Long term puzzles of the CH and CD energetics and related phenomena revisited; solutions sought through REMPI-photofragmentations of bromomethanes

Abstract

Ever since the pioneering work by Herzberg and Johns in 1969 (The Astrophysical Journal, 1969, 158, 399) the spectral assignment and the energetics of the fundamental molecular fragment CH, in the region of 63 000–65 000 cm⁻¹ (7.81–8.06 eV), have remained a puzzle to a large extent. The dissociation of bromoform and deuterated bromoform following two-photon resonance excitations to molecular Rydberg states forms the fragment species CH* and CD* in the excited state A²Δ(v′ =0) as well as carbon and bromine atoms in the ground and first excited states, C/C* and Br/Br*. Further (1_r + 1_i)REMPI of CH* and CD* resonance excites the fragments to the energy region of concern, whereas the atom fragments were identified by further (2_r + 1_i)REMPI. Analysis based on spectral simulations, isotope shifts and comparison with other data allowed spectral identifications, assignments and partial characterization of four highly excited bound states for each of the molecular fragments (CH**/CD**); including the (3)²Π valence state and the (4)²Π Rydberg state, for the first time. Perturbations, shown as line-shifts, line-intensity and/or line-width alterations, due to the level-to-level state interactions between the bound states and predissociations by a repulsive state are recognized. Recording of C⁺ signals in REMPI of several bromomethanes for a one-photon energy of about 40 333 cm⁻¹ allows the clarification of a mystery concerning a broad C⁺ band frequently observed. This work, presented, demonstrates the usefulness of molecular REMPI for fragment analysis.