Two-photon spectroscopy of antiprotonic helium

The precision of laser spectroscopy of antiprotonic helium (a helium atom with one of its electrons replaced by an antiproton) has improved by almost 4 orders of magnitude over its 20 years of history. Experimental transition frequencies can be compared to 3-body QED calculations to derive the antiproton-electron mass ratio. In the latest measurements of the Asacusa experiment at CERN, two-photon transitions of antiprotonic helium were excited using two counter-propagating laser beams. This method reduces the Doppler-broadening caused by the thermal motion of the atoms, and allowed us to measure the transition frequencies with a fractional precision of 2.5–5 parts in 10⁹. From these frequencies, we derived an antiproton-electron mass ratio of 1836.1526736(23). Our precision approaches that of the experimental value of the proton-electron mass ratio, and agrees with the latter within errors. Assuming CPT symmetry (i.e. \(m_{p}=m_{\overline {p}}\) ), we further derived the electron’s atomic mass as m _e = 0.0005485799091(7)u from the more accurately known atomic mass of the proton.     

Precision spectroscopy of antiprotonic helium

We survey recent progress in the theoretical study of vibrational transitions in the antiprotonic helium atom. Along with the latest experiment they allow to achieve a competitive accuracy in determination of the atomic mass of an electron and thus they have been included into the CODATA06 analysis of the fundamental constants. Improved theoretical calculation of the hyperfine structure in $^{4\!}\mbox{He}\bar{p}\/$ atom will be considered as well. We will discuss contributions of order $R_\infty\alpha^4$ to the electron spin-orbit interaction. These corrections are necessary to confirm the latest measurements of the 12.9 MHz intervals of the (n,l)?=?(37,35) state in $^4\mbox{He}^+\bar{p}$ and for precise determination of the antiproton magnetic moment.     

Sub-ppm laser spectroscopy of antiprotonic helium and a CPT-violation limit on the antiprotonic charge and mass

Six laser-resonant transitions have been detected in metastable antiprotonic helium atoms produced at the CERN Antiproton Decelerator. They include UV transitions from the last metastable states in the v = n-l-1 = 0 and 1 cascades. Zero-density frequencies were obtained from measured pressure shifts with fractional precisions between 1.3 x 10(-7) and 1.6 x 10(-6). By comparing these with QED calculations and the antiproton cyclotron frequency, we deduce that the antiproton and proton charges and masses agree to within 6 x 10(-8) with a confidence level of 90%.     

Observation of cold, long-lived antiprotonic helium ions

Cold, two-body antiprotonic helium ions p 4He2+ and p 3He2+ with 100-ns-scale lifetimes, occupying circular states with the quantum numbers ni=28-32 and li=ni-1 have been observed. They were produced by cooling three-body antiprotonic helium atoms in an ultra-low-density helium target at temperature T approximately 10 K by atomic collisions, and then removing their electrons by inducing a laser transition to an autoionizing state. The lifetimes of p 3He2+ against annihilation induced by collisions were shorter than those of p 4He2+, and decreased for larger-ni states.     

High-precision laser spectroscopy of antiprotonic helium atomcules and effects of collisions at high-density conditions

We have performed laser spectroscpy of metastable antiprotonic helium atoms (or “atomcules”) ( He⁺) and have observed a density dependence of the resonance vacuum wavelengths for the known transitions (n,l)=(39,35)→(38,34) and (37,34)→(36,33). They showed linear red-shifts of 0.61±0.01 GHz and 0.22±0.02 GHz per 1 g℞, respectively. With the shift parameters above, the transition vacuum wavelengths were extrapolated to zero-density limits, yielding λ₀ = 597.2570± 0.0003 nm and λ₀ = 470.7220±0.0006 nm, respectively. These values were compared with the result of recent theoretical calculations on the energy of the Coulombic three-body system, including relativistic corrections and the Lamb shift. The agreements between our experimental values and the calculations have become as good as 2×10^-6. This sets a severe constraint on the antiproton charge ( ) and mass ( ) with |Q _p - |/e < 5 × 10^-7 and |M _p - |/M _p < 5 × 10^-7, under a more precisely known constraint on the charge-to-mass ratio. Thus we have opened a new possibility of measuring fundamental constants of the antiproton. This revised version was published online in August 2006 with corrections to the Cover Date.     

