Spontaneous alignment of tetracene molecules in 4He droplets

The polarization dependence of the laser induced fluorescence and absorption was investigated for single tetracene molecules embedded in 4He droplets having N̄4=3×103–3×105 atoms. The measurements revealed an increase in the intensity up to 20% in the spectra, when the laser polarization vector was aligned parallel to the direction of the droplet beam as compared to the perpendicular alignment. This observation suggests a spontaneous alignment of the molecules in the laboratory frame. A mechanism of the molecular alignment in the pick-up process is proposed.