First overtone helium nanodroplet isolation spectroscopy of molecules bearing the acetylenic CH chromophore

High-resolution helium nanodroplet isolation spectra of the first overtone ~2n1) of the acetylenic stretch of several substituted acetylenes ~RC[C–H! at T50.38 K, have been observed for the first time. A tunable 1.5 mm laser is coupled, using a power buildup cavity, to a beam of He droplets seeded with the molecule to be studied. Absorption spectra are recorded by monitoring the beam depletion as a function of laser frequency with a thermal detector. The spectra of hydrogen cyanide ~HCN!, monodeuteroacetylene ~DCCH!, cyanoacetylene ~NCCCH!, propyne ~CH3CCH!, trifluoropropyne ~CF3CCH!, 3,3-dimethylbutyne ((CH3!3CCCH!, and trimethylsilylacetylene ((CH3!3SiCCH! have been recorded. Due to the superfluid nature of the droplet, rotational resolution is achieved despite the presence of some solvent-induced broadening. The spectroscopic constants have been extracted by means of spectral simulations. The resulting rotational constants are smaller than for the bare molecule by a factor which depends on the molecule nonsphericity and its gas-phase moment of inertia. The linewidths are found to be at least twice as large as those of the corresponding fundamental (n1) transitions observed in a helium droplet by Nauta et al. @Faraday Discuss. Chem. Soc. 113, 261 ~1999! and references therein#. The helium-induced spectral shifts are found to be very small, but cannot be easily rationalized. © 2000 American Institute of Physics. @S0021-9606~00!00147-1#