Laser-induced gas-surface interactions

The excitation of surface plasmons (SPs) — collective oscillation of conduction-band electrons in nanostructures — can afford photon, electron and heat energy redistribution over time and space Making use of this ability, plasmon-enhanced molecular spectroscopy (PEMS) techniques with ultra-high sensitivity and surface selectivity have attracted much attention and have undergone considerable development over the past four decades Recently, the development of plasmon-mediated chemical reactions (PMCRs) has shown the potential to have a large impact on the practice of chemistry PMCRs exhibit some obvious differences from and potential advantages over traditional thermochemistry, photochemistry and photocatalysis However, our physicochemical understanding of PMCRs is still far from complete In this Review, we analyse the similarities and distinctive features of PEMS and PMCRs and compare PMCRs with traditional photochemical and thermochemical reactions We then discuss how PMCRs can be improved by rationally designing and fabricating plasmonic nanostructures, selecting suitable surface and interface mediators and teaming them synergistically Surface plasmons can redistribute photoenergy over different time, space and energy scales and have been exploited in new spectroscopic techniques This Review reports on how surface plasmons can also drive chemical reactions by localizing photon, electronic and/or thermal energies

From plasmon-enhanced molecular spectroscopy to plasmon-mediated chemical reactions

This review describes advances which have occurred during the past decade in chemical reaction dynamics using crossed molecular beams. After a brief historical introduction, advances in the generation and state selection of beams of reactants, and in the schemes for product detection, which are at the basis of our increased ability to measure state-averaged and state-resolved reactive differential cross sections in crossed beam experiments, are described. The novel couplings of laser and synchrotron radiation to these experiments are noted. A selection of case studies are considered in some detail, to exemplify recent improvements in our understanding of gas-phase neutral reaction dynamics. The examples include prototype reactions involving three-atom and four-atom systems, and the results are discussed in the light of the most recent, synergistic, theoretical developments for treating both the potential energy surfaces and the reaction dynamics. Progress made in studies of reactions of molecular radicals and of chemically important atoms, such as carbon, nitrogen, and oxygen, with polyatomic molecules, as made possible by recent developments in the classic crossed beam technique with mass spectrometric detection, is emphasized. Some complementary techniques that recently have contributed to our understanding of chemical reactivity are also described briefly.

https://iopscience.iop.org/article/10.1088/0034-4885/63/3/203/pdf

Chemical reaction dynamics with molecular beams

Rotational excitation and spatial alignment in moderate intensity radiation fields are studied numerically and analytically, using time‐dependent quantum mechanics. Substantial rotational excitation is found under conditions typically used in time‐resolved spectroscopy experiments. The broad rotational wave packet excited by the laser pulse is well defined in the conjugate angle space, peaking along the field polarization direction. Both the rotational excitation and the consequent spatial alignment can be controlled by the choice of field parameters. Fragment angular distributions following weak field photodissociation of the rotational wave packet are computed as a probe of the degree of alignment. In the limit of rapid photodissociation the angular distribution is peaked in the forward direction, reflecting the anisotropy of the aligned state. Potential applications of the effect demonstrated range from reaction dynamics of aligned molecules and laser‐control to material deposition and laser‐assisted isotope separation.

Rotational excitation and molecular alignment in intense laser fields

Rotational alignment of product molecules from the reactions Sr+CH3Br, C2H5Br, n-C3H7Br, i-C3H7Br by means of PLIF

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Effect of reagent orientation and rotation upon product state distribution in the reaction Sr+HF (v=1,J) →SrF(v′, J′) +H

The reactions of Ca and Sr with HF and DF are studied using a beam–gas configuration as a function of reagent and product vibration to obtain state‐to‐state reaction rates. A beam of alkaline earth metal atoms enters a scattering chamber containing hydrogen fluoride. The HF and DF reagents are pumped from v=0 to v=1 by a pulsed HF or DF laser; the alkaline earth monofluoride products are detected by a pulsed tunable probe laser. The product internal state distribution is determined from the laser excitation spectrum. While no products could be detected for reactions with DF(v=1) or DF(v=0), the rate constant for the Ca and Sr reactions with HF(v=1) are both at least four orders of magnitude larger than the reactions with HF(v=0). The implications of these results on bond energies and the fate of the excess energy of reaction are discussed.

Ziv Karny

Papers

Effect of reagent orientation and rotation upon product state distribution in the reaction Sr+HF (v=1,J) →SrF(v′, J′) +H

Effect of vibrational excitation on the molecular beam reactions of Ca and Sr with HF and DF

Infrared laser photochemistry: Evidence for heterogeneous decomposition