Modulated molecular beam methods

In analysing data from modulated molecular beam experiments, it is 

The complex convolution integral is reduced to a simple product and it is obvious that an excitation at a frequency f only results in a response at the same frequency. If, as in the case of gas—surface studies, the overall response is determined by a number of processes, then the one of interest may be extracted by deconvolution techniques, which are particularly simple in the frequency domain [53]. In particular, attenuation and phase shift of the signal produced by the flight time of molecules from the modulator to the detector and any non-ideal response of the detector may be taken into account [55]. 

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TMS, (8)]. Both are modulated molecular beam methods with phase sensitive detection, and they allow for accurate measurement of gaseous and condensible species concentrations. The basic differences between the KMS and TMS methods are, the upper pressure A... 

Cycled Feed. The qualitative interpretation of responses to steps and pulses is often possible, but the quantitative exploitation of the data requires the numerical integration of nonlinear differential equations incorporated into a program for the search for the best parameters. A sinusoidal variation of a feed component concentration around a steady state value can be analyzed by the well developed methods of linear analysis if the relative amplitudes of the responses are under about 0.1. The application of these ideas to a modulated molecular beam was developed by Jones et al. ( 7) in 1972. A number of simple sequences of linear steps produces frequency responses shown in Fig. 7 (7). Here e is the ratio of product to reactant amplitude, n is the sticking probability, w is the forcing frequency, and k is the desorption rate constant for the product. For the series process k- is the rate constant of the surface reaction, and for the branched process P is the fraction reacting through path 1 and desorbing with a rate constant k. This method has recently been applied to the decomposition of hydrazine on Ir(lll) by Merrill and Sawin (35). 

Neutral species are best sampled using molecular beam methods in which the neutral beam is modulated between the sampling orifice and the ionization chamber of the mass spectrometer and only the modulated component of the mass spectrometer output is recorded. This approach enables all neutral species, including radicals to be detected with a comparable sensitivity. If modulation techniques are not used, the sensitivity for detecting condensible or reactive species is much less than for non-condensible, non-reactive neutral molecules because of the much larger effective pumping speed for the former in the mass spectrometer chamber. However, the ease of installation of non-line-of-sight non-modulated... 

D. Padowitz, K. PeterUnz, and S. J. Sibener, New modulated molecular beam scattering methods for probing nonlinear and coverage-dependent reaction kinetics at surfaces, Langmuir, vol. 7, pp. 2566-2573, 1991. 

An elegant method of obtaining desorption parameters involves the use of molecular beams. Here, the crystal is maintained at the desorption temperature and the requirement for rapid heating is avoided. Two methods are used. If the beam is chopped, the decay rate of the desorbed particles can be directly monitored [217, 218]. Alternatively, the beam may be modulated and the phase shift between the input and output measured [219], These methods were originally used to determine lifetimes for species desorbing as ions, such as Ba+ and Cs+ from tungsten [217, 219, 220] and for metal atoms [221], but CO, 02 and H2 desorption have also been examined in this way [223—225]. 

The second major section (Section III), comprising the bulk of the chapter, pertains to the studies of IVR from this laboratory, studies utilizing either time- and frequency-resolved fluorescence or picosecond pump-probe methods. Specifically, the interest is to review (1) the theoretical picture of IVR as a quantum coherence effect that can be manifest in time-resolved fluorescence as quantum beat modulated decays, (2) the principal picosecond-beam experimental results on IVR and how they fit (or do not fit) the theoretical picture, (3) conclusions that emerge from the experimental results pertaining to the characteristics of IVR (e.g., time scales, coupling matrix elements, coupling selectivity), in a number of systems, and (4) experimental and theoretical work on the influence of molecular rotations in time-resolved studies of IVR. Finally, in Section IV we provide some concluding remarks. 

Laser isotope separation techniques Laser-based isotope enrichment techniques deploy selective photo-excitation principles to excite a particular isotope as an atom or molecule (Rao 2003). Each device consists of three parts the laser system, the optical system, and the separation module. These methods include the atomic vapor laser isotope separation (AVLIS) that uses a fine-tuned laser beam to selectively ionize vapors of atomic the molecular laser isotope separation (MLIS), and separation of isotopes by laser excitation (SD EX) that use a laser to selectively dissociate or excite molecules. 

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