Electronic Spectroscopy of Molecules in Superfluid Helium Nanodroplets: An Excellent Sensor for Intramolecular Charge Redistribution

Electronic spectra of molecules doped into superfluid He-4 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 on 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.