We describe kinetic control of DNA hybridization: loop complexes are used to inhibit the hybridization complementary oligonucleotides; rationally designed catalysts shown be effective in promoting their hybridization. This is basis a strategy for using as fuel drive free-running artificial molecular machines.

An increasing body of data suggests the existence a unique set criteria characterizing ``ideal'' high-${\mathit{T}}_{\mathit{c}}$ superconductivity. Common to those materials exhibiting these characteristics, i.e., layered cuprates and certain organic superconductors, is reduced dimensionality superfluid density. In cases where carriers are confined two dimensions, parameters specifying superconducting state would include two-dimensional carrier density ${\mathit{n}}_{2\mathrm{D}}$,...

Slow positrons can be obtained by moderating the energetic β+ particles from a radioactive source. We find that solid Ne makes more efficient moderator than any other material known to date. The efficiency ε, defined as number of slow per emitted source, is (0.30±0.02)% for flat layer covering 22Na deposit. In cylindrical geometry, ε (0.70±0.02)%, twice previous best with single-crystal tungsten. energy spectrum has full width at half-maximum 0.58 eV, somewhat broader metal. Moderators made...

A beam of positrons is used to study positronium (Ps) formation at the surfaces clean, single-crystal targets. The Ps probability changes with incident energy $E$ as ${(\frac{E}{{E}_{0}}+1)}^{\ensuremath{-}1}$, ${E}_{0}=915(30), 6000(150), \mathrm{and} 2900(150)$ eV for Si, Ge, and Al 300 K. Since ${E}_{0}$ observed be temperature dependent, it argued that not being formed by backscattering from surface; rather, energetic thermalize in bulk diffuse surface where about half escape free

We report measurements of the transmission 1 - 6-keV energy positrons through films Al, Cu, and Si up to 3000 \AA{} thick. When thickness is expressed in terms mass per unit area, Cu Al found be same within \ifmmode\pm\else\textpm\fi{}10%. Within a \ifmmode\pm\else\textpm\fi{}20% precision we observe no effect having do with crystallinity materials. The median penetration depth vary $E$ as ${E}^{n}$, $n={1.60}_{\ensuremath{-}0.08}^{+0.15} \mathrm{and} {1.43}_{\ensuremath{-}0.11}^{+0.07}$,...

Positronium negative ions ${\mathrm{Ps}}^{\ensuremath{-}}$ are formed by a beam of 420-eV positrons impinging on thin carbon film in vacuum. Measurements the number reaching grid under different acceleration conditions used to determine both decay rate $\ensuremath{\Gamma}=2.09(9)$ ${\mathrm{nsec}}^{\ensuremath{-}1}$ and its initial kinetic energy $T={13}_{\ensuremath{-}10}^{+19}$ eV. The is good agreement with recent calculation Ho.

We have measured the kinetic energy of positronium (Ps) atoms emitted into a vacuum from porous silica film subsequent to positron bombardment, via Doppler spread linewidth Ps ${1}^{3}S$-${2}^{3}P$ transition. find that deeper in target positrons are implanted colder is Ps, an effect we attribute cooling collisions pores as diffuse back surface. observed lower limit mean associated with motion direction laser, ${E}_{x}=42\ifmmode\pm\else\textpm\fi{}3$ meV, consistent conversion confinement...

A beam of slow positrons in the few-eV region is directed at bakable solid targets a vacuum (2-4) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}8}$ Torr. Measurements $\frac{2\ensuremath{\gamma}}{3\ensuremath{\gamma}}$ annihilation-yield ratio indicate conversion incident into positronium. The efficiency depends on target material and positron energy, varying from less than 25% 300 K to 40-85% 900 K.

In this paper we consider the possibilities for producing a dense gas of N\ensuremath{\simeq}${10}^{5}$ triplet positronium (Ps) atoms in vacuum contained small cavity V\ensuremath{\simeq}${10}^{\mathrm{\ensuremath{-}}13}$ ${\mathrm{cm}}^{3}$. We then scenario where polarized Ps may cool through weakly interacting Bose transition. The rates thermalization with wall, equilibrium bulk, and loss polarization by exchange collisions are calculated. A method observing transition is also discussed.

Slow (5 eV) positrons (${e}^{+}$) are observed emanating from clean (submonolayer contaminated) single-crystal metal surfaces which being bombarded by 0.1-3-keV ${e}^{+}$. A portion of the emitted slow ${e}^{+}$ Al(100) has a narrow range energies and temperature-dependent intensity suggests that these thermalized in bulk have negative work function ${\ensuremath{\phi}}_{+}=\ensuremath{-}0.2\ifmmode\pm\else\textpm\fi{}0.1$ eV. higher-energy slow-${e}^{+}$ spectrum temperature-independent...

