### Large-scale atomistic calculations of clusters in intense x-ray pulses

Here, we present the methodology of our recently developed Monte-Carlo/ Molecular-Dynamics method for studying the fundamental ultrafast dynamics induced by high-fluence, high-intensity x-ray free electron laser (XFEL) pulses in clusters. The quantum nature of the initiating ionization process is accounted for by a Monte Carlo method to calculate probabilities of electronic transitions, including photo absorption, inner-shell relaxation, photon scattering, electron collision and recombination dynamics, and thus track the transient electronic configurations explicitly. The freed electrons and ions are followed by classical particle trajectories using a molecular dynamics algorithm. These calculations reveal the surprising role of electron-ion recombination processes that lead to the development of nonuniform spatial charge density profiles in x-ray excited clusters over femtosecond timescales. In the high-intensity limit, it is important to include the recombination dynamics in the calculated scattering response even for a 2- fs pulse. We also demonstrate that our numerical codes and algorithms can make e!cient use of the computational power of massively parallel supercomputers to investigate the intense-field dynamics in systems with increasing complexity and size at the ultrafast timescale and in non-linear x-ray interaction regimes. In particular, picosecond trajectories of XFEL clusters with attosecond time resolution containing millions of particles can be e!cientlymore »

- Publication Date:

- Grant/Contract Number:
- AC02-06CH11357

- Type:
- Accepted Manuscript

- Journal Name:
- Journal of Physics. B, Atomic, Molecular and Optical Physics

- Additional Journal Information:
- Journal Volume: 50; Journal Issue: 10; Journal ID: ISSN 0953-4075

- Publisher:
- IOP Publishing

- Research Org:
- Argonne National Lab. (ANL), Argonne, IL (United States)

- Sponsoring Org:
- USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 74 ATOMIC AND MOLECULAR PHYSICS; light matter interaction; x-ray free-electron laser; multi-photon ionization and excitation; laser cluster interaction; high-performance computing

- OSTI Identifier:
- 1374543

```
Ho, Phay J., and Knight, Chris.
```*Large-scale atomistic calculations of clusters in intense x-ray pulses*. United States: N. p.,
Web. doi:10.1088/1361-6455/aa69e6.

```
Ho, Phay J., & Knight, Chris.
```*Large-scale atomistic calculations of clusters in intense x-ray pulses*. United States. doi:10.1088/1361-6455/aa69e6.

```
Ho, Phay J., and Knight, Chris. 2017.
"Large-scale atomistic calculations of clusters in intense x-ray pulses". United States.
doi:10.1088/1361-6455/aa69e6. https://www.osti.gov/servlets/purl/1374543.
```

```
@article{osti_1374543,
```

title = {Large-scale atomistic calculations of clusters in intense x-ray pulses},

author = {Ho, Phay J. and Knight, Chris},

abstractNote = {Here, we present the methodology of our recently developed Monte-Carlo/ Molecular-Dynamics method for studying the fundamental ultrafast dynamics induced by high-fluence, high-intensity x-ray free electron laser (XFEL) pulses in clusters. The quantum nature of the initiating ionization process is accounted for by a Monte Carlo method to calculate probabilities of electronic transitions, including photo absorption, inner-shell relaxation, photon scattering, electron collision and recombination dynamics, and thus track the transient electronic configurations explicitly. The freed electrons and ions are followed by classical particle trajectories using a molecular dynamics algorithm. These calculations reveal the surprising role of electron-ion recombination processes that lead to the development of nonuniform spatial charge density profiles in x-ray excited clusters over femtosecond timescales. In the high-intensity limit, it is important to include the recombination dynamics in the calculated scattering response even for a 2- fs pulse. We also demonstrate that our numerical codes and algorithms can make e!cient use of the computational power of massively parallel supercomputers to investigate the intense-field dynamics in systems with increasing complexity and size at the ultrafast timescale and in non-linear x-ray interaction regimes. In particular, picosecond trajectories of XFEL clusters with attosecond time resolution containing millions of particles can be e!ciently computed on upwards of 262,144 processes.},

doi = {10.1088/1361-6455/aa69e6},

journal = {Journal of Physics. B, Atomic, Molecular and Optical Physics},

number = 10,

volume = 50,

place = {United States},

year = {2017},

month = {4}

}