The pure rotational spectrum of YbOH has been recorded and analyzed to produce fine and magnetic hyperfine parameters for the 2 (0,0,0) X   state. These parameters are compared with those determined from the optical study [Melville and Coxon, J. Chem. Phys. 115, 6974-6978 (2001)] and with the values for YbF [Dickinson et al. 115, 6979-6989 (2001)]. The results support the existence of an unobserved perturbing state near the ̃2 Π 1/2 state, similar to that previously found in YbF. The precisely determined parameters lay the foundation for laser cooling YbOH, which will aid in the search for new physics beyond the standard model. Introduction Recently, it has been proposed [1] that the linear triatomic molecule YbOH may be a sensitive venue for investigating T-violating physics beyond the standard model (BSM), such as the electron electric dipole moment (EDM) and nuclear magnetic quadrupole moment (MQM) via the heavy Yb atom. When compared to the diatomic molecules currently used in BSM searches, the additional degrees of freedom offered by YbOH provide several advantages. Specifically, YbOH has the coexistence of an electronic structure amenable to laser cooling, and closely spaced opposite parity states useful for systematic error rejection[1], which are not simultaneously offered by diatomic systems suitable for EDM and MQM searches. A precision measurement with laser cooled and trapped YbOH could extend coherence times (the current limitation of molecular beam BSM searches [2-4]) by orders of magnitude, ultimately probing T-violating BSM physics at the PeV scale[1]. The analogous study of laser cooled YbF has been actively pursued for some time [2, 3, 5, 6]. YbOH offers several advantages compared to YbF. First, the metastable (0,1,0) bending mode of YbOH contains closely spaced states of opposite parity, which enables full

Odd-parity energy levels of the neutral europium atom (Eu I) have been investigated by employing both single-colour and two-colour stepwise laser excitation using the technique of resonance ionization spectroscopy in a heat-pipe thermionic diode system. Fifty-two new odd-parity energy levels of Eu I have been found in the energy region 40 575 - 43 410 . The J values for most of these new energy levels have been assigned unambiguously. In addition to this, 19 odd levels which were reported earlier, in the region of our present study, have also been investigated; the assignments of J values to nine of these levels have been confirmed and four levels, which had no unique J assignments, have been assigned definite J values.

In this work, the X2B1 and A2A1 electronic states of the phosphino (PH2) free radical have been studied by dispersed fluorescence and ab initio methods. PH2 molecules were produced in a molecular free-jet apparatus by laser vaporizing a silicon rod in the presence of phosphine (PH3) gas diluted in helium. The laser-induced fluorescence, from the excited A2A1 electronic state down to the ground electronic state, was dispersed and analyzed. Ten (upsilon1upsilon2upsilon3) vibrationally excited levels of the ground electronic state, with upsilon1 < or = 2, upsilon2 < or = 6, and upsilon3 = 0, have been observed. Ab initio potential-energy surfaces for the X2B1 and A2A1 electronic states have been calculated at 210 points. These two states correlate with a 2Pi(u) state at linearity and they interact by the Renner-Teller coupling and spin-orbit coupling. Using the ab initio potential-energy surfaces with our RENNER computer program system, the vibronic structure and relative intensities of the A2A1 --> X2B1 emission band system have been calculated in order to corroborate the experimental assignments.

We report four odd-parity Rydberg series in the spectrum of europium atoms in the energy region 45 000-45 735 cm-1. These four series belong to total angular momentum quantum numbers J = 5/2,7/2,9/2 and 11/2 and converge to the first ionization limit 9S4. The studies were carried out employing the multistep resonance ionization spectroscopy technique using two pulsed dye lasers. For some of the Rydberg series we could observe the members up to n = 67. Different stepwise excitation-ionization schemes helped us to assign a unique J quantum number up to n = 39 and enabled us to separate the series with different values of J. The Rydberg series belonging to J = 5/2 was found to be quite unperturbed and was therefore used to evaluate the first ionization limit of the europium atom.

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