Using continuous-wave excitation to eliminate the problems inherent with pulsed laser measurements of nonlinear transitions, we have measured ${1}^{3}$${\mathit{S}}_{1\mathrm{\ensuremath{-}}}$${2}^{3}$${\mathit{S}}_{1}$ interval in positronium (Ps) be 1 233 607 216.4\ifmmode\pm\else\textpm\fi{}3.2 MHz. The quoted 2.6 ppb (parts per ${10}^{9}$) uncertainty is primarily due determination Ps resonance relative ${\mathrm{Te}}_{2}$ reference line, a 1.5 contribution from recent calibration line...

The hyperfine interval $\ensuremath{\Delta}\ensuremath{\nu}$ of the positronium ground state has been measured with significantly improved accuracy. Using a S${\mathrm{F}}_{6}$ buffer gas we obtain $\ensuremath{\Delta}\ensuremath{\nu}=203.3875\ifmmode\pm\else\textpm\fi{}0.0017$ GHz and...

We have measured the $1^{3}S_{1}\ensuremath{-}2^{3}S_{1}$ interval in positronium (Ps) to be 1 233 607 185 \ifmmode\pm\else\textpm\fi{} 15 MHz, agreement within 1% of ${\ensuremath{\alpha}}^{3}{R}_{\ensuremath{\infty}}$ QED prediction. The quoted 12-ppb uncertainty has equal contributions from measurement Ps resonance relative a ${\mathrm{Te}}_{2}$ absorption line and calibration deuterium $2{S}_{\frac{1}{2}}\ensuremath{-}4{P}_{\frac{3}{2}}$ Balmer line.

An intense pulsed positron source has been developed using a buffer gas trap to accumulate large numbers of positrons and create dense plasma, which may then be bunched spatially focused. Areal densities more than 3×1010e+cm−2 have achieved in subnanosecond pulse producing an instantaneous current 10mA. We describe various aspects the device including detection technique specifically for use with pulses. Two applications are also described as well future experiments such formation...

Recent developments in positron trapping technology have made possible experimentation with dense interacting positronium gases. Along these capabilities comes a need for suitable measurement techniques, and accordingly we developed method to measure lifetimes from single intense burst of positrons. Our is based on recording the anode signal photomultiplier fast oscilloscope following short-time which allows us transitory effects as well high density interactions.

The $2^{3}S_{1}\ensuremath{-}2^{3}P_{2}$ energy splitting of positronium has been observed by inducing rf transitions in a weak magnetic field (\ensuremath{\simeq} 50 G) the absence buffer gas and observing resonant increase Lyman-$\ensuremath{\alpha}$ emission. We obtain 8628.4\ifmmode\pm\else\textpm\fi{}2.8 MHz, which is within 2 standard deviations theoretical value computed to order $\ensuremath{\alpha}^{3}R$ Fulton Martin.

The $1^{3}S_{1}\ensuremath{\rightarrow}2^{3}S_{1}$ transition in positronium has been observed using two-photon Doppler-free excitation. Transitions were induced by a pulsed dye laser, and detected photoionization of the $2^{3}S_{1}$ state. resonance signal linewidth 1.5 GHz signal-to-noise ratio 20:1. frequency agrees with theory to 0.5 GHz. an amplitude proportional square laser intensity displays expected Stark shift higher under applied electric field.

A direct measurement is reported of the velocity positronium (Ps) ejected into a vacuum when 0- to 100-eV positrons (${e}^{+}$) strike negatively biased Cu(111) surface. At 30\ifmmode^\circ\else\textdegree\fi{}C, about half ${e}^{+}$ form Ps with normal energy component $\overline{E}=3.4(3)$ eV. 790\ifmmode^\circ\else\textdegree\fi{}C, most remaining but $\overline{E}=0.14(1)$ eV, and non-Maxwellian thermal distribution. We infer that surface-bound are thermally desorbed extra Ps. These low...

The first observation of Lyman-$\ensuremath{\alpha}$ radiation from positronium is described. Positronium formed in vacuum by a slow positron beam incident on solid target. Single photons transmitted through an \ensuremath{\approx} 20-\AA{}-wide interference filter peaked near the 2430-\AA{} ($2P\ensuremath{\rightarrow}1S$) line are observed coincidence with annihilation $\ensuremath{\gamma}'\mathrm{s}$ ground state positronium. intensity corresponds to production one "atom" excited for...