Detection of low-frequency lambda-doublet transitions of the free ¹² CH and ¹³ CH radicals

Abstract

By Fourier transform microwave spectroscopy, lambda-doubling transitions of 12 CH and 13 CH in the lowest rotational levels of the X 2 ͟1/2 ground state have been directly detected, which has not been done previously. For both radicals, hyperfine-split lines have been measured to an accuracy of better than 1 ppm between 3 and 15 GHz, an improvement of at least 2 orders of magnitude over previous laboratory data. The measured frequencies have been combined with all previous data for CH and 13 CH in the v ‫؍‬ 0 level of the X 2 ͟ electronic state to determine improved hyperfine parameters. The production of CH from various gases also has been studied and, with methanol, the yield of CH relative to OH. Astronomical studies of CH in higher rotational levels and 13 CH can now be undertaken on the basis of the present work.microwave spectroscopy ͉ molecular data ͉ radio astronomy T he CH radical is widely distributed in astronomical sources, observed in planetary atmospheres, comets, HI and HII regions, dark molecular clouds, and planetary nebulae. It was one of the first interstellar molecules identified (1, 2), by means of detection in the optical band of a single line [R 2 (1)] near 4,300 Å of the A 2 ⌬ 3 X 2 ͟ electronic transition. It has now been observed in other parts of the spectrum as follows: (i) in the radio, by detection of lambda-doubling transitions near 3.3 GHz (3-5) in the lowest rotational level (J ϭ 1͞2) of the X 2 ͟ 1/2 state; (ii) near 700 MHz (6) in the lowest rotational level of the upper fine structure ladder (X 2 ͟ 3/2 ); (iii) in the far-infrared (FIR), by detection in both absorption (7) and emission (8) of its fundamental rotational transitions at 149.09 and 149.39 m; and (iv) in the far UV, including the diffuse feature at 1369.13 Å (9). There is little astronomical data on the rare isotopic species 13 CH, but some of its lines have been identified in the solar spectrum (10).Because of the astronomical importance of CH, considerable effort has been devoted in the laboratory to measure its rotational spectrum. Early studies by laser magnetic resonance (ref.11 and references therein) allowed some of the more interesting lines to be predicted to an uncertainty of Ϸ2 MHz. Since then, more accurate FIR spectra (good to 100 kHz; ref. 12) and submillimeter-wave spectra (good to 30-100 kHz; ref. 13) have been obtained, as well as microwave-optical double resonance (MODR) measurements (14, 15) and a few centimeter-wave measurements at fairly high J ϳ 6.5 (16) to better pin down the lambda-doubling intervals. 13 CH has been studied by some of the same techniques (17-19), but owing to the lack of tunable FIR measurements, its rotational spectrum until now could only be predicted to a few megahertz.Although much laboratory spectroscopy has already been done on CH and 13 CH, still more is needed: many of the lambda-doubling transitions have not been measured to high precision, and only a small number of the FIR spin-rotational intervals have been detected at adequately high signal-to-noise. It is parti...

CH radio emission from Heiles Cloud 2 as a tracer of molecular cloud evolution

“…We observed two hyperfine components of the J = 1/2 Λ-type doubling transition of CH (F = 1−1; ν = 3.335482(1) GHz; S μ 2 = 1.4205 Debye 2 : F = 0−1; ν = 3.349193(3) GHz; S μ 2 = 0.7105 Debye 2 ) (McCarthy et al 2006;Müller et al 2005 (CDMS)) toward HCL2. Here, the numbers in parentheses represent the 1 σ error in units of the last significant digits.…”

“…It should be noted that the 1 σ uncertainty of the rest frequencies for the main and satellite hyperfine components are 1 and 3 kHz, respectively, corresponding to velocities of 0.09 and 0.27 km s −1 , respectively (Table 1) (McCarthy et al 2006). Therefore, we cannot definitively identify which velocity component appears as the narrow component among several velocity components known for other molecules (e.g.…”

Method for traveling-wave deceleration of buffer-gas beams of CH

“…1. We note that we are unable to resolve the ground-state hyperfine structure and that parity selection rules prevent measurement of the lambda-doubling in the ground state [20].…”

Sensitivity of rotational transitions in CH and CD to a possible variation of fundamental constants

“…1. Level scheme of the ground state of CH, calculated using the Hamiltonian matrix from Amiot et al [12] and the molecular constants given by McCarthy et al [14]. Indicated are five different types of rotational transitions, labeled I through V. The -doublet splitting is exaggerated by a factor of 10.…”