Verification of long-lived molecular hydrogen anions (Hn-, Dn-, n=2,3) by secondary-ion mass spectrometry

The existence of long-lived (metastable) molecular hydrogen anions H-2(-), D-2(-), H-3(-), and D-3(-) is demonstrated. These anion species were produced by sputtering of TiD2 or HfH2 targets with Cs+ ions and were identified by secondary-ion mass spectrometry (SIMS). From the respective flight times through the spectrometer, minimum lifetimes for H-2(-) and D-2(-) of a few microseconds can be inferred. Correspondingly longer lifetimes can be expected for the triatomic hydrogen molecules. From the SIMS intensities, the following apparent abundance ratios are derived: H-2(-)/H-=3.5x10(-6), H-3(-)/H-=1.7x10(-8), D-2(-)/D-=3.1x10(-5), and D-3(-)/D-=7x10(-9). The kinetic energy spectra of D-2(-) and D- demonstrate that these hydrogen anions are indeed formed by sputtering at the sample surface. According to recent theoretical calculations it is proposed that in the emission event molecular hydrogen species with high rotational and vibrational excitation are formed that are stable on the time scale of the experiment.