Mihai E. Vaida

Femtosecond real-time probing of transition state dynamics in a surface photoreaction: methyl desorption from CH3I on MgO(100).

A novel experimental approach to the investigation of surface adsorbate reaction dynamics is presented. The direct time-resolved monitoring of the surface reaction transition state and product formation dynamics were accomplished via pump-probe mass spectrometry. As an example, methyl iodide molecules adsorbed at submonolayer coverage on an ultrathin magnesia film on Mo(100) were photoexcited to the A-band by ultrafast laser pulse irradiation. Employing time-delayed multiphoton ionization the dynamics of the dissociative methyl iodide transition state and of the emerging methyl photoproduct could be detected with femtosecond resolution. The reaction times deduced from the temporal evolution of the methyl ion mass signal indicate a strong interaction of the methyl fragment with the substrate surface prior to desorption.

Surface‐Aligned Femtochemistry: Real‐Time Dynamics of Photoinduced I2 Formation from CD3I on MgO(100)

The breaking and making of chemical bonds in molecules that are attached to a substrate constitute the elementary steps of a surface chemical reaction and occur on the ultrafast timescale of nuclear motion. Their understanding is fundamental to the perception of chemical reaction mechanisms on surfaces of, for example, catalytic materials. The key to a molecular level insight of a bimolecular reactive encounter, however, is the knowledge about the structure and the dynamics of the transition state of the reaction. 3] Herein, we demonstrate that in contrast to other surface femtochemistry approaches, the transition state and the product formation dynamics of a bimolecular surface reaction can be directly probed by time-, mass-, and velocity-resolved multi-photon ionization on the surface. Starting from a welldefined reactant adsorption geometry representing the initial “collision complex” of the reactive bimolecular encounter, as proposed by Polanyi, 5] this new method of surface pump– probe femtosecond(fs)-laser mass spectrometry provides unprecedented insight into the elementary steps of a complex surface chemical reaction mechanism. Methyl iodide molecules adsorbed on an insulating magnesia thin film were chosen as a photochemical model system and the ultrafast bimolecular reaction of spin-orbit excited iodine atoms (I*) with ground-state iodine (I) emerging after photodissociation on the surface was probed by detection of molecular iodine in the electronically excited B-state and of CD3 radicals via multi-photon ionization. The excitation of methyl iodide to the dissociative A-band by 266 nm photons is well characterized in the gas phase in both the frequency and the time domains (see, e.g. , refs. [7– 12] and references therein). It leads to the formation of methyl radicals and iodine atoms in the P1/2 spin-orbit excited state (I*) as well as in the P3/2 ground state (I ; 19 % for CD3I ), due to a conical intersection between the respective excited states. On the (100) surface of an MgO single crystal, methyl iodide molecules have been found to adsorb at sub-monolayer coverage, with the C–I axis oriented parallel to each other and almost perpendicular to the surface. 14] On magnesia ultrathin films on Mo(100), a similar orientation was reported previously, with the methyl group heading toward the substrate surface (cf. Figure 1 a). In the experiment, both the pump and the probe laser beams directly irradiate the MgO(100)/Mo(100) thin-film substrate with the methyl iodide molecules adsorbed at submonolayer coverage. A static electric field at the surface pushes the ions, once generated, directly into the mass spectrometer (see Experimental Section). A time-of-flight mass spectrum obtained after photodissociation with a 266 nm fs laser pulse (pump) and 2 ps time-delayed ionization of the emerging neutral fragments with a 333.9 nm fs laser pulse (probe) is depicted in Figure 1 b. Two prominent peaks corresponding to the methyl fragment and to molecular iodine are detected. Minor signals are attributed to iodine atoms and methyl iodide molecules. Figure 2 a presents the time evolution of the CD3 + mass signal intensity obtained by pump–probe laser excitation and ionization at various delay times. It should be emphasized that our experiment probes the dynamics of neutral products emerging from photoexcitation and dissociation at the surface. The open circles in Figure 2 a represent the experimental data. The transient signal consists of a peak structure starting at 0 fs, with a maximum reached at about 130 fs followed by a delayed exponential rise. Fitting of a corresponding kinetic model 9] to these data (solid line in Figure 2 a) results in time constants for the rise and the decay of the peak structure of t1(CD3) = t2(CD3) = 90 10 fs. The exponential rise starting with a delay of 170 40 fs exhibits a time constant of t3(CD3) = 680 50 fs. In addition to the mass-resolved detection, the shape and the relative position of a peak corresponding to a particular Figure 1. a) Schematic illustration of the proposed adsorption geometry of methyl iodide on MgO/Mo(100). Note that neither the adsorption site nor the exact tilt angle of the molecules with respect to the surface normal are known. b) Time-of-flight fs laser desorption/ionization mass spectrum recorded at a pump–probe delay time of 2 ps.

