Electronic spectroscopy of 9,10-dichloroanthracene inside helium droplets

Anthracene-Argon Clusters Generated in Superfluid Helium Nanodroplets: New Aspects on Cluster Formation and Microsolvation

About two decades after extensive studies on anthracene-Ar_n clusters in the gas phase, we report corresponding studies in superfluid helium droplets. With AN as a small fluorophore and spectroscopic data from the gas phase and helium droplets, both the formation of clusters and the microsolvation in superfluid helium droplets can be studied. As expected for helium droplets, a significantly higher number of isomeric variants of the respective cluster sizes are obtained, because metastable variants are stabilized by the low temperature and the surrounding helium. Moreover, spectroscopic data recorded in helium droplets reveal cluster configurations with Ar atoms shielded by a helium solvation layer. Surprisingly, AN-Ar_n clusters with more than four Ar atoms do not appear to form rigid configurations. The helium droplet data in combination with the gas phase spectra may serve as a suitable reference for further theoretical investigations on solvation and cluster formation in superfluid helium droplets.

Helium induced fine structure in the electronic spectra of anthracene derivatives doped into superfluid helium nanodroplets

Electronic spectra of organic molecules doped into superfluid helium nanodroplets show characteristic features induced by the helium environment. Besides a solvent induced shift of the electronic transition frequency, in many cases, a spectral fine structure can be resolved for electronic and vibronic transitions which goes beyond the expected feature of a zero phonon line accompanied by a phonon wing as known from matrix isolation spectroscopy. The spectral shape of the zero phonon line and the helium induced phonon wing depends strongly on the dopant species. Phonon wings, for example, are reported ranging from single or multiple sharp transitions to broad (Δν > 100 cm(-1)) diffuse signals. Despite the large number of example spectra in the literature, a quantitative understanding of the helium induced fine structure of the zero phonon line and the phonon wing is missing. Our approach is a systematic investigation of related molecular compounds, which may help to shed light on this key feature of microsolvation in superfluid helium droplets. This paper is part of a comparative study of the helium induced fine structure observed in electronic spectra of anthracene derivatives with particular emphasis on a spectrally sharp multiplet splitting at the electronic origin. In addition to previously discussed species, 9-cyanoanthracene and 9-chloroanthracene will be presented in this study for the first time.

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.

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.