Molecular beam apparatus

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. 

Figure B2.3.7. Schematic apparatus of crossed molecular beam apparatus with synclirotron photoionization mass spectrometric detection of the products [12], To vary the scattering angle, the beam source assembly is rotated in the plane of the detector. (By pemrission from AIP.)...

Yang X, Lin J, Lee Y T, Blank D A, Suits A G and Wodtke A M 1997 Universal crossed molecular beams apparatus with synchrotron photoionization mass spectrometric product detection Rev. Sc/. Instrum 68 3317-26... 

Diagram of ihe rotating disks lhal serve as a velocity selector in a molecular beam apparatus... 

How fast do gas molecules move Molecular speeds can be measured using a molecular beam apparatus, shown schematically in Figure 5-la. Gas molecules escape from an oven through a small hole into a chamber in which the molecular density is very low. A set of slits blocks the passage of all molecules except those moving in the... 

Example describes an experiment with a molecular beam apparatus. 

Mass spectrometers Molecular beam apparatus Ion sources Particle accelerators Electron microscopes Electron diffraction apparatus Vacuum spectographs Low-temperature research Production of thin films Surface physics Plasma research Nuclear fusion apparatus Space simulation Material research Preparations for electron microscopy... 

The laboratory layout consists of a molecular beam apparatus and a laser system. NaK clusters are created in an adiabatic coexpansion of mixed alkali vapour and argon carrier gas through a nozzle of 70 pm diameter into the vacuum. Directly after the nozzle the cluster beam passes a skimmer. Next, the laser beam coming from perpendicular direction irradiates the dimers and eventually excites and ionizes them. The emerging ions are extracted by ion optics, mass selected by QMS and recorded by a computer. 

NMR transitions were first observed by Rabi and co-workers in 1938, using a molecular-beam apparatus.3 NMR transitions in bulk matter were first detected in 1945 by two independent groups, one headed by Bloch, the other by Purcell these two men shared the 1952 Nobel physics prize.4... 

Although theoretical techniques for the characterization of resonance states advanced, the experimental search for reactive resonances has proven to be a much more difficult task [32-34], The extremely short lifetime of reactive resonances makes the direct observation of these species very challenging. In some reactions, transition state spectroscopy can be employed to study resonances through "half-collision experiments," where even very short-lived resonances may be detected as peaks in a Franck-Condon spectrum [35-38]. Neumark and coworkers [39] were able to assign peaks in the [IHI] photodetachment spectrum to resonance states for the neutral I+HI reaction. Unfortunately, transition state spectroscopy is not always feasible due to the absence of an appropriate Franck-Condon transition or due to practical limitations in the required level of energetic resolution. The direct study of reactive resonances in a full collision experiment, such as with a molecular beam apparatus, is the traditional and more usual environment to work. Unfortunately, observing resonance behavior in such experiments has proven to be exceedingly difficult. The heart of the problem is not a... 

Since SFC is in its infancy, the same is true of the hyphenated techniques that involve it. In general, it would be expected that SCF/MS should use interfaces like GC/MS since the supercritical fluid mobile phases become gases when reduced to atmospheric pressure, but the conditions are more severe because of the higher (critical) pressure. OT columns, because of their low pressure drops, are favored. The two interfaces that have been used are a direct fluid injection (DFI) and a molecular beam apparatus. DFI has been used with packed columns26 and with OT columns,27 using both chemical ionization and electron impact ionization. For a more complete discussion of both interfaces, see the chapter on SFC in the ACS Symposium Series edited by Ahuja28 the recent review on LC/MS25 also contains considerable information about SFC/MS. 

Figure 3 Fritz Haber Institut molecular beam apparatus, (a) Experimental chamber, (b) schematic representation of the different probes converging toward the sample surface (from Libuda et al. [51]).

Fig. 3. A schematic view of a crossed-molecular beam apparatus used for studying the reactions of chlorine atoms with halogen molecules. The mass spectrometer detector is rotatable about the scattering centre for measuring the angular distributions of the reaction products whose recoil velocities are determined by time-of-flight analysis. (Reproduced from ref. 558 by permission of the authors and the American Institute of Physics.)...

