Photoionization experiment

Our experimental techniques have been described extensively in earlier papers (2, 13). The gamma ray irradiations were carried out in a 50,000-curie source located at the bottom of a pool. The photoionization experiments were carried out by krypton and argon resonance lamps of high purity. The krypton resonance lamp was provided with a CaF2 window which transmits only the 1236 A. (10 e.v.) line while the radiation from the argon resonance lamp passed through a thin ( 0.3 mm.) LiF window. In the latter case, the resonance lines at 1067 and 1048 A. are transmitted. The intensity of 1048-A. line was about 75% of that of the 1067-A. line. The number of ions produced in both the radiolysis and photoionization experiments was determined by measuring the saturation current across two electrodes. In the radiolysis, the outer wall of a cylindrical stainless steel reaction vessel served as a cathode while a centrally located rod was used as anode. The photoionization apparatus was provided with two parallel plate nickel electrodes which were located at equal distances from the window of the resonance lamp. 

Detailed structural calculations have been carried out for this system. This is because the neutral isomer, C2HsO, which is implicated in the thermochemistry of ethanol, is of interest in pollution control, atmospheric chemistry, and combustion. Also, there is new information available from photoionization experiments with which to compare theoretical calculations. For details of these comparisons, see Curtiss et al.73 In the earlier theoretical studies of Nobes et al.,74 calculations were performed at the MP2 and MP3 levels with basis sets of double plus polarization (6-13G ) with electron correlation. These studies revealed four stable minima for the system protonated acetaldehyde, CHj-C H-OH <-> CH3-CH=0+H the methoxymethyl cation, CH3OCH2 protonated oxirane, (CH2)2OH+ and vinylox-... 

In photoionization experiments, there is evidence that the thermalization length increases with the photon energy. 

There are two alternative methods for determining internal-state distributions in single-source photoionization experiments, depending on the prevailing type of ionization process. 

Utilizing ionization efficiency curves to determine relative populations of vibrationally excited states (as in the photoionization experiments) is a quite valid procedure in view of the long radiative lifetime that characterizes vibrational transitions within an electronic state (several milliseconds). However, use of any ionization efficiency curve (electron impact, photon impact, or photoelectron spectroscopic) to obtain relative populations of electronically excited states requires great care. A more direct experimental determination using a procedure such as the attenuation method is to be preferred. If the latter is not feasible, accurate knowledge of the lifetimes of the states is necessary for calculation of the fraction that has decayed within the time scale of the experiment. Accurate Franck -Condon factors for the transitions from these radiating states to the various lower vibronic states are also required for calculation of the modified distribution of internal states relevant to the experiment.991 102... 

The HSCC equations have been solved for various Coulomb three-body processes, such as photoionization and photodetachment of two-electron systems and positronium negative ions [51, 105-111], electron or positron collisions [52, 112-115], ion-atom collisions [116-119], and muon-involving collision systems [103, 114, 120-125]. Figures 4.6, 4.7, 4.8, 4.9, and 4.10 are all due to HSCC calculations. Figure 4.12 illustrates the good agreement between the results of HSCC calculations [51] and the high-resolution photoionization experiment on helium [126]. See Ref. [127] for further detailed account of the comparison between the theory and experiment on QBSs of helium up to the threshold of He+(n = 9). 

In the last few years, first measurements have been performed determining the length of the attosecond soft X-ray bunches. Paul et al. measured 250 attosecond pulses in a two-color photoionization experiment [5]. Making use of the fact that in a train of soft X-ray bunches the frequency spectrum consists of odd multiples ( harmonics ) of the frequency of the driver laser, they studied the ionization of Ar atoms by simultaneous photo-absorption from the soft X-ray bunch and the fundamental color of the femtosecond laser. Measured... 

Goulet T, Jay-Gerin J-P. (1992) On the reactions of hydrated electrons with OH and HjO+. Analysis of photoionization experiments. J Chem Phys 96 5076-5087. 

On the basis of 193 nm photoionization experiments [45], the lifetime of the propene radical cation formed in the heterolysis step is <20 ns. As expected on the basis of the lower acidity of sulfonic acid compared with sulfuric, sulfonates are weaker leaving groups than sulfates, e.g. ... 

The lifetime of the radical cation is <20 ns, as deduced from 193 nm photoionization experiments of the cyclohexene in aqueous solution. 

The classical structures of propan-1-yhum ion (1-propyl cation) are not minima on the potential energy surface of the C3Hy manifold. They are some 20kcalmol above 4, as determined computationally. This result is in fair agreement with gas-phase photoionization experiments on the w-propyl radical by Schultz,and Dyke and co-workers.These structures are transition states involved in the hydrogen scrambling of 4 in superacidic media. ° ... 

E14.11 (b) F is formed when F2 loses an antibonding electron, so we would expect Fj" to have a shorter bond than F2, The difference in equilibrium bond length between the ground state (Fi) and excited state (F, + e ) of the photoionization experiment leads us to expect some vibrational excitation in the upper state. The vertical transition of the photoionization will leave the molecular ion with a stretched bond relative to its equilibrium bond length. A stretched bond means a vibrationally excited molecular ion, hence a stronger transition to a vibrationally excited state than to the vibrational ground state of the cation. 

Warneck has carried out an extensive series of experiments in which he used a pulsed nitrogen-spark light source to determine drift velocities, diffusion coefficients, and kinetic temperatures as well as reaction rates of ions in gases. These have been the only photoionization experiments in which a pulse technique has been effectively used in the study of ion-molecule reactions. The reactant ions were formed by repetitive light pulses of about 0.5-//sec average duration, which (in all cases except hydrogen) was sufficiently short to justify the approximation of instanta-... 

In the mid-1960s, the availability of tunable ultraviolet radiation laboratory sources led to the replacement of electron beams with photon sources. Rare gas resonance lamps producing continuum sources of radiation that could then be passed through monochromators allowed the field of photoionization mass spectrometry to develop. The determination of photoion yield as a function of ionization energy, the photoionization efficiency curve, led to determination of ionization potentials with accuracies exceeding those of electron impact methods. Modern photoionization experiments often utilize laser or synchrotron light sources with narrow bandwidths and may employ collimated molecular beam sources that reduce the effects of... 

We first report on a two-laser photoionization experiment of benzene where the stepwise excitation was accomplished by frequency doubled dye lasers. The first laser pumped the molecule to a selected vibronic level of its first excited siglet state from where it was ionized... 

The simplest signal to be measured in a time-resolved photoionization experiment is the total ion yield following the action of pump and probe pulses. It is given by the integral population of the ionization continua in the limit t —> oo. ... 

Photoionization experiments are conveniently performed with monochro-matized synchrotron radiation. However, if a high temporal resolution is required, harmonics provide unique possibihties, as recently demonstrated [9.269]. 

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