Rydberg–valence interactions in CH2Cl→CH2+Cl photodissociation: Dependence of absorption probability on ground state vibrational excitation

Abstract

A strong enhancement of absorption to the lowest A12 state is observed for vibrationally excited chloromethyl radicals. It is demonstrated that this enhancement is due to a significant increase in both electronic and vibrational Franck–Condon factors. Electronic structure calculations of potential energy surfaces (PESs) and transition dipole moments for the ground and the two lowest excited states of A1 symmetry, the 1 2A1 valence and 22A1 Rydberg states, reveal the origin of this effect. The shelflike shape of the 1 2A1 PES in the Franck–Condon region and the strong dependence of the electronic transition dipole moment on C–Cl distance are responsible for the enhancement. Analysis of the shape of the electron density distribution demonstrates that Rydberg–valence interaction in the two lowest excited states causes the changes in the shape of PESs and transition dipoles with C–Cl distance.

Interacting Rydberg and valence states in radicals and molecules: experimental and theoretical studies

“…When the photodissociation of CH 2 Cl from its lowest excited state (1 2 A 1 ) was studied in a molecular beam [26], it was noticed that [112] in spite of efficient vibrational and rotational cooling by carrier gases, a large fraction of dissociation products appeared with c.m. kinetic energies exceeding the energy allowed for vibrationally unexcited ground state radicals.…”

“…In order to ascertain that products with kinetic energy in excess of the maximum allowed by the thermochemistry were correlated with vibrationally excited radicals, experiments with Ar as a carrier gas, which is known to relax rotational states efficiently, were carried out [26]. Figure 4 shows the c.m.…”

“…In CH 2 Cl specific orbital overlap confers partial double-bond character to the C-Cl bond, resulting in strongly absorbing valence bands, as opposed to strong transitions to Rydberg states in CH 2 F [21]. Moreover, the valence and Rydberg orbitals in CH 2 Cl are mixed in a way that depends sensitively on molecular geometry [26]. In CH 2 OH, on the other hand, multiple conical intersections couple the Rydberg states to the ground state in two simultaneous dissociative coordinates [22][23][24][25].…”

High-resolution infrared studies in slit supersonic discharges: CH2 stretch excitation of jet-cooled CH2Cl radical

“…12 An important step into the gas phase was made by Endo et al who recorded the first pure rotational spectrum of chloromethyl radical using microwave absorption methods, in which A-type transitions were observed with nearly fully resolved fine and hyperfine components for both the 35 Cl and 37 Cl isotopic species in the ground vibrational state. 13 More recently, Reisler and co-workers have probed the photodissociation dynamics of the gas-phase chloromethyl radical using ion imaging techniques, 14,15 with accompanying theoretical studies by Levchenko et al. 15 Self-association rate constants for CH 2 Cl+ CH 2 Cl have been established, 16 and combined experimental/ theoretical investigations have probed the radical-radical reaction dynamics for O͑ 3 P͒ +CH 2 Cl.…”

“…13 More recently, Reisler and co-workers have probed the photodissociation dynamics of the gas-phase chloromethyl radical using ion imaging techniques, 14,15 with accompanying theoretical studies by Levchenko et al. 15 Self-association rate constants for CH 2 Cl+ CH 2 Cl have been established, 16 and combined experimental/ theoretical investigations have probed the radical-radical reaction dynamics for O͑ 3 P͒ +CH 2 Cl. 17 In their microwave studies of chloromethyl radical, Endo et al calculated a small positive inertial defect, ⌬ 0 = 0.0333 amu Å 2 , from the rotational constants.…”