Zeeman quantum beat observed by IR–UV double resonance LIF spectroscopy of acetylene in the 3ν+ν and 3ν3′+ν4′ungerade vibrational states

The Ã1Au state of acetylene: ungerade vibrational levels in the region 45,800–46,550 cm−1

“…We note that the 1−2 cm −1 splitting of the J ≥ 2 levels of the 3 3 6 1 , K′=1 state reported in previous work [13,21] is not the result of triplet perturbations, but represents an avoided crossing with another vibrational level within the S 1 -trans state. There are triplet perturbations in addition to the avoided crossing, but these are much smaller, and are centred on the 3 3 6 1 , K=1 state rather than its perturbing partner, B 5 , K=2 (II).…”

Spectral Signatures of Inter-System Crossing Mediated by Energetically Distant Doorway Levels: Examples from the Acetylene S1 State

“…Because the geometry change from in-plane to out-ofplane occurs along the torsional coordinate, a simple model based on FranckÀCondon factors would suggest that increased excitation in the torsional (ν 4 0 ) mode would lead to increased S 1 ∼ T 3 vibrational overlap. However, recent experimental 19,76 evidence and theoretical 32,33 calculations are not in agreement with this simple model. IR-UV double resonance LIF/SEELEM spectroscopy 60 of the S 1 combination levels 3 3 4 1 K 0 = 0 and 3 3 6 1 K 0 = 0 could be used to separately address the role of modes 4 and 6 in promoting vibrational overlap with T 3 levels, and avoid complications due to Darling-Dennison resonances and a-type Coriolis interaction.…”

“…All ISC in the à 1 A u ( S 1 ) state of acetylene is described by doorway-mediated interaction. ,,, The model is supported by Zeeman anticrossing spectroscopy, ,, LIF Zeeman quantum beat spectroscopy, − high-resolution LIF spectroscopy, simultaneous LIF and SEELEM spectroscopy, − and photoelectron spectroscopy. , The doorway model treats matrix elements between vibrational levels of the S 1 and T 1,2 electronic states as negligible, and stipulates that all mixing between S 1 and T 1,2 levels is induced indirectly by nonzero S 1 ∼ T 3 and T 3 ∼ T 1,2 spin–orbit matrix elements.…”

Stretch-bend combination polyads in the Ã1Au state of acetylene, C2H2

“…Much of the emphasis is on the lowest singlet excited state, S 1 ( à 1 A u ), where ab initio calculations [1][2][3][4][5][6] indicate that there is a comparatively low barrier to trans-cis isomerization and, at higher energy, a conical intersection with the second singlet excited state, S 2 . Perturbations by triplet states are widespread in the S 1 state, [7][8][9][10][11][12][13][14] where the density of such perturbations depends on the specific vibrations that are excited, and the onset of predissociation [15,16] leads to interest in the mechanisms of the near-threshold photodissociation of acetylene. [2,3,17,18] Moreover, the à 1 A u # X 1 $ + g transition is very…”