Molecular beams beam attenuation

Details of the photoexcitation and dissociation of seeded supersonic molecular beams of I2 by 514.5 nm radiation from a CW Ar laser have been provided by Bernstein and co-workers.A number of measurements were made, including LIF, I2 beam attenuation, and I2 TOF distributions as functions of carrier-gas pressure and nozzle temperature. A value for the direct photodissociation cross-section for I2 was determined to be (2.4 0.5) x 10 cm from measurements of the laser-induced beam loss. Use of additional spectroscopic information enabled calculation of the fraction of molecules excited on the basis of a simple model, and comparisons of the degree of excitation for different beam conditions and beam/laser geometries were made. 

Moerkerken et al. (1970) applied an RF field to H 2-molecules of a molecular beam passing through a fairly conventional arrangement of A-, B- and C-fields. Molecules in a well-defined rotational state undergo a transition into a state with different Zeeman-effect when they pass through the C-field where the RF field is applied (see Fig. 2). This combination of deflecting fields and spectroscopic techniques permits the production of a beam of preferentially oriented non-polar molecules. The scattering chamber is also shown in Fig. 2 where the beam of selected molecules can be attenuated for determination of the total collision cross section. 

Figure 20.1 Schematic view of the beam attenuation of a molecular beam, with intensity Ja, in a gas cell with target B. Theintensityisgiven/A =...

From the foregoing discussion, it is evident that IR dichroic ratio of polypropylene is related to molecular orientation of polypropylene tape or fiber when the 1256 cm band is chosen, whereas crystalline orientation can be obtained using the 1220 cm band. The polypropylene tape orientation can be determined using IR transmission spectra. Tape mounting arrangement on a Perkin-Elmer 881 IR spectrophotometer is illustrated in Fig. 17. A microspecimen holder can be used for sample presentation that enables 2 or 3-mm wide single tape to be presented to the instrument. A Willis 4 X beam attenuator is used to minimize the loss of intensity in the transmissive mode. 

Because IR absorption measurements discussed in this book are all performed in optically extremely dilute samples, such as those occurring in molecular beams or tandem mass spectrometers, action spectroscopy is employed instead of direct absorption IR spectroscopy. In conventional direct absorption IR spectroscopy, the attenuation of the light beam transmitted through the sample is measured as a function of the frequency of the IR radiation. By measuring the IR laser beam intensity before and after the sample, the absorbance as a function of the frequency of the IR photons is obtained according to the Lambert-Beer law ... 

Since the intersection path of the laser beam and the molecular beam is typically less than 1 cm and the density of absorbing molecules is low 10l2/cm ) the relative attenuation of the laser intensity may be below 10 10 for molecular transitions with small absorption cross section. Sensitive detection schemes are therefore demanded to obtain a sufficiently good signal to noise ratio. 

Fluctuations in the dielectric properties near the interface lead to scattering of the EW as well as changes in the intensity of the internally reflected wave. Changes in optical absorption can be detected in the internally reflected beam and lead to the well-known technique of attenuated total reflectance spectroscopy (ATR). Changes in the real part of the dielectric function lead to scattering, which is the main topic of this review. Polarization of the incident beam is important. For s polarization (electric field vector perpendicular to the plane defined by the incident and reflected beams or parallel to the interface), there is no electric held component normal to the interface, and the electric field is continuous across the interface. For p polarization (electric field vector parallel to the plane defined by the incident and reflected beams), there is a finite electric field component normal to the interface. In macroscopic electrodynamics this normal component is discontinuous across the interface, and the discontinuity is related to the induced surface charge at the interface. Such discontinuity is unphysical on the molecular scale [4], and the macroscopic formalism may have to be re-examined if it is applied to molecules within a few A of the interface. 

Every molecular species is capable of absorbing its own characteristic frequencies of electromagnetic radiation, as described in Figure 24-5. This process transfers energy to the molecule and results in a decrease in the intensity of the incident electromagnetic radiation. Absorption of the radiation thus attenuates the beam in accordance with the absorption law described later. 

Infrared spectroscopy (IR) is a fairly simple in situ method. Since the absorption coefficients of molecular vibrations are rather low, it is impossible to detect the IR absorption of a molecule adsorbed or bonded to the semiconductor surface, merely by an ordinary vertical transmission measurement. This problem was solved by using attenuated total reflection (ATR) spectroscopy, as introduced by Harrick [17], and first applied to semiconductor-liquid junctions by Beckmann [18,19]. In this technique, the incident IR light beam is introduced via a prism into a semiconductor, at such an angle that total internal reflection occurs at the semiconductor-liquid interface, as illustrated... 

The El source precedes the FD source and is used as a collision cell. The gas has a major role in collisions collision gas pressure required to attenuate the main beam by 2/3 varies as a function of the products, their molecular weights and the type of gas chosen. 

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