This work investigates laser optimally controlled fragmentation in a series of chemically related halomethane compounds in order to determine the extent to which regular trends in objective yields are observed upon variation of both the laser pulse shape and the chemical substrate. Three control objectives defined by ratios of fragment ions were considered, where families of shaped pulses were found that produced optimal objective yields. For the objective defined by the ratio of a halogen ion to a methyl halide ion, a systematic correlation between the optimal objective yield and chemical composition was revealed. Furthermore, some of the optimal shaped pulses were found to successfully control the same objective in closely related molecular substrates. These results provide a basis to expect systematic chemical responses from optimally shaped laser pulses acting as “photonic reagents” in analogy with the action of traditional chemical reagents.

Optimal control of molecular fragmentation with homologous families of photonic reagents and chemical substrates

When plasmonic nanoparticles are incorporated into nanostructures and they are exposed to external optical fields, plasmonic coupling causes electric near-field enhancement which is significantly larger than that of isolated nanoparticles. We report on the plasmonic coupling in arrays of gold nanospheres (20 ± 3 and 50 ± 4 nm) prepared by colloidal chemistry and self-organization. This yields field enhancement in arrays with areas of several mm2 and provides an alternative approach to lithographic methods for preparation of nanostructures for plasmonic applications. Gold nanospheres are surface-functionalized by organic ligands, which define the interparticle distance in the array upon self-organization of the nanoparticles. The experiments are accompanied by finite-difference time-domain simulations, which quantify the dependence of the field enhancement on the interparticle distance.

Plasmonic electric near-field enhancement in self-organized gold nanoparticles in macroscopic arrays

The growing use of ultrafast laser pulses to induce chemical reactions prompts consideration of these pulses as "photonic reagents" in analogy to chemical reagents. This work explores the prospect that photonic reagents may affect systematic trends in dissociative ionization reactions of a homologous family of halomethanes, much as systematic outcomes are often observed for reactions between homologous families of chemical reagents and chemical substrates. The experiments in this work with photonic reagents of varying pulse energy and linear spectral chirp reveal systematic correlations between observable ion yields and the following set of natural variables describing the substrate molecules: the ionization energy of the parent molecule, the appearance energy of each fragment ion, and the relative strength of carbon-halogen bonds in molecules containing two different halogens. The results suggest that reactions induced by photonic reagents exhibit systematic behavior analogous to that observed in reactions driven by chemical reagents, which provides a basis to consider empirical "rules" for predicting the outcomes of photonic reagent induced reactions.

Systematic trends in photonic reagent induced reactions in a homologous chemical family.

Femtosecond-induced resonance-enhanced multiphoton-ionization photoelectron spectroscopy (REMPI-PS) suffers from poor spectral resolution due to the large laser spectral bandwidth. In this paper, we present a scheme to achieve high-resolution REMPI-PS in the potassium atom by shaping the ultrashort femtosecond laser pulse. Our results show that the REMPI-PS bandwidth can be narrowed by several tens of times with a cubic spectral phase modulation, and thus high-resolution REMPI-PS and the fine structure of the energy-level diagram for multiple excited states can be obtained in spite of the broadband femtosecond laser pulse. Furthermore, we explain the physical control mechanism of the narrowband REMPI-PS by considering the two-photon power spectrum.

https://faculty.ecnu.edu.cn/picture/article/1867/b4/4f/3b1f73ef48c8a5ea92a9dd8693b1/6178dc36-d7eb-414b-a8c1-8d811769cddd.pdf.x

Achieving high-resolution photoelectron spectroscopy from a broadband femtosecond laser pulse

Signatures of transient resonance heating in photoemission from free NaCl nanoparticles in intense femtosecond laser pulses

We report on the coherent control of the ultrafast ionization and fragmentation dynamics of the bromochloroalkanes C2H4BrCl and C3H6BrCl using shaped femtosecond laser pulses. In closed-loop control experiments on bromochloropropane (C3H6BrCl) the fragment ion yields of CH2Cl+, CH2Br+, and C3H3+ are optimized with respect to that of the parent cation C3H6BrCl+. The fragment ion yields are recorded in additional experiments in order to reveal the energetics of cation fragmentation, where laser-produced plasma radiation is used as a tunable pulsed nanosecond vacuum ultraviolet radiation source along with photoionization mass spectrometry. The time structure of the optimized femtosecond laser pulses leads to a depletion of the parent ion and an enhancement of the fragment ions, where a characteristic sequence of pulses is required. Specifically, an intense pump pulse is followed by a less intense probe pulse where the delay is 0.5 ps. Similarly optimized pulse shapes are obtained from closed-loop control experiments on bromochloroethane (C2H4BrCl), where the fragment ion yield of CH2Br+ is optimized with respect to that of C2H4BrCl+ as well as the fragment ion ratios C2H2+/CH2Br+ and C2H3+/C2H4Cl+. The assignment of the underlying control mechanism is derived from one-color 804 nm pump–probe experiments, where the yields of the parent cation and several fragments show broad dynamic resonances with a maximum at Δt = 0.5 ps. The experimental findings are rationalized in terms of dynamic ionic resonances leading to an enhanced dissociation of the parent cation and some primary fragment ions.

Coherent control of the ultrafast dissociative ionization dynamics of bromochloroalkanes.

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Coherent control of the ultrafast dissociative ionization dynamics of bromochloroalkanes.