Molecular beam expansion

The advent of supersonic molecular beam expansion techniques and high spectral-and time-resolved laser spectroscopic probing has transformed experimental... 

We limit the presentation of experimental data to those experiments utilizing supersonic molecular beam expansions in combination with UHV techniques. Supersonic beam experiments control the incident kinetic energy and angle of the molecule to a high degree, and thus are generally easier to interpret at the microscopic dynamical level than those experiments utilizing effusive beams. 

For example, let us contrast the dissociations of OCS and H202 in a molecular beam. Because of the strong rotational cooling present in molecular beam expansions, the parent angular momentum will be very small. Thus, in the case of OCS, the dissociation can be considered to occur in a single plane. The OCS bending motion, which provides the torque to give... 

Clusters are generally observed in molecular beam expansions. The low-temperature characteristic of this environment permits the condensation of molecules into dimers, trimers and, if the cooling is sufficient, veritable snowballs with thousands of monomer units. These broad distributions of clusters in a beam cause major problems in detection. While it is possible to control the expansion so that dimers are the dominant cluster (in the presence of a large excess of monomers), the trimers can only be studied in the presence of an excess of dimers, etc. Thus a major experimental problem in the study of neutral clusters is the detection of a particular cluster in the presence of many other sized clusters. 

In many respects the development of tunable infrared and ultraviolet laser sources when combined with molecular beam expansions, mrurked the start of the modern or contemporary period of cluster studies. First, it offered the opportunity to selectively excite specific rovibrational or rovibronic levels in a complex. Second, variations in the spectra (linewidth, intensity and frequency) gave insight into dynamical behavior and the presence of nearby perturbing states. 3 Finally, the availability of widely tunable sources has enabled the experimentalist to select quantum states that would provide the maximum information content on a cluster system, an impetus that continues to drive the development of new lasers and laser systems. As this is an extremely wide field of research, primary emphasis in this chapter will be placed on vibrational spectroscopic studies of neutral and ionic clusters. 

The small amount of radiation that leaks out of the cavity is measured with an appropriate detector. The decay lifetime is affected by the presence of an absorbing species within the cavity. The high finesse, resulting from extremely well fabricated high reflective coatings on the cavity mirrors, translates into an effective path length of tens of meters. If a molecular beam expansion is performed within the cavity, the direct absorption by molecular clusters can be observed. Considerable attention has been paid to the theory associated with the use of both pulsed and cw lasers. This method, while still in its infancy, promises to be another useftil tool in the characterization of hydrogen-bonded neutral molecular clusters. 

As virtually all biological molecules possess low vapour pressures gas-phase molecular spectroscopy methods to investigate isolated neutral biomolecules require volatilization methods other than thermal evaporation. The combination of laser desorption with a supersonic molecular beam expansion together with the selectivity of IR and UV double resonance methods opened up the possibility of characterizing isolated, neutral biomolecules and their clusters with the biological environment. 

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)...

Additionally, the UV spectra might reveal the cmiformational abundances present in the molecular beam expansion. However, this information ought to be interpreted with care, since specific detection schemes may introduce a quantitative bias. For REMPI detection, the photo-ionization efficiency can be conformer-dependent [75], decreasing for instance in the presence of NH-jt intramolecular interactions [76]. When employing fluorescence detection, strongly fluorescent conformers may be overestimated, while poorly fluorescent crmformers are underestimated. 

One of the most useful applications of peak widths in the TOF distributions is in distinguishing an ionization from a dissociative ionization process. This is particularly useful in the case of photoionization of clusters. An example of such a study is the case of ArCO", which can arise from a variety of process in a molecular beam expansion of Ar and CO gases, including those shown in Equations [18]-[20]. 

As with most methods for studying ion-molecule kinetics and dynamics, numerous variations exist. For low-energy processes, the collision cell can be replaced with a molecular beam perpendicular to the ion beam [106]. This greatly reduces the thennal energy spread of the reactant neutral. Another approach for low energies is to use a merged beam [103]. In this system the supersonic expansion is aimed at the tluoat of the octopole, and the ions are passed tluough... 