The antiprotonic helium atom

The Born-Oppenheimer approximation is used to discuss the high rotational and vibrational state of the (He p⁻)e⁻ system in the electronic 1sσ and 2pσ states. Very high angular momentum states, in which the antiproton is well outside the electron orbit, have a Rydberg-like character. states in which the antiproton is within the 1sσ electron orbit have enhanced radiative lifetimes due to the polarization of the 1sσ state by the antiproton. This effect may account for the long-lived component observed in antiproton destruction in He. Preliminary results on the effect of coupling to the 2pσ well, in which the polarization effects enhance the decay rate, are also presented. Some consequences for the suggestion that metastable antiprotonic He atoms may be used to promote antihydrogen formation are discussed.     

Metastable antiprotonic helium atomcules

The present status and future scope of the metastabe antiprotonic helium atomcules ¯pHe⁺ are described. Recently developed laser resonance spectroscopy has not only established the atomcule model but also triggered high-precision calculations of these atomcules.     

X-rays from antiprotonic helium in helium gas

X-ray transitions to the 4F, 3D, and 2P atomic levels of p?He have been observed with antiprotons stopped in He gas at 4 and 1.1 atm NT. The population by radiative transitions of the 3D level in gas Y_M(4atm) = (28±14)% and Y_M(1.1 atm) = (43±22)% exceeds by more than one order of magnitude that measured in liquid He. The annihilation width of the 3D level Γ^a_3D = 2.8±1.0 × 10^?3eV is determined from the ratio between the numbers of X-rays feeding and depopulating the 3D level. The strong-interaction shift of the 2P level ε(2P) = ?14±6 eV is obtained by inputting the $\text{p}\text{He}$ experimental X-ray yields into a cascade calculation, the results of which are in good agreement with well-established data from muonic, pionic, and kaonic helium.     

Auger decay rates of antiprotonic helium

Auger decay rates of the metastable antiprotonic helium ^3,4He e are calculated. The variational method and solution of coupled differential equations are combined to determine the initial metastable state wave function. Besides metastable states, the calculation reveals specific short-lived states of the antiprotonic helium with an essentially different structure of the wave function. An effect of mixture of the wave functions is taken into account to calculate the decay rate for a few metastable states, which are close in energy to the short-lived ones. This revised version was published online in August 2006 with corrections to the Cover Date.     

Metastable states of antiprotonic helium atoms

We discuss the latest theoretical achievements in calculations of energy transitions in the antiprotonic helium He⁺ p-0304; atoms. New variational calculations of the nonrelativistic energies with precision of ∼10^-10 a.u. and relativistic and QED corrections to the energy levels of mα ⁵ order are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Non-adiabatic high-precision calculation of antiprotonic helium atomcules

The energy levels of the metastable states of Coulombic exotic three-body systems were calculated with the non-adiabatic coupled rearrangement channel method. The resonant boundary condition of the metastable levels is taken into account. Relativistic and QED corrections on the energies were made. Excellent agreement between corrected wavelengths and experimental data was achieved. This agreement gave the best limit of the antiproton mass with 10^-7 uncertainty. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fine and hyperfine structure of antiprotonic helium

We consider the fine and hyperfine splitting of the energy levels of antiprotonic helium pHe⁺ in highly excited rotational states in connection with the interpretation of the recently discovered delayed annihilation of antiprotons in helium, based on the Condo model.     

Measurement of the hyperfine structure of antiprotonic helium

This paper discusses the level splitting of the antiprotonic helium atomcule, an exotic atom consisting of a helium nucleus, an electron, and an antiproton which occupies highly excited states with angular momentum of L ≈ 30–35. The observation of a splitting in a laser transition between two levels is presented, and an experiment in preparation at the forthcoming AD facility at CERN is described which aims at directly measuring the level splitting by using a 2-laser microwave triple resonance technique. This experiment has the potential to determine the magnetic moment of the antiproton with higher precision than currently known. This revised version was published online in August 2006 with corrections to the Cover Date.     

