"Soft" Molecular desorption for Molecular Dynamics

1. Introduction

Placing atoms or molecules into the gas phase, under vacuum, is a fundamental prerequisite to a multitude of important analytical and/or investigative techniques in diverse scientific fields. The most common method is to simply heat a volatile organic molecule[A], possessing a non-negligible vapour pressure, in the presence of some inert carrier gas. This gas is then allowed to expand into vacuum, carrying the target species with it. Often, the molecules are then allowed to pass into a second ultra-high vacuum[B] chamber through a smaller aperture to produce a  well collimated molecular beam. However, this method is not applicable to many large biologically relevant molecules which are non-volatile and simply decompose upon heating before releasing any significant numbers into the gas-phase.

Currently two main techniques exist for placing non-volatile molecules into the gas phase, electrospray ionisation (ESI[C]) and matrix assisted laser desorption/ionisation (MALDI[D]), both of which shared in the award of the 2002 Nobel prize in Chemistry. Despite the success of these methods, they both possess a number of drawbacks which limit their use for molecular dynamics studies. Both methods employ the use of solvents and/or small matrix molecules which are also carried into vacuum and hence contaminate any measured spectrum. ESI is generally only applicable to polar molecules which have the potential to undergo protonation/deprotonation during the solvation. Unwanted reactions with the matrix molecules are also possible with MALDI. Additionally, both methods produce significant numbers of charged products which precludes the possibility of studying the neutral form.

Figure 1: Illustration of the standard LIAD scheme. A pulsed laser is focused onto the back of a thin metal foil. Laser ablation generates an acoustic wave which travels through the foil and causes molecular desorption from a layer of target molecules deposited on the front surface.

The goal of this projects is to develop the potential of a new method of placing nonvolatile molecules into the gas phase, known as laser-induced acoustic desorption[F] [1,2] (LIAD). A schematic of the typical LIAD configuration is depicted in Fig. 1. In this configuration, the target species is deposited on the front surface of a thin metallic foil ( typically ~10-15 µm thick). The rear of the foil is then irradiated with nanosecond laser pulses, with peak intensities between 108-109 W/ cm2. This pulse initiates an acoustic wave which then causes molecules to desorb from the front surface. From the initial studies performed on this process, the mechanism for molecular desorption was assumed to occur by mechanical "shake-off" initiated by the arrival of the acoustic wave at the front surface [5]. Further investigations, e.g. by measurements of the velocity of the desorbed molecules, have indicated that this cannot be the primary mechanism of desorption[6,7,8]. Additionally, because of the large variety molecules which have been observed to survive the process with negligible decomposition, a purely thermal driven process has also been ruled out. Although a better understanding of the mechanics of the molecular desorption process is clearly required, and could lead to greater improvements in desorption efficiency, the generality and gentleness of the LIAD process make it a promising method of liberating molecules into the gas phase.

This project plays a key role within the overall research program of the group, which aims to understand the relationship between the structure of molecules and their photodynamical behaviour. A prominent case being to understand the mechanisms of ultraviolet "self-protection" at work in the DNA molecule. In this context, there have been a number of studies carried out on the DNA bases, adenine, thymine and cytosine, but almost no work on guanine, which is very difficult to place into the gas phase. An important outcome of this project will be the ability to perform the first extensive measurements on guanine, and other large nonvolatile molecules. Critically, this will enable a great leap in understanding the dynamics of a large molecules such as DNA, through a series of  measurements in which the DNA backbone is systematically built onto the base (see Fig. 2).

Figure 2: Molecular structure of some guanine based systems which will be accessible to study with the development of the LIAD sources in our laboratory.




2. Experimental Set up

This project being carried out in collaboration with Queens University Belfast (QUB[E]) who have demonstrated that LIAD is capable of producing plumes of neutral molecules with average densities approaching 1010 molecules/cm3[3]; sufficient for studying ultrafast dynamics in biological molecules[4]. Based on this approach, we are designing a new velocity map imaging spectrometer[9] (VMIS) incorporating a LIAD foil target into the repeller electrode(LIAD-VMIS). A cutaway drawing of our design is shown in Fig. 3. The laser-molecule interaction region is situated 2.5mm the foil, allowing us to sample the highest possible density of desorbed molecules. The design allows for rotation of the foil target to expose fresh areas of sample to the desorption laser. This feature is necessary to counteract the depletion of the desorption signal from a single that site, which typically occurs after ~1000 laser shots[3,7], and to allow for the long acquisition times required for time-resolved photoelectron spectroscopy (TRPES) studies. This novel design will enable the powerful technique of photofragment imaging[G], to be applied to a range of  previously unstudied species.

Figure 3: Section view of our velocity map imaging spectrometer (VMIS) design which incorporates a rotating LIAD foil target into the repeller electrode. The design has been guided by SIMION calculations to optimise the velocity resolution, while maintaining a small distance between the foil surface and the laser-molecule interaction region. To maintain the cylindrical symmetry of the electrostatic fields, the design tolerances are specified to ensure that the foil maintains a perpendicularity of ±3° with respect to the axis of the VMIS. The design is expected to be UHV compatible and will allow imaging of ions and electrons.

A key goal of the project will be to develop a molecular beam (MB) source based on the LIAD principle (LIAD-MB). A LIAD-MB source would greatly increase the flexibility and utility of LIAD as it could be coupled to any existing instrument/experiment designed to work with conventional molecular beams.  One possible scheme for such a source, based on a modified pulsed valve design is depicted in Fig. 4. Such a scheme could significantly cool the LIAD produced molecules via collisional cooling occurring during the expansion into vacuum[10]. Another alternative approach for a LIAD-MB source would be to employ a focusing aerodynamic lens [11]. While this method would not exhibit significant cooling, it could increase the transmission efficiency into the MB, by active collimation of the desorbed molecules.

Figure 4: Conceptual drawing of a LIAD based pulsed molecular beam source.


3. Further Reading

[1] V. Golovlev, et al., Int. J. Mass Spectr. and Ion Proc. 169-170, 69–78. (1997), doi:10.1016/S0168-1176(97)00209-7.
[2] A. M. Dow, et al., Eur. J. Mass Spectrometry, 18(2),77–92, (2012), doi: 10.1255/ejms.1162.
[3] C. Calvert, et al., Phys. Chem. Chem. Phys.,18(14),6289, (2012), doi: 10.1039/c2cp23840c.
[4] L. Belshaw, et al., J. Phys. Chem. Lett., 3 (24), 3751–3754, (2012), doi: 10.1021/jz3016028.
[5] B. Linder, U. Seydel, Anal. Chem.57 (4),895–899, (1985), doi: 10.1021/ac00281a027.
[6] A. V. Zinovev, et al., Anal. Chem. 79(21), 8232–41, (2007), doi: 10.1021/ac070584o.
[7] R. C. Shea, et al. Anal. Chem., 79 (7), 2688–2694, (2007), doi: 10.1021/ac061597p.
[8] A. V. Zinovev, et al., AIP Conf. Proc. 1104, 200 (2009), doi: 10.1063/1.3115603.
[9] A. T. J. B. Eppink & D. H. Parker, Rev. Sci. Instrum. 68, 3477 (1997), doi: 10.1063/1.1148310
[10] Atomic and Molecular  Beam  Methods  Vol.  1,  ed.  G.  Scholes, Oxford  University  Press,(1988). doi: 10.1016/0168-583X(90)90032-P
[11] P. Liu, et al., Aerosol Sd. Tech., 22: 293–313, (1995), doi: 10.1080/02786829408959748