Emission spectra of alkyl substituted diacetylene radical cations: CH3–C≡C–C≡C–R+ with R=H, D, CH3, and CD3; CD3–C≡C–C≡C–R+ with R=H, D, CD3 and C2H5–C≡C–C≡C–R+ with R=H and C2H5. Competition of the radiative à (π−1) → X̃ (π−1) and fragmentation decay of their à (π−1) states

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Abstract
Optical emission spectra of the radical cations given in the title have been detected in the gaseous phase, using low energy electrons for the excitation. The band systems are assigned to the à (π−1) →X̃ (π−1) electronic transitions of these cations by reference to the photoelectron spectroscopic data. A vibrational analysis of the emission bands yields the frequencies of some of the totally symmetric fundamentals for the X̃ states and of one or two for the à states that are supplemented by the frequencies inferred from the photoelectron spectra. The title cations excited to their à states exhibit the rare decay behavior in that the radiative relaxation is detected even though fragmentation also takes place. The lifetimes of these cations in the lowest vibrational levels of the à states have been measured. The rates of the radiative and radiationless processes depleting the à states are estimated from these lifetimes and the upper limits of the quantum yields of emission obtained from the cation breakdown curves. Also reported are some other alkyl diacetylene cations for which the radiative pathways could not be detected.

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