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Strong-field effects in the photo-induced dissociation of the hydrogen molecule on a silver nanoshell

Natalia E. Koval, J. Iñaki Juaristi, Maite Alducin

TL;DR

This paper tackles whether plasmon resonance or strong-field effects drive photo-induced dissociation of H2 on a silver nanoshell. Using real-time TDDFT with Ehrenfest dynamics, the authors simulate H2 on a Ag$_{55}^{L1}$ nanoshell under Gaussian light pulses across frequencies around the plasmon resonance and at high intensities. They find that strong-field nonlinearities lead to multiphoton ionization and subsequent dissociation, often dominated by ionization rather than plasmonic excitation, especially when continuum states are included via ghost atoms. The work cautions against interpreting high-intensity RT-TDDFT-EMD results as plasmon-driven chemistry and highlights the need to consider continuum and ionization effects when modeling plasmonic catalysis.

Abstract

Plasmonic catalysis is a rapidly growing field of research, both from experimental and computational perspectives. Experimental observations demonstrate an enhanced dissociation rate for molecules in the presence of plasmonic nanoparticles under low-intensity visible light. The hot-carrier transfer from the nanoparticle to the molecule is often claimed as the mechanism for dissociation. However, the charge transfer time scale is on the order of few femtoseconds and cannot be resolved experimentally. In this situation, ab initio non-adiabatic calculations can provide a solution. Such simulations, however, have their own limitations related to the computational cost. To accelerate plasmonic catalysis simulations, many researchers resort to applying high-intensity external fields to nanoparticle-molecule systems. Here, we show why such an approach can be problematic and emphasize the importance of considering strong-field effects when interpreting the results of time-dependent density functional theory simulations of plasmonic catalysis. By studying the hydrogen molecule dissociation on the surface of a silver nanoshell and analyzing the electron transfer at different field frequencies and high intensities, we demonstrate that the molecule dissociates due to multiphoton absorption and subsequent ionization.

Strong-field effects in the photo-induced dissociation of the hydrogen molecule on a silver nanoshell

TL;DR

This paper tackles whether plasmon resonance or strong-field effects drive photo-induced dissociation of H2 on a silver nanoshell. Using real-time TDDFT with Ehrenfest dynamics, the authors simulate H2 on a Ag nanoshell under Gaussian light pulses across frequencies around the plasmon resonance and at high intensities. They find that strong-field nonlinearities lead to multiphoton ionization and subsequent dissociation, often dominated by ionization rather than plasmonic excitation, especially when continuum states are included via ghost atoms. The work cautions against interpreting high-intensity RT-TDDFT-EMD results as plasmon-driven chemistry and highlights the need to consider continuum and ionization effects when modeling plasmonic catalysis.

Abstract

Plasmonic catalysis is a rapidly growing field of research, both from experimental and computational perspectives. Experimental observations demonstrate an enhanced dissociation rate for molecules in the presence of plasmonic nanoparticles under low-intensity visible light. The hot-carrier transfer from the nanoparticle to the molecule is often claimed as the mechanism for dissociation. However, the charge transfer time scale is on the order of few femtoseconds and cannot be resolved experimentally. In this situation, ab initio non-adiabatic calculations can provide a solution. Such simulations, however, have their own limitations related to the computational cost. To accelerate plasmonic catalysis simulations, many researchers resort to applying high-intensity external fields to nanoparticle-molecule systems. Here, we show why such an approach can be problematic and emphasize the importance of considering strong-field effects when interpreting the results of time-dependent density functional theory simulations of plasmonic catalysis. By studying the hydrogen molecule dissociation on the surface of a silver nanoshell and analyzing the electron transfer at different field frequencies and high intensities, we demonstrate that the molecule dissociates due to multiphoton absorption and subsequent ionization.
Paper Structure (9 sections, 1 equation, 7 figures)

This paper contains 9 sections, 1 equation, 7 figures.

Table of Contents

  1. Introduction
  2. Methods
  3. Geometry optimization
  4. Real-time time-dependent density functional theory calculations of the absorption spectrum
  5. Ehrenfest molecular dynamics simulations
  6. Results and Discussion
  7. Field intensity and frequency dependence of H$_2$ dissociation on Ag$_{55}^{L1}$
  8. Strong-field effects with augmented basis set
  9. Conclusions

Figures (7)

  • Figure 1: (a) Atomic structure of the relaxed Ag$_{55}^{L1}$ nanoshell with H$_2$ (interatomic distance of 0.75 Å) at a distance of 3 Å from the nanoshell facet. (b) Absorption spectrum of Ag$_{55}^{L1}+$H$_2$. (c) Time-dependent field strength of the external field pulse with a Gaussian envelope ($\sigma = 5$ fs, $t_0 = 18$ fs) with $\hbar\omega_0 = \hbar\omega_\textrm{p} =3.15$ eV. The maximum field strength $E_0 =1.94$ V/Å (0.038 a.u.) corresponds to the maximum intensity $I_\mathrm{max} = 1\times 10^{14}$ W/cm$^2$.
  • Figure 2: H-H bond length as a function of time for the three chosen field frequencies. Field intensity is (a) $I_\mathrm{max} = 2\times 10^{13}$ W/cm$^2$ and (b) $I_\mathrm{max} = 1\times 10^{14}$ W/cm$^2$. The maximum of the external field arrives at 18 fs.
  • Figure 3: Time evolution of the Mulliken (lines) and Bader (symbols) population change [$\Delta N\mathrm{e} = N_\mathrm{e}(t) - N_\mathrm{e}(t=0)$] on (a,c) Ag$_{55}^{L1}$ and (b,d) H$_2$ for the three studied field frequencies. Field intensity is (a,b) $I_\mathrm{max} = 2\times 10^{13}$ W/cm$^2$ and (c,d) $I_\mathrm{max} = 1\times 10^{14}$ W/cm$^2$.
  • Figure 4: Time evolution of the Ag$_{55}^{L1}$+H$_2$ orbital populations induced by an external field with intensity (left panels, (a,b,c)) $I_\mathrm{max} = 2\times 10^{13}$ W/cm$^2$ and (right panels, (d,e,f)) $I_\mathrm{max} = 1\times 10^{14}$ W/cm$^2$. For each $I_\mathrm{max}$, the field frequency is: (a,d) $\hbar \omega_0 =$ 2 eV, (b,e) $\hbar\omega_0 =$ 3.15 eV, and (c,f) $\hbar\omega_0 =$ 4.1 eV. Orbital populations are calculated every 0.2 fs as sums of the squares of the projections of the time-dependent occupied MOs on the initially unoccupied orbitals. Only populations with maximum values $> 0.1$ are plotted.
  • Figure 5: Time-dependent electric dipole moment for (a) $I_\mathrm{max} = 2\times 10^{13}$ W/cm$^2$ and (b) $I_\mathrm{max} = 1\times 10^{14}$ W/cm$^2$.
  • ...and 2 more figures