Quenching of metastable states of antiprotonic helium atoms by collisions with H2 molecules

Abstract

Laser resonance transitions between normally metastable states of antiprotonic helium atoms were induced making use of state dependent quenching eects 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 = 3 6 ; 37; 38 and (38; l ) ; l = 3 5 ; 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 = 3 9 w ere found to be of the order of the geometrical cross section for pHe + -H2 collisions (210 15 cm 2 ), 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. PACS number(s): 34.90.+q, 42.62.Fi

Indication of the Wigner threshold law in collisions between antiprotonic helium and hydrogenic molecules

“…For comparison, the right side of Fig. 1 shows σ q of the state (38, 37) measured previously [6,10], which does follow Eq. (3).…”

“…1 shows the logarithm of the newly and previously [6,7,10] measured quenching cross sections of the state (39, 35) versus the inverse temperature (Arrhenius plot). One can clearly see that σ q increases with decreasing temperature, both with H 2 and D 2 .…”

“…This quenching phenomenon is particularly interesting because i) the quenching remains strong even below 30 K, ii) the B. Juhász quenching cross section σ q strongly depends on the quantum numbers (n, l) of the pHe + atom [4,5,6], and iii) σ q is usually a factor of ∼1.5 smaller for D 2 than for H 2 [7,8].…”

Quantum tunnelling effects revealed in collisions of antiprotonic helium with hydrogenic molecules at low temperatures

“…Besides the measured decay rates, we also used the theoretically calculated decay rate in pure helium (n adm ¼ 0), which is 0.62 ls À1 for all three states [23]. As in the previous measurements, an error of 15% was assigned to this value [11,13]. As an example of such fits, Fig.…”

“…The lifetimes of the metastable states were found to decrease when hydrogen molecules were added to the helium gas [2,7,8] suggesting that reaction (1) does take place. This quenching phenomenon is of great interest because: (i) strong quenching persists at low temperatures below 30 K where the activation-type Arrhenius cross-section should vanish and (ii) the quenching cross-section in collisions with H 2 strongly depends on the quantum numbers ðn; lÞ of the p pHe þ atom [9][10][11]. A similar state dependence was also found in the case of D 2 , and interestingly, the quenching cross-section with D 2 was found to be $1.5 times smaller than with H 2 [12,13].…”