Ultraviolet and Infrared Spectroscopy of Synthetic Foldamers, Aib Homopeptides, and Solvated 1,2-Diphenylethane in the Gas Phase

The work presented here implements a supersonic jet expansion source to funnel the population of model peptides and solvated-bichromophore clusters into their low lying structural minima and to collisionally cool these minima to their respective zero-point vibrational levels. Single-conformation ultraviolet and infrared spectroscopy techniques are then used to probe these systems and investigate their electronic properties and uncover their intrinsic conformational preferences in the gas phase. Model β/γ-peptides known as synthetic foldamers and aminoisobutyric acid (Aib) homopeptides incorporate structural constraints that are designed/known to impose particular structural motifs. Here the ability of a β/γ-dipeptide to replicate the backbone length of an α-tripeptide and subsequently form the first portion of an α-helix is presented. Additionally, tests of the propensity for (Aib)n homopeptides to form 310-helices, in spite of accumulation of a macrodipole moment are shown. In fact, Aib is a strong 310-helix former, but there appears to be a point around n = 6 where competing forces funnel the population into a competing conformational family. 1,2-Diphenylethane (DPE) is a model, flexible bichromophore comprising two phenyl rings bound by an ethane bridge. By complexing DPE with (H 2O)n (n = 1-3) the effects of step-wise solvation on the electronic spectroscopy (i.e. the impact of water on vibronically coupled chromophores) and the conformational preferences of both the DPE monomer and the (H2O)n structures were investigated. In three of the four resultant clusters, the water molecule(s) were found to bind symmetrically to the DPE host, and an S0-S2 origin transition was not observed. However, in the fourth case, in which the anti conformation of the DPE monomer serves as host, localization is observed, and the S0-S2 origin is detected. Also of note is that the water-trimer, which almost always adopts a cycle geometry, was found to exclusively adopt a chain-geometry in the presence of DPE

ASSESSING THE IMPACT OF BACKBONE LENGTH AND CAPPING AGENT ON THE CONFORMATIONAL PREFERENCES OF A MODEL PEPTIDE: CONFORMATION SPECIFIC IR AND UV SPECTROSCOPY OF 2-AMINOISOBUTYRIC ACID

2-Aminoisobutyric acid (Aib) is an achiral, ${alpha}$-amino acid having two equivalent methyl groups attached to C$_{alpha}$. Extended Aib oligomers are known to have a strong preference for the adoption of a 3$_{10}$-helical structure in the condensed phase.footnote{Toniolo, C.; Bonora, G. M.; Barone, V.; Bavoso, A.; Benedetti, E.; Di Blasio, B.; Grimaldi, P.; Lelj, F.; Pavone, V.; Pedone, C., Conformation of Pleionomers of $alpha$-Aminoisobutyric Acid. textit{Macromolecules} textbf{1985}, textit{18}, 895-902.} Here, we have taken a simplifying step and focused on the intrinsic folding propensities of Aib by looking at a series of capped Aib oligomers in the gas phase, free from the influence of solvent molecules and cooled in a supersonic expansion. Resonant two-photon ionization and IR-UV holeburning have been used to record single-conformation UV spectra using the Z-cap as the UV chromophore. Resonant ion-dip infrared (RIDIR) spectroscopy provides single-conformation IR spectra in the OH stretch and NH stretch regions. Data have been collected on a set of Z-(Aib)$_{n}$-X oligomers with n = 1, 2, 4, 6 and X = -OH and -OMethyl. The impacts of these capping groups and differences in backbone length have been found to dramatically influence the conformational space accessed by the molecules studied here. Oligomers of n=4 have sufficient backbone length for a full turn of the 3$_{10}$-helix to be formed. Early interpretation of the data collected shows clear spectroscopic markers signaling the onset of 3$_{10}$-helix formation as well as evidence of structures incorporating C7 and C14 hydrogen bonded rings

Communication: Time-domain measurement of high-pressure N2 and O2 self-broadened linewidths using hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering

The direct measurement of self-broadened linewidths using the time decay of pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps RCARS) signals is demonstrated in gas-phase N2 and O2 from 1–20 atm. Using fs pump and Stokes pulses and a spectrally narrowed ps probe pulse, collisional dephasing rates with time constants as short as 2.5 ps are captured with high accuracy for individual rotational transitions. S-branch linewidths of N2 and O2 from ∼0.06 to 2.2 cm−1 and the line separation of O2 triplet states are obtained from the measured dephasing rates and compared with high-resolution, frequency-domain measurements and S-branch approximations using the modified exponential gap model. The accuracy of the current measurements suggests that the fs/ps RCARS approach is well suited for tracking the collisional dynamics of gas-phase mixtures over a wide range of pressures

A Threshold-Based Approach to Calorimetry in Helium Droplets: Measurement of Binding Energies of Water Clusters

Helium dropletbeam methods have emerged as a versatile technique that can be used to assemble a wide variety of atomic and molecular clusters. We have developed a method to measure the binding energies of clusters assembled in helium droplets by determining the minimum droplet sizes required to assemble and detect selected clusters in the spectrum of the dopeddropletbeam. The differences in the droplet sizes required between the various multimers are then used to estimate the incremental binding energies. We have applied this method to measure the binding energies of cyclic waterclusters from the dimer to the tetramer. We obtain measured values of D0 that are in agreement with theoretical estimates to within ∼20%. Our results suggest that this threshold-based approach should be generally applicable using either mass spectrometry or optical spectroscopy techniques for detection, provided that the clusters selected for study are at least as strongly bound as those of water, and that a peak in the overall spectrum of the beam corresponding only to the cluster chosen (at least in the vicinity of the threshold) can be located

