Rotationally Resolved Infrared Spectroscopy of the Hydroxymethyl Radical (CH₂OH)

Abstract

Infrared spectra of CH 2 OH have been recorded in the molecular beam using a combination of depletion and double resonant ionization detected IR (DRID-IR) spectroscopy via the 3p z Rydberg state. With DRID-IR spectroscopy, IR transitions are detected by exciting CH 2 OH in a selected vibrational level in the ground state to a Franck-Condon favorable level in 3p z from which it is ionized. Rotationally resolved spectra of the fundamental CH symmetric stretch (ν 3 ), CH asymmetric stretch (ν 2 ), OH stretch (ν 1 ), and the first OH-stretch overtone (2ν 1 ) are obtained. The rovibrational structure is analyzed with the aid of ab initio calculations and asymmetric rotor simulations. The OH and CH symmetric stretch fundamentals are hybrid a/b-type bands. On the other hand, a pure b-type transition is observed for the CH asymmetric stretch fundamental. The spectrum of the first overtone of the OH stretch is rotationally resolved, indicating that intramolecular vibrational redistribution (IVR) is not extensive. The rotational linewidth, however, is ∼0.8 cm -1 , greater than the laserlimited linewidth observed for the fundamental transitions (∼0.4 cm -1 ). Reasons for the increase in linewidth are discussed. † Part of the special issue "Richard Bersohn Memorial Issue".

IR/UV double resonant spectroscopy of the methyl radical: Determination of ν3 in the 3pz Rydberg state

“…One would initially think that IR and UV laser frequencies have to be tuned simultaneously to acquire the IR+ UV DRIE-IR spectrum, as was reported for the study of hydroxymethyl radical. 38 Limited by the symmetry of CH 2 OH, the energy of the UV transition for DRIE-IR is different if different rovibronic levels in the ground electronic state are prepared by IR radiation. 38 CH 3 has a planar geometry with D 3h symmetry, however, no matter which K or N levels in 3 ͑X ͒ are populated by IR radiation; the UV transition used in DRIE-IR, i.e., the Q band of the 3 1 1 3p z ← X transition, does not change position.…”

“…38 Limited by the symmetry of CH 2 OH, the energy of the UV transition for DRIE-IR is different if different rovibronic levels in the ground electronic state are prepared by IR radiation. 38 CH 3 has a planar geometry with D 3h symmetry, however, no matter which K or N levels in 3 ͑X ͒ are populated by IR radiation; the UV transition used in DRIE-IR, i.e., the Q band of the 3 1 1 3p z ← X transition, does not change position. Our results demonstrate that IR+ UV REMPI combined with DRIE-IR spectroscopy provides an important mechanism for observing and assigning new vibronic transitions that are inaccessible by conventional REMPI from the 0͑X ͒ state.…”

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). I. A theoretical study

“…Gas-phase spectroscopic studies of vibrational levels such as the symmetric and antisymmetric CH stretch fundamental vibrations of CH 2 OH, as well as excitation of 1-4 quanta of the OH stretch (ν 1 mode) were reported previously. [11][12][13] Perhaps the most intriguing result of the previous studies was the observation of overtone-induced vibrational predissociation of CH 2 OH upon excitation of the 4ν 1 vibrational level. 13 In these studies, the excited OH bond coincided with the bond that was broken.…”

Unimolecular processes in CH2OH below the dissociation barrier: O–H stretch overtone excitation and dissociation

“…Previously, we reported spectroscopic studies in the region of the CH and OH fundamental transitions and first overtone of the OH-stretch vibration in CH 2 OH, and compared the experimental results to ab initio calculations. 26 Rotationally resolved spectra were recorded by double resonance ionization detection ͑DRID͒ via the 3p z Rydberg state and showed line broadenings in the 2 1 spectrum that were thought to arise from low-order resonances. Here, we extend our studies to levels up to 4 1 ͑13 600 cm −1 ͒, thereby approaching the dissociation barrier.…”