Two-Dimensional (2+n) REMPI of CH₃Br: Photodissociation Channels via Rydberg States

Abstract

(2+n) resonance enhanced multiphoton ionization (REMPI) spectra of CH(3)Br for the masses H(+), CH(m)(+), (i)Br(+), H(i)Br(+), and CH(m)(i)Br(+) (m = 0-3; i = 79, 81) have been recorded in the 66 000-81 000 cm(-1) resonance energy range. Signals due to resonance transitions from the zero vibrational energy level of the ground state CH(3)Br to a number of Rydberg states [Ω(c)]nl;ω (Ω(c) = 3/2, 1/2; ω = 0, 2; l = 1(p), 2(d)) and various vibrational states were identified. C((3)P) and C*((1)D) atom and HBr intermediate production, detected by (2+1) REMPI, most probably is due to photodissociation of CH(3)Br via two-photon excitations to Rydberg states followed by an unusual breaking of four bonds and formation of two bonds to give the fragments H(2) + C/C* + HBr prior to ionization. This observation is supported by REMPI observations as well as potential energy surface (PES) ab initio calculations. Bromine atom production by photodissociation channels via two-photon excitation to Rydberg states is identified by detecting bromine atom (2+1) REMPI.

Resonance-Enhanced Multiphoton Ionization of CH2Br2: Rydberg States, Photofragmentation, and CH Spectra

“…Comparison of this structure with the one-photon absorption spectrum, , which has been interpreted as being due to transitions of halogen lone-pair pπ electrons to sσ Rydberg orbitals, shows no clear correspondence between the two spectra. This suggests that the Rydberg states resonantly excited by two photons do not correspond to sσ Rydberg orbitals but rather to pπ and dπ Rydberg orbitals, analogous to what is found for resonance excitation of the monohalomethanes. , …”

“…In addition to being of interest for fundamental studies of photofragmentation processes in molecules, these compounds and their photofragmentation species are of great importance in both atmospheric chemistry and astrochemistry. 10−14 Our recent REMPI studies of methyl bromide 4 and CF 3 Br 6 confirmed and added information relevant to their Rydberg state structures and dissociation dynamics. The data revealed clear photodissociation processes to form bromine atoms in both cases.…”

“…Recently we have undertaken the task of making use of as well as developing the one-color mass-resolved resonance-enhanced multiphoton ionization (REMPI) technique to perform photofragmentation studies of molecules. − This paper is a continuation of an ongoing series of spectroscopy and photofragmentation studies of halogenated methyl derivatives by REMPI for resonance excitations into medium- to high-energy Rydberg state regions of the molecules. In addition to being of interest for fundamental studies of photofragmentation processes in molecules, these compounds and their photofragmentation species are of great importance in both atmospheric chemistry and astrochemistry. − Our recent REMPI studies of methyl bromide and CF 3 Br confirmed and added information relevant to their Rydberg state structures and dissociation dynamics. The data revealed clear photodissociation processes to form bromine atoms in both cases.…”

“…The data revealed clear photodissociation processes to form bromine atoms in both cases. Interactions between Rydberg and ion-pair states are believed to play an important role in the photodynamics of both molecules. ,, Unusual bond breaking and C/C* atom formation processes were observed for CH 3 Br …”

Photofragmentations, state interactions, and energetics of Rydberg and ion-pair states: Two-dimensional resonance enhanced multiphoton ionization of HBr via singlet-, triplet-, Ω = 0 and 2 states

“…Thus it will resemble analogous findings for other Br-containing compounds. 32 The three strong bromine atomic lines observed in the spectral region discussed here (see above) are all observed in an excitation region corresponding to the low energy tail of the weak A-band spectrum where one-photon absorption cross section is very low, of the order 10 −21 cm 2 molecule −1 at room temperature (to be compared with the cross section of about 2.5 × 10 −18 cm 2 molecule −1 for the maximum of the A-band 33 ) (see Fig. 1).…”