298 research outputs found
Probing weak dipole-dipole interaction using phase-modulated non-linear spectroscopy
Phase-modulated non-linear spectroscopy with higher harmonic demodulation has
recently been suggested to provide information on many-body excitations. In the
present work we theoretically investigate the application of this method to
infer the interaction strength between two particles that interact via weak
dipole-dipole interaction. To this end we use full numerical solution of the
Schr\"odinger equation with time-dependent pulses. For interpretation purpose
we also derive analytical expressions in perturbation theory. We find one can
detect dipole-dipole interaction via peak intensities (in contrast to
line-shifts which typically are used in conventional spectroscopy). We provide
a detailed study on the dependence of these intensities on the parameters of
the laser pulse and the dipole-dipole interaction strength. Interestingly, we
find that there is a phase between the first and second harmonic demodulated
signal, whose value depends on the sign of the dipole-dipole interaction.Comment: 12 pages, 8 figures, Supporting information provided with the source
file
Vibronic Lineshapes of PTCDA Oligomers in Helium Nanodroplets
Oligomers of the organic semiconductor PTCDA are studied by means of helium
nanodroplet isolation (HENDI) spectroscopy. In contrast to the monomer
absorption spectrum, which exhibits clearly separated, very sharp absorption
lines, it is found that the oligomer spectrum consists of three main peaks
having an apparent width orders of magnitude larger than the width of the
monomer lines. Using a simple theoretical model for the oligomer, in which a
Frenkel exciton couples to internal vibrational modes of the monomers, these
experimental findings are nicely reproduced. The three peaks present in the
oligomer spectrum can already be obtained taking only one effective vibrational
mode of the PTCDA molecule into account. The inclusion of more vibrational
modes leads to quasi continuous spectra, resembling the broad oligomer spectra
Coherent multidimensional spectroscopy in the gas phase
Recent work applying multidimentional coherent electronic spectroscopy at
dilute samples in the gas phase is reviewed. The development of refined
phase-cycling approaches with improved sensitivity has opened-up new
opportunities to probe even dilute gas-phase samples. In this context, first
results of 2-dimensional spectroscopy performed at doped helium droplets reveal
the femtosecond dynamics upon electronic excitation of cold, weakly-bound
molecules, and even the induced dynamics from the interaction with the helium
environment. Such experiments, offering well-defined conditions at low
temperatures, are potentially enabling the isolation of fundamental processes
in the excitation and charge transfer dynamics of molecular structures which so
far have been masked in complex bulk environments.Comment: Invited Review Articl
Alkali Atoms Attached to He Nanodroplets
We have experimentally studied the electronic excitation of Na
atoms attached to He droplets by means of laser-induced fluorescence as
well as beam depletion spectroscopy. From the similarities of the spectra
(width/shift of absorption lines) with these of Na on He droplets, we
conclude that sodium atoms reside in a ``dimple'' on the droplet surface and
that superfluid-related effects are negligible. The experimental results are
supported by Density Functional calculations at zero temperature, which confirm
the surface location of Na, K and Rb atoms on He droplets. In the case of
Na, the calculated shift of the excitation spectra for the two isotopes is in
good agreement with the experimental data.Comment: 6 pages, 3 figures, sent to JLT
Wave packet dynamics in triplet states of Na2 attached to helium nanodroplets
The dynamics of vibrational wave packets excited in Na2 dimers in the triplet
ground and excited states is investigated by means of helium nanodroplet
isolation (HENDI) combined with femtosecond pump-probe spectroscopy. Different
pathways in the employed resonant multi-photon ionization scheme are
identified. Within the precision of the method, the wave packet dynamics
appears to be unperturbed by the helium droplet environment
- …