Alkwin Slenczka

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.

Inhomogeneous broadening of the zero phonon line of phthalocyanine in superfluid helium droplets

The electronic origin band of the S1←S0 transition of monomer phthalocyanine doped into liquid helium droplets consist of a single zero phonon line (ZPL) and a structured phonon wing. The latter reflects the low frequency modes of the helium droplet. At very high resolution (1 MHz) the asymmetric spectrum of the ZPL of phthalocyanine provides no indication of a rotational substructure. Changes in the asymmetry and the peak position of the ZPL with variation of the average droplet size are in very good agreement with an inhomogeneous model line shape.

Inhomogeneous line shape theory of electronic transitions for molecules embedded in superfluid helium droplets

A model is developed for the calculation of the inhomogeneous line shapes of electronic transitions of molecules embedded in superfluid liquid helium droplets. This model takes account of the cluster size distribution, and the size dependence of the capture cross section and the resonance frequency. Fits to experimental spectra yield very good agreement and reasonable values for the model parameters.

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.

Impulsive laser induced alignment of molecules dissolved in helium nanodroplets.

We show that a 450 fs nonresonant, moderately intense, linearly polarized laser pulse can induce field-free molecular axis alignment of methyliodide (CH(3)I) molecules dissolved in a helium nanodroplet. Time-resolved measurements reveal rotational dynamics much slower than that of isolated molecules and absence of the sharp transient alignment recurrences characteristic of gas phase molecules. Our results presage a range of new opportunities for exploring both molecular dynamics in a dissipative environment and the properties of He nanodroplets.

Hybridization of rotor states in parallel electric and magnetic fields

Abstract We demonstrate in a spectroscopic experiment the hybridization of rotor states within the X and A electronic states of ICl, induced by parallel magnetic and electric fields of up to 0.7 T and 50 kV/cm. For a polar paramagnetic electronic state, such as the A state, the electric and magnetic fields conjoin in producing hybridization for one half of the molecular ensemble while they counteract for the other half. In the latter case the electric and magnetic effects can be tuned to balance out exactly; the concomitant field-induced hyperfine splitting remains in place, however, despite the resulting free rotation of the molecule.

Electronic spectroscopy of molecules in superfluid helium nanodroplets: an excellent sensor for intramolecular charge redistribution.

Electronic spectra of molecules doped into superfluid (4)He nanodroplets reveal important details of the microsolvation in superfluid helium. The vibrational fine structure in the electronic spectra of phthalocyanine derivatives and pyrromethene dye molecules doped into superfluid helium droplets have been investigated. Together with previous studies on anthracene derivatives [J. Chem. Phys.2010, 133, 114505] and 3-hydroxyflavone [J. Chem. Phys.2009, 131, 194307], the line shapes vary between two limiting cases, namely, sharp Lorentzians and nonresolved vibrational fine structure. All different spectral signatures are initiated by the same effect, namely, the change of the electron density distribution initiated by the electronic excitation. This change can be quantified by the difference of the electrostatic moments of the molecule in the electronic ground state and the corresponding Franck-Condon point in the excited state. According to the experimental data, electronic spectroscopy suffers from drastic line broadening when accompanied by significant changes of the charge distribution, in particular, changes of the dipole moment. Vice versa, the vibrational fine structure in electronic spectra of molecules doped into helium droplets is highly sensitive to changes of the electron density distribution.

Microsolvation of anthracene inside superfluid helium nanodroplets

This article reports on the microsolvation of anthracene in superfluid helium nanodroplets as revealed by electronic spectroscopy. Among the polyacene molecules benzene, naphthalene, anthracene, tetracene and pentacene only anthracene and tetracene have been found to exhibit multiplet splitting of the electronic and vibronic transitions which can not be explained by a rotational fine structure. The experimental approach for the investigation of the multiplet is the combined investigation of the fluorescence excitation spectrum and dispersed emission spectra. New experimental data are presented on the microsolvation of anthracene in helium droplets. A detailed analysis of the anthracene data will be contrasted to corresponding data on tetracene. These data together with those reported for benzene, naphthalene and pentacene might serve to test and develop theoretical models which are needed to understand microsolvation of single molecules in superfluid helium droplets.

Microsolvation in superfluid helium droplets studied by the electronic spectra of six porphyrin derivatives and one chlorine compound

After almost two decades of high resolution molecular spectroscopy in superfluid helium droplets, the understanding of microsolvation is still the subject of intense experimental and theoretical research. According to the published spectroscopic work including microwave, infrared, and electronic spectroscopy, the latter appears to be particularly promising to study microsolvation because of the appearance of pure molecular transitions and spectrally separated phonon wings. Instead of studying the very details of the influence of the helium environment for one particular dopant molecule as previously done for phthalocyanine, the present study compares electronic spectra of a series of non-polar porphyrin derivatives when doped into helium droplets consisting of 10(4)-10(5) helium atoms. Thereby, we focus on the helium-induced fine structure, as revealed most clearly at the corresponding electronic origin. The interpretation and the assignment of particular features obtained in the fluorescence excitation spectra are based on additional investigations of dispersed emission spectra and of the saturation behavior. Besides many dopant-specific results, the experimental study provides strong evidence for a particular triple peak feature representing the characteristic signature of helium solvation for all seven related dopant species.

Polarization Spectroscopy of Pendular Molecules in the Gas Phase

The orientation or alignment of pendular mols. gives rise to an optical anisotropy. The sparse resonance structure of polarization spectra of pendular mols. can be explored to elucidate their structure. A quant. simulation of the polarization spectra is possible at any field strength. Both the exptl. procedure and the simulation are demonstrated for the ICl mol., and an extension to large mols. is discussed.