Photofragmentation and electron detachment of aromatic phosphonate, sulfonate and phosphate oxyanions

Abstract

The photodetachment energy threshold, as well as vibrationally-resolved spectral signatures of the lower lying excited states and dipole bound states in model aromatic phosphonate, sulfonate and phosphate oxyanions have been investigated using a photofragmentation spectrometer equipped with a cold ion trap. The effect of the laser excitation was monitored by mass-selective detection of product ion fragments or, alternatively, measuring the yield of the complementary neutral radicals discriminated according to their kinetic energy. The anions phenylphosphate, phenylsulfonate and p-toluenesulfonate evidenced the expected behaviour, characterised by the predominance of ionic fragmentation processes, at low energies, rapidly evolving to a scenario controlled by the electron photodetachment channel at higher energies. Surprisingly for such a similar system, the phenylphosphonate anion does not have any ionic fragmentation channels and only exhibits the presence of dipole bound states.

Loss of CO2 from Monodeprotonated Phthalic Acid upon Photodissociation and Dissociative Electron Detachment

“…Moreover, it has been observed that electron detachment is a very efficient process when the photon energy is larger than the ADE. This has been observed for naphthoate, 10 as well as for aromatic phosphonate, sulfonate and phosphate oxyanions, 19 for which the differences between electron detachment and ionic fragmentation are quite easy to discriminate. For naphthoate, the radical fragments are involved in an impulsive mechanism leading to a multimodal TOF profile (Figure 1e), with an appearance rather different to that of a statistical profile, while for phosphonate and sulfonate the radicals are stable and photodetachment leads to a narrow peak, as in Figure 1b,c.…”

“…The same behavior has already been observed in other deprotonated aromatic acids such as sulfonic acids. 19 The spectrum is not well resolved, and only two bands are distinguished at 284.0 nm (4.37 eV) and 275.6 nm (4.50 eV). The gas phase spectrum of monodeprotonated phthalic acid appears slightly red-shifted from the corresponding aqueous solution spectrum at pH = 8, but displays the same absorption pattern.…”

“…We have found this situation by exciting deprotonated p-toluenesulfonic acid at 260 nm (Figure 1c). 19 Here, a statistical product energy distribution is observed at the base of the signal, accounting for a fragmentation process in the anion excited state, which is superimposed upon a narrow central component that results from a nondissociative electron detachment event. (d) Two squared functions convolved with the apparatus function, with no sign of a central narrow peak: such an observation implies that the kinetic (translational) energy released is split into two quasi-isotropic distributions (impulsive KER), which likely indicates the presence of a dissociative detachment process.…”

“…A detailed description of the apparatus is already available in former publications. [18][19][20] Photodissociation action spectroscopy is widely used to determine the photophysical properties of isolated molecular ions. [21][22][23][24][25][26][27] The action signal, usually originating from photodissociation or electron photodetachment events, can easily be assessed by collecting and measuring the yield of product particles as a function of the excitation energy.…”

“…The TOF distribution that characterizes the signal of uncharged particles is symmetrized by using the procedure explained in recent publications. 10,19,20 No smoothing algorithms were applied to the data, and the observed noise results principally from the source instability and the statistical uncertainty of the measure.…”