Interference-free gas-phase thermometry at elevated pressure using hybrid femtosecond/picosecond rotational coherent anti- Stokes Raman scattering

Rotational-level-dependent dephasing rates and nonresonant background can lead to significant uncertainties in coherent anti-Stokes Raman scattering (CARS) thermometry under high-pressure, lowtemperature conditions if the gas composition is unknown. Hybrid femtosecond/picosecond rotational CARS is employed to minimize or eliminate the influence of collisions and nonresonant background for accurate, frequency-domain thermometry at elevated pressure. The ability to ignore these interferences and achieve thermometric errors of \u3c5% is demonstrated for N2 and O2 at pressures up to 15 atm. Beyond 15 atm, the effects of collisions cannot be ignored but can be minimized using a short probe delay (~6.5 ps) after Raman excitation, thereby improving thermometric accuracy with a time- and frequency-resolved theoretical model

Communication: Hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering thermometry using a narrowband time-asymmetric probe pulse

A narrowband, time-asymmetric probe pulse is introduced into the hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering (fs/ps RCARS) technique to provide accurate and precise single-shot, high-repetition-rate gas-phase thermometric measurements. This narrowband pulse-generated by inserting a Fabry-Pérot étalon into the probe-pulse beam path-enables frequency-domain detection of pure-rotational transitions. The unique time-asymmetric nature of this pulse, in turn, allows for detection of resonant Raman-active rotational transitions free of signal contamination by nonresonant four-wave-mixing processes while still allowing detection at short probe-pulse delays, where collisional dephasing processes are negligible. We demonstrate that this approach provides excellent single-shot thermometric accuracy (1 error) and precision (∼2.5) in gas-phase environments

Energy deposition during electron-induced dissociation

AbstractWe report studies of the internal energy deposited during activation of mass-selected ions through electron-ion collisions. Characteristic fragmentations of the molecular ion of limonene and W(CO)n+ (n = 1-6) indicate that electron-induced dissociation in a Fourier transform ion cyclotron resonance mass spectrometer proceeds via multiple collisions and that the average internal energy deposited during the activation process can be selected to be similar to that associated with electron-impact ionization. Control of the degree of ion excitation through selection of the electron energy, flux, and interaction time with the ions of interest is demonstrated, and advantages of this promising activation technique are discussed

Single-shot gas-phase thermometry using purerotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering

High-repetition-rate, single-laser-shot measurements are important for the investigation of unsteady flows where temperature and species concentrations can vary significantly. Here, we demonstrate singleshot, pure-rotational, hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps RCARS) thermometry based on a kHz-rate fs laser source. Interferences that can affect nanosecond (ns) and ps CARS, such as nonresonant background and collisional dephasing, are eliminated by selecting an appropriate time delay between the 100-fs pump/Stokes pulses and the pulse-shaped 8.4-ps probe. A time- and frequency-domain theoretical model is introduced to account for rotational-level dependent collisional dephasing and indicates that the optimal probe-pulse time delay is 13.5 ps to 30 ps. This time delay allows for uncorrected best-fit N2- RCARS temperature measurements with ~1% accuracy. Hence, the hybrid fs/ps RCARS approach can be performed with kHz-rate laser sources while avoiding corrections that can be difficult to predict in unsteady flows

All-diode-pumped quasi-continuous burst-mode laser for extended high-speed planar imaging

An all-diode-pumped, multistage Nd:YAG amplifier is investigated as a means of extending the duration of high-power, burst-mode laser pulse sequences to an unprecedented 30 ms or more. The laser generates 120 mJ per pulse at 1064.3 nm with a repetition rate of 10 kHz, which is sufficient for a wide range of planar laser diagnostics based on fluorescence, Raman scattering, and Rayleigh scattering, among others. The utility of the technique is evaluated for image sequences of formaldehyde fluorescence in a lifted methane–air diffusion flame. The advantages and limitations of diode pumping are discussed, along with long-pulse diode-bar performance characteristics to guide future designs

Micro-Optical Initiation of Nanoenergetic Materials Using a Temporally Tailored Variable-Pulse-Width Laser

Nanoenergetic materials can provide a significant enhancement in the rate of energy release as compared with microscale materials. The energy-release rate is strongly dependent not only on the primary particle size but also on the level of agglomeration, which is of particular interest for the inclusion of nanoenergetics in practical systems where agglomeration is desired or difficult to avoid. Unlike studies of nanoparticles or nanometer-size aggregates, which can be conducted with ultrafast or nanosecond lasers assuming uniform heating, microscale aggregates of nanoparticles are more sensitive to the thermophysical time scale of the heating process. To allow control over the rate of energy deposition during laser initiation studies, a custom, temporally tailored, continuously variable-pulse-width (VPW) laser was employed for radiative heating of nanoenergetic materials. The laser consisted of a continuous-wave master oscillator, which could be sliced into desired pulses, and a chain of amplifiers to reach high peak power. The slicer allowed control over the time profile of the pulses via the combination of an arbitrary waveform generator and acousto-optic modulator (AOM). The effects of utilizing flat-top or ramped laser pulses with durations from 100 ns to 150 ls and energies up to 20 mJ at 1064 nm were investigated, along with a broad range of heating rates for single particles or nanoparticle aggregates up to 100-lm diameter. In combination with an optical microscope, laser heating of aggregates consisting of 70-nm diameter Al nanoparticles in a Teflon matrix showed significant dependence on the heating profile due to the sensitivity of nanoenergetic materials to heating rate. The ability to control the temporal pulse-intensity profile leads to greater control over the effects of ablative heating and the resulting shockwave propagation. Hence, flexible laser-pulse profiles allow the investigation of energetic properties for a wide size range of metal/metal-oxide nanoparticles, aggregates, and composites