Steering the outcome of photoionization in a molecule

An important step towards the understanding and control of photoinduced fragmentation processes in molecules has been achieved in an experiment on the H₂ molecule taking advantage of the unique properties of the FERMI free-electron laser source in the vacuum ultraviolet (VUV) photon energy range.
Molecular dissociation, i.e., the breaking of a chemical bond, is governed by the coupling of electronic and nuclear motion and, once understood and controlled in large systems, e.g., by utilizing ultrashort light pulses, has the potential to impact tremendously photochemical and biochemical applications. A team of both experimentalists and theoreticians from France (CNRS, Université Paris-Sud, Université de Bordeaux), Spain (Universidad Autónoma de Madrid), Germany (European XFEL), and Italy (Elettra-Sincrotrone Trieste) has demonstrated that the outcome of dissociative (DI) and nondissociative (NDI) photoionization in the simplest of all molecules, H₂, can be controlled exploiting nonlinear two-photon ionization with intense femtosecond pulses in the VUV.
The FERMI seeded free-electron laser is currently the only light source worldwide that provides external users access to bright femtosecond pulses at wavelengths in the VUV up to 100 nm, the energy regime required for studying nonlinear two-photon single-ionization in H₂. The high spectral resolution and precise tunability of the 100-fs pulses provided by FERMI made it possible to selectively excite single vibrational levels in the neutral intermediate B state of H₂ (blue line in Fig. 1). Absorption of a second VUV photon then leads to NDI or DI into the ionic H₂⁺ ground state (green in Fig. 1) or to DI into the first excited H₂⁺2p continuum (orange in Fig. 1). In single-photon single-ionization of H₂, the yield of DI is very low – less than 2%. By contrast, recent ab initiocalculations suggest that the ratio of DI/NDI can be increased significantly in resonance-enhanced two-photon ionization and that it can be controlled by varying the pulse duration between 2 and 10 fs.
 

Figure 1.  (a) Schematic of resonant two-photon ionization viathe B intermediate state (12.51 eV). The grey shaded area shows the Franck-Condon region for one-photon absorption from the H2electronic ground state. The dashed purple arrows visualize the range for the absorption of the second FEL photon. The green (red) horizontal line shows the ionization threshold at 15.43 eV (dissociation limit at 18.08 eV). (b) The experimental photoelectron spectrum shows a clear separation of electrons correlated to NDI and DI. For DI, it is close to the prediction of the Condon-reflection approximation, i.e., the projection of the vibrational wavefunction onto the dissociative 2p continuum state. The infinite-time limit calculation (grey line for the convolution of the contributions from the two first ionization continua) reproduces the main features of the spectrum. The differences between experiment and calculation indicates that at FERMI a timescale between ultrafast dynamics and steady-state excitation is probed.
 

Utilizing longer 100-fs pulses, which are spectrally narrow enough to populate a single vibrational level in the B intermediate state,the experiment at FERMI demonstrated that the ratio of DI/NDI can be increased by almost two orders of magnitude. Varying the vibrational level in the intermediate state from v=8-11 shows a general increasing trend of the DI/NDI ratio, which was extracted consistently from time-of-flight (TOF) mass spectra and photoelectron spectra obtained with the combined velocity map imaging (VMI) and TOF spectrometer of FERMI’s low-density matter (LDM) beamline. New calculations for pulse durations up to 40 fs, in which the time-dependent Schrödinger equation was solved to second order of the perturbation theory, and calculations for infinitely long pulse durations showed that the experiment probes a timescale at the transition between ultrafast dynamics and steady state excitation. The photoelectron spectra reveal that in the resonant ionization scheme via the B intermediate state, which expands from 2.7 to 3.2 Å for v=8-12, ionization at large internuclear distances plays an important role for both NDI and DI. Near the outer-turning point, the repulsive H₂⁺2p continuum becomes easily accessible and therefore, besides the involvement of autoionizing states to DI, contributes significantly to the substantial enhancement of DI. By selectively exciting a single vibrational level, the nuclear DOF in the intermediate state are controlled and, in consequence, the outcome of DI and NDI of H₂ molecules. This study on the benchmark molecule H₂ vis the first step towards developing strategies to achieve quantum control in complex molecular systems.
 

Figure 2.  (a) Experimental and theoretical DI/NDI ratio as a function of the photon energy. The point at 25.5 eV shows the ratio for one-photon ionization (≈0.02). The black (blue) comb indicates the position of the vibrational levels of the B (C) intermediate state. (b) TOF mass spectrum for a photon energy of 12.76 eV. Reprinted figure with permission from F. Holzmeier et al., Phys. Rev. Lett. 121, 103002 (2018). Copyright (2018) by the American Physical Society.

 

This research was conducted by the following research team:

F. Holzmeier^1,2, R. Y. Bello³, M. Hervé¹, A. Achner⁴, T. M. Baumann⁴, M. Meyer⁴, P. Finetti⁵, M. Di Fraia⁵, D. Gauthier⁵, E. Roussel⁵, O. Plekan⁵, R. Richter⁵, K. C. Prince⁵, C. Callegari⁵, H. Bauchau⁶, A. Palacios^3,7, F. Martín^3,8,9, and D. Dowek¹

¹ Institut des Sciences Moléculaires d’Orsay CNRS, Univ Paris-Sud, Paris, France
² Synchrotron SOLEIL, Gif-sur-Yvette, France
³ Departamento de Química, Universidad Autónoma de Madrid, Madrid, Spain
⁴ European XFEL, Hamburg, Germany
⁵ Elettra-Sincrotrone Trieste SCpA, Trieste, Italy
⁶ Centre des Lasers Intenses et Applications, CNRS, Université de Bordeaux, Talence, France
⁷ Institute for Advanced Research in Chemical Sciences, Universidad Autónoma de Madrid, Madrid, Spain
⁸ Instituto Madrileño de Estudios Avanzados en Nanosciencia, Madrid, Spain
⁹ Condensed Matter Physics Center, Universidad Autónoma de Madrid, Madrid Spain

Contact persons:

Fabian Holzmeier, e-mail:
Danielle Dowek, e-mail: