Spectral Signatures of Ground- and Excited-State Wavepacket Interference after Impulsive Excitation.

Impulsive transient absorption spectroscopy is used to track the formation and evolution of vibrational coherences in cresyl violet perchlorate under different excitation conditions. Resonant and off-resonant pump pulses result in the selective formation of excited (S1)- and ground (S0)-state wavepackets. Partially resonant and broadband excitation conditions lead to the simultaneous formation of wavepackets in the ground and excited states. The wavepackets are characterized by the phase-flips in the coherent signal associated with wavepacket motion across the absorption and emission maxima and by a red shift of 2-10 cm-1 in the Raman features of the excited state compared to the ground-state wavepacket. We observe that, when wavepackets are simultaneously excited on the ground- and excited-state surfaces, interference on a picosecond timescale between coherent oscillations in the two wavepackets gives rise to features that cannot be attributed to the passage of a wavepacket through a conical intersection, such as shifting phase-flips and zero-amplitude nodes. Wavepacket filtering using windowed Fourier transforms highlights these interference effects and demonstrates that special care must be taken in order to properly interpret data that have been processed in this manner.

Formation of Copper(I) Oxide- and Copper(I) Cyanide-Polyacetonitrile Nanocomposites through Strong-Field Laser Processing of Acetonitrile Solutions of Copper(II) Acetate Dimer.

Irradiation studies of acetonitrile solutions of copper(II) acetate dimer ([Cu(OAc)2]2) using high energy, simultaneously spatially and temporally focused (SSTF) ultrashort laser pulses are reported. Under ambient conditions, irradiation for relatively short periods of time (10-20 s) selectively produces relatively small, narrowly size-dispersed (3.5 ± 0.7 nm) copper(I) oxide nanoparticles (Cu2O NPs) embedded in CuCN-polyacetonitrile polymers generated in situ by the laser. The Cu2O NPs become embedded in a CuCN-polyacetonitrile network as they form, stabilizing them and protecting the air-sensitive material from oxygen. Laser irradiation of acetonitrile causes fragmentation into transient radicals that initiate and terminate polymerization of acetonitrile. Control and mechanistic investigations reveal that HCN formed during laser irradiation reacts rapidly to reduce the Cu(II) centers in [Cu(OAc)2]2, leading to the formation of CuCN or, in the presence of water, Cu2O nanoparticles that bind and cross-link CuCN-polyacetonitrile chains. The acetate-bridged Cu(II) dimer unit is a required structural feature that functions to preorganize and direct the Cu(II) reduction and selective formation of CuCN and Cu2O nanoparticles. This study illustrates how rapid deposition of energy using shaped, ultrashort laser pulses can initiate multiple photolytic and thermal processes that lead to the selective formation of composite nanoparticle/polymer materials for applications in electronics and catalysis.

Gold Nanotriangle Formation through Strong-Field Laser Processing of Aqueous KAuCl4 and Postirradiation Reduction by Hydrogen Peroxide.

Femtosecond laser irradiation of aqueous KAuCl4 followed by postirradiation reduction with hydrogen peroxide (H2O2) is investigated as a new approach for the synthesis of gold nanotriangles (AuNTs) without any added surfactant molecules. Laser irradiation was applied for times ranging from 5 to 240 s, and postirradiation reduction of the solutions was monitored by UV-vis spectroscopy. Laser processing of aqueous KAuCl4 for 240 s, where the full reduction of Au(III) occurred during irradiation, produced spherical gold nanoparticles (AuNPs) with an average size of 11.4 ± 3.4 nm. Irradiation for shorter times (i.e., 15 s) resulted in the formation of laser-generated AuNP seeds (5.7 ± 1.8 nm) in equilibrium with unreacted KAuCl4 after termination of laser irradiation. The postirradiation reduction of these solutions by H2O2 produced a mixture of spherical and triangular AuNPs. Decreasing the laser irradiation time from 45 to 5 s significantly reduced the number of laser-generated Au seeds, the amount of H2O2 produced, and the rate of postirradiation reduction, resulting in the formation of a large number of AuNTs with sizes increasing from 29.5 ± 10.2 to 125 ± 43.2 nm. Postirradiation reduction is kinetically inhibited in the absence of laser-generated AuNP seeds.

Elucidating Strong Field Photochemical Reduction Mechanisms of Aqueous [AuCl4](-): Kinetics of Multiphoton Photolysis and Radical-Mediated Reduction.

