Selective spectroscopy of rigid and fluxional carbazole-argon clusters

Two size-selective spectroscopic techniques were used to experimentally differentiate between nearly rigid (solid-like) and highly fluxional (liquid-like) carbazole⋅Arn (n=4–6) clusters produced and cooled in supersonic molecular beams: (1) ionization potential selective resonant two-photon ionization (IP selective R2PI) spectroscopy; and (2) spectral hole-burning with R2PI detection. For each cluster size, separate and qualitatively very different electronic spectra were obtained by IP selective R2PI, depending on total ionization energy. At low ionization energies, broad bands of halfwidth ≊50 cm−1 (FWHM) were obtained, which are interpreted as due to fluxional clusters of high internal energy. When ionizing slightly above an abrupt step in the ionization efficiency curve, additional narrow (Δν≊5 cm−1) features appear superimposed on the semicontinuous spectra; these are interpreted as due to (near) rigid clusters with low internal energy. The spectral hole-burning experiments support this interpretation in that deep holes of ≊5 cm−1 width could be burned in the rigid cluster spectra, but no spectral holes could be observed in the broad bands. The latter fact is interpreted as due to spectral diffusion of the fluxional subpopulation on a 10−8 s or faster time scale. These results are in good agreement with conclusions drawn from recent numerical simulations.