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Ionisation and fragmentation dynamics of laser desorbed polycyclic aromatic hydrocarbons using femtosecond and nanosecond post-ionisation

Robson, L. and Tasker, A.D. and Ledingham, K.W.D. and McKenna, P. and McCanny, T. and Kosmidis, C. and Tzallas, P. and Jaroszynski, D.A. and Jones, D.R. (2002) Ionisation and fragmentation dynamics of laser desorbed polycyclic aromatic hydrocarbons using femtosecond and nanosecond post-ionisation. International Journal of Mass Spectrometry, 220 (1). pp. 69-85. ISSN 1387-3806

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Abstract

Nanosecond laser desorption/femtosecond laser mass spectrometry (LD/FLMS) incorporating a reflectron time-of-flight mass spectrometer has been used to study the ionisation/fragmentation of polycyclic aromatic hydrocarbons (PAHs) in intense laser fields (7.0×1014 to 9.3×1015 W cm−2). Pulses of 80 fs, 800 nm have been used to post-ionise the PAHs anthracene, tetracene and pentacene. For each molecule strong singly and doubly charged parent ions are observed accompanied by fragmentation. In addition, strong triply charged parent ions (M3+) are observed for anthracene and weaker M3+ signals for tetracene and pentacene are also observed. Nanosecond post-ionisation (266 nm, 16 ns) spectra of the molecules have been recorded and are included for comparison with the femtosecond data. Similarities in the observed fragmentation pattern of low-mass fragments of the nanosecond and low intensity femtosecond spectra are highlighted. In addition, as the laser intensity increases, it is observed that fragmentation pathways preferentially switch from CmH3+ ion yield to Cm+ production for m=2-5 at a critical intensity which is molecule dependent.