12308 Phys. Chem. Chem. Phys., 2013, 15, 12308--12313 This journal is c the Owner Societies 2013 Cite this: Phys. Chem. Chem. Phys., 2013, 15, 12308 Comparison of high-order harmonic generation in uracil and thymine ablation plumes Christopher Hutchison, a Rashid A. Ganeev, ab Marta Castillejo, c Ignacio Lopez-Quintas, c Amelle Zaı ¨r, a Se ´bastien J. Weber, ad Felicity McGrath, a Zara Abdelrahman, a Malte Oppermann, a Margarita Martin, c Dang Yuan Lei, ae Stefan A. Maier, a John W. G. Tisch a and Jonathan P. Marangos* a We present studies of high-order harmonic generation (HHG) in laser ablation plumes of the ribonucleic acid nucleobase uracil and its deoxyribonucleic acid counterpart thymine. Harmonics were generated using 780 nm, 30 fs and 1300 nm, 40 fs radiation upon ablation with 1064 nm, 10 ns or 780 nm, 160 ps pulses. Strong HHG signals were observed from uracil plumes with harmonics emitted with photon energies >55 eV. Results obtained in uracil plumes were compared with those from thymine, which did not yield signs of harmonic generation. The ablation plumes of the two compounds were examined by collection of the ablation debris on a silicon substrate placed in close proximity to the target and by time-of-flight mass spectrometry. From this evidence we conclude that the differences in HHG signal are due to the different fragmentation dynamics of the molecules in the plasma plume. These studies constitute the first attempt to analyse differences in structural properties of complex molecules through plasma ablation-induced HHG spectroscopy. 1 Introduction The interaction of atoms with intense ultrashort laser pulses can lead to high-order harmonic generation (HHG) resulting in the emission of coherent extreme ultraviolet radiation. 1 HHG is often described by the semi-classical three step model. 2,3 In this picture the strong electric field of the laser pulse causes an electron wave packet initially bond to the molecule to quantum tunnel out of the bound state and be driven away from its parent ion by the strong laser field. Once the laser field reverses, the electron is slowed down to a stop and then reaccelerated in the opposite direction towards the parent ion. There is a possibility that the electron will recombine with the host and return to the initial electronic state, resulting in the emission of the coherent short wavelength photons. The energy of these photons will be equal to the sum of the kinetic energy gained during reacceleration and the ionisa- tion potential of the state from which it tunnelled out. Over the past two decades, HHG has proved to be a powerful tool for the generation of short bursts of coherent X-rays including attosecond pulses, and for studies of ultra-fast atomic and molecular dynamics. 4,5 Further advances in HHG have come through the use of weakly ionised plasma plumes as the generating media, instead of the traditional atomic or molecular gases. The use of plasma plumes has increased the number of materials available for study and has led to the discovery of new features of high-order harmonic spectroscopy. Examples include electronic resonance induced enhancement of individual harmonics 6 and HHG from ions leading to the appearance of a second plateau in the harmonic spectra. 7 Previous studies of laser plasma HHG have been mostly confined to ablation targets constituted by a single element, such as graphite or silver, or by two-element targets, for example gallium arsenide. 8 The extension of the technique to more complex species, including biological molecules, is of high interest, as these studies may help unravel some of the ultrafast electronic dynamics that take part in processes such as deoxyribonucleic acid (DNA) damage. To approach the application of laser plasma HHG spectroscopy 9 to large molecules, we have examined in this work the ribonucleic acid (RNA) base uracil and have established a comparison with its DNA counterpart, thymine. Uracil and thymine have been previously studied using laser ablation to produce molecular beams 10–12 in Fourier transform microwave a Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ, UK. E-mail: j.marangos@imperial.ac.uk b Voronezh State University, Voronezh 394006, Russia c Instituto de Quı ´mica Fı ´sica Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain d CEA-Saclay, IRAMIS, Service des Photons, Atomes et Molecules, 91191 Gif-sur-Yvette, France e Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China Received 14th March 2013, Accepted 31st May 2013 DOI: 10.1039/c3cp00004d www.rsc.org/pccp PCCP PAPER Published on 31 May 2013. Downloaded by CEA Saclay on 04/12/2013 14:57:35. View Article Online View Journal | View Issue