Rotationally resolved excitation spectra of the acetylene à 1Au←X̃ 1∑g+ transition were recorded by measuring separately the total emission in two spectral regions: UV (200–400 nm) and NIR (0.9–1.5 μm). UV emission is due to Ã→X̃ transitions. The source of the NIR signal is suggested by the observation that the NIR:UV intensity ratio is approximately constant throughout the V02K01 subband. (V denotes the trans-bending mode.) This is consistent with C2H à 2Π→X̃ 2∑+ NIR transitions occurring as a result of resonance-enhanced two-photon excitation of predissociated Rydberg states. The V03K01 subband shows significant fluctuations in the NIR:UV intensity ratio. The increase in trans-bending excitation from 2ν3′ to 3ν3′ enables accidental resonances with background states (both triplet and S1) to alter the NIR emission intensity. Triplet perturbers can facilitate Tn (n>1)→T1 NIR transitions or modify the resonance enhancement of C2H Ã̃-state production. Extra lines in the V03K01 subband, due to an S1 anharmonic perturber, show dramatically diminished NIR but enhanced UV emission intensity. This indicates that the perturbing state is anomalously resistant to photodissociation.

Direct absorption and dispersed fluorescence (DF) spectra sample the 12C2H2 X̃ 1Σ+g potential energy surface via profoundly different zero-order bright states. Despite the complementary nature of the data sets, a polyad model based on the approximate quantum numbers, Ns=v1+v2+v3, Nr=5v1+3v2+5v3+v4+v5, l=l4+l5, accounts for the energies (±0.35 cm−1) and relative intensities for all transitions into pure bending levels (Ns=0) at Evib≤12 000 cm−1 and Nr≤15, l=0 and 2. The parameters that define this model are obtained by fitting 41 pure bending levels, of which 9 are J=0 energies derived from components of the [Ns=0, Nr=8, 10, 12, l=0, g] polyads observed in the DF spectrum. This polyad model provides a basis for extrapolating a description of otherwise indescribably complex spectra and dynamics to the Evib≊16 000 cm−1 region of the barrier to acetylene ↔ vinylidene isomerization, and could therefore provide a basis for detecting the pattern-breaking signature of the onset of isomerization.

We have recorded a high-resolution dispersed fluorescence (DF) spectrum of the (A) over tilde (1)A(u) --> (X) over tilde (1) Sigma(g)(+) transition of acetylene-h(2), utilizing the zero-point level of the (A) over tilde (1)A(u) state, Here we present only the analysis of the vibrational levels of the (X) over tilde-state with E(vib) < 10 000 cm(-1). By comparing the observed and calculated spectral intensity patterns, we have estimated the previously undetermined pure bend vibrational constant, s(45)(h2), to be -11(2) cm(-1). Unlike previously recorded DF spectra, this DF spectrum is uniquely suited for comparison with an effective Hamiltonian since (1) the Franck-Condon envelope facilitates observation of levels at lower E(vib) in the (X) over tilde-state, (2) the improved resolution is sufficient to observe intramolecular vibrational redistribution (IVR) at lower E(vib), and (3) spectral features are no longer absent because of nodal patterns in the Franck-Condon envelope. Our comparison shows that our current effective Hamiltonian model, (H) over cap(eff)(R), can qualitatively describe the IVR pathways on the (X) over tilde-state for chromostates (zero-order bright states) which contain high excitation in the trans-bend (upsilon(4) less than or equal to 12). The failure of our (H) over cap(eff)(R) model for chromostates which contain excitation in both the CC stretch (upsilon(2)) and the trans-bend (upsilon(4) > 8) may be attributed to a new stretch-bend or stretch-only resonance. Comparison of the present DF spectrum to prior SEP spectra has allowed us to make plausible rotational and vibrational assignments for previously unassigned transitions in the SEP spectra.

Dispersed Fluorescence spectra of acetylene have been recorded and unzipped, through a fortuitous coincidence of FC active modes and active anharmonic resonances, thereby pictorially revealing the key factors controlling the vibrational dynamics on the acetylene So surface. When energy is placed into very high excitation of the trans-bend and moderate excitation of the CC stretch, three approximately conserved quantities survive, i.e., no trace of the expected total breakdown of vibrational constants of motion due to energetic access to the vinylidene isomer is observed for vibrational energies up to at least 16400 cm(-1). Unzipping the DF spectrum reveals the FC intensity profiles for the acetylene ($) over tilde A <-> ($) over tilde X system with two FC active modes. Also, the unzipped DF spectrum clearly demonstrates that IVR increases within a polyad by increasing trans-bend, nu(4), and by decreasing CC stretch, nu(2). Increasing IVR with increasing nu(4) or decreasing nu(2), are both cases where the first few resonance tiers play the dominant role in controlling IVR for the initial states prepared in these experiments, very high excitation in the trans-bend and moderate excitation in the CC stretch.