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Register a new userUV frequency metrology on CO (a3Pi); isotope effects and sensitivity to a variation of the proton-to-electron mass ratio
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UV frequency metrology has been performed on the a3Pi - X1Sigma+ (0,0) band of various isotopologues of CO using a frequency-quadrupled injection-seeded narrow-band pulsed Titanium:Sapphire laser referenced to a frequency comb laser. The band origin is determined with an accuracy of 5 MHz (delta u / u = 3 * 10^-9), while the energy differences between rotational levels in the a3Pi state are determined with an accuracy of 500 kHz. From these measurements, in combination with previously published radiofrequency and microwave data, a new set of molecular constants is obtained that describes the level structure of the a3Pi state of 12C16O and 13C16O with improved accuracy. Transitions in the different isotopologues are well reproduced by scaling the molecular constants of 12C16O via the common mass-scaling rules. Only the value of the band origin could not be scaled, indicative of a breakdown of the Born-Oppenheimer approximation. Our analysis confirms the extreme sensitivity of two-photon microwave transitions between nearly-degenerate rotational levels of different Omega-manifolds for probing a possible variation of the proton-to-electron mass ratio, mu=m_p/m_e, on a laboratory time scale.
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Recently, methanol was identified as a sensitive target system to probe variations of the proton-to-electron mass ratio $mu$ [Jansen emph{et al.} Phys. Rev. Lett. textbf{106}, 100801 (2011)]. The high sensitivity of methanol originates from the interplay between overall rotation and hindered internal rotation of the molecule -- i.e. transitions that convert internal rotation energy into overall rotation energy, or vice versa, give rise to an enhancement of the sensitivity coefficient, $K_{mu}$. As internal rotation is a common phenomenon in polyatomic molecules, it is likely that other molecules display similar or even larger effects. In this paper we generalize the concepts that form the foundation of the high sensitivity in methanol and use this to construct an approximate model which allows to estimate the sensitivities of transitions in internal rotor molecules with $C_{3v}$ symmetry, without performing a full calculation of energy levels. We find that a reliable estimate of transition sensitivities can be obtained from the three rotational constants ($A$, $B$, and $C$) and three torsional constants ($F$, $V_3$ and $rho$). This model is verified by comparing obtained sensitivities for methanol, acetaldehyde, acetamide, methyl formate and acetic acid with a full analysis of the molecular Hamiltonian. From the molecules considered, methanol appears to be the most suitable candidate for laboratory and cosmological tests searching for a possible variation of $mu$.
Molecular transitions recently discovered at redshift z_abs=2.059 toward the bright background quasar J2123-0050 are analysed to limit cosmological variation in the proton-to-electron mass ratio, mu=m_p/m_e. Observed with the Keck telescope, the optical echelle spectrum has the highest resolving power and largest number (86) of H_2 transitions in such analyses so far. Also, (seven) HD transitions are used for the first time to constrain mu-variation. These factors, and an analysis employing the fewest possible free parameters, strongly constrain mus relative deviation from the current laboratory value: dmu/mu =(+5.6+/-5.5_stat+/-2.9_sys)x10^{-6}, indicating an insignificantly larger mu in the absorber. This is the first Keck result to complement recent null constraints from three systems at z_abs>2.5 observed with the Very Large Telescope. The main possible systematic errors stem from wavelength calibration uncertainties. In particular, distortions in the wavelength solution on echelle order scales are estimated to contribute approximately half the total systematic error component, but our estimate is model dependent and may therefore under or overestimate the real effect, if present. To assist future mu-variation analyses of this kind, and other astrophysical studies of H_2 in general, we provide a compilation of the most precise laboratory wavelengths and calculated parameters important for absorption-line work with H_2 transitions redwards of the hydrogen Lyman limit.
Far infrared fine-structure transitions of CI and CII and rotational transitions of CO are used to probe hypothetical variations of the electron-to-proton mass ratio mu = m_e/m_p at the epoch of reionization (z > 6). A constraint on Delta mu/mu = (mu_obs - mu_lab)/mu_lab = (0.7 +/- 1.2)x10^-5 (1sigma) obtained at <z> = 6.31 is the most stringent up-to-date limit on the variation of mu at such high redshift. For all available estimates of Delta mu/mu ranging between z = 0 and z = 1100, - the epoch of recombination, - a regression curve Delta mu/mu = k_mu (1+z)^p, with k_mu = (1.6 +/- 0.3) x10^-8 and p = 2.00 +/- 0.03, is deduced. If confirmed, this would imply a dynamical nature of dark matter/dark energy.
Astrophysical molecular spectroscopy is an important means of searching for new physics through probing the variation of the proton-to-electron mass ratio, $mu$. New molecular probes could provide tighter constraints on the variation of $mu$ and better direction for theories of new physics. Here we summarise our previous paper citep{19SyMoCu.CN} for astronomers, highlighting the importance of accurate estimates of peak molecular abundance and temperature as well as spectral resolution and sensitivity of telescopes in different regions of the electromagnetic spectrum. Whilst none of the 11 astrophysical diatomic molecules we investigated showed enhanced sensitive rovibronic transitions at observable intensities for astrophysical environments, we have gained a better understanding of the factors that contribute to high sensitivities. From our results, CN, CP, SiN and SiC have shown the most promise of all astrophysical diatomic molecules for further investigation, with further work currently being done on CN.
Molecular hydrogen (H2) absorption features observed in the line-of-sight to Q2348-011 at redshift zabs = 2.426 are analysed for the purpose of detecting a possible variation of the proton-to-electron mass ratio mu=mp/me. By its structure Q2348-011 is the most complex analysed H2 absorption system at high redshift so far, featuring at least seven distinctly visible molecular velocity components. The multiple velocity components associated with each transition of H2 were modeled simultaneously by means of a comprehensive fitting method. The fiducial model resulted in dmu/mu = (-0.68 +/- 2.78) x 10^-5, showing no sign that mu in this particular absorber is different from its current laboratory value. Although not as tight a constraint as other absorbers have recently provided, this result is consistent with the results from all previously analysed H2-bearing sight-lines. Combining all such measurements yields a constraint of dmu/mu < 10^-5 for the redshift range z = (2--3).
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