We have looked for a violation of time-reversal symmetry (T) in the molecule thallium fluoride using a rotationally cold beam from a jet source. Our method was to search for a frequency shift of the thallium nuclear magnetic resonance when an external electric field of 29.5 kV/cm was reversed with respect to a magnetic quantization axis. The measured shift, (1.4+2.4) X 10 Hz, was interpreted as a null result and constitutes a tenfold improvement in sensitivity over the previous measurement. Accordingly, our measurement has reduced the upper limits on the proton and electron electric dipole moments and on other T-violating weak couplings that can be deduced from the frequency shift.

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We have made a very precise test of time-reversal invariance using nuclear-spin resonance in a thallium fluoride molecular beam. We searched for a shift of the 120-kHz Tl resonance when an external electric field was reversed relative to the nuclear spin and found it to be ( -2.2 ± 2.1 )x 10 ~3 Hz, less than 0.02 ppm and a fivefold improvement over any previous measurement. Our result imposes new constraints on the proton electric dipole moment and on CP nonconservation in strong and weak interactions.PACS numbers: 35.20.My, ll.30.Er.33.25.FsThe KQ system provides strong evidence that time reversal (T) is not an exact symmetry of nature, but the origin of the asymmetry remains obscure even today, twenty-three years after its discovery. l Perhaps the best hopes for enlightenment lie in the ongoing searches for other examples of T asymmetry. These include several very sensitive attempts to detect a permanent electric dipole moment (edm) in a nondegenerate system: If it is not zero, both T and P symmetry must be violated. No edm has been found, but the null results to date have nevertheless been important because of the strong limits that they impose on theory. 2 At present, the best limits on possible P-and T-asymmetric interactions come from measurements on neutrons, 3 atoms, 4 and molecules. 5 In this Letter we report the first results from a new apparatus designed to look for T asymmetry in the thallium fluoride (TIF) molecule. As was first pointed out by Sandars, 6 this heavy polar molecule is a sensitive system in which to search because it is much more easily polarized than an atom by a supposed T-odd, P-odd interaction. Many different mechanisms are able in principle to induce an effect of the kind we have tried to detect. These include P-and T-odd nucleon-nucleon or electron-nucleon interactions, as well as the "intrinsic" nucleon edm measured by the neutron experiments. The results described here, which constitute a fivefold improvement over any previous measurement in a molecule, are both complementary to and competitive with the limits derived from the neutron and from atoms. We are able to place new limits on several fundamental quantities.Our experiment is most simply viewed as the search for a .P-odd, T-odd hyperfine interaction between the spin-y Tl nucleus (spin direction