Utilization of both ion and neutral beams for such studies has been reported. Toennies [150] has performed measurements on the inelastic collision cross section for transitions between specified rotational states using a molecular beam apparatus. T1F molecules in the state (J, M) were separated out of a beam traversing an electrostatic four-pole field by virtue of the second-order Stark effect, and were directed into a noble-gas-filled scattering chamber. Molecules which were scattered by less than were then collected in a second four-pole field, and were analyzed for their final rotational state. The beam originated in an effusive oven source and was chopped to obtain a velocity resolution Avjv of about 7 %. The velocity change due to the inelastic encounters was about 0.3 %. Transition probabilities were calculated using time-dependent perturbation theory and the straight-line trajectory approximation. The interaction potential was taken to be purely attractive ... 

In 1950, Norman Ramsey, a member of the team that developed the magnetic resonance method, made a basic modification to Rabi s molecular beam apparatus that significantly enhanced... 

I.r. laser spectroscopy and quadrupole mass spectrometry were used by Fischer et al. to study vibrational predissociation of clusters of C2H4, and CsHg, but-l-ene, cis- and trans-but-2-ene, and isobutene. They obtained spectra in the range 2900—3200 cm and for C2H4 clusters predissociation was observed to result from excitation near the v-i, and vg fundamentals and the i 2 + V12 combination band. The vibrational bands were observed to have Lorentzian lineshapes with IWHM of ca. 5 cm. A homogeneous broadening mechanism was assumed and the widths were used to calculate excited-state lifetimes. Valentini and co-workers studied the predissociation of C2H4 clusters at 950 cm in a crossed laser/molecular beam apparatus. 

The scattering dynamics experiments done in our laboratory utilize the coupling of a laser detonation source (described above) with a crossed molecular beams apparatus (Fig. 0). 42-144 pulsed beam containing energetic species (oxygen atoms or inert species, such as Ar and N2) is... 

The experimental arrangement consists of a simple molecular beam apparatus, a pulsed tunable dye laser for exciting fluorescence, and a gated optical detection system. 

Fig. 1.— The molecular beam apparatus (o) top view, (6) side view. In (6) the laser beam is perpendicular to the plane of the figure and the secondary beam source has been rotated by 45° about...

We thank Frank Tittel, Y. Liu and Q. Zhang for helpful discussions, encouragement and technical support. This research was supported by the Army Research Office and the Robert A. Welch Foundation, and used a laser and molecular beam apparatus supported by the NSF and the US Department of Energy. H.W.K. acknowledges travel support provided by SERC, UK. J.R.H. and S.C.O B. are Robert A. Welch Predoc-toral Fellows. 

FIGURE 14.1 Schematic top view of the crossed molecular beam apparatus. The two pulsed beam source chambers and the detector (electron impact + quadrupole mass filter) rotating chamber are visible. In the case of the CN radical beam source, the carbon rod holder and the incident laser beam are also sketched. The chopper wheel and the cold shield are also shown. 

The major features of the crossed molecular beams apparatus used in these studies have been described elsewhere (21-22). However, several important modifications were made specifically for these studies. The major objectives were to reduce the velocity spread of the reactant beams in order to resolve the product vibrational states as distinct peaks in time of-flight measurements and to reduce the background of mass 20 in the detector, especially near the F atom beam. A scheoiatic top cross sectional view of the experimental arrangement is shown in Figure 2. 

Fig. 1. Schematic diagram of a crossed molecular beam apparatus. (1) primary beam oven. (2) velocity selector. (3) secondary beam oven. (4) velocity analyser for the secondary beam, (5) detector for measuring the differential cross section, (6) monitor detector. (7) detector for measuring the total cross section.

In 1951, Purcell, Pound and Ramsey [21] did some NMR experiments with inverted populations of the nuclear spin systems in LiF and noted that the spin systems were at negative absolute temperatures and that they were intrinsic amplifiers rather than absorbers. The first suggestions to use systems with inverted populations as practical amplifiers or oscillators were made independently in the early 1950 s by Townes [15,22], Weber [15] and Bassov and Prokhorov [15]. The first such amplifier was a molecular beam apparatus operating on the NH3 inversion states and built by Gordon, Zeiger and Townes [22] and was called a microwave amplifier by stimulated emission of radiation (MASER). 

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