Millisecond pulsed radio frequency glow discharge time of flight mass spectrometry: Temporal and spatial variations in molecular energetics

Abstract

The internal energy distributions, P(epsilon), of a millisecond pulsed radio frequency glow discharge plasma were investigated using tungsten hexcarbonyl W(CO)(6) as a "thermometer molecule". Vapor of the probe molecule, W(CO)(6), was introduced into the plasma and subjected to various ionization and excitation processes therein. The resultant molecular and fragment ions were monitored using a Time-of-Flight mass spectrometer. Ion abundance data were utilized in combination with the known energetics of W(CO)(6) to construct the P(epsilon) plots. The P(epsilon) of W(CO)(6) exhibited strong temporal dependence over the pulse cycle: Distinct internal energy distributions were found at the discharge breakdown period (prepeak), the steady state period (plateau), and the post-pulse period (afterpeak). Spatial variation in P(epsilon) was also observed, especially during the plateau regime. The observations suggest that this pulsed glow discharge affords excellent energy tunability that can be used to perform selective ionization and fragmentation for molecular, structural, and elemental information. Parametric studies were performed to evaluate the effects of discharge pressure and operating power on P(epsilon). These studies also provided insight into the correlation of the observed P(epsilon)s with the fundamental ionization and excitation mechanisms in the plasma. The temporal and spatial variations in P(epsilon) were hence attributed to changes in the dominant energy transfer processes at specific times in specific regions of the plasma. These data will be useful in future efforts to optimize the analytical performance of this source for chemical speciation.

Pulsed r.f.‐glow‐discharge time‐of‐flight mass spectrometry for fast surface and interface analysis of conductive and non‐conductive materials

“…This way, the plasma can be periodically switched on and off and accordingly sputter and ionisation processes will change from sputtering and hard ionisation (plasma on) to soft ionisation processes during the afterglow occurring right after switching off the power supply. 11 The possibility to operate the ToF-MS with high extraction frequencies together with the high speed data acquisition makes it possible to split a single r.f.-generator pulse into several ToF spectra. For example, with a ToF extraction frequency of 50 kHz and a pulse frequency of 1 kHz for the r.f.-generator, it is possible to record 50 spectra within a single GD pulse.…”

Volatile organic compound analysis by pulsed glow discharge time of flight mass spectrometry as a structural elucidation tool

“…Pulsed glow discharges, typically, generate three time regions along the pulse period (prepeak, plateau and afterglow) where the ionization mechanisms cited above have different importance. Electron ionization, added to the energy transferred to the molecules because of the breakdown potential applied during the prepeak, usually is strong enough to dissociate the compounds in their atoms . The plateau corresponds to the next time interval where the generated particles (ions, photons, etc.)…”

“…When the pulse ends (afterglow region), the recombination of electrons and argon ions leads to metastable argon atoms, which give rise to Penning ionization. The limited energies of metastable argon atoms (11.55 and 11.62 eV) are, in some cases, not enough to create a complete fragmented molecule, and, so, the ions typically generated are molecular ions …”

Investigation of the afterglow time regime in pulsed radiofrequency glow discharge time‐of‐flight mass spectrometry

“…It is commonly considered that processes like electron ionization and charge‐transfer (dominant mechanisms in the prepeak and plateau, respectively) disappear once the pulse has finished. On the contrary, Penning ionization grows up in this region because the population of excited metastable argon atoms is increased due to recombinations occurring between thermalized electrons and argon ions 8–11. A characteristic of argon Penning ionization is its selectivity because only species with ionization energy below 11.72 eV can be ionised through such mechanism.…”