Rotational spectra of the complex HCN-BF3 have been observed using pulsed-nozzle Fourier transform microwave spectroscopy. Spectra have been observed for the l l B and loB species with both HC14N and HCISN, and the hyperfine structure has been analyzed. The complex has the expected C3" structure, with the nitrogen end of the H C N toward the boron, but the observed BN bond length of 2.473(29) A is notably shorter than that in the related weakly bound systems NGN-BF~ and NGC-CEN-BF~.The out-of-plane distortion of the BF3 cannot be determined accurately but is probably less than 3 O . We compare the structure, force constants, quadrupole coupling constants, and binding energy of the complex with those of other species formed from BF3 and a variety of nitrogen donors. Despite the rather short B-N bond length, the force constant for the intermolecular bond, as well as the boron and nitrogen nuclear quadrupole coupling constants, is comparable to those of a truly weakly bound system.

Microwave and tunable farinfrared laser spectroscopy of the ammonia-water dimerA tunable far-infrared difference frequency spectrometer has been used to examine the fully protonated form of malonaldehyde in the region near the ground-state tunneling frequency (21 cm -I). An extremely dense and complex spectrum is observed in which the strongest features have been assigned as pure rotational lines involving high values of J and K _ I' These transitions, which occur within the individual rotational manifolds of the two halves of the ground-state tunneling doublet, have been analyzed simultaneously with existing microwave data for this species. The value of the ground-state tunneling splitting, determinable indirectly from analysis of vibration-rotation interactions, is 21.58476(17) cm-I , and is in close agreement with that similarly obtained in previous microwave work. A thorough treatment of the centrifugal distortion in this system significantly extends the range of rotational states whose energies may be reliably calculated, and should therefore be valuable in the future direct measurement of the tunneling frequency. Aspects of the far-infrared spectrum of this species, and of the indirect method of determining the tunneling splitting, are discussed. h)Present address:

We report a precise determination of the 19 Ne half-life to be T 1/2 = 17.262 ± 0.007 s. This result disagrees with the most recent precision measurements and is important for placing bounds on predicted right-handed interactions that are absent in the current Standard Model. We are able to identify and disentangle two competing systematic effects that influence the accuracy of such measurements. Our findings prompt a reassessment of results from previous high-precision lifetime measurements that used similar equipment and methods.PACS numbers: 24.80.+y, 27.20.+n, 12.15.Hh, 29.40.Mc Precise measurements of decay rates and angular correlations in semi-leptonic processes are known to be excellent probes for interactions that are predicted by extensions of the Standard Model [1]. For example, the measured lifetime and electron asymmetry in neutron β decay [2] are used to probe for right-handed currents and obtain a precise value of V ud , the up-down element of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix, in a relatively simple system that is free of nuclear structure effects. However, in spite of this compelling advantage, precision neutron β decay experiments are challenging. Current results from independent neutron decay measurements show large discrepancies that need to be addressed before conclusive interpretations can be made from the data [2]. In this regard Nature offers a fortuitous alternative in 19

The two Σ and four Π states of the weakly bound complex Ar–NH3 correlating to j=2, k=±1 ammonia have been observed by tunable far infrared difference frequency-microwave sideband spectroscopy. The results have been combined with published data to determine a new angular potential energy surface for the system. The barrier to threefold internal rotation of the NH3 about its C3 axis in the complex is estimated to be 25.606(24) cm−1 near the minimum energy (T-shaped) configuration. The potential also exhibits maxima at both symmetric top configurations, with energies approximately 53 and 31 cm−1, respectively above that of the global minimum. The location and splitting between the symmetric and antisymmetric Σ states are indicative of a strong interaction with another pair of unobserved states, most likely the first excited intermolecular stretch built on j=1, k=±1 Ar–NH3.