Determination of Autoionization Lifetimes by Ion Cyclotron Reonances Linewidths

Abstract

The half-width and half-height for power absorption in the ICR spectrometer is shown to depend upon the time during which an ion absorbs power in the detection region of the instrument. In the case of negative ions, it is shown that autoionization can limit resolution, and conversely that the observed resolution for a given negative ion can be used to determine autoionization lifetimes. The SF6-ion is observed in the ICR spectrometer, and a lifetime of ~500 JLsec is calculated for this ion. The C,F 8-ion is observed, and a lifetime of ~200 JLsec is calculated for that ion. The lifetimes are discussed with respect to other measured and calculated values, which show a wide variation, and it is suggested that the autoionization lifetimes may be strongly dependent on the energy of the impacting electrons. In this study, both ions are shown to be produced by electrons which are trapped in the ICR cell as opposed to being produced by beam electrons.

Measurement of the autodetachment lifetime of SF6−* as a function of electron energy in a free jet expansion

“…Any of the SF 6 Ϫ* states surviving transit down the drift tube are registered as signal. In addition, any of these states which detach during transit down the drift tube will yield fast SF 6 neutrals which will also lead to signals on the microchannel plate detector.…”

“…The ion signal is amplified ͑Philips Scientific Model 6039͒, acquired by a 125 MHz digitizing oscilloscope ͑LeCroy Model 9400͒ and averaged at least 200 times. By measurement of the total ion signal versus time spent in the free jet, it is possible to determine the lifetime of the short-lived SF 6 Ϫ* states. The precise techniques for doing this will be discussed in the following section.…”

“…Values reported with time-of-flight experiments [1][2][3][4][5] are typically several tens of microseconds while the order of magnitude given by ion cyclotron resonance methods 6-9 is milliseconds or more. Using the technique of ion cyclotron resonance, Odom et al 7 found that the autodetachment lifetime of SF 6 Ϫ* varies as a function of observation time and surmized that SF 6 Ϫ* is generated in multiple states which autodetach with different rates, i.e., with different lifetimes. This hypothesis also explains the wide range of reported lifetimes.…”

“…The variation of the temperature of neutrals prior to the electron capture is now established as being one way to control the distribution of these multiple states. In the present paper, we report the determination of the lifetime of SF 6 Ϫ* as a function of electron energy in a very low-temperature (T trans Ͻ10 K) and low-pressure environment obtained with a free-jet expansion. As will be shown, with cooled SF 6 targets lying in their vibrational ground state, we observe SF 6…”

Electron ionization time‐of‐flight mass spectrometry: Historical review and current applications

“…Likewise, Foster and Beauchamp (1975) rationalized their observation of stable SF 6 À in an ICR mass spectrometer to radiative stabilization. Henis and Mabie (1970) also used an ICR to measure a lifetime of $300 msec for SF 6 À * formed by slow electron attachment. In support of the increase of lifetime for a packet of ions due to radiative decay it should be noted that emission of one IR quantum of radiation of SF 6 À * ions made by electron attachment to a ground state SF 6 0 will be stable.…”