Schematic experimental setup for collecting the visible fluorescence signal either dispersed or undispersed, from the photofragments produced by synchrotron excitation of the molecules.
Absorption spectra [panels (a) and (b)] and fluorescence excitation spectra [panels (c) and (d)] of propyne and allene, respectively; in the insert of panels (c) and (d), an enlarged view of the fluorescence excitation signal is shown. All signals are in arbitrary units.
Comparison between the VUV absorption spectra below the first ionization potential for (a) acetylene (taken from Ref. 33) and (b) propyne (this work) and (c) ethylene (taken from Ref. 32) and (d) allene (this work).
Three dimensional representation of the dispersed visible fluorescence spectra of the photofragments produced by the photolysis of propyne [panel (a)] and allene [panel (b)] as a function of the excitation energy above IP. The main observed and assigned emission features are indicated. Note that below , the weak emission line originates from second order light effect (see the text).
Integrated fluorescence signals in arbitrary units for different emission bands of the photofragments as a function of the excitation energy. In propyne: (a) and lines and (b) and and in allene: (c) and lines and (d) and .
Schematic energy diagram showing the thresholds for formation in propyne and allene. The vertical arrows shows the observed apparition thresholds or sudden increases of the signal reported in Figs. 5(a) and 5(c). The three dissociation energies relevant in the explored region are labeled (5)–(7) referring also to Eqs. (5)–(7) in the text. Horizontal arrows indicate either dissociation or isomerization occurring without any barrier (see the text). The region of interest for is embedded in the ionization continua (displayed by the dashed areas) of both isomers. Neutral superexcited states and cation electronic states are located on the diagram from Ref. 11 (and references therein) for propyne and from Refs. 7 and 37 for allene. Dissociative ionization thresholds occur in the same region (see Ref. 10), but are not shown in this figure for the sake of clarity since they do not exhibit any correspondence with the evolution of the signal in Figs. 5(a) and 5(c).
Thermodynamic thresholds (in eV) determined from Ref. 34 for the different fragmentation channels of . The thresholds are given with a accuracy.
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