N. Morita

Hyperfine structure of the metastable p̄He+ atomcule revealed by a laser-induced (n, l) = (37, 35) → (38, 34) transition

Abstract A precise scan of the previously discovered laser-induced transition ( n, l ) = (37, 35) → (38, 34) in pHe + revealed a doublet structure with a separation of Δ ν HF = 1.70 ± 0.05 GHz. This new type of “hyperfine” splitting is ascribed to the interaction of the antiproton orbital angular momentum and the electron spin.

Laser resonance studies of the interactions of metastable antiprotonic helium atomcules p4He+ with surrounding H2 molecules

Abstract We have employed a laser resonance method to study the interactions of individual states of metastable antiprotonic atomcules p He + with surrounding H 2 molecules. We have found that the lifetimes of the ( n , 1) = (37,34) and (39,35) states are shortened by small admixtures of H 2 molecules in quite different ways; the observed quenching cross section for the upper (39,35) state in helium medium at 1 bar and 30 K is (2.4 ± 1.0) × 10 −15 cm 2 , a factor of 24 larger than that for the lower (37,34) state.

QUENCHING OF METASTABLE STATES OF ANTIPROTONIC HELIUM ATOMS BY COLLISIONS WITH H2 MOLECULES

Laser resonance transitions between normally metastable states of antiprotonic helium atoms were induced making use of state dependent quenching effects caused by trace admixtures of H2 to the target helium gas. With this method of “H2-assisted inverse resonances” the decay rates of the states (n,l)=(39,l),l=36,37,38, and (38,l),l=35,36,37 of pHe+ were determined as a function of the H2 admixture. The quenching cross sections at 30 K deduced therefrom for the states with n=39 were found to be of the order of the geometrical cross section for pHe+–H2 collisons (2⋅10−15 cm2), with a moderate decrease with increasing l. Within a given cascade with constant v=n−l−1, the quenching cross sections for states with n=38 are smaller by a factor of 4–6 than those for states with n=39.

Analog measurement of delayed antiproton annihilation time spectra in a high intensity pulsed antiproton beam

Abstract An analog detection system has been developed to measure delayed antiproton annihilation time spectra for laser resonance spectroscopy of metastable antiprotonic helium atoms using the high-intensity pulsed beam of antiprotons from LEAR at CERN.

Instrumentation for laser-induced annihilation spectroscopy of metastable antiprotonic helium atoms

We have established a technique for laser resonance spectroscopy of metastable antiprotonic helium atoms (pHe+) by successfully observing laser-induced annihilation. In this paper, we describe the full instrumentation of the experiment in detail, namely, the particle detectors, the laser system, the cryostat and the helium gas target, the data acquisition and analysis method. For an effective laser triggering selective to 3% metastable antiprotons, a highly efficient detection system for antiproton annihilation was required, with a suppressed inefficiency of less than a percent. Guided by simulations, we designed a system of seven lead-scintillator sandwich counters to detect the charged pions and π0-decay gamma rays from antiproton annihilation, and achieved 99.7±0.1% efficiency for the detection of annihilation. We employed two identical sets of excimer-pumped dye laser systems with 2–5 mJ output energy per pulse, which were triggered by every metastable antiproton formed in a helium gas target of 0.3–1.0 bar at 4.5–10 k. The resonant deexcitation of the metastable states was detected as a spike-like forced annihilation in the time spectrum, thus revealing the level structure of this exotic atom. This powerful technique enabled us to study also the lifetimes and populations of specific metastable states, by changing the timing and the wavelengths of the laser pulses.

Visible observation of metastable helium atoms confined in an infrared/visible double resonance trap

Using a visible probe laser exciting the 2p3P2→3d3D3 transition (λ=588 nm), we have demonstrated a visible observation of metastable He atoms magnetooptically trapped by an infrared laser resonant with the 2s3S1→2p3P2 transition (λ=1083nm). Detecting visible resonance fluorescence, only 104 atoms confined in a trap with a diameter of 1 mm have been sensitively observed even with the naked eye. It has been found that under a given condition the visible laser operates as a trapping laser and considerably extends the capture range of infrared laser detuning.

Laser spectroscopic measurements of fine-structure changing cross sections of Ca + and Sr + ions in collisions with He atoms

Producing Ca+ and Sr+ ions by laser ablation of pure metal samples and detecting their laser induced fluorescences, cross sections of collision induced transitions between fine-structure levels in the 4p 2PJ state of Ca+ and the 5p 2PJ state of Sr+ due to collisions with He atoms at room temperature (298 K) have been measured. The cross sections determined are σ(Ca+: 4p 2P3/2→4p 2P1/2)= 1.17±0.05 A2, σ(Ca+: 4p 2P1/2→4p 2P 3/2) = (7.92±0.44)×10-1 A2 and σ(Sr+: 5p 2P3/2→5p 2P 1/2) = (1.44±0.10)×10-2 A2.

Interaction of antiprotonic helium atoms with H2 molecules

The interaction of antiprotonic helium atoms with surrounding H 2 molecules was studied microscopically using laser spectroscopy. The state-dependent quenching effects observed were employed to induce laser resonance transitions between normally metastable states.

Laser spectroscopic measurements of fine structure changing cross sections of alkali earth ions in collisions with molecular hydrogen (H2, D2)

Production of alkali earth ions (Ca+, Sr+ and Ba+) by means of a laser ablation technique and measurement of their laser-induced fluorescence, cross sections of collision-induced transitions between the np 2PJ fine structure levels (n= 4, 5 and 6 for Ca+, Sr+ and Ba+, respectively) due to collisions with hydrogen molecules at room temperature (298 K) has been performed. The cross sections thus determined for the collisions with H2 and D2 are σ(Ca+: 4p 2P3/2 → 4p 2P1/2) = 20.5 ± 0.5, 27.1 ± 1.3 A2, σ(Ca+: 4p 2P1/2→ 4p 2P3/2) = 13.2 ± 0.6, 18.7 ± 0.8 A2, σ(Sr+: 5p 2P3/2→ 5p 2P1/2) = 22.7 ± 0.4, 18.0 ± 0.4 A2 and σ(Ba+: 6p 2P3/2→ 6p 2P1/2) = 10.4 ± 0.1, 3.4 ± 0.2 A2, respectively.

Antiproton trapping in various helium media: report of the HELIUMTRAP experiment at LEAR

Abstract HELIUMTRAP (PS205) investigates the recently discovered anomalously long-lived states of antiprotons in various helium media. An overview is given of experiments stopping antiprotons in several phases of helium performed at LEAR in the last two years.