Helium photoionization

R. Moccia, P. Spizzo, Helium photoionization between the N = 2 and N = 3 thresholds including angular distribution and resonance properties A k-matrix L2 basis-set calculation, Phys. Rev. A 43 (1991) 2199. 

The pulsed discharge detector (PDD) is a universal and highly sensitive nonra-dioactive and non-destructive detector, also known as a helium photoionization detector. It is based on the principle of the photoionization by radiation... 

The FDD chromatograms show a great similarity to the classical FID detector and offers comparable performance without the use of a flame, radioactive emitter or combustible gases. The FDD in helium photoionization mode is an excellent replacement for FIDs in petrochemical or refinery environments, where the flame and use of hydrogen can be problematic. In addition, when the helium discharge gas is doped with a suitable noble gas, such as argon, krypton or xenon (depending on the desired cut-off point), the FDD can function as a specific photoionization detector for selective determination of aliphatics, aromatics, amines, as well as other species. 

The cluster reactor is attached to the pulsed cluster source s condensation channel, as shown in Figure 6. (16) To it is attached a high-pressure nozzle from which a helium/hydrocarbon mixture is pulsed into the reactor at a time selected with respect to the production and arrival of the clusters. The effect of turbulent mixing with the reactant pulse perturbs the beam, but clusters and reaction products which survive the travel from the source to the photoionization regime ( 600y sec) and the photoionization process are easily detected. 

The aromatic method uses as its primary analytical column a packed column of 5% SP-1200 + 1.75% Bentone 34 on Supelcoport (100-120 mesh). The carrier gas is helium at a flow rate of 30 mL/min. The temperature is programmed as follows (for lower boiling compounds) 50 °C isothermal for 2 min, then 6 °C/min, then 6 °C/min to 90 °C, and then held until all compounds have eluted (for a higher boiling range of compounds) 50 °C isothermal for 2 min, then 3 °C/min to 110 °C, and then held until all compounds have eluted. A photoionization detector with a 10.2-eV lamp is used for measurement. 

The apparatus has been already described in Ref. [7]. The clusters are produced by laser vaporization of a sodium rod, with helium at about 5 bars as a carrier gas and a small amount of SF6. The repetition rate is 10 Hz. In this configuration, the vibrationnal temperature of the formed clusters is roughly 400 K,[10] that gives 85% of C2V geometry and 15% of C3V for a Boltzman distribution. The laser beams are focused onto the cluster beam between the first two plates of an axial Wiley Mac-Laren Time-Of-Flight mass spectrometer with a reflectron. The photoionization efficiency curve as well as the photoabsorption spectrum determined by a photodepletion experiement are displayed on Fig. 1(b) and 1(c) respectively. The ionization threshold is at 4.3 eV, close to the 4.4 eV calculated for the C3V isomer and 4.9 eV for the C2V isomer (see the Fig. 1 (b)). The conclusion arising out of the photodepletion spectrum shown on Fig 1(c) and from ab initio calculations of the excited states, [5] is that the observed... 

Figure 19- Photoionization efficiency curves for production of H2+ and HeH+ from mixture of hydrogen and helium. Threshold energies for formation of Hj" in vibrationally excited states are indicated at top of figure.85 ...

The excitation profile for two-color 1+1 photoionization of 2,2 -bithiophene seeded in a supersonic helium expansion showed that its two-photon photoionization is significantly enhanced by resonance with the Si state, but the observed dynamics clearly show that the dominant channels for photoionization involve long-lived triplet state into which Si decays (94JPC4990). To fit the observed temporal profiles, sequential decay of Si through two triplet states must be invoked. 

XPS. As indicated in Figure 1, core-level B.E. s are characteristic of individual atoms and so one obtains an atomic identification directly from the determined B.E. value. In addition, the core-level photoionization cross-sections are reasonably well-established, both theoretically (13) and experimentally (14), and since the core-levels are atomic in nature, there are no significant variations with chemical environment of the atoms, which means that the atom composition analysis can be made quantitative. All elements which have core-levels, i.e., everything but hydrogen and helium can be detected, though the magnitude of the cross-sections and hence the relative sensitivities to the different elements varies by 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). 

J.-Z. Tang, I. Shimamura, Mechanism of the enhancement of some high-lying resonance series in the photoionization spectra of excited helium, Phys. Rev. A 50 (1994) 1321. 

A.S. Kheifets, I. Bray, Photoionization with excitation and double photoionization of the helium isoelectronic sequence, Phys. Rev. A 58 (1998) 4501. 

A.K. Kazansky, P. Selles, L. Malegat, Hyperspherical time-dependent method with semi-classical outgoing waves for double photoionization of helium, Phys. Rev. A 68 (2003) 052701. 

Section 6.2). An example of such a calibration measurement is shown in Fig. 1.18 where Is photoionization in helium with the energy-independent p value of p = 2 was selected. The angle-dependent intensity varies between a maximum and a minimum value. Within these limits, at a certain angle partial cross section. The above relation requires for this case that... 

Figure 4.37 Spectrum of electrons ejected from helium after photoionization with mono-chromatized Al Ka radiation. The main Is photoline and (magnified by a factor of 20) discrete (n = 2, 3,4) and continuous satellites (above the threshold indicated at 79 eV) are shown as well as structures resulting from the inelastic scattering of Is photoelectrons in the source volume. Reprinted from J. Electron Spectrosc. Relat. Phenom. 47, Svensson et al., 327 (1988) with kind permission from Elsevier Science - NL, Sara Burgerhartstraat 25, 1005 KV Amsterdam, The Netherlands.

Depending on the individual orbital angular momenta /a and tb involved, the summation can go up to infinity. Such a situation occurs for double photoionization in helium where the 1P° state of the continuum pair wavefunction can be obtained by an unlimited coupling of individual orbital momenta (esep, sped, edef,...). However, in the case of photon-induced two-step double ionization the formulation... 

Table 4.1. The lowest A-coefficients neededfor the description of double photoionization in helium according to equ. (4.68).

Starting in a manner similar to the treatment of single photoionization described in Section 2.1, double photoionization in helium caused by linearly polarized light will be treated first with uncorrelated wavefunctions. A calculation of the differential cross section for double photoionization then requires the evaluation... 

Figure 4.43 Energy- and angle-resolved triple-differential cross section for direct double photoionization in helium at 99 eV photon energy. The diagram shows the polar plot of relative intensity values for one electron (ea) kept at a fixed position while the angle of the coincident electron (eb) is varied. The data refer to electron emission in a plane perpendicular to the photon beam direction for partially linearly polarized light (Stokes parameter = 0.554) and for equal energy sharing of the excess energy, i.e., a = b = 10 eV. Experimental data are given by points with error bars, theoretical data by the solid curve.

The content of this angular factor is in agreement with two selection rules derived rigorously for vanishing intensity in helium double photoionization [MBr93] ... 

The matrix element Mfi derived so far for the differential cross section of double photoionization in helium is based on uncorrelated wavefunctions in the initial and final states. For simplicity the initial state will be left uncorrelated, but electron correlations in the final state will now be included. The significance of final state correlations can be inferred from Fig. 4.43 without these correlations an intensity... 

Starting from a different treatment of double photoionization in helium, based on properties of the wavefunctions in the threshold region, and special coordinates (hyperspherical coordinates) for the description of the correlated motion of the electrons, different predictions for this 0 parameter have been obtained (see [HSW91, KOs92] with references therein).)... 

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