Ultra short-time dynamics
Exploration of the real-time dynamics of molecules and nanostructures inside helium nanodroplets. |
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Wanted: Diploma students for the following projects:
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The project
The goal of this project is to experimentally investigate and control the dynamics of atoms, molecules and molecular aggregates embedded in superfluid helium nanodroplets. The dynamics is studied using the pump-probe technique in combination with various detection schemes. In this approach, a dynamical process is induced by absorption of one or several photons from a first femtosecond laser pulse (pump pulse), and the temporal evolution is probed by absorption of photons from a second laser pulse (probe pulse). The total population of the final state is recorded as a function of the delay time between the two laser pulses by measuring the fluorescence light from this state or, in the case of photo ionization, by counting the number of ions or electrons.
Alkali atoms, molecules and clusters that are weakly bound to superfluid helium nanodroplets are intriguing systems at the borderline between the gas-phase and condensed matter physics. Probing the dynamics of these systems gives insight into the nature and dynamics of solvation of impurities in a quantum fluid at a temperature of 0.4 K [1]. Using one-color resonant pump-probe photoionization (PI), the dynamics of vibrational wave-packets in diatomic and triatomic molecules [2], of exciplex formation [3], and of cluster fragmentation can be studied. The internal dynamics of molecules that are strongly bound inside the helium droplets, such as I2, NaI, and organic complexes, will also be probed.
[1] Frank Stienkemeier, Kevin K. Lehmann, Spectroscopy and Dynamics in Helium Nanodroplets (Topical Review), J. Phys. B. 39, R127–R166 (2006); J.P. Toennies, A. F. Vilesov; Angew. Chem. 43, 2622 (2004).
[2] B. Grüner et al., PCCP 13, 6816 (2011); Ch. Giese et al., PCCP 13, 18769 (2011); Mudrich et al. Phys. Rev. A 80, 042512 (2009); P. Claas et al., J. Phys. B: At. Mol. Opt. Phys. 39, S1151 (2006). P. Claas et al., J. Phys. Chem. A. 111 (31), 7537-7541 (2007)
[3] M. Mudrich et al., Phys. Rev. Lett. 100 , 023401 (2008); G. Droppelmann et al. Phys. Rev. Lett. 93, 023402 (2004); C.P. Schulz et al., Phys. Rev. Lett. 87 (15), 153401 (2001).
Experimental setup
The experimental setup for femtosecond pump-probe spectroscopy of alkali-doped helium nanodroplets is sketched below. The helium droplet beam is doped with one or many atoms by pickup when flying through a heated oven cell. Further downstream, the doped droplets interact with the pump and probe pulses from a femtosecond laser. The photoionization products are detected mass selectively using a quadrupole mass spectrometer (QMS) or time of flight (TOF) ion detector. The pump and probe pulse pairs are generated using a Mach-Zehnder interferometer setup. The optical path length of one arm of the interferometer can be varied using a precision translation stage such that delay time increments in the 100 attosecond range can be realized.
Recent results
| Droplet-induced ignition of a nanoplasma
Doped He nanodroplets are widely used as inert, transparent, and cold matrix for spectroscopy of embedded molecules and clusters. When exposed to strong laser fields, however, a few dopant atoms are sufficient to “ignite” avalanche-like ionization that turns the whole droplet into a strongly absorbing nanoplasma [1]. As a result, the complete He droplet fully ionizes and explodes. Highly energetic He+ and He++ ions are released (see top right). The ionization dynamics of such a nanoplasma measured by the pump-probe technique at varius laser intensities features a pronounced maximum as a result of a Mie plasmon resonance that occurs as the nanoplasma expands (bottom right). [1] A. Mikaberidze et al., PRL 102, 128102 (2009); S.R. Krishnan et al., PRL 107, 173402 (2011) |
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Imaging detection of photoions and electrons using a position sensitive detector gives detailed insight into the kinematics and energetics of the ionization process. As an example, the Fig. to the right shows the distribution of Rb+ ions created by resonant ionization of Rb-doped He nanodroplets by a nanosecond laser pulse. The anisotropic ion velocity distribution reflects the fact that during the laser pulse the Rb atom is excited, ejected away from the droplet surface, and subsequently ionized [1]. The narrow stripe comes from Rb atoms in an effusive beam that accompanies the doped He droplet beam. [1] L. Fechner et al., PCCP accepted (arXiv:1109.2298) |
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The excitation of a coherent superposition of vibrational states in a molecule by a short laser pulse initiates the propagation of a vibrational wave packet. Classically speaking, the short pulse triggers the vibrational motion just like two masses coupled by a spring, spured by a short kick. The vibrational dynamics can be visualized using the pump-probe technique, in which the first pulse creates the wave packet motion and the second pulse stroboscopically probes the wave packet at a certain position determined by favorable Franck-Condon regions. The figure to the top left shows the pump-probe photoionization trace of rubidium dimers Rb2 in comparison with a theoretical calculation including damping of the vibrational motion due to coupling to the He droplet [1]. From Fourier analysis of these traces the contributing electronic and vibrational levels can be determined. Moreover, delay time-dependent Fourier transformation gives insight in the time evolution of the vibrational spectra (bottom left). The round dots at ~36cm-1 reflect the periodic dispersion and rephasing of the vibrational wave packet in the electronically excited state, which is not present for ground state wave packets (stripes at 13, 26cm-1). [1] B. Grüner et al., PCCP 13, 6816 (2011); M. Mudrich et al. Phys. Rev. A 80, 042512 (2009). |
