Avoided-crossing molecular-beam spectroscopy of methane

Publisher Website
Google Scholar
Abstract
An avoided‐crossing RF resonance technique has been used to study methane (12CH4) in the (J=2) rotational state with a conventional molecular‐ beam magnetic resonance spectrometer. In zero external field the (I=1) levels are about 8 MHz above the (I=0) levels because of the tensor centrifugal distortion interaction. The (I=1) levels with negative nuclear Zeeman effect can be made nearly degenerate with corresponding (I=0) states by the application of a magnetic field of about 2 kG. Mixing of the levels by nuclear hyperfine interactions gives rise to avoided crossings. Electric and magnetic dipole transitions were observed near these avoided crossings. The method is discussed in detail. Values were obtained for the scalar and tensor spin–rotation constants ca and cd, and for the tensor spin–spin coupling constant d. By using a value of the tensor centrifugal distortion splitting Δ2=60 (DT+6 H4T+36 L4T) calculated from a combined double‐resonance and microwave absorption set of distortion constants, a value of the rotational g‐factor gJ was obtained. Alternatively, the value of gJ measured in studies of the rotational‐moment molecular‐beam spectrum can be used, in conjunction with the present work, to obtain Δ2. The values and their estimated uncertainties (90% confidence limits) are ca=10.372 (83) kHz, cd=18.370 (23) kHz, d=21.17 (32) kHz, gJ=0.31317 (28), and Δ2=7970.30 (66) kHz. These results for (J=2) are in agreement with previous determinations, which (except for Δ2) represent averages over many values of J. The application of the present measurements to calculations of the nuclear magnetic shielding constants and of the molecular magnetic susceptibility is discussed.