Laser-Driven Acoustic Desorption of Organic Molecules from Back-Irradiated Solid Foils

Abstract

Laser-induced acoustic desorption (LIAD) from thin metal foils is a promising technique for gentle and efficient volatilization of intact organic molecules from surfaces of solid substrates. Using the single-photon ionization method combined with time-of-flight mass spectrometry, we have examined the neutral component of the desorbed flux in LIAD and compared it to that from direct laser desorption. These basic studies of LIAD, conducted for molecules of various organic dyes (rhodamine B, fluorescein, anthracene, coumarin, BBQ), have demonstrated detection of intact parent molecules of the analyte even at its surface concentrations corresponding to a submonolayer coating. In some cases (rhodamine B, fluorescein, BBQ), the parent molecular ion peak was accompanied by a few fragmentation peaks of comparable intensity, whereas for others, only peaks corresponding to intact parent molecules were detected. At all measured desorbing laser intensities (from 100 to 500 MW/cm2), the total amount of desorbed parent molecules depended exponentially on the laser intensity. Translational velocities of the desorbed intact molecules, determined for the first time in this work, were of the order of hundreds of meters per second, less than what has been observed in our experiments for direct laser desorption, but substantially greater than the possible perpendicular velocity of the substrate foil surface due to laser-generated acoustic waves. Moreover, these velocities did not depend on the desorbing laser intensity, which implies the presence of a more sophisticated mechanism of energy transfer than direct mechanical or thermal coupling between the laser pulse and the adsorbed molecules. Also, the total flux of desorbed intact molecules as a function of the total number of desorbing laser pulses, striking the same point on the target, decayed following a power law rather than an exponential function, as would have been predicted by the shake-off model. To summarize, the results of our experiments indicate that the LIAD phenomenon cannot be described in terms of simple mechanical shake-off or direct laser desorption. Rather, they suggest that multistep energy-transfer processes are involved. Two possible (and not mutually exclusive) qualitative mechanisms of LIAD that are based on formation of nonequilibrium energy states in the adsorbate-substrate system are proposed and discussed.

Laser-Induced Acoustic Desorption Atmospheric Pressure Photoionization via VUV-Generating Microplasmas

“…LIAD is accomplished by irradiating the backside of a metal foil~5-20 μm thick with a nanosecond laser pulse of power density~10 8 W/cm 2 . After absorption of this pulse, molecular species are ejected from the opposite side of the foil with relatively little energy transferred to the desorbed species [11,12]. Since LIAD is known to generate mostly neutral and intact molecules, this technique is well suited for softly producing pulses of low-polarity thermally-labile gas-phase molecules [11,13,14].…”

“…Initial studies describing the LIAD process suggested an acoustic wave mechanism for volatilization [15]. However, subsequent studies suggest a more complex mechanism where laser-induced stresses in the foil cause cracks in deposited molecular crystals and repulsive non-equilibrium electronic surface states, thus creating desorption sites from which analytes are ejected [12,16]. Though now considered to be misleading, we continue to use the term LIAD here due to familiarity.…”

Interplay Between Mechanochemistry and Sonochemistry

“…Atomistic explanations remain largely obscure in view of the frequency mismatch between SAWs and the vibrational states of the adsorbed species. The acoustic enhancement of surface diffusion has recently been suggested [38], which might also account for well-known cases of surface catalysis [39,40] and molecular desorption [41,42]. The experimental observation of ultrasonically-induced birefringence is also relevant [43,44].…”

Molecular Desorption by Laser–Driven Acoustic Waves: Analytical Applications and Physical Mechanisms

“…The LIAD volatilization method was successfully coupled with Fourier transform ion cyclotron resonance mass spectrometer and alanylglycine (Reid, Tichy et al 2001), saturated hydrocarbons (Campbell, Crawford et al 2004), polyethylene (Campbell, Fiddler et al 2005) and even petroleum distillates were analyzed. The great advantages of this technique were the ability to efficiently volatilize various organics and the simplicity of its incorporation into different classes of MS instruments, such as Linear Quadrupole Ion Trap MS (Habicht, Amundson et al 2010) and Time-ofFlight MS (Zinovev, Veryovkin et al 2007). Various approaches for the ionization of the desorbed molecules were successfully employed, among them: electron impact and chemical ionization , single-photon ionization (SPI) (Zinovev, Veryovkin et al 2007), as well as Elecro-Spray Ionization (ESI) (Cheng, Huang et al 2009).…”