Beam - Transient Species

Molecular Beam—Transient Species.—In their molecular beam-AES study of C2H4 adsorption on Ni(llO), Zuhr and Hudsonfound that there was an initial interaction process between C2H4 and Ni. In addition, a secondary non-dissociated adsorbed phase was found to co-exist with the initial phase formed at temperatures below 350 °C. They say that this may well be the active phase in olefin reactions on transition metals. 

The authors consider adsorptions below 350 °C occurring upon exposures in the range 1 to 20 x 10 molecules per cm. Using AES signals they deduced 

Their results can be expressed as In = In tq + SWJkT where AH(desorp-tion) is 11.9 kcal mol and tq is 1 x 10 ° s. These values indicate mean lifetimes of a few hundredths of a second at room temperature. For a molecular impingement rate on to the surface of 1 x 10 molecules cm s this gives an equilibrium concentration on the surface of 10 molecules cm , or about 10 monolayer. The authors go on to express the view that the firmly-held chemisorbed species could not contribute to further chemical reaction and that the species involved in catalysis is probably the loosely-bound phase. 

When the Ni surface used by the authors was covered by a graphitic layer, formed above 300 °C, the signal variations indicating a secondary species were lost. 

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Figure 20.2. Schematic outline of typical pump-probe-detect experiments with femtosecond pulses, a molecular beam source, and mass spectrometric detection of transient species. Computer control and data processing instruments, as well as various optical components, are not shown. The time separation Af between pump and probe pulses is dictated by the difference in optical path lengths. Ad, traversed by the two components of the original pulse.

The femtosecond transient absorption studies were performed with 387 nm laser pulses (1 khz, 150 fs pulse width) from an amplified Ti Sapphire laser system (Model CPA 2101, Clark-MXR Inc). A NOPA optical parametric converter was used to generate ultrashort tunable visible pulses from the pump pulses. The apparatus is referred to as a two-beam setup, where the pump pulse is used as excitation source for transient species and the delay of the probe pulse is exactly controlled by an optical delay rail. As probe (white light continuum), a small fraction of pulses stemming from the CPA laser system was focused by a 50 mm lens into a 2-mm thick sapphire disc. A schematic representation of the setup is given below in Fig. 7.2. 2.0 mm quartz cuvettes were used for all measurements. 

The most commonly used sources of radiation are the 60 Co gamma source for continuous irradiation and pulsed high-energy (>1 MeV) electron beams for fast kinetic studies. Detailed descriptions of several such sources and accelerators are given in numerous books, as are the various methods used by radiation chemists for dosimetry, sample preparation and irradiation, and common product analysis. Several new developments in the analytical procedures, both in the determination of final products and in the direct observation of transient species, will be discussed below. 

Electrons are stabilized by the surrounding water molecules to form hydrated electrons in less than 1 ps. The yields at this stage are defined as initial yields. The resulting transient species such as e, H, OH are distributed locally along the track where the energy deposits, called spur. The spatial distribution gready depends on the LET of the incident beam. Then these products diffuse randomly and either react together or escape into the bulk solution. After the completion of spur processes, which take place within s, the products... 

The cover image depicts the spirit of pulse radiolysis experiment. The white arrows represent the pulsing of the radiation beam on the target (grey circle) leading to formation of transient species (maroon circle) whose spectrum is exhibited by the colored lines. The picture is redrawn from the mural at the National Centre for Free Radical Research, Department of Chemistry, University of Pune, Pune 411007, India. 

Significantly more information is available when the probe pulse is a white-light continuum and a spectrograph is used to disperse the spectral distribution of the beam transmitted by the sample. Under these conditions, one obtains an absorption spectrum of transient species formed, or ground-state species removed by the ini-... 

Once the transient species has been formed, it has to be monitored by some form of kinetic spectroscopy, typically with ultraviolet-visible absorption or emission, infrared (time-resolved infrared or TRIR) (74), or resonance Raman (time-resolved resonance Raman or TR3) (80) methods of detection. The transient is usually tracked by a probe beam at a single characteristic frequency, thereby giving direct access to the kinetic dimension. Spectra can then be built up point by point, if necessary, with an appropriate change of probe frequency for each point, although improvements in the sensitivity of multichannel detectors may be expected to lead increasingly to the replacement of the laborious point-by-point method by full two-dimensional methods of spectroscopic assay (that is, with both spectral and kinetic dimensions). 

The detection of short-lived transient species is often achieved by flash photolysis where an extremely short flash of UV/Vis radiation from a laser generates a high concentration of transient species, and a second probe beam monitors any changes that occur after the flash. Traditionally, UVA is spectroscopy has been used as a detection method. However, time-resolved infrared spectroscopy (TRIR), a combination of UV flash photolysis and fast IR detection, also has a long history. There are several different approaches to fast IR spectroscopy and the method of choice depends upon the timescale of the reaction. Measurements on the nanosecond to millisecond timescale are obtained using point-by-point techniques or by step-scan FTIR. In the point-by-point approach, a continuous wave IR laser (GO or diode) or globar is used as the IR source, which is tuned to one particular IR frequency (Figure 3). ... 

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