Knowledge Bank

The $A~2\Pi \leftrightarrow X~2\Pi$ transitions of the ions $H-(C\equiv C)n-H+(n=3-5)a,H-(C\equiv C)n-C\equiv N+(n=2-3)b$, and $N\equiv C-(C\equiv C)n-C\equiv N+(n=1-3)c-d$ are studied in neon matrices through Fourier-transform absorption, emission and laser excitation spectroscopies. The ions are produced from simple precursors diluted in neon using a source combining a self-igniting discharge with a pulsed supersonic expansion recently developed in our laboratory. For each of the three ion families, the red shift of the $A~2\Pi \leftrightarrow X~2\Pi$ transition with increasing chain length is quite accurately predicted by a one-dimensional particle in a box model. Stretching vibrational information for both the ground and excited states is extracted from the very sharp and well-resolved spectra. As clearly seen in our three-dimensional (emission, excitation, emission intensity) spectra, these linear-chain ions all show subsidiary emission/excitation bands for the bluer of the two discrete sites, which are best interpreted as due to the nominally forbidden $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 12\leftrightarrow \mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 32$ transitions.