LIAD-fs scheme for studies of ultrafast laser interactions with gas phase biomolecules

Abstract

Laser induced acoustic desorption (LIAD) has been used for the first time to study the parent ion production and fragmentation mechanisms of a biological molecule in an intense femtosecond (fs) laser field. The photoacoustic shock wave generated in the analyte substrate (thin Ta foil) has been simulated using the hydrodynamic HYADES code, and the full LIAD process has been experimentally characterised as a function of the desorption UV-laser pulse parameters. Observed neutral plumes of densities >10(9) cm(-3) which are free from solvent or matrix contamination demonstrate the suitability and potential of the source for studying ultrafast dynamics in the gas phase using fs laser pulses. Results obtained with phenylalanine show that through manipulation of fundamental femtosecond laser parameters (such as pulse length, intensity and wavelength), energy deposition within the molecule can be controlled to allow enhancement of parent ion production or generation of characteristic fragmentation patterns. In particular by reducing the pulse length to a timescale equivalent to the fastest vibrational periods in the molecule, we demonstrate how fragmentation of the molecule can be minimised whilst maintaining a high ionisation efficiency.

Ultrafast Charge Dynamics in an Amino Acid Induced by Attosecond Pulses

“…Phenylalanine was deposited on a 10-µm-thick stainless steel foil, mounted onto the repeller electrode of a time of flight (TOF) mass spectrometer [31]. To evaporate the sample, the reverse side of the foil was irradiated with a CW diode laser operating at a wavelength of 960 nm, with a spot diameter of 6 mm and power in the range 0.3 -0.4 W. The distance from the foil to the focal point of the ionizing laser pulses was approximately 3 mm.…”

Ultrafast non-radiative decay of gas-phase nucleosides

“…We have overcome these issues, however, through the use of a "soft" laser desorption method based on back-irradiation of thin foil targets coated with the molecular samples of interest [42] [43]. In the present study we have sought to remedy this situation by obtaining the first reported gas-phase measurements of ππ* excited state decay in all of the DNA nucleosides following UV excitation.…”

Attosecond Electron Spectroscopy in Molecules

“…These amino acids were chosen as model molecules for charge transfer because they each have a radical cation with two charge-acceptor sites at approximately the same binding energy located on the phenyl and amine groups, separated by two singly bonded carbon. Clean plumes of isolated, neutral molecules were produced by a laser induced acoustic desorption (LIAD) technique [46]: a UV desorption laser was focused on the uncoated side of a thin (thickness of 10 µm) tantalum foil, the acoustic wave produced by the UV laser volatilized the sample on the other side of the foil, thus creating a molecular plume in the source region of a Time of Flight (TOF) mass spectrometer. Compared to direct laser desorption methods, LIAD leads to less energy coupling into the molecules, thus giving the possibility to produce fragile neutral molecules with lower internal temperature [46].…”