High-resolution spectroscopy in superfluid helium droplets. Investigation of vibrational fine structures in electronic spectra of phthalocyanine and porphyrin derivatives

摘要

Superior to supersonic jet experiments where rotational temperatures of 1 K are easily reached while vibrational temperatures of 10 K may persist, superfluid helium droplets incorporating a molecule cool all degrees of freedom of the dopant to the same temperature safe in the sub-Kelvin regime at 0.37 K. Due to the dissipative medium, simplification of electronic spectra is achieved since only very few rotational levels remain occupied. Though, comparison with isolated molecules in supersonic jet expansions revealed significant deviations concerning electronic spectra of certain molecules. Phenomena such as line splitting, phonon wings (PWs) or dramatic line broadening clearly point to a not yet entirely understood involvement of guest-host interactions. In general, mechanisms that may cause line broadening in electronic spectra either depend on damping of nuclear rearrangement or disturbance of the electronic excitation process. Phthalocyanines and porphyrins are known to possess a rather rigid molecular backbone. Therefore, interaction of the molecule with the superfluid helium droplet can be assumed to be mainly mediated via differing substituent groups. Since a considerably large variety of substituted compounds is commercially available and the electronic excitation spectra fit well into the spectral range covered by the continuous wave dye laser used for this study several porphyrin and phthalocyanine derivatives substituted with different types and numbers of alkyl and aryl groups were chosen as molecular probes. This enables enquiring for a direct or indirect effect of low energy vibrational modes on line widths in electronic spectra of organic molecules doped into superfluid helium droplets.ududRecording fluorescence excitation and dispersed emission spectra revealed exclusively sharp transitions for all species. Together with an intensity peaking at the electronic origin, the spectra indicate that the molecular structure remains unchanged upon electronic excitation. Although, PWs dominating in their intensities over the zero phonon lines (ZPLs), even below any saturation limit, were observed when probing tetra-methyl-porphyrin (TMP) with a pulsed dye laser. This is an indication for a displacement of the equilibrium configuration of the solvation complex in S0 and S1. This means considerable rearrangement of the helium atoms surrounding the molecular dopant. But, considering experiments performed with the continuous wave dye laser, which provides a photon flux that is by about 5 to 6 orders of magnitude lower than for pulsed dye lasers, the ZPL was the dominating spectral feature of the electronic origin for all molecules within this study with the exception of TMP. The fluorescence excitation spectrum of the latter was found to consist of contributions from two species. This spectroscopic finding was also confirmed by an HPLC analysis of our TMP sample. The electronic origin of one compound within the TMP sample not only presented with a spectral shape that was never observed before in electronic spectra of organic molecules in superfluid helium droplets: a series of several sharp lines separated by about 0.3 cm-1 from each other was observed with increasing intensity from the first to the fourth and finally merging of the sharp lines into a broad unstructured band. It also exhibited a PW that was at least of comparable intensity to the ZPL even at the low photon flux provided by the continuous wave dye laser.ududEven though sharp lines have been observed in the electronic spectra for all molecules investigated for this study, a result that is generally expected considering the cold helium droplet environment, several molecules in recent studies exhibitedudsignificant line broadening in their electronic spectra when doped into superfluid helium droplets. At this, large amplitude motions were possible via e.g. methyl or phenyl groups for some species and therefore a correlation of line broadening and intramolecular nuclear rearrangement seemed to be a plausible explanation. Taking into account an even larger set of experimental data, a recent attempt to interpret those findings proposes that broadening is a result of intramolecular charge redistribution initiated by the electronic excitation. Precisely, a change of the molecule's electrostatic moments, primarily and most effectively, a change of the molecular dipole moment regarding both magnitude and orientation, was identified as the main contribution for line broadening effects. The ability of a molecule to allow for large amplitude motions is thus not mandatory for line broadening since rigid molecules like e.g. 3-hydroxyavone or fluorazene exhibit broadened spectra as well. Furthermore, not all molecules with substituents that can undergo torsional motions or the like show line broadening in their electronic spectra, e.g. the molecules investigated for this study. For the latter, the existence of a molecular dipole moment is excluded due to an inversion center of the symmetrically substituted compounds. Therefore, experiments performed for this study support the model that line broadening in electronic spectra of organic molecules in superfluid helium droplets is caused by intramolecular charge redistribution following the electronic excitation. Hence, the solvation shell of helium atoms tightly adapted to the guest molecule needs to change its configuration in order to adapt to the new charge density distribution. This adaption procedure obviously manifests in broadened transitions since the potential energy curves of S0 and S1 are considerably shifted against each other.ududApart from the sharp lines presented in their fluorescence excitation spectra, theudphthalocyanine derivatives investigated for this study, namely chloro-aluminium-phthalocyanine (AlClPc) and tetra-tertbutyl-phthalocyanine (TTBPc), exhibited more than one emission spectrum. Replacing one helium atom of the molecule's solvation shell by another rare gas atom like e.g. argon, i.e. forming AlClPc-Ar1 van der Waals clusters within the superfluid helium droplet, one aims to gain further insight into the dynamics of solvation structures next to the guest molecule. Recording dispersed emission spectra of those complexes revealed again more than one emission spectrum. Yet, vibronic transitions were not just split into two spectra but multiplets consisting of three and more signals and explained with a multiple level system representing different solvation configurations of the first helium layer around the dopant.ududWithin the course of this study also a replacement of the formerly used flow-through cryostat of the continuous droplet machine by a closed-cycle cryostat unit was performed. This made an adjustment procedure of the nozzle unit to the droplet beam axis defined by the openings of the skimmer and the pick-up cell necessary. It revealed that the helium droplets can be reflected into the detection chamber via collision with the skimmer if the nozzle is not adjusted to the actual droplet beam axis.ud