Pulsed molecular beam studies

In the pulsed molecular beam studies, the results were used to calculate an impact parameter for the photodissociation process. From these impact parameters it was concluded that the recoiling CN fragment did not take the lowest energy path when it was departing from the halogen atom. Rather, because of strong impulsive motion, the trajectory that the CN rsdical... 

The reduction of NO by CO was studied on small supported palladium clusters with sizes up to 30 atoms [468]. Combined TDS and pulsed molecular beam studies showed small clusters to stay indeed active during a catalytic process and to be catal3dically active at temperatures 100 K below those observed for larger palladium particles [441,442,454] and bulk systems [459]. In... 

Previous workers had used the molecular beam TOF technique (134) and the VUV flash photolysis LIF technique (135). Ling and Wilson (136) had suggested that either the A(2n) state of CN is produced in the original photolysis process or that I atoms were produced in the Pi/2 and 3/2 states. It had been previously shown (135), by collisional quenching studies, that the A state of CN was not produced. This earlier work has been reviewed by Baronvaski (137) but recently both he and others have done further work on this molecule using excimer laser sources in both static gases and pulsed molecular beams. 

Wittig and his co-workers (138) have studied the photolysis at 266, 320, and 355 nm in a static gas cell as well as in a pulsed molecular beam. At each of these wavelengths they report only on the CN(x2 +, v"= 0) radical product. 

Several groups (138,144,152-154) have reported on LIF studies of CN following the photolysis of BrCN in the A continuum at 193 nm and longer wavelengths. These studies have been done in a pulsed molecular beam as well as in a static gas cell. A symmary of the results of these studies is given in Table 6. 

Other than the earlier work reviewed by Ashfold et al. (3), only three studies on the photodissociation dynamics have been reported for this molecule (153,154,158). The first study reported the quantum state distribution of the CN radical obtained in an effusive molecular beam and in a static gas cell, while the second study reported the observations in a pulsed molecular beam. The dynamics remains the same despite the fact that the initial internal state distribution of the C1CN molecule changes. This of course shows that hot bands are not important in the photodissociation of this molecule at this wavelength. 

The LIF studies of Hawkins and Houston were done in a static gas cell and with a pulsed molecular beam. Even though there is almost 20,000 cm-l of available energy, very little rotational excitation is observed. The rotational temperatures that were observed under both experimental conditions are summarized in Table 11. 

Linear hydrocarbon radicals have been the subject of intensive laboratory spectroscopic and radio-astronomical research since the early 1980s. In recent years, a considerable number of rotational spectroscopic studies of medium to longer hydrocarbon chains such as C5H, CeH, CgH, and ChH have been carried out using a pulsed molecular beam FTMW spectrometer. The high resolution offered by such a spectrometer allowed the detection of the hyperfine sphtting of rotational transitions. These measurements improved fine and hyperfine coupling constants and provided rest frequencies with accuracies better than 0.30 km s in equivalent radial velocity up to 50 GHz. Indeed, some of the small C H radicals with n < 9 have subsequently been detected in space, in molecular cloud cores, and in certain circumstellar shells. These hydrocarbon chains are among the most abundant reactive space molecules known. 

Mass spectrometry is one physical technique that does not (at least directly) involve electromagnetic radiation. However, some sample desorption and ionization processes do use high intensity pulses of laser light in techniques such as MALDI (Matrix-Assisted Laser Desorption Ionization) that have proved very useful in mass analysis of proteins and other biologic macromolecules. High resolution mass spectrometry derives from atomic/molecular beam studies in which the trajectories of ionized particles in a vacuum can be manipulated by static... 

A systematic view of the relevant elements is depicted in Figure 17.10. The deposited clusters can be exposed to different reactant gases by two kinds of valves. First, they can be exposed isotropically to e.g. O2 by a commercial, ultra-high vacuum (UHV) compatible, variable leak valve. Second, reactant molecules (e.g. CO) can be introduced via a pulsed molecular beam produced by a piezo-electric driven, pulsed valve. This pulsed valve has a high pulse-to-pulse stability (time profile), and allows the study of catalytic processes on supported clusters at relatively high pressures (up to 10 mbar). Furthermore, a stainless steel tube is attached to the pulsed nozzle in order to collimate the molecular beam and to expose the reactant molecules to the substrate only. The pulse duration at the position of the sample can, in principle, be varied from 1 ms up to continuous operation. For the experiments described below a constant pulse duration of about 100 ms was used. The repetition rate of the pulsed valve can be up to 100 Hz. The experiments were carried out at 0.1 Hz the 10 s interlude allows the reactant gas to be pumped completely. 

