Helium resonance radiation

A common result of all the experiments is that most molecules quench the alkali resonance radiation very effectively with total cross sections ranging from 10 A2 to over 200 A2. However, if the molecule BC is replaced by a rare-gas atom, the quenching cross sections become very small at thermal energies. They are probably below 10 2 A2 for quenching by helium, neon, argon, krypton, and xenon.55 The latter result is easily understood in terms of Massey s adiabatic criterion.67 If Ar is a characteristic interaction range, v the impact velocity, and AE the energy difference between initial and final electronic states E(3p) and E(3s), respectively, then we must have a Massey parameter... 

A very interesting technique has been used by Walker and Back124 in which the photolyses of methane, ethylene, and ethane have been carried out in a windowless system at 584 A., the helium resonance line. Since this is a study which has crossed the line into the realm of radiation chemistry, no discussion of the work will be presented. It does, however, represent a radiation chemical study using monochromatic ionizing radiation and it deserves, therefore, the attention of interested researchers. 

The helium atom is a classically chaotic system. In Chapter 10 we saw that chaos chooses the way of overlapping resonance series to make its presence felt. The existence of overlapping series in the helium atom was confirmed experimentaUy by Domke et al. (1995). Using synchrotron radiation, helium resonances up to the N = 9 threshold were investigated. At the same time good agreement with theoretical complex rota-... 

In low-energy PES, usually termed ultraviolet PES or UPS, far-UV radiation is used for excitation. The most common source for UPS is the helium resonance lamp, which provides radiation with an energy of 21.2 eV and a half-width down to 10 meV (see Section III for further... 

Double-resonance spectroscopy involves the use of two different sources of radiation. In the context of EPR, these usually are a microwave and a radiowave or (less common) a microwave and another microwave. The two combinations were originally called ENDOR (electron nuclear double resonance) and ELDOR (electron electron double resonance), but the development of many variations on this theme has led to a wide spectrum of derived techniques and associated acronyms, such as ESEEM (electron spin echo envelope modulation), which is a pulsed variant of ENDOR, or DEER (double electron electron spin resonance), which is a pulsed variant of ELDOR. The basic principle involves the saturation (partially or wholly) of an EPR absorption and the subsequent transfer of spin energy to a different absorption by means of the second radiation, leading to the detection of the difference signal. The requirement of saturability implies operation at close to liquid helium, or even lower, temperatures, which, combined with long experimentation times, produces a... 

We locate the EBIT source and calibration source inside the Rowland circle by design. Bragg diffraction angles of calibration lines are in the range 29-45° while the helium-like resonances are observed around 39°. The plane of crystal dispersion is parallel to the electron beam axis. The crystal acts as a polarizer at Bragg angles near 45° and radiation polarised perpendicular to the electron beam axis is the dominant diffracted component. 

The first stage was the production of a pulsed free-jet molecular beam of helium containing 20% CO, which was then crossed with an electron beam to produce ionisation. The ions were produced close enough to the beam nozzle for cooling to occur downstream. Some 4 cm from the nozzle the beam entered a confocal Fabry-Perot cavity where it was exposed to millimetre wave radiation close to 120 GHz in frequency. Following microwave excitation, when on resonance, the beam was probed with a Nd YAG pumped dye laser beam with the frequency chosen to drive rovibronic components of the A 2 U-X 2 + band system. Figure 11.54 shows two recordings of a spin component of the lowest rotational transition the line shown in (a) is... 

Ward ER, Hedlund LW, Kurylo WC, Wheeler CT, Gofer GP, Dewhirst MW, Marks LB, Vujaskovic Z. Proton and hyperpolarized helium magnetic resonance imaging of radiation-induced lung injury in rats. Int J Radiat Oncol Biol Phys 2004 58(5) 1562-1569. 

The same system has been investigated by Moller and coworkers in helium droplets. At wavelengths above 100 nm (12.41 eV), which is below the ionization energy threshold for H2O at 98.4nm (12.61 eV), the fluorescence excitation spectrum is remarkably similar to that observed for the free molecules (See Fig. 7.11). In this experiment the well known Rydberg transitions of H2O were excited and the emission of OH (channel (1)) at 315 nm was recorded. No significant emission was observed either from resonant VUV emission or of the continuous visible radiation expected if the... 

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