Quantum Interference Spectroscopy of Rubidium-Helium Exciplexes Formed on Helium Nanodroplets

Femtosecond multiphoton pump-probe photoionization is applied to helium nanodroplets doped with rubidium (Rb). The yield of ${\mathrm{Rb}}^{+}$ ions features pronounced quantum interference (QI) fringes demonstrating the coherence of a superposition of electronic states on a time scale of tens of picoseconds. Furthermore, we observe QI in the yield of formed RbHe exciplex molecules. The quantum interferogram allows us to determine the vibrational structure of these unstable molecules. From a sliced Fourier analysis one cannot only extract the population dynamics of vibrational states but also follow their energetic evolution during the RbHe formation.

Figure 1

(a) Yield of photoionized Rb atoms excited on the surface of helium nanodroplets as a function of the delay between femtosecond pump and probe pulses. The inset shows that by choosing a step increment of 220 attoseconds the QI oscillation is fully resolved. (b) QI fringes of ionized RbHe exciplexes detected on mass 89 amu. The amplitude modulation comprehends the vibrational structure of the RbHe molecule.

Figure 2

Pump-probe excitation schemes showing the involved electronic states of Rb atoms (left side) as well as the vibronic states after having formed a RbHe exciplex [23]. The spatial distributions of the electron orbitals in corresponding states are also sketched.

Figure 3

(a) Fourier transforms of the amplitude functions of the quantum interferograms of the Rb and RbHe data shown in Fig. 1. The spectrum of Rb is dominated by the frequency component $E_2-E_1$, whereas the RbHe spectrum displays a vibrational progression. (b) Morse potential reconstructed from a fit of the vibrational spectrum in comparison with an ab initio potential [23].

Figure 4

Spectrogram of the RbHe QI oscillation showing the time evolution of the energy and the population of the coherently excited states.