With the “Cluster Lab II” apparatus it is possible to dope high melting metal atoms and clusters to helium nanodroplets (HeN). Currently we dope the droplets with chromium (Cr) atoms (see our publications) in a crossed beam orientation shown in the figure below.
To characterize the interaction and behaviour of the Cr with the ultracold droplets, photoionization experiments are carried out (see our publications). A tuneable pulsed dye laser with 20 ns pulse duration is used to facilitate a two step resonance-enhanced multiphoton ionization (REMPI) where both photons have the same colour. With an amazing relaxation mechanism occurring, intermediate states are populated and a subsequent ionization into the continuum produces detectable ions. As a special feature, also transitions to autoionizing states can be observed. It has been already predicted in 1983  that such ionization paths are more efficient than the simple ionization into continuum states which can be shown by these experiments. Furthermore, asymmetric line shapes give hints about Fano resonances which occur when a discrete state interacts with continuum states .
 M. Pellin, D. Gruen, T. Fisher, and T. Foosnaes,J. Chem. Phys. 79, 5871 (1983).
 U. Fano,Phys. Rev.124, 1866 (1961).
Further characterization using the techniques of laser induced fluorescence (LIF) and beam depletion (BD) yielded a deeper insight into the relaxation mechanism occurring with the ejection of the Cr dopant from the droplet (see our publications). With our LIF experiments, we were also the first to measure fluorescence from transition metal atoms in HeN. BD is the technique of reducing the amount of doped droplets reaching a quadrupole mass detector (QMS) at the end of the measurement chamber after a resonant electronic excitation. The latter is achieved by a pulsed laser guided antiparallel to the droplet beam to increase the interaction path. The shifted and broadened excitation from the photoionization experiments could be confirmed and was similarly found for a lower lying state that is not accessible with a one color two photon ionization experiment. Despite on the laser wavelength within these broad structures, fluorescence emission was only observed from the energetically lowest substate showing a strong trend of relaxation due to the surrounding helium. Interesting differences were found for the population after nonradiative relaxation to energetically lower states.
This figure shows the principle of optically detected magnetic resonance (ODMR) exploiting magnetic circular dichroism (MCD).
Direct detection of an ESR transition, as commonly accomplished in ESR, is not possible because the droplet beam is “optically” not dense enough.
In the old days of Rabi and coworkers this problem was overcome by exploding the effect of spatial separation of different spin states in inhomogeneous magnetic (or electric) fields (upper image of Fig. 2). The first inhomogeneous field prepares a certain spin state and successful manipulation is detected indirectly after the second inhomogeneous field by an increase of the particle flux into the detector.
Fig. 2: Rabi scheme molecular (atomic) beam magnetic resonance (upper image). Optically detected magnetic resonance (ODMR, lower image).
In modern experiments tunable, single mode lasers are used to prepare and probe spin states (lower image of Fig. 2). Energetic splitting of different spin states in a homogeneous magnetic field (Zeeman splitting) in combination with the high energetically resolution in atomic (molecular) beam experiments make selective spin manipulation possible. An optical pumping scheme is used to prepare a certain spin state and an ESR transition is detected optically by means of laser induced fluorescence (LIF). This is ODMR.
In contrast to the sharp free atom transitions, on helium droplets electronic transitions are significantly broadened and shifted, both by around 50 cm-1 or more (Fig. 3); conventional ODMR is not possible.
Fig. 3: Laser induced fluoresence spectra of single K and Rb atoms on helium droplets. The free atomic transitions are indicated as vertical bars.
Nevertheless one can use circular polarized light in combination with the corresponding selection rules to selectively address spin states (Fig. 4). This is the principle of MCD. In other words the MCD sceme is used to overcome the problem of unresolved Zeeman splittings on helium droplets.
Fig. 4: Energy level diagram of alkali-metal atoms. Although the Zeeman is not resolved on helium droplets (typically 1/3 cm-1 Zeeman splitting compared to ~50 cm-1 line broadening) the spin states can be addressed selectively using a magnetic circular dichroism (MCD) scheme.
All images © TU Graz/Institute of Experimental Physics