Rudolf Lehnig

Spectroscopic investigation of the solvation of organic molecules in superfluid helium droplets

The spectroscopy of molecules doped into superfluid helium nanodroplets provides valuable information on the process of solvation in superfluid helium. In continuation of an earlier report on emission spectra of various phthalocyanines showing a splitting of all molecular transitions in the range of about 5-12 cm(-1), the emission spectra of tetracene, pentacene, and perylene in superfluid helium droplets are presented. The new spectra and the results obtained for the phthalocyanines are explained by an empirical model which accounts for the existence of different metastable configurations of a nonsuperfluid solvation layer around the guest molecule.

Quantum solvation of phthalocyanine in superfluid helium droplets

We have measured quantum states of the solvent-solute system of phthalocyanine in superfluid helium droplets in a high resolution pump-probe experiment. This provides evidence for the attribution of a splitting effect in the emission spectra of phthalocyanine in helium droplets to the relaxation of the first helium layer upon electronic excitation, measured recently by us. Our experimental results are a strong indication for the first helium layer playing a key roll for the solvation of molecules in helium droplets and, thus, for their spectroscopic features.

Photochemistry of 3-hydroxyflavone inside superfluid helium nanodroplets

3-Hydroxyflavone is a prototype system for excited state intramolecular proton transfer which is one step of a closed loop photocycle. It was intensively studied for the bare molecule and for the influence of solvents. In the present paper this photocycle is investigated for 3-hydroxyflavone and some hydrated complexes when doped into superfluid helium droplets by the combined measurement of fluorescence excitation spectra and dispersed emission spectra. Significant discrepancies in the proton transfer behavior to gas phase experiments provide evidence for the presence of different complex configurations of the hydrated complexes in helium droplets. Moreover, for bare 3-hydroxyflavone and its hydrated complexes the proton transfer appears to be promoted by the helium environment.

Evidence for an energy level substructure of molecular states in helium droplets

The pure tunneling inversion transition of ammonia embedded in (4)He droplets was investigated in the microwave frequency range. We observed a spectrum that consists of a sharp peak, only 15 MHz wide, on top of a broad feature. The peculiar line shape could be simulated with an empirical model and is a clear experimental evidence for an energy level substructure of molecular states in doped helium droplets.

Observation of a high lying van der Waals mode in the intermolecular potential of Ar-CO

A continuous two-dimensional jet expansion in combination with an infrared diode laser spectrometer has been used to record a new subband of the weakly bound complex Ar-CO in the 2168cm−1 region. Twenty-two transitions were assigned to a thus far unobserved K a = 0 van der Waals state at 36.765 cm−l above the ground state. This is the highest van der Waals mode of Ar-CO reported so far. These high lying energy levels will provide a very sensitive test for future ab initio and Semiempirical potential energy surfaces.

Infrared spectroscopy of van der Waals modes in the intermolecular potential of Ar-CO: The K a, = 0 combination of stretch and bending

Using our diode laser spectrometer a new subband of Ar−CO in the 2167 cm−1 region has been detected. We have assigned 18 rotational transitions to a thus far unobserved K a = 0 van der Waals mode at 23.927cm−1 above the ground state. An exact analysis yields a Coriolis coupling of this new observed state to a previously detected van der Waals state at 26.187cm−1 [10]. The measurements and a complete analysis are presented.

Rotational spectroscopy of single carbonyl sulfide molecules embedded in superfluid helium nanodroplets.

The pure rotation spectrum of carbonyl sulfide embedded in superfluid helium nanodroplets was measured in the frequency range from 4 to 15.5 GHz. Four lines, corresponding to the J = 1-0, J = 2-1, J = 3-2, and J = 4-3 transitions, were found. The line widths of the transitions increase with increasing rotational quantum number J, which is indicative of a distribution of the effective B rotational constant. A comparison of the pure rotational spectrum with the microwave-infrared double resonance spectrum [S. Grebenev, M. Havenith, F. Madeja, J. P. Toennies, and A. F. Vilesov, J. Chem. Phys., 2000, 113(20), 9060] reveals that the double resonance measurement scheme probes predominantly rotational transitions within the vibrationally excited state.

Microsolvation of molecules in superfluid helium nanodroplets revealed by means of electronic spectroscopy

The empirical model explaining microsolvation of molecules in superfluid helium droplets proposes a non-superfluid helium solvation layer enclosing the dopant molecule. This model warrants an empirical explanation of any helium induced substructure resolved for electronic transitions of molecules in helium droplets. Despite a wealth of such experimental data, quantitative modeling of spectra is still in its infancy. The theoretical treatment of such many-particle systems dissolved into a quantum fluid is a challenge. Moreover, the success of theoretical activities relies also on the accuracy and self-critical communication of experimental data. This will be elucidated by a critical resume of our own experimental work done within the last ten years. We come to the conclusion that spectroscopic data and among others in particular the spectral resolution depend strongly on experimental conditions. Moreover, despite the fact that none of the helium induced fine structure speaks against the empirical model for solvation in helium droplets, in many cases an unequivocal assignment of the spectroscopic details is not possible. This ambiguity needs to be considered and a careful and critical communication of experimental results is essential in order to promote success in quantitatively understanding microsolvation in superfluid helium nanodroplets.