Surface-aligned femtochemistry: Photoinduced reaction dynamics of CH3I and CH3Br on MgO(100)

Methyl iodide and methyl bromide molecules were adsorbed at submonolayer coverages on an ultrathin MgO(100) film on Mo(100) and photoexcited by 266 nm femtosecond-laser irradiation. The subsequent photodissociation and desorption dynamics were probed by time delayed multi photon ionization mass spectrometric detection of the emerging reaction products. The pronounced difference in the appearance times of the methyl radical fragments from methyl iodide and bromide is discussed on the basis of the different molecular adsorption geometries on magnesia. The surface adsorption structure also defines the alignment of the encounter complex for the observed bimolecular formation of the halogen molecules I2 and Br2 within about 1 and 2 ps, respectively. Finally, photoexcitation of co-adsorption layers of CH3I and CH3Br resulted in a heteronuclear bi-molecular reaction yielding IBr molecules.

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

Summary 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.

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

A detailed account of the experimental methodology of surface pump-probe femtosecond-laser mass spectrometry is presented. This recently introduced technique enables the direct time-resolved investigation of surface reaction dynamics by monitoring the mass and the relative velocity of intermediates and products of a photoinduced surface reaction via multiphoton ionization. As a model system, the photodissociation dynamics of methyl iodide adsorbed at submonolayer coverage on magnesia ultrathin films is investigated. The magnesia surface preparation and characterization as well as the pulsed deposition of methyl iodide are described. The femtosecond-laser excitation (pump) and, in particular, the resonant multiphoton ionization surface detection (probe) schemas are discussed in detail. Results of pump-probe time-resolved methyl and iodine atom detection experiments are presented and the potential of this method for velocity-resolved photofragment analysis is evaluated.

Nonmetal to Metal Transition and Ultrafast Charge Carrier Dynamics of Zn Clusters on p-Si(100) by fs-XUV Photoemission Spectroscopy

Understanding the electronic structure and charge carrier dynamics of supported clusters is important due to their many potential applications in photochemistry and catalysis. In this investigation, photoemission spectroscopy, in conjunction with femtosecond extreme ultraviolet (XUV) laser pulses, is used to investigate the electronic structure and ultrafast charge carrier dynamics at a Si(100) surface decorated with Zn clusters. Static photoemission spectroscopy is used to investigate the changes in the electronic structure as the dimensionality of the Zn is increased from small clusters composed of a very few atoms to metallic Zn particles. Furthermore, femtosecond optical-pump XUV-probe photoemission spectroscopy is employed to induce a charge transfer from the p-Si(100) substrate to the Zn clusters and to measure in real time the charge trapping at the Zn cluster as well as the subsequent charge relaxation. The ultrafast charge carrier dynamics are also investigated for small clusters and metallic Zn particles. Significant transient charging of the Zn clusters after excitation of the Si(100) substrate by 800 nm light is observed for Zn coverages greater than 0.12 ML Zn, which coincides with the formation of a Schottky barrier at the interface between the Zn particle and the p-Si(100) substrate. The transient signals show that the charge trapping time at the Zn cluster varies with the cluster size, which is rationalized based on the electronic structure of the cluster as well as the band energy alignment at the Zn cluster-Si(100) junction.

Femtosecond Two Photon Photoemission Spectroscopy of Methyl Iodide Adsorbed on a Gold Surface

The adsorption behavior and the electronic structure of methyl iodide molecules on thin gold films grown on Mo(100) have been investigated by temperature programmed desorption spectroscopy and two photon photoemission spectroscopy. The repulsive first order thermal desorption of methyl iodide from the Au surface indicates that the majority of the molecules adsorbs without decomposition and with the molecular dipole axis oriented parallel to each other at submonolayer coverages. A decrease of the surface work function due to methyl iodide deposition is detected in the photoemission spectra. This supports a positive outward oriented dipole moment of the molecules. Time resolved two photon photoemission transients are presented and the measured time constants are interpreted in terms of the different excited states of the Au/Mo(100) substrate that are probed as a function of the detection wavelength and the methyl iodide coverage.

Surface-Aligned Femtochemistry: Molecular Reaction Dynamics on Oxide Surfaces

In this contribution the application of ultrafast laser pulses to reveal the dynamics of chemical reactions on metal oxide surfaces will be discussed. A combination of an optical pump-probe configuration with time-of-flight mass spectrometry is employed to monitor the mass and the relative velocity of intermediates and products of a photoinduced surface reaction in real time. Starting from a well defined reactant adsorption geometry representing the initial “collision complex” of the reactive encounter, this approach enables the observation of the coherent nuclear motion through the transition state to the emerging reaction products and thus provides insight into the elementary steps of complex surface chemical reaction mechanisms. Results will be presented for the application of this technique to the photodissociation dynamics of methyl iodide and methyl bromide adsorbed on magnesia ultrathin films on a Mo(100) single crystal surface.