Properties of matrix‐assisted laser desorption. Measurements with a time‐to‐digital converter

Some properties of matrix‐assisted laser desorptio have been studied using single‐ion‐counting methods and a time‐to‐digital converter. The methods allow examination of the process for irradiances near the reported threshold for observation with a transient recorder. All measurements were made using bovine insulin as a test compound. We presernt direct evidence that an irradiance threshold near 10 6 W cm −2 exists for ion production , and that the process in a collective effect, either involving a large number of molecular ions (∼10 4 )in a successful event or none at all. Above the threshold, the yield is found to scale with a high power (4th to 6th) of the irradiance. Measurements of initial velocity distributions indicate an axial velocity spread corresponding to ∼50 eV and a radial velocity spread corresponding to ∼2.4 eV. Thus the ejection or extraction mechanism appears to be strongly asymmetric.