Formation of even-numbered hydrogen cluster cations in ultracold helium droplets.
Neutral hydrogen clusters are grown in ultracold helium nanodroplets by successive pickup of hydrogen molecules. Even-numbered hydrogen cluster cations are observed upon electron-impact ionization with and without attached helium atoms and in addition to the familiar odd-numbered H(n)(+). The helium matrix affects the fragmentation dynamics that usually lead to the formation of overwhelmingly odd-numbered H(n)(+). The use of high-resolution mass spectrometry allows the unambiguous identification of even-numbered H(n)(+) up to n approximately = 120 by their mass excess that distinguishes them from He(n)(+), mixed He(m)H(n)(+), and background ions. The large range in size of these hydrogen cluster ions is unprecedented, as is the accuracy of their definition. Apart from the previously observed magic number n=6, pronounced drops in the abundance of even-numbered cluster ions are seen at n=30 and 114, which suggest icosahedral shell closures at H(6)(+)(H(2))(12) and H(6)(+)(H(2))(54). Possible isomers of H(6)(+) are identified at the quadratic configuration interaction with inclusion of single and double excitations (QCISD)/aug-cc-pVTZ level of theory.
Jaksch
S
S
Institut für Ionenphysik und Angewandte Physik, Leopold Franzens Universität, Technikerstr. 25, A-6020 Innsbruck, Austria.
Mauracher
A
A
Bacher
A
A
Denifl
S
S
da Silva
F Ferreira
FF
Schöbel
H
H
Echt
O
O
Märk
T D
TD
Probst
M
M
Bohme
D K
DK
Scheier
P
P