Electronic Spectroscopy of Phthalocyanine and Porphyrin Derivatives in Superfluid Helium Nanodroplets

Phthalocyanine and porphyrin were among the first organic compounds investigated by means of electronic spectroscopy in superfluid helium nanodroplets. Superfluid helium nanodroplets serve as a very gentle host system for preparing cold and isolated molecules. The uniqueness of helium nanodroplets is with respect to the superfluid phase which warrants the vanishing viscosity and, thus, minimal perturbation of the dopant species at a temperature as low as 0.37 K. These are ideal conditions for the study of molecular spectra in order to analyze structures as well as dynamic processes. Besides the investigation of the dopant species itself, molecular spectroscopy in helium droplets provides information on the helium droplet and in particular on microsolvation. This article, as part of a special issue on phthalocyanines and porphyrins, reviews electronic spectroscopy of phthalocyanine and porphyrin compounds in superfluid helium nanodroplets. In addition to the wide variety of medical as well as technical and synthetical aspects, this article discusses electronic spectroscopy of phthalocyanines and porphyrins in helium droplets in order to learn about both the dopant and the helium environment. View Full-Tex

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 104–105 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

Vibrations and hydrogen bonding in porphycene

Combined use of IR, Raman, neutron scattering and fluorescence measurements for porphycene isolated in helium nanodroplets, supersonic jet and cryogenic matrices, as well as for solid and liquid solutions, resulted in the assignments of almost all of 108 fundamental vibrations. The puzzling feature of porphycene is the apparent lack of the N–H stretching band in the IR spectrum, predicted to be the strongest of all bands by standard harmonic calculations. Theoretical modeling of the IR spectra, based on ab initio molecular dynamics simulations, reveals that the N–H stretching mode should appear as an extremely broad band in the 2250–3000 cm−1 region. Coupling of the N–H stretching vibration to other modes is discussed in the context of multidimensional character of intramolecular double hydrogen transfer in porphycene. The analysis can be generalized to other strongly hydrogen-bonded systems

Detection of \"Cold\" Spectra from a Room-Temperature Ensemble: Magnetic Rotation Spectroscopy with Simple Interpretation in Terms of Molecular Pendular States

The anisotropic orientation of the figure axis of pendular mols. is directly detected by polarization spectroscopy for ICl in the excited A 3P1 state. A simple geometric model is outlined that simulates all spectral features over a wide range of exptl. parameters, without any fitting procedure. The exptl. technique, closely related to magnetic rotation spectroscopy, strongly favors low rotational states and is also selective for DJ within a rotational band. Spectra obtained using a gas cell at room temp. thereby become even sparser and simpler than can be attained by cooling mols. to a few kelvin. It provides a technique for local measurement of elec. or magnetic field strengths without mech. devices

Zustandsselektive Molekularstrahluntersuchungen totaler integraler und reaktiver Streuprozesse von Alkalimolekülen mit Lasern

Scattering processes f alkali atoms and mols. are state selectively investigated in a scattering expt. with two crossed mol. beams. The aim of the investigation is to probe and improve the potential hypersurface. Exptl. there are measured state selective cross sections using laser spectroscopy. The system Li2(v,J)+Na is ideal for prepn. of vibrational excited states of Li2 near up to the dissocn. limit as well as for state selective measurement of scattered Li2 and the reaction product NaLi(V,J) from the endothermic reaction channel. Investigations of the total integral cross section in dependence on the vibrational excitation of the Li2 mol. probes the long range part of the interaction potential. For low vibrational states the total integral cross section shows const. values while for highly excited states Li2(v=20) the cross section is 30% greater. Comparison with quasi-classical trajectory calcns. on a LEPS potential surface shows qual. agreement but quant. disagreement. Just so the results of reactive measurements do not agree with the theor. postulates. The LEPS potential surface is not able to fit the exptl. data. In a second scattering expt. the resonance exchange interaction of differently excited like atoms was investigated. This interaction is a long range dipole-dipole interaction and scales with the inverse third power of the distance of the atoms. Classical ests. show that the exponent of the dependence on the distance of the long range potential can be extd. from the measurement of the total integral cross section with variation of the exptl. resoln. The exptl. result for Li atoms in the ground state and the first excited state verifies the expected result and agrees with quantum mech. calcns. on the well known potential hypersurfaces of the first excited state of Li2

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

Microsolvation of phthalocyanines in superfluid helium droplets

Exptl. and theor. studies of the spectroscopy of mols. in superfluid He droplets provide evidence for the key role of the 1st He layer surrounding the dopant mol. in detg. the mols. spectroscopic features. Recent studies of emission spectra of phthalocyanine in He droplets revealed a doubling of all transitions. Herein, the authors present the emission spectra of Mg-phthalocyanine and of phthalocyanine-Ar clusters in He droplets, which confirm the splitting as a general effect of the He environment. A scheme of levels is deduced from the emission spectra and attributed to quantized states of the 1st He layer surrounding the dopant mol