Abstract
We investigate the photoinduced relaxation dynamics of Cr atoms embedded into superfluid helium nanodroplets. One- and two-color resonant two-photon ionization (1CR2PI and 2CR2PI, respectively) are applied to study the two strong ground state transitions z7P2,3,4° ← a7S3 and y7P2,3,4° ← a7S3. Upon photoexcitation, Cr* atoms are ejected from the droplet in various excited states, as well as paired with helium atoms as Cr*–Hen exciplexes. For the y7P2,3,4° intermediate state, comparison of the two methods reveals that energetically lower states than previously identified are also populated. With 1CR2PI we find that the population of ejected z5P3° states is reduced for increasing droplet size, indicating that population is transferred preferentially to lower states during longer interaction with the droplet. In the 2CR2PI spectra we find evidence for generation of bare Cr atoms in their septet ground state (a7S3) and metastable quintet state (a5S2), which we attribute to a photoinduced fast excitation–relaxation cycle mediated by the droplet. A fraction of Cr atoms in these ground and metastable states is attached to helium atoms, as indicated by blue wings next to bare atom spectral lines. These relaxation channels provide new insight into the interaction of excited transition metal atoms with helium nanodroplets.
Highlights
The advent of helium nanodroplets (HeN) has spawned many new vistas in the field of matrix isolation spectroscopy.[1,2]Various fascinating spectroscopic experiments have been enabled by helium nanodroplet isolation spectroscopy, among them the study of the phenomenon of superfluidity from a microscopic perspective[3,4] or the investigation of high-spin molecules.[5−7] Helium droplets as well as bulk superfluid helium[8] offer a unique spectroscopic matrix because of the weak interaction with dopants
We propose two different scenarios for the production of ground state and metastable Cr atoms and Cr−Hen. (i) Septet states are absent in the concerning spectral regime, but states with other multiplicities lie in the vicinity of the XeCl laser photon energy
Chromium atoms doped to superfluid helium nanodroplets are investigated with one- and two-color resonant two-photon ionization spectroscopy (1CR2PI and 2CR2PI, respectively) via the y7P2°,3,4 resonant intermediate states and with 2CR2PI via the z7P2°,3,4 states
Summary
The advent of helium nanodroplets (HeN) has spawned many new vistas in the field of matrix isolation spectroscopy.[1,2]. Calculations for coinage metals show that the binding energy rises with increasing number of helium atoms attached to the metal atom.[44] For the ground state, the coinage metals with their completely filled d-orbitals and one electron in the s-orbital, and chromium with its half-filled d-orbitals and one s-electron are very similar in their interaction with He atoms, which is dictated mainly by the electron in the s-orbital.[43] the observed spectrum suggests that larger Cr−Hen complexes are formed upon UV excitation, followed by droplet-mediated relaxation via various routes into the electronic septet and quintet (and probably into the triplet) ground states Note that this process must be completed in less than 20 ns, the pulse duration of the synchronized excitation and ionization lasers. The formation of Cr dimers is observed in helium nanodroplets,[22] and their spectra will be explored in the near future
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