Ultraviolet-Optical Double-Resonance study of the predissociated state of acetylene
Résumé
Ultraviolet-Optical Double-Resonance (UVODR) spectra were recorded via individually selected rotation-vibration intermediate levels in the Ã1A u state of acetylene. These spectra reveal a previously unobserved predissociated electronic state, the π ∗(a g) 2 ← π(a u) 2 doubly excited C̃' 1A g state. Owing to the changes in the HCC bond angle from 120° to 103°, and in the CC bond-order from 2 in the Ã1A u state to 1 in the C̃' 1A g state, long progressions in the CC stretch ( ν2) and the trans-bending ( ν3) vibrations and their combinations are observed. Although the C̃' 1A g state is predissociated, the rotational simplification provided by UVODR permits complete resolution of rotational structure in the lowest energy vibrational level. The principal observations for the HCCH and DCCD isotopomers include: T0 = 62 294(4) and 62 237(2) cm -1, ν2 = 884(10) and 822(10) cm -1, ν3 = 1303(20) and 935(10) cm -1, A = 7.90(7) and 4.65(5) cm -1, B = 0.880(7) and 0.773(5) cm -1, C = 0.792(7) and 0.641(5) cm -1, respectively. From the rotational constants the geometry of this 1Ag state is deduced to be r CC = 1.65(2) Å, r CH = 1.14(3) Å, and θHCC = 103(4)°.