Under strong electromagnetic excitation, electron–hole (e-h) pairs may
be generated in solids, which are subsequently driven to high energy
and high momentum, producing high harmonics (HH) of the ...driving field.
The HH efficiency depends on the degree of coherence between the
driven electron and hole created by the laser field. Here, we disrupt
this e-h coherence in an atomically thin semiconductor by photodoping
via incoherent e-h pairs. We observe a strong, systematic harmonic
order-dependent intensity reduction. This trend is explained by an
exponential decay of the inter-band polarization, proportional to the
sub-cycle excursion time of the e-h pair. Our study provides a
platform to probe the importance of many-body effects, such as
excitation density-dependent decoherence time for strongly driven
electrons without the need of ultrashort laser pulses.
We have observed the ultrafast conformational dynamics of electronically excited N-methyl piperidine (NMP) using time-resolved Rydberg fingerprint spectroscopy. Optical excitation at various ...wavelengths ranging from 212 nm to 229 nm leads to the 3s or 3p Rydberg states and induces coherent oscillatory motions with periods of about 700 fs. Furthermore, these coherent motions survive the internal conversion from 3p to 3s but then dephase on a time scale of a few oscillations. Intramolecular vibrational energy redistribution on a picosecond time scale leads to an equilibrium between two conformeric structures that are separated in binding energy by 0.09 eV. Model calculations using the DFT-SIC method are in excellent agreement with the experiments and identify the conformers as the chair and twist structures of NMP. The analysis of the equilibrium parameters at long time delays as a function of excitation wavelength allows for the extraction of thermodynamic parameters for the conformeric transformation. We derive an enthalpy of the chair to twist reaction in the 3s excited state of 62 meV with an entropy of 19.70 J mol–1 K–1. An activation energy of 276 meV is also obtained with a kinetic model.
•Line coupling and line mixing in CH3I spectra perturbed by N2 and O2 have been calculated.•The semi-classical method correctly takes into account the k-degeneracy of the transitions.•It leads to a ...reasonable agreement with the available data, including the T-dependence.•New experiments at higher pressures are suggested to check the description of inter-doublets couplings.
Calculations of the N2-, O2-, and air-broadened widths, together with their temperature dependence exponents have been made for transitions of CH3I in the ν5 and ν6 bands. The calculations are based on a semi-classical line shape formalism developed by the current authors through modifying and refining the Robert-Bonamy formalism. In recent years, we have applied this formalism for linear molecules, symmetric-top molecules with inversion symmetry, and asymmetric-top molecules. For symmetric-top molecules with the k degeneracy such as CH3I, the formalism has a new feature. In this case, one should consider each of the CH3I transitions labeled by ki or f ≠ 0 as a doublet. Then, one needs to consider the effects of the line mixing process between these two components. Comparisons of our theoretical predictions with some data available demonstrate a very reasonable agreement. Finally we propose new experiments at higher perturber pressures that would enable one to check the theoretically calculated relaxation matrices and to extend the analysis to the inter-doublet mixing effects.
Here,iIonically conductive, hydrated polyelectrolyte nanocomposites are prepared from iodomethane-treated poly(2-vinylpyridine) (mPV)-grafted cellulose nanocrystals (MxG-CNC-g-mPV). These ...polyelectrolyte-grafted nanoparticle (PEGN) films exhibit an order-of-magnitude higher iodide ion conductivity relative to mPV films and a high in-plane/through-plane anisotropy. The PEGN architecture prevents CNC aggregation, maximizing the CNC/polyelectrolyte interface. PEGN films were prepared with varying polymer graft density (0.03-0.12 chains/nm2) and molecular weight (7k-30k g/mol). The greatest ion conductivity enhancement is observed with lower molecular weight, higher density grafts: ca. 89 ± 6 mS/cm (140 ±10 mS/cm accounting for the volume of the CNC) versus 3.3 ± 0.4 mS/cm for ungrafted mPV. Poly(styrene-block-2-vinylpyridine)-grafted CNCs were prepared in which the insulating polystyrene block or the conducting mPV block was directly attached to the CNCs; only the films with the mPV block closest to the CNCs exhibited an enhancement in conductivity relative to mPV. Together, these data point to beneficial CNC/polyelectrolyte interfacial effects, resulting in significant ionic conductivity enhancement along the length of the CNCs in these films.
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