Direct, multiphoton photolysis of aqueous metal complexes is found to play an important role in the formation of nanoparticles in solution by ultrafast laser irradiation. In situ absorption spectroscopy of aqueous [AuCl4](-) reveals two mechanisms of Au(0) nucleation: (1) direct multiphoton photolysis of [AuCl4](-) and (2) radical-mediated reduction of [AuCl4](-) upon multiphoton photolysis of water. Measurement of the reaction kinetics as a function of solution pH reveals zeroth-, first-, and second-order components. The radical-mediated process is found to be zeroth-order in [AuCl4](-) under acidic conditions, where the reaction rate is limited by the production of reactive radical species from water during each laser shot. Multiphoton photolysis is found to be first order in [AuCl4](-) at all pHs, whereas the autocatalytic reaction with H2O2, the photolytic reaction product of water, is second order.

Filament-Assisted Impulsive Raman Spectroscopy of Ozone and Nitrogen Oxides.

The filament-assisted impulsive Raman spectra of ozone, nitric oxide, and nitrogen dioxide are presented. The Raman response as a function of ozone concentration scales as N(2), where N is the number of oscillators in the interaction region. The system described has a detection limit of ∼300 ppm for gas-phase ozone. Ozone produced via the strong field chemistry occurring within the filament pump was also detected. The measurements reveal spectral interference in the Raman features. Simulations show the spectral fringing results from interference of the Raman signal with pump-induced cross-phase modulation. The fringes are used to classify the symmetric mode of the low concentration filament-generated ozone.

High-energy noncollinear optical parametric amplifier producing 4 fs pulses in the visible seeded by a gas-phase filament.

We report on the design and characterization of a short-pulse-pumped, single-stage noncollinear optical parametric amplifier (NOPA) that achieves high pulse energies in the few-cycle pulse regime. Optimal pulse-front tilting and temporal compression of the short (35 fs) pump pulse are achieved using a 4f grating compressor, while spatial chirp at the NOPA crystal is eliminated with proper imaging using a pair of reflective telescopes. Gas-phase filamentation in an open-ended argon-filled cell provides a bright, stable seed source with little residual chirp that is suitable for temporal overlap with the short pump pulse without dispersion precompensation. Two seeding geometries are explored, and pulses as short as 3.5 fs are obtained by seeding with the entire filament bandwidth. Fourier-transform-limited 4 fs pulses are obtained by filtering the IR portion of the spectrum.

Triangular gold nanoplate growth by oriented attachment of Au seeds generated by strong field laser reduction.

The synthesis of surfactant-free Au nanoplates is desirable for the development of biocompatible therapeutics/diagnostics. Rapid Δ-function energy deposition by irradiation of aqueous KAuCl4 solution with a 5 s burst of intense shaped laser pulses, followed by slow addition of H2O2, results in selective formation of nanoplates with no additional reagents. The primary mechanism of nanoplate formation is found to be oriented attachment of the spherical seeds, which self-recrystallize to form crystalline Au nanoplates.

Spectral-to-temporal amplitude mapping polarization spectroscopy of rotational transients.

A new implementation of pump-probe polarization spectroscopy is presented where the revivals of an impulsively excited rotational wavepacket are mapped onto a broad-band, chirped continuum pulse to measure a long temporal window without the need for delay scanning. Experimental measurements and a theoretical framework for spectral-temporal amplitude mapping polarization spectroscopy (STAMPS) as applied to impulsive rotational motion are presented. In this technique, a femtosecond laser pulse is used to prepare a rotational wavepacket in a gas-phase sample at room temperature. The rotational revivals of the wavepacket are then mapped onto a chirped continuum (400-800 nm) pulse created by laser filamentation in argon. Nearly single-shot time-resolved rotational spectra are recorded over a 65 ps time window. The transient birefringence spectra are simulated by including terms for polarization rotation of the probe as well as cross-phase modulation. Measurements and simulations are presented for the cylindrically symmetric N2, O2, and CO2 molecules. The long time window of the method allows measurement of rotational spectra for asymmetric top molecules, and here we present measurements for ethylene and methanol.

Filament-driven impulsive Raman spectroscopy.

Vibrational Raman spectroscopy is performed in the gas phase using a femtosecond laser pulse undergoing filamentation as an impulsive excitation source. The molecular coherence induced by the filamentary pulse is subsequently probed using a narrowband, sub-picosecond laser pulse to produce Raman spectra of gas phase species in a few tens of milliseconds (~10 laser shots). Pulse shortening with concomitant spectral broadening during filamentation results in a pulse that is both sufficiently short and of sufficient spectral power density to impulsively excite the highest energy ground state vibrations (up to 4158 cm(-1) corresponding to H(2)). Gas phase detection of chloroform, methylene chloride, cyclohexane, toluene, pentane, triethylamine, ammonia, nitromethane, and gasoline is performed.