencies was searched using approximately 120 individual laser lines; double-resonance signals were observed with about 25% of the lines used. The double-resonance data obtained were sufficient to determine rotational, distortion, fine, and hyperfine constants for the vibrational states (0, 0, 0), (1, 1, 0), and (0, 0, 1) of ³⁵ClO₂ and (1, 0, 0) and (0, 0, 1) for ³⁷ClO₂. The constants for the (0, 0, 0) state of ³⁵ClO₂ and ³⁷ClO₂ and for (0, 1, 0) of ³⁵ClO₂ were refined and those for the (0, 1, 0) state of ³⁷ClO₂ were determined with data obtained from double-resonance and conventional measurements. The rotational constants obtained allowed equilibrium structures for both³⁵ClO₂ and ³⁷ClO₂ to be calculated. The structural parameters obtained in each case are in excellent agreement: $\text{r}{}_{\mathrm{e}}\text{= 1.4700(1}\text{A}\text{?}\text{)}$, θ_e = 117° 23.3′(7). The centers of the bands ν₁ and ν₁ + ν₂ ? ν₂ of ³⁵ClO₂ were precisely determined and the data available were sufficient to check the reliability of the values obtained. The band origins were also determined for ν₃, ³⁵ClO₂ and ³⁷ClO₂, and with less precision for ν₁ for ³⁷ClO₂.