Predissociation of HCN/DCN in Two Lowest-Lying Singlet Excited States:  Effect of Fermi Resonances on Spectra and Dynamics

Abstract

The three-dimensional potential energy surfaces of the 1A‘ ‘ and 21A‘ states and the corresponding transition dipole surfaces from the ground electronic (11A‘) state of HCN have been obtained from high-level ab initio calculations. Using these surfaces, we investigate the predissociation dynamics of HCN and DCN with a recursive quantum mechanical method. Whereas the perpendicular transition to the 11A‘ ‘ state leads to narrow predissociative resonances, the absorption spectrum of the parallel 21A‘ ← 11A‘ transition is dominated by relatively broad peaks associated with rapid fragmentation. Detailed analysis suggests that the 11A‘ ‘ spectrum of DCN is affected by a 1:2 Fermi resonance between the C−N stretching and bending modes. Both theoretical and experimental evidence suggests that the so-called β band should be reassigned to the 11A‘ ‘ absorption. In the 21A‘ state, a strong isotopic effect is found in the fragment internal state distribution and attributed to a Fermi resonance between the H−C and C−N vibrational modes.

THEORETICAL STUDIES OF $\tilde{A}{}^1 A^{\prime\prime}\to\tilde{X}{}^1 A^\prime$ RESONANCE EMISSION SPECTRA OF HCN/DCN USING SINGLE LANCZOS PROPAGATION METHOD

“…Recently, we reported accurate ab initio PESs for two low-lying excited electronic states of HCN and transition dipole surfaces. 16,44,45 The calculated absorption spectra and predissociation dynamics on these surfaces agreed very well with available experimental data and provided much insight into the dynamics on the excited states. 44,45 In this work, we continue our investigation of the HCN system by reporting resonance emission spectra of HCN and DCN using our newly calculated excited (Ã 1 A ) state PES and transition dipole moment, via a recursive method.…”

“…The changes in thẽ A(0, 0, 1) emission may be related to the strong Fermi resonance in DCN(Ã 1 A ), as discussed recently in our work. 44,45 As in the HCN, the emission shows very weak intensities to the delocalized levels in DCN.…”

Photodissociation of HCN and HNC isomers in the 7-10 eV energy range

“…7, and can be compared with that in Fig. 4 in Ref.[44]. The differences are attributed to differences in the PES’s and the transition electric dipole used.…”

“…The differences are attributed to differences in the PES’s and the transition electric dipole used. For example, Xu et al [44] neglected the overall rotation and the 2 1 A′ ← 1 1 A ′ transition was approximated by a simple sum of the x and y contributions. Also, here we use as zero of energy the CN(A 2 Π, v = 0, j = 0) + H threshold, and the autocorrelation function is multiplied by a decaying exponential function which introduces a broadening in the lower peaks, below 1 eV of kinetic energy, of approx.…”

“…Later they used this PES’s to study the vibrational structure of the 1 1 Σ − ← X 1 Σ + and 1 1 Δ ← X 1 Σ + [49] and 1 1 Π ← X 1 Σ + [42] transitions. More recently, Xu et al obtain the 1 1 A ″[43, 44] and 2 1 A ′[45] PES, and use them to study the predissociation dynamics of the HCN/DCN. These PES’s do not include the region of the hydrogen isocyanide HNC.…”