Threshold ionization mass spectrometry

Using the same threshold ionization mass spectrometry setup, Perrin et al. [317] have measured the temporal decay of radical densities in a discharge afterglow. From these experiments the coefficient p for the radical SiH. has been determined to be 0.28, which is in agreement with already known results from other (indirect) experimental approaches [136,137,318]. For the Si2H5 radical is determined to be between 0.1 and 0.3. The coefficient p for atomic hydrogen on a-Si H lies between 0.4 and 1, and is thought to represent mainly surface recombination to H. ... 

Using threshold ionization mass spectrometry and in situ ellipsometry, Schroder and Bauer [555] have shown that the Si2H4 radical may well be the species responsible for deposition, rather than SiH3 as in PECVD. This larger and less mobile precursor is thought to be the cause of the observed differences in the deposition conditions required in HWCVD and PECVD to obtain device quality material. 

M. Shiratani, J. Jolly, H. Videlot, J. Perrin Surface reaction kinetics of CH3 in CH4 rf discharge studied by time-resolved threshold ionization mass spectrometry. Jpn. J. Appl. Phys. 36, 4752 (1997)... 

Childs et al. (1992) measured the CH3 density as a function of the fraction of CH4 in the input gas and found that the CHj radicals increased linearly from 0 in pure H2 to about 3 x 10 CH3 radicals/cm at 4% CH4 in the input feed gas and were nearly constant at 3 x lO CH3 radicals/cm as the CH4 fraction in the input gas increased from 4 to 7%. Diamond growth occurs only for CH4 fractions less than 2%, in the input gas. For CH4 fractions greater than 2%, other forms of carbon are grown rather than diamond films. It is clear that something other than low CH3 density causes the diamond growth to cease for input CH4 fractions above 2%. These results of Childs et al. (1992) on CH3 density were very similar to the results obtained by Hsu (1991) using near-threshold ionization mass spectrometry. 

Considerable work is directed at the study of H-atom recombination relevant to RF plasma discharges (Kae-Nune et al, 1996 Perrin et al, 1998). Use is made of temperature-programmed desorption (TPD) (Zangwill, 1988 Bruch et al, 1997) in conjunction with threshold ionization mass spectrometry. The surfaces studied... 

Stults, J. (1995). Matrix-assisted Laser-desorption Ionization Mass Spectrometry (MALDI-MS), Current Opinion in Structural Biology 5 691-698. The review contains a host of references for special topics, such as differentiation of sulfate and phosphate groups, MALDI with infrared lasers and succinic acid matrix, subfem-tomolar detection thresholds with a thin layer matrix, highly sensitive protein detection in the attomolar range, and high-resolution mass spectrometry. 

For molecules containing atoms of high electron affinity the photocurrent at the first threshold may be due to a dissociation into ions. This phenomenon was already demonstrated in the early 1930 s by Terenin and Popov28 for TIHal vapors, which split into T1+ + Hal - as a primary photoprocess. Such was the first instance of the application of mass spectrometry to the study of the photoionization of gases. A similar process has been later shown by Morrison et al.8 for Br2 and I2, the first threshold, corresponding to a pre-ionization, accompanied by the dissociation into Hal+ + Hal-. 

Synchrotron radiation of 115 to 170 nm has been used to dissociate SiH4 in a pulsed supersonic free jet, and the abundance of SiH2 was measured by quadrupole mass spectrometry using 11 V sub-ionization threshold electron-impact energy301. The possible detection of SiH2 in the outer envelope of a stellar object has been reported302. 

Another characterization procedure of the excited clusters can be obtained by ionization by a second photon and detected by mass spectrometry (processes VI and VII). Tuning this second photon, the first one being fixed on the - S0 transition of a given cluster, allows one to determine the ionization threshold of this cluster. The ionization potentials of AH and A- being significantly different, the ionization process A - HB + - A HB + + e will occur at lower energies than the AH B - AH+ - - B + e process. The two-photon ionization techniques can provide a mass selective way of detecting proton transfer in clusters. 

A combination of mass spectrometry studies and ab initio calculations was used to identify the products of dissociative electron ionization of selenophene <2005JMP796>. Cobalt-chalcogenophene ion complexes were generated in a mass spectrometer and subsequently irradiated to study their photodissociation thresholds and measure their bond energies <1997JMP475>. 

The ultraviolet photoelectron spectra of diatomic alkali halide molecules are reviewed and interpreted. Data for lithium halide dimers, 112X2> are presented and it is shown that the dimers have significantly larger ionization thresholds than the corresponding monomers. Some historical controversies regarding the presence of dimers and their ionization energies are clarified. Photoionization mass spectrometry is used to determine the adiabatic ionization potential of lithium chloride trimer, in order to probe the trend of I.P. with cluster size. The predictions of Hartree-Fock, Xa and ionic model calculations on this point are presented. 

Figure 4.2. Experimental set-up for kinetic studies of SiHj reactions by excimer laser photolysis in a tubular slow-flow reactor with time-resolved detection by molecular beam sampling, near-threshold ionization electron impact mass spectrometry...

MATI Mass-analyzed threshold ionization— the mass spectrometry of ions produced in coincidence with zero-kinetic energy (ZEKE) electrons. 

The majority of ionic species are formed by the removal (or the addition) of an electron from (or to) a stable atom or molecule. As a result, ionic species are highly reactive. Because the environment in which ionic species are created is often chemically complex, special techniques for the preparation and handling of such transients are required for reliable determination of ionization and appearance energies, energetic thresholds for chemical reactions, and unambiguous measurements of chemical reaction cross sections and rates. The general techniques of mass spectrometry form the basis for experimental methods that provide information on ion energetics and kinetics. 

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