Microscopic Cluster Formation during the Laser Desorption of Chrysene-d 12 Steven M. Hankin and Phillip John* Department of Chemistry, Heriot-Watt UniVersity, Riccarton, Edinburgh EH14 4AS, U.K. ReceiVed: March 17, 1999 The laser ablation of chrysene-d 12 has been investigated by laser desorption/post-ionization time-of-flight mass spectrometry (L2ToFMS). The early stages of plume expansion were probed by locating the focus of the post-ionization laser to within 50 μm of the surface. A spatial and temporal study of the desorption plume was carried out by recording positive ion time-of-flight mass spectra as a function of delay time and position. When the ionization laser focus was moved to within 50 μm of the surface, the appearance of the chrysene- d 12 parent ion signals changed from being sharply defined with a mass resolution of ∼700 to broad signals of reduced intensity and a significantly lower resolution. Furthermore, the broad ionization signals were observed at longer delay times on increasing the desorption laser power. We attribute the observed phenomena to the laser desorption of molecular clusters, their transient survival, and ultimate evaporation to discrete molecules. Introduction A detailed understanding of the mechanism of high power pulsed laser desorption of molecules has not emerged despite a considerable number of theoretical 1-7 and experimental 8-18 studies. Extensive experimental studies of laser ablation plumes have been conducted using a variety of techniques including laser-induced fluorescence imaging, 19-21 post-ionization using X-rays 22 or focused lasers 9,11-13,15-18 coupled with time-of-flight mass spectrometry. Recent molecular dynamics studies 1-3 have predicted the presence of clusters as well as discrete molecules within the evolving plume. Conventionally, desorption plumes resulting from high power laser irradiation of solids have been imaged at distances extending to several centimeters 19-21 from the surface. However, these studies preclude probing the initial stages of plume evolution occurring close to the surface. In the present study, the ability to locate the post-ionization laser focus as close as 50 μm to the surface with a precision of (10 μm has allowed experiments to be conducted on monitoring collisional processes within the desorption plume. Two-laser time-of-flight mass spectrometry (L2ToFMS) has been em- ployed to probe the desorbed plume generated from the laser irradiation of the polyaromatic hydrocarbon chrysene-d 12 . We report on laser ionization mass spectral observations of phe- nomena which can be attributed to the presence of molecular clusters within the expanding laser generated plume. Experimental Section The construction of the L2ToF mass spectrometer has been described previously. 23,24 Alignment of the desorption and ionization lasers with respect to the surface of the sample was achieved by viewing the fluorescence from laser irradiation of a thin layer of chrysene-d 12 (98 atom % D purity; Aldrich, Gillingham, U.K.) using an optical microscope (Olympus, BH2- DO) attached to a color video camera (Hitachi, VK-C150ED). Movement of the samples on the x, y and z-axes of the rotary sample stage was achieved using precision translation microme- ters. The sample was translated relative to the fixed desorption laser. The ionization laser focus could be positioned >30 μm above the sample using the micrometer translation stage of the focusing lens. The focus was translated in the direction of the desorbing molecules (z-axis), parallel to the sample surface, to spatially profile the desorption plume of chrysene-d 12 neutral molecules. The area profiled extended to 300 μm from the point of desorption in the x-y plane and from 30 μm to 600 μm along the ion axis. The fourth harmonic output (266 nm, 10 ns pulse width, 1 Hz) of a Nd:YAG laser (Quanta-Ray DCR-11, Spectra- Physics) was used for desorption. Achromatic focusing of the desorption beam to a spatial resolution of 1-2 μm normal to the sample surface was accomplished using a Cassegrain reflecting objective lens (focal length ) 14 mm). The desorbed neutrals were ionized at 266 nm using a 10 ns pulse from a second Nd:YAG laser (Quanta-Ray DCR-11, Spectra-Physics; <10 μJ pulse -1 ). The beam was focused within the ablation plume using a fused silica UV-grade biconvex lens (focal length ) 150 mm, diameter ) 25 mm). The beam waist at the focus of the ionization laser was estimated to be ca. 15 μm. Synchronous firing of the two Nd:YAG lasers was controlled using a variable time delay unit (0-50 ms with a jitter of <20 ns). The chrysene-d 12 parent ion peak intensity at m/z 240 was measured as a function of the delay time between the desorption and ionization pulses. Spectra were collected after establishing signal uniformity for ∼50 single shots at 1 Hz. The post ionization signal was contingent on the presence of both laser pulses; removal of either eliminated the signal. The ions were mass separated (nominal mass resolution m/Δm ) 700 at m/z 240) using a 2 meter flight tube fitted with a reflectron. The ions were detected by a dual microchannel plate (Galileo 3025MA) detector capable of operation from 2 to 20 kV. The amplified signal was fed to a 175 MHz transient digitizer (LeCroy 9400A) and transferred to a PC for display and analysis. Chrysene-d 12 was used without further purification. The thin film of chrysene-d 12 , used for alignment of the desorption and ionization laser beams, was prepared by depositing 50 μL of a solution of chrysene-d 12 (8 × 10 -4 mol dm -3 , HPLC grade toluene) onto an aluminum stub and evaporating the solvent at room temperature. Samples for laser desorption studies were prepared from a suspension of chrysene-d 12 in 250 μL of HPLC * E-mail: p.john@hw.ac.uk. Fax: +44 (0)131 451 3180. 4566 J. Phys. Chem. B 1999, 103, 4566-4569 10.1021/jp990950a CCC: $18.00 © 1999 American Chemical Society Published on Web 05/13/1999