Laser-induced dispersed vibration–rotation fluorescence of acetylene: Spectra of ortho and para forms and partial trapping of vibrational energy

Abstract

The laser-induced dispersed vibration–rotation fluorescence method has been developed further when compared with a previous publication [Saarinen et al., J. Chem. Phys. 110, 1424 (1999)]. More than one order of magnitude better signal-to-noise ratio has been achieved in the wave-number region 2900–3500 cm−1 by taking advantage of directionality of the fluorescence signal. The improvement has been applied to overtone spectroscopy of normal acetylene where for high CH stretching excitations separate spectra of ortho and para forms are obtained containing basically just single CH stretching vibrational quantum transitions from the pumped antisymmetric vibrational (ν1+3ν3(Σu+) and ν2+3ν3(Σu+)) and close-lying symmetric vibrational local mode (4ν3(Σg+) and ν1+ν2+2ν3(Σg+)) states. No nuclear spin conversion is observed in these spectra. Two new symmetric vibrational states (ν1+2ν2+4ν40(Σg+)(29%) and (50%)) have been observed and the precision of the spectroscopic parameters of previously published symmetric states has been improved by an order of magnitude. Unexpected fluorescence originating from the antisymmetric CH stretching fundamental state ν3 and some associated states of acetylene have also been observed. These spectra are characterized by both ortho and para forms in normal abundance and by unusual intensity patterns due to strong reabsorption of the fluorescence by ground state acetylene molecules in the sample cell. A simple collisional step-down mechanism is proposed to account for the appearance of the ν3 fluorescence band system. The excess vibrational energy in the sample volume is partly trapped in the form of ν3 mode energy and it decays from the system by radiation.