Laser Desorption Jet-Cooling of Organic Molecules Cooling Characteristics and Detection Sensitivity

Laser desorption followed by jet-cooling allows wavelength-selective as well as massselective detection of molecules desorbed from a surface without fragmentation. The cooling characteristics and detection sensitivity of laser desorption jet-cooling of organic molecules are investigated. From the rotational contour of the electronic origin of the $1 ~-So transition of laserdesorbed anthracene, rotational cooling to 5-10 K is demonstrated. Vibrational cooling is studied for laser-desorbed diphenylamine, a molecule with low-energy vibrations, and a vibrational temperature below 15 K is found. The absolute detection sensitivity is determined for the perylene molecule. Using two-color (1 + 1) resonance enhanced multi-photon ionization (with a measured ionization efficiency of 0.25) for detection, it is found that one ion is produced in the detection region for every 2 x 105 perylene molecules evaporated from the desorption laser spot. A two-color (1 + I) REMPI spectrum (400 points) of perylene is recorded using only 30 picogram of material. PACS: 35.80s, 82.80Bi, 47.45Nd In the laser desorption jet-cooling technique, molecules originally adsorbed on a surface are descrrbed with a laser pulse close to the orifice of a pulsed jet expansion. The laser-desorbed molecules are entrained in the jet and their internal energy is cooled by multiple collisions in the expansion region. In the collision-free region of the expansion the molecular beam is probed with one or more ionization laser(s), and ions are massand wavelengthselectively detected [1-6]. Cooling is important both for simplifying the spectroscopy of the desorbed molecules [7] and for increasing the detection efficiency; one can take advantage of resonance-enhanced multi-photon ionization (REMPI) spectroscopy to achieve sensitive and selective detection [8]. Even when non-resonant ionization techniques are used to detect the entrained molecules, cooling has the advantage that it results in less fragmentation following ionization. When compared with the more commonly used laser desorption technique in which ionization of the desorbates is performed close to the surface, selectivity and ionization efficiency are higher with jet-cooling. The fraction of the desorbed molecules brought into the detection region, however, is expected to be smaller for the latter technique [9]. In this report our laser desorption jet-cooling apparatus is described in detail. Measurements are presented from which the degree of both rotational and vibrational cooling is deduced. The overall detection sensitivity, i.e. the number of molecules that need to be desorbed from the surface to be able to see one perylene ion in the detection region, is determined for perylene. The various factors that contribute to the overall detection sensitivity are discussed and, where possible, measured independently. In particular the ionization efficiency for jet-cooled perylene is measured independently. This enables us to give a molecule-independent apparatus constant for the overall detection efficiency.