Tunable diode laser spectroscopy of helium clusters

Helium clusters, He_N-X, containing a probe molecule, X, are studied by infrared spectroscopy for the size range N≈1∼100. Spectra are observed using a supersonic jet expansion and a tunable diode laser source operating in a rapid-scan (sweep integration) mode. The pulsed jet uses a dilute gas mixture of the probe molecule in helium, with relatively high backing pressures (5–50 bar), and a cooled (80–295 K) nozzle. Sensitivity is enhanced by multi-passing the laser beam through the jet with a toroidal mirror system. The clusters are larger than van der Waals dimers and trimers, but smaller than those encountered in the field of helium nanodroplets (N≈10³–10⁵). Furthermore, individual cluster sizes are resolved here, but not with nanodroplets, and infrared absorption is detected directly (change in transmitted laser intensity), rather than indirectly (change in cluster fragmentation). Trends in the spectra are described for five probe molecules, X=CO, SiH₄, OCS, N₂O, and CO₂. Superfluid effects dominate for clusters larger than N≈8. Notable results include the unexpected observation of broad oscillations in the effective rotational constants as a function of cluster size. PACS 33.20.Ea; 34.30.+h; 36.40.Mr; 42.55.Px     

Isotopic effect in the decay of antiprotonic helium

An important decay channel of recently discovered metastable antiprotonic helium atoms is the emission of an electron, i.e., the Auger transition. The rate of this process for a number of states of the ³He\barpe, ⁴He\barpe and ⁶He\barpe systems is calculated and a substantial isotopic effect is found. While the reduced mass of heavy particles increases slightly with increasing nucleus mass, the Auger transition rate decreases by about a factor of three. The explanation of this strong amplification is given and the role of this effect in experiments is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spin structure of antiprotonic helium in a magnetic field

The spin structure of the (37,35) energy level of antiprotonic helium-4 in permanent homogenous external magnetic field has been calculated in first order of perturbation theory. For weak magnetic fields the effect is observed as an additional broadening of the spectral lines. For magnetic fields of the order of kG and higher, the magnetic and hyperfine interactions become comparable to each other, and the hyperfine spectrum is rearranged.     

Collisional effects on the HFS transitions of antiprotonic helium

Collisions of metastable antiprotonic helium with ordinary He atoms can induce transitions between hyperfine structure (HFS) sublevels and thus relaxation of the populations as well as shift and broadening of M1 microwave spectral lines. Previously we have considered these effects, using quantum close-coupling method for the collisions and the model interatomic interaction involving tensor term (Korenman and Yudin, J Phys B At Mol Opt Phys 39:1473–1484, 2006). In this paper we extend the consideration in two lines: (i) collisional transition rates, shifts and broadenings of M1-lines are considered in relation to the strength of the tensor interaction; (ii) time evolution of the density matrix of HFS states is considered using the generalized master equation with the calculated elementary characteristics. A good agreement with the recent experimental data on the time dependence of optimum signal-to-noise ratio and of linewidth is obtained using the strength of tensor interactions as a free parameter.     

High-accuracy calculation of meta-stable states of antiprotonic helium atoms

The variational calculation of energy levels for the antiprotonic helium atomcule ¯p^3,4He⁺ based on molecular expansion of the wave function is presented. We have taken into account the , and components of the molecular wave function that correspond to azimuthal magnetic momentam=0, 1, 2, respectively. It has been discovered that the contribution of the component to energy levels of the atomcule is about 5 × 10^–4 a.u. and the contribution of the component is about 10^–7 a.u. The test of convergence enables us to estimate the accuracy of the obtained results to about 10^–7 a.u. which is significantly better than all previous theoretical results.     

Density shift and broadening of dipole transitions in antiprotonic helium

The phase composition of carbon nanotubes (CNTs) with encapsulated iron atoms was examined by ⁵⁷Fe M?ssbauer spectroscopy. It was shown that iron atoms were stabilized in thermodynamically stable iron carbide and oxide phases and phases that are not usual under synthesis conditions (γ-Fe and γ-Fe₂O₃).