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

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.

Cluster-assistant generation of multiply charged atomic ions in nanosecond laser ionization of seeded methyl iodide beam

“…Under the assumption of point charges, the total Coulomb potential of the precursor can be estimated through [10,11] …”

“…The use of tunable dye lasers to produce resonantly enhanced multiphoton ionization (REMPI) provides a powerful method for investigating molecular energy levels [1][2][3][4][5][6]. On the other hand, the study of photoionization and fragmentation processes induced by ultra-intense lasers in atoms and molecules has been the subjects of recent researches [7][8][9][10][11][12][13][14][15][16][17]. The observation of highly charged atomic ions produced by intense laser beams has been widely reported [7][8][9][10][11][12][13][14].…”

Femtosecond laser photoionization time-of-flight mass spectrometry of nitro-aromatic explosives and explosives related compounds

“…In addition, at 795 nm and a power density of 3.5 ϫ 10 14 W/cm 2 , the presence of a competition between multiphoton ionization (MPI) and Coulomb explosion (CE) channels is revealed by peak shape analysis, and is thought to be operative under these conditions for all of the molecules investigated. [5][6][7][8][9][10][11][12], and polyatomic aromatic organic molecules [13][14][15][16][17][18][19][20][21][22][23], has been the subject of intense investigation. These studies have focused on the fundamental interaction of the laser pulse with the atomic or molecular system, and have generally found a number of operative ionization and dissociation mechanisms present, which are sensitive to the laser wavelength and power density.…”

Interaction Mechanism of Some Alkyl Iodides with Femtosecond Laser Pulses

“…It may be argued that the observed multiphoton processes are taking place in the spatial wings of the focused laser beam as we have proposed in some previous publications. 36,37 However, in the present set of experiments, a narrow slit (1 mm) was inserted before the ion optics in order to eliminate the contribution from the wings; in any case this dependence was not observed for 1-iodopropane and 1-iodobutane mass spectra recorded by the same experimental setup. …”