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

To date, the IR-CRLAS studies have concentrated on water clusters (both FI2O and D2O), and methanol clusters. Most importantly, these studies have shown that it is in fact possible to carry out CRLAS in the IR. In one study, water cluster concentrations in the molecular beam source under a variety of expansion conditions were characterized [34]- hr a second study OD stretching bands in (020) clusters were measured [35]. These bands occur between 2300... 

Figure Bl.7.3. Schematic diagram of a molecular beam generator nozzle (1) expansion region (2) skinmrer (3) and molecular beam (4).

The proposed technique seems to be rather promising for the formation of electronic devices of extremely small sizes. In fact, its resolution is about 0.5-0.8 nm, which is comparable to that of molecular beam epitaxy. However, molecular beam epitaxy is a complicated and expensive technique. All the processes are carried out at 10 vacuum and repair extrapure materials. In the proposed technique, the layers are synthesized at normal conditions and, therefore, it is much less expansive. The presented results had demonstrated the possibility of the formation of superlattices with this technique. The next step will be the fabrication of devices based on these superlattices. To begin with, two types of devices wiU be focused on. The first will be a resonant tunneling diode. In this case the quantum weU will be surrounded by two quantum barriers. In the case of symmetrical structure, the resonant... 

Supersonic molecular beam (SMB) mass spectrometry (SMB-MS) measures the mass spectrum of vibra-tionally cold molecules (cold El). Supersonic molecular beams [43] are formed by the co-expansion of an atmospheric pressure helium or hydrogen carrier gas, seeded with heavier sample organic molecules, through a simple pinhole (ca. 100 p,m i.d.) into a 10 5-mbar vacuum with flow-rates of 200 ml. rn in. In SMB, molecular ionisation is obtained either through improved electron impact ionisation, or through hyperthermal surface ionisation... 

The molecular beam is formed by the supersonic expansion of gas through a pulsed nozzle. It is then collimated by two skimmers, and enters... 

The fragment recoil velocity resolution depends on the divergence of the molecular beam, molecular beam velocity distribution in the direction of the molecular beam axis, and the distance of fragments expanded in the velocity axis of the two-dimensional detector. If the divergence of the molecular beam is small and the fragment recoil velocity is much larger than the velocity difference of parent molecules, the recoil velocity resolution can be simply expressed as AV/V = s/L, where L is the length of expansion of... 

Another method is to measure the disappearance rate of the excited parent molecules, that is, the intensity changes of the disk-like images at various delay times (therefore, at various photolysis laser positions) along the molecular beam. This is very useful when the dissociation rate is slow and the method described above cannot be applied. This measurement requires a small molecular beam velocity distribution and a large variable distance between the crossing points of the pump and probe laser beams with the molecular beam. The small velocity distribution can be obtained through adiabatic expansion, and the available distances between the pump and probe laser beams depend on the design of the chamber. For variable distances from 0 to 10 cm in our system and AV/V = 10% molecular beam velocity distribution, dissociation rates as slow as 3 x 103 s 1 under collisionless condition can be measured. 

A technique which is not a laser method but which is most useful when combined with laser spectroscopy (LA/LIF) is that of supersonic molecular beams (27). If a molecule can be coaxed into the gas phase, it can be expanded through a supersonic nozzle at fairly high flux into a supersonic beam. The apparatus for this is fairly simple, in molecular beam terms. The result of the supersonic expansion is to cool the molecules rotationally to a few degrees Kelvin and vibrationally to a few tens of degrees, eliminating almost all thermal population of vibrational and rotational states and enormously simplifying the LA/LIF spectra that are observed. It is then possible, even for complex molecules, to make reliable vibronic assignments and infer structural parameters of the unperturbed molecule therefrom. Molecules as complex as metal phthalocyanines have been examined by this technique. 

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