Photodissociation dynamics in Titan's atmosphere

Photodissociation dynamics of molecules relevant to understanding Titan’s atmosphere (diacetylene, cyanoacetylene and heptane isomers) are carried out under collisionless condition using the DC slice imaging technique. In diacetylene photodissociation, two-photon processes dominate at 243 nm and 212 nm whereas at 121.6 nm, a one-photon dissociation process dominates. Direct measurement of the lifetime of metastable triplet diacetylene confirms sub-microsecond lifetimes. Photodissociation of cyanoacetylene at 193.3 nm proceeds on the S1 potential energy surface with an exit barrier. In heptane photodissociation, the dissociation occurs on the ground state or low-lying triplet states with nonradiative electronic relaxation. Time-of-flight mass spectroscopy studies in this system yield the relative ionization efficiencies of 1- and 2-butyl and propyl radicals at 157 nm

Toward Label-Free Super-Resolution Microscopy

We propose and implement a far-field spectroscopic system for imaging below the diffraction limit without the need for fluorescence labeling. Our technique combines concepts from Stimulated Emission Depletion (STED) microscopy and Femtosecond Stimulated Raman Spectroscopy (FSRS). The FSRS process generates signal through the creation of vibrational coherences, and here we use a toroidal-shaped decoherence pulse to eliminate vibrational signal from the edges of the focal spot. The nonlinear dependence on decoherence pulse power enables subdiffraction imaging. As in STED, the resolution is in theory infinitely small given infinite decoherence pulse power. Here, we first experimentally demonstrate that the photophysical principles behind our super-resolution Raman imaging method are sound. We then prove that addition of the decoherence pulse significantly improves the spatial resolution of our microscope, achieving values beyond the diffraction limit. We discuss future directions for this technique, including methods to reach resolution on the order of ten nanometers

Determination of Resonance Raman Cross-Sections for Use in Biological SERS Sensing with Femtosecond Stimulated Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a promising technique for <i>in vivo</i> bioanalyte detection, but accurate characterization of SERS biosensors can be challenging due to difficulties in differentiating resonance and surface enhancement contributions to the Raman signal. Here, we quantitate the resonance Raman cross-sections for a commonly used near-infrared SERS dye, 3,3′-diethylthiatricarbocyanine (DTTC). It is typically challenging to measure resonance Raman cross-sections for fluorescent dye molecules due to the overwhelming isoenergetic fluorescence signal. To overcome this issue, we used etalon-based femtosecond stimulated Raman spectroscopy, which is intrinsically designed to acquire a stimulated Raman signal without strong fluorescence or interference from signals resulting from other four-wave mixing pathways. Using this technique, we found that the cross-sections for most of the resonantly enhanced modes in DTTC exceed 10^–25 cm²/molecule. These cross-sections lead to high signal magnitude SERS signals from even weakly enhancing SERS substrates, as much of what appears to be a SERS signal is actually coming from the intrinsically strong resonance Raman signal. Our work will lead to a more accurate determination of SERS enhancement factors and SERS substrate characterization in the biologically relevant near-infrared region, ultimately leading to a more widespread use of SERS for biosensing and bioimaging applications

Lanthanum-Mediated C–H Bond Activation of Propyne and Identification of La(C₃H₂) Isomers

η²-Propadienylidenelanthanum [La­(η²-CCCH₂)] and deprotio­lanthana­cyclobutadiene [La­(HCCCH)] of La­(C₃H₂) are identified from the reaction mixture of neutral La atom activation of propyne in the gas phase. The two isomers are characterized with mass-analyzed threshold ionization spectroscopy combined with electronic structure calculations and spectral simulations. La­(η²-CCCH₂) and La­(HCCCH) are formed by concerted 1,3- and 3,3-dehydrogenation, respectively. Both isomers prefer a doublet ground state with a La 6s-based unpaired electron, and La­(η²-CCCH₂) is slightly more stable than La­(HCCCH). Ionization of the neutral doublet state of either isomer produces a singlet ion state by removing the La-based electron. The geometry change upon ionization results in the excitation of a symmetric metal–hydrocarbon stretching mode in the ionic state, whereas thermal excitation leads to the observation of the same stretching mode in the neutral state. Although the La atom is in a formal oxidation state of +2, the ionization energies of these metal–hydrocarbon radicals are lower than that of the neutral La atom. Deuteration has a very small effect on the ionization energies of the two isomers and the metal–hydrocarbon stretching mode of La­(η²-CCCH₂), but it reduces considerably the metal–ligand stretching frequencies of La­(HCCCH)