Multiply Charged Ions from Aromatic Molecules Following Irradiation in Intense Laser Fields

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Abstract

It is well-known that aromatic molecules, when irradiated by UV laser beams in the nanosecond pulse width regime, yield exclusive parent ions at laser intensities of 107 W cm-2 (soft ionization). As the laser intensities increase up to 109 W cm-2, however, extensive fragmentation takes place such that small mass fragments dominate the spectra at the expense of parent signature ions. The reason for this is that the dissociative lifetimes are shorter than the laser pulse width and ladder-switching fragmentation takes place. With the development of high power femtosecond lasers, these dissociative lifetimes can often be bypassed. Presently, at laser intensities up to the order of 1015 W cm-2 with pulse widths as short as 50 fs at near-infrared (IR) wavelengths (790 nm), soft ionization again takes place. This has been seen using a technique known as femtosecond laser mass spectrometry (FLMS). Under such conditions, stable multiply charged parent and adjacent satellite ions are observed and the fragmentation is minimal. In this paper, these effects are described for the medium-mass aromatic molecules benzene, monodeuterated benzene, toluene, and naphthalene. Other studies have shown that for diatomic and triatomic ions, the absorption of many photons produces transient highly ionized parent species which subsequently fragment on a time scale of femtoseconds, leading to multicharged atomic species. This so-called “Coulomb explosion” model has proved effective in describing the fragmentation of small molecules but seems a less attractive model to explain the results which are presented here, at the above-mentioned beam intensities. In contrast, the polyatomic molecules studied presently using IR FLMS display atomic-like characteristics.

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