Fig. 1.52. Typical experimental setup for a pulsed molecular beam experiment for studying the catalytic properties of size-selected clusters on surfaces. It mainly consists of a pulsed valve for the generation of a pulsed molecular beam and a differentially pumped, absolutely calibrated quadrupole mass spectrometer. The length of the valve extension tube is adjusted to obtain a beam profile of similar dimensions as the sample under investigation. A typical time profile is also shown. It can be adjusted up to continuous operation. The pulse-to-pulse stability is better than 1%...

Catalytic Reactivity. The size-dependent cluster catalysis was first studied by pulsed molecular beams. In these experiments, a nitric oxide molecular pulse is injected onto the cluster catalysts and the product molecules CO2 and N2 are quantitatively detected by a mass spectrometer as a function of cluster size, temperature, and CO background pressure [468]. The catal3dic formation of CO2 on Pdso and Pd is shown in Fig. 1.95a and b for selected temperatures and for a constant CO partial pressure of 5 x 10 mbar and an NO effective pressure of 1 x 10 mbar. The width of the NO pulse was 100 ms. Pdg and Pdgo show almost no catalytic reactivity up to about 390 K. For Pdso, maximal reactivity is observed at 420 K whereas Pdg is most reactive at 450 K. At higher temperatures, the formation of CO2 decreases. The CO2 formation on both cluster sizes at temperature of maximal reactivity is stable even after hundreds of NO pulses. 

One of the most sophisticated techniques in the study of the dynamics of ion—molecule reactions is the simultaneous measurement of the velocity and angular distributions using beams for both reactant ions and neutrals (crossed beams). Herman et al. [103] developed such an apparatus in which the neutral reactant is formed into a pulsed molecular beam at 55°C. Figure 8 shows their apparatus schematically. The ion beam intersects the molecular beam at a fixed 90° angle in a carefully screened field-free region (D). Ions from the collision zone pass through an energy analyser (F), a 60°-sector mass spectrometer (G) and are detected by an... 

These few examples will have demonstrated that the combination of pulsed lasers, pulsed molecular beams, and time-of-flight mass spectrometry represents a powerful technique for studying the selective excitation, ionization, and fragmentation of wanted molecules out of a large variety of different molecules or species in a molecular beam [487 89, 500-504]. The technique, refined by Boesl... 

Figure 1 presents a schematic overview of a typical molecular beam time-of-flight mass spectrometer equipped with a laser desorption source. In the studies presented in this book, the sample bar is made from graphite. Accurate positioning of the sample bar with respect to the nozzle is required for optimal performance. It is typically mounted on a double translation stage (Fig. 1). The vertical travel (x-direction) with a typical accuracy better than 0.01 mm allows for optimal cooling with minimal distortion of the molecular beam expansion. The sample bar is typically positioned about 0.1 mm below the aperture of the pulsed molecular beam valve. Travel in the horizontal direction (y-axis) of 50 mm (length of the sample bar) with a position accuracy of about 0.1 mm ensures desorption of fresh sample at every laser shot. Both positioning options can be controlled under operating conditions. Finally, the distance along the molecular beam (z-axis)...

The setup used for crossed beam experiments is basically the same apparatus used in the H2O photodissociation studies but slightly modified. In the crossed beam study of the 0(1D) + H2 — OH + H reaction and the H + HD(D2) — H2(HD) + D reaction, two parallel molecular beams (H2 and O2) were generated with similar pulsed valves. The 0(1D) atom beam was produced by the 157 photodissociation of the O2 molecule through the Schumann-Runge band. The 0(1D) beam was then crossed at 90° with the... 

Temporal analysis of products (TAP) reactor systems enable fast transient experiments in the millisecond time regime and include mass spectrometer sampling ability. In a typical TAP experiment, sharp pulses shorter than 2 milliseconds, e.g. a Dirac Pulse, are used to study reactions of a catalyst in its working state and elucidate information on surface reactions. The TAP set-up uses quadrupole mass spectrometers without a separation capillary to provide fast quantitative analysis of the effluent. TAP experiments are considered the link between high vacuum molecular beam investigations and atmospheric pressure packed bed kinetic studies. The TAP reactor was developed by John T. Gleaves and co-workers at Monsanto in the mid 1980 s. The first version had the entire system under vacuum conditions and a schematic is shown in Fig. 3. The first review of TAP reactors systems was published in 1988. 

In conclusion, we present a spectroscopic study of nn excitation in trans-Stilbene in a molecular beam experiment. The excitation involves a 1+1 REMPI scheme following the interaction of the molecule with femtosecond UV laser pulses. When the excitation is resonant with the origin of the intermediate Si state, the measured photoelectron distribution reveals a maximum probability for the 0-0 transition. For higher photon energies (266nm) the photoelectron spectrum exhibits a rather complex distribution, due to the excitation of an alternate (C-C) stretching mode. 

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