Ion cluster desorption from frozen NH3 induced by impact of fast multi-charged ions

Ion cluster desorption yields due to the impact of 252Cf fission fragments on condensed ammonia are determined. Identification and abundance of the ion desorbed species were provided by a time-of-flight mass spectrometer. Ammonia ice thin layers deposited on Au film were heated from 25 K to complete sublimation. The typical pressure during heating process was 10−8 mbar. Measurements in the 25-150 K temperature range show emission of (NH3)nNHm± ion clusters, where n varies from 0 to ∼30 and m varies from 0 to 3 (for negative ions) or to 5 (for positive ions). The distribution is described by the sum of two decaying exponentials when n increases, which is interpreted by the simultaneous occurrence of two cluster formation regimes: (i) recondensation of sublimated molecules from the projectile track and (ii) emission of pre-formed bulk fragments from the periphery of the impact site. Moreover, the cluster yields of positive and negative ion series are approximately proportional to each other, suggesting that the ion cluster distribution shape is dictated by the neutral molecules but absolute yield is controlled by the emission of the ion species. In the 125-140 K range, sublimation rate increases, the ice thickness decreases rapidly and all the mentioned cluster series tend to disappear. The series (HCN)nCN− then appears, originating in the clustering of residual hydrocarbon contaminants on the metal substrate surface. The residual gases are analyzed as a function of the NH3 ice target temperature and their partial pressure compared to the ion desorption yield.