RADIATIVELY SELF-COOLED PENNING-TRAPPED ELECTRONS - A NEW WAY TO MAKEGAS-PHASE NEGATIVE-IONS FROM NEUTRALS OF LOW ELECTRON-AFFINITY

We present a new method for attachment of self-cooled electrons tb gas -phase neutrals of potentially low electron affinity. The cyclotron fr equency of electrons in a Penning trap operated at 3 T is sufficiently high that 2 eV electrons injected into the trap from a hot filament c ool themselves to room temperature (similar to 0.02 eV) in similar to 1 s by classical radiative emission. In fact, we show that radiative c ooling can be much more efficient than cooling by electron-neutral or electron-electron collisions, Such cold electrons have high cross-sect ion for attachment to gaseous neutrals of quite low electron affinity (< 0.5 eV). Because both electrons and negative ions can be trapped si multaneously in a Penning trap, the negative ions so formed may be det ected by PT-ICR mass spectrometry in the same Penning trap, Compared t o conventional chemical ionization, the present method for producing n egative ions requires no additional chemical reagent and dan be conduc ted at relatively low pressure by use of pulsed-gas introduction of th e sample, The method is demonstrated by successful electron attachment to polycyclic aromatic hydrocarbons with electron affinities as low a s 0.49 eV (anthracene) and 0.45 eV (pyrene).