Knudsen cell, source

Generally, vaporization source temperatures are very difficult to monitor or control in a precise maimer. Since the vaporization rate is very temperature-dependent, this makes controlling the deposition rate by controlling the source temperature very difficult. In molecular beam epitaxy (MBE), the deposition rate is controlled by careful control of the temperature of a well-shielded Knudsen cell source using embedded thermocouples. 

MetallorganicMBE (MOMBE). tire solid source Knudsen cells in conventional MBE are replaced witli gaseous beams of organometallic precursors, directed toward a heated substrate in UHV. Compared to MOCVD, MOMBE eliminates gas phase reactions tliat may complicate tire deposition surface reactions, and provides lower growtli temperatures. 

The enthalpies of phase transition, such as fusion (Aa,s/f), vaporization (AvapH), sublimation (Asut,//), and solution (As n//), are usually regarded as thermophysical properties, because they referto processes where no intramolecular bonds are cleaved or formed. As such, a detailed discussion of the experimental methods (or the estimation procedures) to determine them is outside the scope of the present book. Nevertheless, some of the techniques addressed in part II can be used for that purpose. For instance, differential scanning calorimetry is often applied to measure A us// and, less frequently, AmpH and AsubH. Many of the reported Asu, // data have been determined with Calvet microcalorimeters (see chapter 9) and from vapor pressure against temperature data obtained with Knudsen cells [35-38]. Reaction-solution calorimetry is the main source of AsinH values. All these auxiliary values are very important because they are frequently required to calculate gas-phase reaction enthalpies and to derive information on the strengths of chemical bonds (see chapter 5)—one of the main goals of molecular energetics. It is thus appropriate to make a brief review of the subject in this introduction. 

Figure 2.34 Schematic of an electron ionization source with Knudsen cell.

Figure 5.38 Knudsen cell mass spectrometer with e beam ion source. (K. Hilpert, Fresenius J. Anal. Chem. 370, 471 (2001). Reproduced by permission of Springer Sciences and Business Media.)...

We summarize what is special with these prototype fast ion conductors with respect to transport and application. With their quasi-molten, partially filled cation sublattice, they can function similar to ion membranes in that they filter the mobile component ions in an applied electric field. In combination with an electron source (electrode), they can serve as component reservoirs. Considering the accuracy with which one can determine the electrical charge (10 s-10 6 A = 10 7 C 10-12mol (Zj = 1)), fast ionic conductors (solid electrolytes) can serve as very precise analytical tools. Solid state electrochemistry can be performed near room temperature, which is a great experimental advantage (e.g., for the study of the Hall-effect [J. Sohege, K. Funke (1984)] or the electrochemical Knudsen cell [N. Birks, H. Rickert (1963)]). The early volumes of the journal Solid State Ionics offer many pertinent applications. 

Rather different spectra are obtained when the equilibrium vapor above metallic selenium (160°C) is ionized by a field ion source (2.5-8.0 kV), resulting in the ions Se + (n = 2 and 5-8) neither Se+ nor Se3+ and Se4+ have been observed (these species usually result from fragmentation processes), but occasionally traces of Seg+ have been found. Using a Knudsen cell, Se6+ is the most abundant species, followed by Se8+ and Se7+, which all originate from the corresponding neutral, cyclic molecules (56). Field evaporation (20-100°C) of whiskers of selenium (prepared by field condensation of Se vapor) produces mainly Se5+, but small amounts of molecular ions with one, two, and four positive charges up to Se33 have been identified. However, these species may have chainlike structures (56). 

The model molecules which are solid at room temperature (except for benzene), were evaporated from a source consisted of a borosilicate glass container with a small opening, like the design of a Knudsen cell. The source was mounted inside a heatable and coolable copper shaft, that could be inserted to the vaccum system through a load-lock arrangement. Upon heating, the pressure was... 

The pressure calibration factor k takes into account losses of species caused for example by the intensity distribution of the molecular beam from the Knudsen cell or the transmission of the ion source and the analyzer. Three different methods are generally used [86] ... 

A valve between the chambers of the Knudsen cell and the ion source is very advantageous according to our experience. Special constructions are available (see e.g. Refs. 86, 126), which practically do not lead to a decrease of the sensitivity due to the slight increase of the distance between ion source and Knudsen cell. The valve renders possible a fast change of the material in the cell... 

This equation describes the solid-angle dependent emission of mass from sources with rotationally symmetric lobe-shaped vapour cloud characteristics. With n = 1, Knudsen-cell evaporator, it is identical with other published [253,254] calculations. With n = 0, point source or spherical source, there are slight differences with other published values which generally consider evaporation in the total space to = 4n, whereas here, evaporation is only considered in the half-space above the vapour source. 

A ideal Knudsen cell holds the source in an isothermal box with a relatively small exit orfice. This can be implemented by placing the crucible and filament inside a radiation shield without making contact between the three elements. The filament heats the crucible radiatively (there is no gas or direct mechanical connection to conduct heat), and this multiple reflection and conduction inside the crucible homogenizes the temperature along the crucible length. This configuration also increases the temperature that can be achieved for a given power input, since less heat is lost to the environment via radiation. The maximum temperature which can be achieved by a K-cell is limited by the metal elements used to fabricate the cell and the thermocouples, and is typically 1200-1400°C. 

Abbreviations used in the tables calc = calculated value PT = photodetachment threshold using a lamp as a light source LPT = laser photodetachment threshold LPES = laser photoelectron spectroscopy DA = dissociative attachment attach = electron at-tachment/detachment equilibrium e-scat = electron scattering kinetic = dissociation kinetics Knud=Knudsen cell CT = charge transfer CD = collisional detachment and ZEKE = zero electron kinetic energy spectroscopy. 

In this method the sample is vaporized in a micro-oven placed in the ion source or out of a Knudsen-cell. Many metals can be analyzed qualitatively and quantitatively by this technique as metal organic compounds (see Refs. ). The metal chelates have lower volatilities than the metals and in many cases the mass spectra reveal higher sensitivities for these compounds compared with the analysis using direct evaporation of the metal. The latter technique of direct metal analysis by EIMS is only applied if the ionization energies of the metals are too high for thermal ionization mass spectrometry . 

---