Time-resolved spontaneous and laser-induced unimolecular fragmentation of perylene cations (C 20 H 12 + ) has been measured on timescales up to 2 s in a cryogenic electrostatic ion beam storage ring. We elaborate a quantitative model, which includes fragmentation in competition with radiative cooling via both vibrational and electronic (recurrent fluorescence) de-excitation. Excellent agreement with experimental results is found when sequential fragmentation of daughter ions co-stored with the parent perylene ions is included in the model. Based on the comparison of the model to experiment, we constrain the oscillator strength of the D 1 → D 0 emissive electronic transition in perylene (f RF = 0.055 ± 0.011), as well as the absolute absorption cross section of the D5 ← D 0 excitation transition (σ abs > 670 Mb). The former transition is responsible for the laser-induced and recurrent fluorescence of perylene, and the latter is the most prominent in the absorption spectrum. The vibrational cooling rate is found to be consistent with the simple harmonic cascade approximation. Quantitative experimental benchmarks of unimolecular processes in polycyclic aromatic hydrocarbon ions like perylene are important for refining astrochemical models.
Inverse internal conversion followed by recurrent fluorescence was observed as a fast decay (10 μs range) in the time profile of neutral yields from photo-excited C4(-) molecular ions. We also elucidated the contribution of such electronic radiative cooling to the C4(-) ions with internal energy far below the detachment threshold by an alternative novel approach, observing the laser wavelength and storage time dependence (ms range) of the total yield of the photo-induced neutrals.
The radiative cooling of highly excited carbon cluster cations of sizes N = 8, 10, 13-16 has been studied in an electrostatic storage ring. The cooling rate constants vary with cluster size from a maximum at N = 8 of 2.6 Â 10 4 s À1 and a minimum at N = 13 of 4.4 Â 10 3 s À1 . The high rates indicate that photon emission takes place from electronically excited ions, providing a strong stabilizing cooling of the molecules.
SynopsisThe potentiometric titration of poly(L-glutamic acid) was performed under conditions of varied ionic strength and concentration of added divalent cations. From these titration curves, the amount of divalent cations, especially magnesium, bound to poly(Lglutamic acid) was determined using a new method of analysis based on polyelectrolyte t,heory. By comparison with the polyelectrolyte, poly(acry1ic acid), it was found that there are no specific interactions between metal ion and poly(L-glutamic acid) in either the helical or random coil conformation. The effect of these divalent cations on the conformation of poly( L-glutamic acid) was also discussed.
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