Pulsed molecular-beam experiments

One of the gas mixtures was used in a pulsed molecular beam experiment. The result of the experiment is shown below. Which of the two gas samples, A or B, was used for this experiment ... 

Heiz further examined the oxidation of CO in a recent paper using dual pulsed molecular beam experiments over Pd (n=8, 13, 30) [59]. The mole fractions of CO and O could be varied across the entire range a maximum in CO production occurring at a mole fraction x =0.6 at 478 K. As the temperature is... 

Fig. 17.10. Experimental scheme of the relevant elements for performing pulsed molecular beam experiments show/ing the UHV chamber with the pulsed piezo-electric valve, the variable leak valve and the absolutely calibrated mass spectrometer. The cluster...

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%...

VUV lasers offer the most effective means for PI product detection. Although their average intensity is not particularly high (ca. 10 photons/sec), when their pulsed structure is combined with pulsed molecular beam experiments, the photons concentrated in a few nanoseconds increase the effective intensity by orders of magnitude. That is, the peak intensity of a VUV laser in a typical 5-nsec pulse is on the order of 10 9 photons/sec, which means that a sample can be completely ionized. However, only those molecules which happen to be in the laser interaction region during those 5 nsec can be detected. 

W. R. Gentry, Pulsed molecular beam experiments, in "Electronic and Atomic Collisions. Invited Papers and Progress Reports",... 

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. 

They only observed an LIF spectra at 266 nm. The experiments were done in a static gas cell as well as in a pulsed molecular beam. In the static gas cell experiments, the CN radicals are formed in the v" = 0, 1, and 2 levels. The highest rotational level observed for the v" = 0 and v" = 1 levels of the X state... 

In a recent series of papers C. Wittig and his group have reported on the photolysis of NCNO in its first absorption band (540 - 900 nm). These experiments have been done both in a static gas cell (165) and in a pulsed molecular beam (166). 

Ethylene has no dipole moment and a center of symmetry and therefore the Raman spectrum is an important source of structural information. After the early work on the rotational (Dowling and Stoicheff, 1959) and rovibrational Raman spectrum (Feldman et ah, 1956) these spectra were thoroughly studied in a series of publications (Hills and Jones, 1975 Hills et ah, 1977 Foster et ah, 1977). Overtones and combination bands were measured in an intracavity Raman experiment by Knippers et ah (1985). The Q-branch of the U2 band was resolved by pulsed CARS spectroscopy in a molecular beam experiment (Byer et ah, 1981). 

Experiments are performed in an ultra-high vacuum (UHV) chamber with a base pressure of better than 5 x 10 T. A pulsed molecular beam is used to dose gases on a thin film single-crystal surface of 200-nm thickness. Each pulse, which... 

Following the TPR experiments discussed earlier, Judai et al. examined the reaction between CO and NO on size-selected Pd clusters using a pulsed molecular beam [73]. In these experiments, CO and N formation are monitored when a constant pressure of 5 x 10 mbar of CO is maintained in chamber while delivering 100 ms pulses of NO (equivalent to an effective pressure of 1 x 10 mbar) to the surface of the sample held at a constant temperature. Although TPR experiments indicated a... 

A Newly Designed Pulsed Valve for Molecular Beam Experiments... 

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. 

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. 

Table 1.5. Reaction temperatures for maximal formation of CO2 and N2 measured in single cycle TPR experiments and measured under steady-state conditions or using pulsed molecular beams...

Fig. 3.12. Principle of the measurement of the global adsorption probability in a molecular beam experiment. The desorption pulse is composed by a fast component ( S fast) corresponding to molecules reflected or desorbing from the substrate and a slow component (5siow) corresponding to (chemisorbed) molecules desorbing from the metal clusters. The relative amplitude of the slow component gives the global adsorption probability...

I- 1 REMPI of S( P2,i,o> D2)IC H S. The experimental setup and procedures used to measure the electronic S( P2.i,o, 2) state distribution formed in the 193-nm photodissociation of organosulfur species have been described in Section II.A [58-60]. In this experiment, a pulsed molecular beam of neat thiophene is produced by supersonic expansion through a pulsed valve (nozzle diameter = 0.5 mm, temperature 298 K, stagnation pressure = 90 Torr). 

Figure 5.13 The stimulated emission pumping (SEP) experimental set-up. The pulsed molecular beam is located in the center of the experiment and is oriented perpendicular to the page. The pump (255 nm) and dump (375—600 nm) beams enter from the left, while the VUV fluorescence excitation (FE) laser enters from the right. Taken with permission from Choi et al. (1992).

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