Direct observation of the symmetric stretching modes of ùA_u acetylene by pulsed supersonic jet laser induced fluorescence

Abstract

Rotational analyses are reported for the and bands of the transition of C₂H₂ near 45,000 cm⁻¹ (+2800 cm⁻¹ relative to T ₀) from jet-cooled laser-induced fluorescence spectra. While the band is unperturbed and straightforward to assign, the 1¹ level is strongly perturbed by interactions with the 2¹ B ² polyad, where υ _B ′ = υ₄′ + υ₆′. In order to assign the lines of this band, a population-labelling technique was used, employing an infrared laser to deplete the population in selected ground state rotational levels before probing with the ultraviolet laser. Deperturbation of the 1¹/2¹ B ² interaction leads to the value cm⁻¹ for the fundamental symmetric C–H stretching frequency. Assignments are also reported for the 2³ and 1¹2¹ levels, completing all assignments of levels containing excitation in only the totally symmetric vibrational modes up to +4500 cm⁻¹. The reassignment of implies that some of currently accepted assignments above 47,000 cm⁻¹ are in error and suggests that the interpretation of some aspects of the near-threshold photodissociation measurements of Mordaunt et al. [J. Chem. Phys. 108, 519 (1998)] may need to be revisited.

Keywords:

• vibrational spectroscopy
• acetylene
• vibration–rotation analysis
• excited states

Acknowledgements

For partial support of this work, AJM thanks the Natural Sciences and Engineering Research Council of Canada. At M.I.T. the work was supoprted by DOE grant DE-FG0287ER13671.