Electron pulse fine structure

The first experimental measurements of the time dependence of the hydrated electron yield were due to Wolff et al. (1973) and Hunt et al. (1973). They used the stroboscopic pulse radiolysis (SPR) technique, which allowed them to interpret the yield during the interval (30-350 ps) between fine structures of the microwave pulse envelope (1-10 ns). These observations were quickly supported by the work of Jonah et al. (1973), who used the subharmonic pre-buncher technique to generate very short pulses of 50-ps duration. Allowing... 

In order to identify the spin multiplicity of the tris(carbene), field-swept two-dimensional electron spin transient nutation (2D-ESTN) spectroscopy was used. This technique is based on pulsed fourier transform (FT) EPR spectroscopic methods and is capable of elaborating straightforward information on electronic and environmental strucmres of high-spin species even in amorphous materials, information that conventional CW EPR cannot provide. The nutation spectra unequivocally demonstrated that the observed fine structure spectrum is due to a septet spin state. 

The absolute frequency of the fundamental IS — 2S transition in atomic hydrogen has now been measured to 1.8 parts in 1014, an improvement by a factor of 104 in the past twelve years. This improvement was made possible by a revolutionary new approach to optical frequency metrology with the regularly spaced frequency comb of a mode locked femto-second multiple pulsed laser broadened in a non-linear optical fiber. Optical frequency measurement and coherent mixing experiments have now superseded microwave determination of the 2S Lamb shift and have led to improved values of the fundamental constants, tests of the time variation of the fine structure constant, tests of cosmological variability of the electron-to-proton mass ratio and tests of QED by measurement of g — 2 for the electron and muon. 

Another aspect of pulse radiolysis which has been improved is the pulse duration. For most experiments of interest to the physical organic chemist the common machines with pulse durations of 10 7-10-5 s are quite satisfactory, though for certain reactions, such as those involving protonation, examination on a shorter time scale can be of value. Several accelerators which supply nanosecond pulses are currently in use, but they are employed mostly with microsecond detection systems. Work in the 10-12-10-1° s region has recently become possible by the stroboscopic technique utilizing the fine structure pulses from a linear accelerator (Bronskill et al., 1970). More recently, a system which produces a single pulse of 40 picoseconds has been constructed (Ramler et al., 1975) and utilized for the observation of hydrated electrons at very short times (Jonah et al., 1973). 

Pulse radiolysis systems capable of picosecond time resolution use the fine structure of the output from the electron linear accelerator. Electrons in the accelerating tube respond to positive or negative electric field of the radiofrequency, and they are eventually bunched at the correct phase of the radiofrequency. Thus the electron pulse contains a train of bunches or fine structures with their repetition rate being dependent on the frequency of the radiofrequency (350 ps for the S-band and 770 ps for the L-band). 

The rotational selection rule for electronic transitions between states at the same Hund s case (see Section 3.2.1) limit is A J = AN = 1,0. If either electronic state is not at a Hund s limiting case or if the limiting cases are not identical in the upper and lower electronic states, less restrictive rotational selection rules apply. However, the number of rotational eigenstate J or N components present in a (to) created by a typical spectroscopically realizable pluck (i.e., a single, non-chirped, not-saturating excitation pulse) is small, typically 2 or 3. The possibility that each of these rotational components is split by spin fine structure (see Section 3.4) is neglected in the following discussion. 

In order to achieve more accurate values for the ratio of electron mass to proton mass from which the fine-structure constant can be derived, a frequency comb in the infrared region was developed which can be used to measure vibrational-rotational transitions in HD+-ions. The frequency distance q between the modes of the comb, which equals the repetition rate of the femto-second pulses, is stabilized on the resonance frequency of a cryogenic ultra-stable sapphire resonator. For the measurement of the transitions in the HD+-ion diode lasers with external cavity and a grating for achieving single mode operation are used. Their frequency is stabilized onto the centre of the molecular transition and is then compared with the adjacent mode of the frequency comb. 

Collision-induced transitions between fine-structure components where the relative orientation of the electron spin with respect to the orbital angular momentum is changed have been studied in detail by laser-spectroscopic techniques [13.64]. One of the methods often used is sensitized fluorescence, where one of the fine-structure components is selectively excited and the fluorescence of the other component is observed as a function of pressure [13.65]. Either pulsed excitation and time-resolved detection is used [13.66] or the intensity ratio of the two fine-structure components is measured under cw excitation [13.67]. 

Figure 37 (a) Fast measurement cell 1,3 conductors 2 Teflon 4 spacer 5,6,12 ceramic disks 7 metal gasket 8 cell body 9,13 electrodes 10 thermo couple 11 guard ring 14 filling tube (Redrawn from Doldissen, W., Report B 137, Hahn-Meitner-Institut, Berlin, 1980.) (b) response of the cell filled with tetramethylsilane to the fine structure electron pulses of a linear accelerator. 

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