Kansas State University

Dr. Ming-Fu Lin

Linac Coherent Light Source, SLAC National Accelerator Laboratory

 

Probing Ultrafast Light-Induced Electronic and Molecular Structural
Dynamics: from gas phase molecule to solid-state nanomaterials

102 Cardwell Hall

Understanding light-matter interaction and subsequent dynamics are important steps toward the manipulation of photochemical reactions. In this colloquium, I will present ultrafast laser-induced molecular structural dynamics of vinyl bromide (CH₂=CHBr), and electronic and structural dynamics of two-dimensional nanomaterials (PbI₂ and MoSe₂), respectively. At first, I used a tabletop extreme ultraviolet (XUV) light source from high-harmonic generation (HHG) to explore strong-field generated dissociative ionization dynamics of vinyl bromide.¹ Strong-field ionization often produces complex mixtures of ionic states of different internal energy, and corresponding dissociation pathways are too complicated to unveil. The broadband XUV photon source directly probes the real-time ultrafast C-Br bond dissociation specifically associated with the first ionic excited à state.¹ On the other hand, the produced ionic ground state ions are stable with a lifetime longer than tens of picoseconds. From this study, I also show that the tabletop XUV transient absorption core-level spectroscopy has element and chemical specificities with femtosecond temporal resolution.

In addition to vinyl bromide, I also explored the ultrafast carrier relaxation dynamics of lead iodide (PbI₂) and identifying the subsequent energy redistribution pathway.² An optical pump beam at 400 nm was utilized to produce electron and hole populations in the conduction and valence bands, respectively, followed by a delayed broadband XUV light source that measures these two distinct charges at the same time.^3,4 Lead iodide is a precursor for synthesizing organic-inorganic hybrid perovskite that has been shown to have efficient solar energy conversion for solar cell applications. Using PbI₂ as a model case, I showed that the tabletop core-level spectroscopy with transitions associated with iodine atoms could be used to directly measure electron and hole populations separately and simultaneously. This carrier-specific detection has potential applications to selectively probe charge transfer dynamics at the interfaces and junctions of stacked materials. Here, in PbI₂ the photogenerated electron-hole pair recombines nonradiatively in consistent with further measurements using Mega electron-volt ultrafast electron diffraction (MeV-UED)⁵ that probes atomic motion directly. The recombined charge carriers converts the absorbed energy to lattice thermal vibration, causing a temperature jump of ~100 K on a time scale of several picoseconds.

This nonradiative channel is also an important pathway for initiating a possible structure phase transformation. In order to directly quantify the efficiency of this specific dynamics, a technique that measures atomic/lattice motions is required. In the final section, I will present our studies of ultrafast nonradiative dynamics of atomically thin MoSe₂ using MeV-UED method.⁶ 2D transition metal dichalcogenides (TMDC) such as MoSe₂ are emerging materials for many potential applications in optoelectronics and solar fuels. However, most studies of TMDC concentrated on the mechanisms of efficient extraction of photogenerated excitons, free carriers and radiative channels of LED. In this experiment, I observed an optical-induced subpicosecond decay of Bragg peak intensity, which corresponds to Debye-Waller response of MoSe₂. We derived a high quantum yield of nonradiative channel, indicating a very efficient electron-phonon coupling pathway. The nonadiabatic quantum molecule dynamics simulations (NAQMD) suggests that the carrier-driven phonon softening accelerates the electron-phonon coupling channel, specifically located at the vibrational motion along the partial Mo-Mo dimerization coordinates for a new conductive structure phase of MoSe₂. Interestingly, this light-induced structure dynamics of MoSe₂ crystal is analogous to the unimolecular internal conversion of forming hot molecule that may isomerize and dissociate in the ground electronic state.

References: 

1. Lin, M.-F., Neumark, D. M., Gessner, O. & Leone, S. R. Ionization and dissociation dynamics of vinyl bromide probed by femtosecond extreme ultraviolet transient absorption spectroscopy. J. Chem. Phys. 140, 64311 (2014).
2. Lin, M.-F. et al. Carrier-Specific Femtosecond XUV Transient Absorption of PbI₂. J. Phys. Chem. C 121, 27886–27893 (2017).
3. Lin, M.-F., Verkamp, M. A., Ryland, E. S., Zhang, K. & Vura-Weis, J. Impact of spatial chirp on high harmonic extreme ultraviolet absorption spectroscopy of thin films. J. Opt. Soc. Am. B 33, 1986–1992 (2016).
4. Zhang, K. et al. Shrinking the Synchrotron: Tabletop Extreme Ultraviolet Absorption of Transition Metal Complexes. J Phys Chem Lett 7, 3383–3387 (2016).
5. Weathersby, S. P. et al. Mega-electron-volt ultrafast electron diffraction at SLAC National Accelerator Laboratory. Rev. Sci. Instrum. 86, 73702 (2015).
6. Lin, M.-F. et al. Ultrafast non-radiative dynamics of atomically thin MoSe₂. Nat. Commun.8, 1745 (2017).