Surface pump-probe femtosecond-laser mass spectrometry: time-, mass-, and velocity-resolved detection of surface reaction dynamics

Understanding the Effect of an Amorphous Surface on the Ultrafast Dynamics of a Heterogeneous Photoinduced Reaction: CD3I Photoinduced Reaction on Amorphous Cerium Oxide Films

In this work, to understand how an amorphous surface influences the dynamics of surface photoinduced reactions, pump-probe spectroscopy in conjunction with mass spectrometry is employed to track the ultrafast evolution of intermediates and final products with time, mass, and energy resolution. As a model system, the photoinduced reaction of CD₃I adsorbed on amorphous cerium oxide films is investigated. A fraction of the first intermediates produced on a freshly prepared surface is trapped to passivate the surface. After the A-band excitation, the minimum dissociation time of CD₃I indicates that CD₃I adsorption geometries with either CD₃ or I facing the gas phase exist; however, the transient data suggest that most molecules are adsorbed with the I atom facing the surface. CD₃ and I are consumed to form I₂ and reform CD₃I, which are produced at a steady rate only after the intermediates have lost the excess translational energy released from photodissociation.

Nonmetal-to-Metal Transition of Magnesia Supported Au Clusters Affects the Ultrafast Dissociation Dynamics of Adsorbed CH3Br Molecules

The detection of intermediate species and the correlation of their ultrafast dynamics with the morphology and electronic structure of a surface is crucial to fully understand and control heterogeneous photoinduced and photocatalytic reactions. In this work, the ultrafast photodissociation dynamics of CH₃Br molecules adsorbed on variable-size Au clusters on MgO/Mo(100) is investigated by monitoring the CH₃⁺ transient evolution using a pump-probe technique in conjunction with surface mass spectrometry. Furthermore, extreme-UV photoemission spectroscopy in combination with theoretical calculations is employed to study the electronic structure of the Au clusters on MgO/Mo(100). Changes in the ultrafast dynamics of the CH₃⁺ fragment are correlated with the electronic structure of Au as it evolves from monomers to small nonmetallic clusters to larger nanoparticles with a metallic character. This work provides a new avenue to a detailed understanding of how surface-photoinduced chemical reactions are influenced by the composition and electronic structure of the surface.

Photoionization and photofragmentation in mass spectrometry with visible and UV lasers

Ever since the introduction of laser technology to the field of mass spectrometry, several disciplines evolved providing solutions to challenging scientific and analytical tasks in research and industry. Among these are techniques involving multiphoton ionization such as Resonance-Enhanced Multiphoton Ionization (REMPI, R2PI) and Mass-Analyzed Threshold Ionization (MATI) spectroscopy, a variant of Zero Kinetic Energy (ZEKE) spectroscopy, that possess the ability to selectively ionize certain preselected compounds out of complex mixtures, for example, environmental matrices, with a high level of efficiency. Another key feature of multiphoton ionization techniques is the ability to control the degree of fragmentation, whereas soft ionization is most highly appreciated in most applications. In cases where rich fragmentation patterns are desired for diagnostic purposes, Photodissociation mass spectrometry (PD-MS) is applied successfully. PD-MS allows for the cleavage of selected chemical bonds. With the introduction of chromophoric labels in PD-MS, it became possible to target certain molecules or groups within a molecule. In this review article, an overview of the basic principles and experimental requirements of REMPI and MATI spectroscopy and PD mass spectrometry are given. By means of selected examples, the latest developments and application possibilities in this field over the past decade with special focus on the German research landscape are pointed out. © 2018 Wiley Periodicals, Inc. Mass Spec Rev 38: 202-217, 2019.

Femtosecond time-resolved photodissociation dynamics of methyl halide molecules on ultrathin gold films

The photodissociation of small organic molecules, namely methyl iodide, methyl bromide, and methyl chloride, adsorbed on a metal surface was investigated in real time by means of femtosecond-laser pump-probe mass spectrometry. A weakly interacting gold surface was employed as substrate because the intact adsorption of the methyl halide molecules was desired prior to photoexcitation. The gold surface was prepared as an ultrathin film on Mo(100). The molecular adsorption behavior was characterized by coverage dependent temperature programmed desorption spectroscopy. Submonolayer preparations were irradiated with UV light of 266 nm wavelength and the subsequently emerging methyl fragments were probed by photoionization and mass spectrometric detection. A strong dependence of the excitation mechanism and the light-induced dynamics on the type of molecule was observed. Possible photoexcitation mechanisms included direct photoexcitation to the dissociative A-band of the methyl halide molecules as well as the attachment of surface-emitted electrons with transient negative ion formation and subsequent molecular fragmentation. Both reaction pathways were energetically possible in the case of methyl iodide, yet, no methyl fragments were observed. As a likely explanation, the rapid quenching of the excited states prior to fragmentation is proposed. This quenching mechanism could be prevented by modification of the gold surface through pre-adsorption of iodine atoms. In contrast, the A-band of methyl bromide was not energetically directly accessible through 266 nm excitation. Nevertheless, the one-photon-induced dissociation was observed in the case of methyl bromide. This was interpreted as being due to a considerable energetic down-shift of the electronic A-band states of methyl bromide by about 1.5 eV through interaction with the gold substrate. Finally, for methyl chloride no photofragmentation could be detected at all.