Stimulated Raman versus Inverse Raman: Investigating Depletion Mechanisms for Super-Resolution Raman Microscopy.

Super-resolution fluorescence microscopy has been critical in elucidating the nanoscale structure of biological systems. However, fluorescent labels bring difficulties such as perturbative labeling steps and photobleaching. Thus, label-free super-resolution techniques are of great interest, like our group's 2016 stimulated Raman scattering (SRS) technique, stimulated Raman depletion microscopy (SRDM).

Molecular Insights into the Binding of Linear Polyethylenimines and Single-Stranded DNA Using Raman Spectroscopy: A Quantitative Approach.

Establishing how polymeric vectors such as polyethylenimine (PEI) bind and package their nucleic acid cargo is vital toward developing more efficacious and cost-effective gene therapies. To develop a molecular-level picture of DNA binding, we examined how the Raman spectra of PEIs report on their local chemical environment. We find that the intense Raman bands located in the 1400-1500 cm region derive from vibrations with significant CH scissoring and NH bending character.

Femtosecond stimulated Raman evidence for charge-transfer character in pentacene singlet fission.

Singlet fission is a spin-allowed process in which an excited singlet state evolves into two triplet states. We use femtosecond stimulated Raman spectroscopy, an ultrafast vibrational technique, to follow the molecular structural evolution during singlet fission in order to determine the mechanism of this process. In crystalline pentacene, we observe the formation of an intermediate characterized by pairs of excited state peaks that are red- and blue-shifted relative to the ground state features.

Low-Energy Photoelectron Imaging Spectroscopy of La(benzene) (n = 1 and 2).

La(benzene) (n = 1 and 2) are formed in a pulsed laser-ablation molecular beam source and characterized by low-energy photoelectron imaging spectroscopy. The photoelectron spectrum of La(benzene) displays a strong origin band, a short metal-ligand stretching progression, and a weak ring deformation band. Four isomers are considered for La(benzene), and the preferred structure is an inverse sandwich with two La atoms residing on the opposite sides of the benzene ring.

Mass-analyzed threshold ionization spectroscopy of lanthanum-hydrocarbon radicals formed by C-H bond activation of propene.

La(CH) and La(CH) are observed from the reaction of laser-vaporized La atoms with propene by photoionization time-of-flight mass spectrometry and characterized by mass-analyzed threshold ionization spectroscopy. Two isomers of La(CH) are identified as methyl-lanthanacyclopropene [La(CHCCH)] (C) and lanthanacyclobutene [La(CHCHCH)] (C); La(CH) is determined to be H-La(η-allyl) (C), a C-H bond inserted species. All three metal-hydrocarbon radicals prefer a doublet ground state with a La 6s-based electron configuration.

La-Activated Bicyclo-oligomerization of Acetylene to Naphthalene.

We report the first example of metal-mediated acetylene bicyclopentamerization to form naphthalene in the gas phase. The bicyclic aromatic compound was observed in a complex with La. The La(naphthalene) complex was formed by the reaction of laser-ablated La atoms with acetylene molecules in a molecular beam source and was characterized by mass-analyzed threshold ionization spectroscopy.

Excited state structural evolution during charge-transfer reactions in betaine-30.

Ultrafast photo-induced charge-transfer reactions are fundamental to a number of photovoltaic and photocatalytic devices, yet the multidimensional nature of the reaction coordinate makes these processes difficult to model theoretically. Here we use femtosecond stimulated Raman spectroscopy to probe experimentally the structural changes occurring following photoexcitation in betaine-30, a canonical intramolecular charge-transfer complex. We observe changes in vibrational mode frequencies and amplitudes on the femtosecond timescale, which for some modes results in frequency shifts of over 20 cm(-1) during the first 200 fs following photoexcitation.

Lanthanum-mediated C-H bond activation of propyne and identification of La(C₃H₂) isomers.

η(2)-Propadienylidenelanthanum [La(η(2)-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(η(2)-CCCH2) and La(HCCCH) are formed by concerted 1,3- and 3,3-dehydrogenation, respectively.

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 in vivo 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.

Photodissociation of heptane isomers and relative ionization efficiencies of butyl and propyl radicals at 157 nm.

We report an ion imaging and time-of-flight mass spectroscopy study of the photodissociation of a variety of heptane isomers using 157 nm dissociation and ionization. Time-of-flight mass spectra show that C(3)H(7) + C(4)H(9) is the dominant detected product channel following one-color 157 nm dissociation/ionization of heptanes. The results further allow determination of the relative ionization efficiencies of 1- and 2-butyl and propyl radicals at 157 nm.

Photodissociation of spatially aligned acetaldehyde cations.

Photofragment translational energy and angular distributions are reported for the photodissociation of acetaldehyde cations in the wavelength range 354-363 nm obtained using the DC slice ion imaging technique. Vibrationally selected parent ions were produced by 2+1 resonance-enhanced multiphoton ionization (REMPI) via the 3s<--n Rydberg transition, with photodissociation resulting from absorption of a fourth additional photon. Three product channels were observed: HCO+, CH3CO+, and CH4+.

S(1D2) atomic orbital polarization in the photodissociation of OCS at 193 nm: Construction of the complete density matrix.

The absolute velocity-dependent alignment and orientation for S(1D2) atoms from the photodissociation of OCS at 193 nm were measured using the dc slice imaging method. Three main peaks ascribed to specific groups of high rotational levels of CO in the vibrational ground state were found, with rotationally resolved rings in a fourth slow region ascribed to weak signals associated with excited vibrational states of CO. The observed speed-dependent beta and polarization parameters support the interpretation that there are two main dissociation processes: a simultaneous two-surface (A' and A") excitation and the initial single-surface (A') excitation followed by the nonadiabatic crossing to ground state.