30 research outputs found
Supercontinuum pulse shaping in the few-cycle regime
The synthesis of nearly arbitrary supercontinuum pulse forms is demonstrated
with sub-pulse structures that maintain a temporal resolution in the few-cycle
regime. Spectral broadening of the 35 fs input pulses to supercontinuum
bandwidths is attained in a controlled two-stage sequential filamentation in
air at atmospheric pressure, facilitating a homogeneous power density over the
full spectral envelope in the visible to near infrared spectral range. Only
standard optics and a liquid crystal spatial light modulator (LC-SLM) are
employed for achieving pulse compression to the sub 5 fs regime with pulse
energies of up to 60 μJ and a peak power of 12 GW. This constitutes the
starting point for further pulse form synthesis via phase modulation within
the sampling limit of the pulse shaper. Transient grating frequency-resolved
optical gating (TG-FROG) allows for the characterization of pulse forms that
extend over several hundred femtoseconds with few-cycle substructure
mechanistic insight into the oxidation of propene on the V4O11− cluster
An experimental methodology for a mechanistic analysis of gas phase chemical
reactions is presented in the context of structure–reactivity relationships of
metal oxide clusters relevant to photocatalysis. The spectroscopic approach is
demonstrated with the investigation of the photoinduced oxygenation of propene
on the V4O11−cluster, where the thermal activation and subsequent
photoreaction are deduced with the information from (i) the temperature
dependency of the aggregation kinetics in the propene-seeded helium atmosphere
of an ion-trap reactor; (ii) the fluence dependency in the yield of different
product channels of the photoreaction and (iii) the intensity dependency in
the fragmentation of neutral reaction products that are probed via in situ
multi-photon ionization. For the thermal reaction, selective hydrogen
abstraction from the allylic position of propene accompanied by the linkage to
the cluster at the dioxo moiety is postulated as the mechanism in the
aggregation of propene on the V4O11−cluster. In accordance with an insightful
neutralization–reionization study (Schröder et al., J. Mass. Spectrom., 2010,
301, 84), the subsequent photoinduced reaction is defined by an allylic
oxidation in the formation of acrolein from the initial allyloxy radical
photoproduct. The relevance of the observed selectivity is discussed in view
of the electronic structure and bond motifs offered by high valence oxide
systems such as the V4O11−cluster
Photo-oxidation by laser pulse induced desorption of phthalocyanines
Photo-oxidation of iron(II)-phthalocyanine (PcFe) has been observed in matrix
assisted laser desorption/ionization (MALDI) and laser desorption/ionization
(LDI) and is interpreted by theoretical molecular dynamics simulations. The
two ionization methods show different amounts of μ-oxo-bridged PcFe-dimer and
deliver evidence that MALDI produces less mechanical stress on the analyte.
The typical proton-transfer in the MALDI-process does not occur which leads to
the assumption of a released electron of the delocalized π-system
a step towards modulating precipitation?
We review the recent results about laser-induced condensation based on self-
guided filaments generated by ultrashort laser pulses. After recalling the
physico-chemistry of cloud particle formation in the atmosphere and the
physics of laser filamentation, we discuss experimental results on laser-
induced condensation and its relevance for modulating precipitation
Photoassociation and coherent transient dynamics in the interaction of ultracold rubidium atoms with shaped femtosecond pulses - I. Experiment
We experimentally investigate various processes present in the
photoassociative interaction of an ultracold atomic sample with shaped
femtosecond laser pulses. We demonstrate the photoassociation of pairs of
rubidium atoms into electronically excited, bound molecular states using
spectrally cut femtosecond laser pulses tuned below the rubidium D1 or D2
asymptote. Time-resolved pump-probe spectra reveal coherent oscillations of the
molecular formation rate, which are due to coherent transient dynamics in the
electronic excitation. The oscillation frequency corresponds to the detun-ing
of the spectral cut position to the asymptotic transition frequency of the
rubidium D1 or D2 lines, respectively. Measurements of the molecular
photoassociation signal as a function of the pulse energy reveal a non-linear
dependence and indicate a non-perturbative excitation process. Chirping the
association laser pulse allowed us to change the phase of the coherent
transients. Furthermore, a signature for molecules in the electronic ground
state is found, which is attributed to molecule formation by femtosecond
photoassociation followed by spontaneous decay. In a subsequent article [A.
Merli et al., submitted] quantum mechanical calculations are presented, which
compare well with the experimental data and reveal further details about the
observed coherent transient dynamics
Coherent control with shaped femtosecond laser pulses applied to ultracold molecules
We report on coherent control of excitation processes of translationally
ultracold rubidium dimers in a magneto-optical trap by using shaped femtosecond
laser pulses. Evolution strategies are applied in a feedback loop in order to
optimize the photoexcitation of the Rb2 molecules, which subsequently undergo
ionization or fragmentation. A superior performance of the resulting pulses
compared to unshaped pulses of the same pulse energy is obtained by
distributing the energy among specific spectral components. The demonstration
of coherent control to ultracold ensembles opens a path to actively influence
fundamental photo-induced processes in molecular quantum gases
Infrared Photodissociation Spectroscopy of C2n+1N− Anions with n = 1 – 5
The gas phase vibrational spectroscopy of cryogenically cooled C2n + 1N−
anions with n = 1 − 5 is investigated in the spectral range of the C≡C and C≡N
stretching modes (1850–2400 cm–1) by way of infrared photodissociation (IRPD)
spectroscopy of messenger-tagged C2n+1N–· mD2 complexes. The IRPD spectra are
assigned based on a comparison to previously reported anharmonic and harmonic
CCSD(T) vibrational frequencies and intensities. Experimentally determined and
predicted anharmonic vibrational transition energies lie within ± 21 cm–1. For
the harmonic CCSD(T)/vqz+ vibrational frequencies a scaling factor of 0.9808
is determined, resulting in comparable absolute deviations. The influence of
the D2-messenger molecules on the structure and the IRPD spectrum is found to
be small. Compared to the results of previous IR matrix isolation studies
additional, in particular weaker, IR-active transitions are identified
PCF-Based Cavity Enhanced Spectroscopic Sensors for Simultaneous Multicomponent Trace Gas Analysis
A multiwavelength, multicomponent CRDS gas sensor operating on the basis of a compact photonic crystal fibre supercontinuum light source has been constructed. It features a simple design encompassing one radiation source, one cavity and one detection unit (a spectrograph with a fitted ICCD camera) that are common for all wavelengths. Multicomponent detection capability of the device is demonstrated by simultaneous measurements of the absorption spectra of molecular oxygen (spin-forbidden b-X branch) and water vapor (polyads 4v, 4v + δ) in ambient atmospheric air. Issues related to multimodal cavity excitation, as well as to obtaining the best signal-to-noise ratio are discussed together with methods for their practical resolution based on operating the cavity in a “quasi continuum” mode and setting long camera gate widths, respectively. A comprehensive review of multiwavelength CRDS techniques is also given
Argon physisorption as structural probe for endohedrally doped silicon clusters
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