Multiphoton resonance ionization of molecules desorbed from surfaces by ion beams

ABSTRACT It is known that high molecular weight, thermally labile molecules can be desorbed intact using keY ion beams. This knowledgehas led to numerous applications of fast atom bombardment (FAB) and secondary ion mass spectrometry (SIMS) by mass spec-trometric detection of the desorbed ions. Here we show that these measurements can be significantly enhanced by using reso-nanceenhancedlaser ionization to softly ionize the neutral component ofthe desorbed flux. This experimental configuration canproduce sensitivity improvements of several orders of magnitude over SIMS while adding a certain degree of selectivity to theionization process itself. Examples of this perfonnance will be presented using a wide variety of molecules including aromatichydrocarbons, a number ofbiologically relevant compounds and organic polymer substrates. In some cases, detection limits inthe attomole range can be achieved. 1. INTRODUCTION The application ofenergetic ion bombardment to the analysis ofnonvolatile, thermally labile compounds is a rapidly expandingresearch field. Akey to this expansion began with the discovery thatthermally labile molecules can be desorbed directly from thesolid state using energetic particles (1,2). The ion bombardment approach offers several unique advantages when compared toother related methods. In particular, ion beam desorption, as opposed to laser or thermal desorption, is extremely efficient atselectively removing material from only the top layer of a solid (3). This characteristic means that the signals are specific to thesurface concentration of the analyte. The molecules can be ionized by collisions during the desorption event or subsequentlyusing electron impact, chemical ionization or laser excitation (4). The separation of the desorption event from the ionizationevent is important to improve sensitivity since most of the desorbed flux consists of neutral particles (5).Moreover,matrixionization effects are not nearly as severe when using postionization techniques (6).Our efforts have been focused on utilizing multiphoton resonance ionization (MPRI) spectroscopy to ionize particles desorbedfrom surfaces using keY ion beams (7). MPRI detection offers selective ionization using relatively low power pulsed lasers (8).The low power requirement allows the use of a large ionization volume. This aspect of the technique results in more efficientsampling than ionization schemes based on electron impact or focused lasers. Next, the pulsed nature of the radiation adds an-other dimension ofselectivity provided by the built—in availability ofa time-of—flight (TOF) mass spectrometer detector. Thesedetectors exhibit inherently very high transmission efficiency (9). Finally, MPRI can be an extremely efficient means of produc-ing ions. For the case ofatomic species, we have already demonstrated ionization efficiencies of several percent, yielding detec-tion limits ofjust a few hundreds atoms spread over a 1 cm2 surface (8).Special problems arise when attempting to utilize this technology for the detection of molecules on surfaces using ion beaminduced desorption. Computer simulations (10) as well as a limited number of experimental studies (1 1,12) suggest that themolecules desorb in a variety of highly excited vibrational and rotational states, with the largest population remaining in theground state electronic manifold. Two approaches have been successfully pursued in connection with other desorption schemeswhich will likely have relevance to the ion beam induced desorption technique. In one scheme, the molecules are first entrainedand cooled by a supersonicjet ofa noble gas (13). The advantage ofthis step is that the target molecules are cooled into a singlespectroscopic state. The MPRI process can then be highly selective and efficient. A disadvantage is that only a very smallfraction of the molecules can be trapped into thejet, dramatically reducing sensitivity. In a second approach, the molecules areionized directly by a laser tuned to a wavelength associated with an electronic transition of the molecule. Since there are a highdensity ofoccupied vibrational androtational states in the ground state manifold and since each ofthese states can connect with areal level in the excited state manifold, a degree of selectivity is lost. This simplifl&l scheme, however, offers major advantagesin sensitivity due to the increased sampling efficiency.2