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

High-resolution helium nanodroplet isolation spectra of the first overtone (2ν1) of the acetylenic stretch of several substituted acetylenes (RC≡C–H) at T=0.38 K, have been observed for the first time. A tunable 1.5 μm 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 (ν1) 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.