Resonant beam apparatus

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... 

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... 

Various methods (1-1) have used to determine the dynamic mechanical properties of polymers. Many of the instruments described are well known and are widely used (torsional pendulum, rheovibron, vibrating reed, and Oberst beam ASTM D4065-82). Newer instruments like the torqued cylinder apparatus (4), resonant bar apparatus (5) and Polymer Laboratories Dynamic Mechanical Thermal Analyzer (6) are becoming more popular in recent times. 

Fig. 3. Schematic of the dual laser beam apparatus. The sample flows vertically as indicated by the vertical arrows. The photolyzing beam pumps the photochemistry, and the probe beam is utilized to obtain a resonance-enhanced Raman spectrum of some intermediate with an optimal concentration corresponding to At (inset). Details are described in the original work. (From Marcus and Lewis. )...

The possibilities of molecular beam spectroscopy can be enhanced by allowing for spectrally resolved fluorescence detection or for resonant two-photon ionization in combination with a mass spectrometer. Such a molecular beam apparatus is shown in Fig. 4.5. The photomultiplier PMl monitors the total fluorescence /r(A.l) as a function of the laser wavelength Xl (excitation spectrum. Sect. 1.3). Photomultiplier PM2 records the dispersed fluorescence spectrum excited at a fixed laser... 

Fig. 1. Schematic diagram of a triple resonance atomic beam apparatus. Between source S (a hot oven) and detector D, magnets A and B produce inhomogeneous deflecting fields, and act as polarizer and analyzer magnet C produces a homogeneous field in which magnetic resonance transitions occur at loops A, B and C. Resonance is detected by the deflection of atoms away from the detector D, if the gradients in magnets A and B are in opposite directions.

This thesis will be organized as follows The Chap. 2 describes the H atom Rydberg tagging time-of-flight crossed molecular beam apparatus and experimental methods in our laboratory the Chap. 3 shows the research of resonance phenomenon in the F -I- H2 reaction in the Chap. 4, the breakdown of the B-O approximation in the F - - H2 reaction is introduced. 

Vohralik and Millerl used a crossed molecular beam apparatus to study HF-HF collisions. Laser excitation was used to state select one beam, and the depletion of the excited state gave evidence of resonant rotational energy transfer. Using a kinetic model involving the reasonable assumption that exactly resonant R-R processes dominate the depletion process, they obtained cross sections for one- and two-quantum resonant R-R processes, HF(ji) + HF(j2) - HF(j2) + HF(ji). Their results are given in Table VI. 

Fig. 7.21. Schematic diagram of the droplet beam apparatus used by the group of the late Roger Miller at the University of North Carolina to study isomerization of HCN HF van der Waals complexes. The HCN and HF molecules are first picked up separately to form oriented van der Waals complexes inside the droplets. The CH or HF stretch vibrations are used to excite the complex and after a flight time of 175 ns to probe the isomers formed. The corresponding laser resonances are monitored by the increase or depletion of the droplet signal at the downstream bolometer detector. Adapted from Ref. 57.

Polyamide is a hydrophilic polymer and has three relaxations between 1(X)°C and — 180°C that are strongly affected by sorbed water. Figure 23 represents the damping and resonant frequency for four poly(hexamethylene adipamide) measured by Woodward et al. using a transverse beam apparatus [21], showing that the resonant frequency is related to the dynamic modulus. The dry sample (0% H2O) has an a-relaxation at 370°K, and the damping peak shifted to lower temperatures as the water content in the polymer was increased. When the polymer absorbs water, the / -relaxation appears at... 

These atoms will not be refocussed at the detector in this so-called flop-out arrangement of the polarizer and analyser fields. Consequently when the detector current is recorded as a function of oJq for a fixed value of B, a series of sharply defined minima will be observed. The width of these magnetic resonance lines is often as narrow as 300 Hz and permits very precise measurements of the spins and magnetic moments of both atoms and nuclei. We shall now discuss some aspects of the atomic beam apparatus in more detail before considering the application of this technique to hyperfine structure measurements in one-electron atoms. 

Ultrafast TRCD has also been measured in chemical systems by incoriDorating a PEM into the probe beam optics of a picosecond laser pump-probe absorjDtion apparatus [35]. The PEM resonant frequency is very low (1 kHz) in these experiments, compared with the characteristic frequencies of ultrafast processes and so does not interfere with the detection of ultrafast CD changes. 

---