Photodissociation of methanol at 193.3 nm: Translational energy release spectra

Abstract

Translational energy distributions of the products of the 193 and 157 nm photodissociation of chloroethylenesCenter-of-mass translational energy distributions of the dominant primary products resulting from 193.3 nm excitation of jet-cooled CH 3 0H, CH 3 0D, and CD 3 0H were obtained by using the high-n Rydberg time-of-flight (HRTOF) technique. The appearance threshold in the HRTOF spectrum yields a bond dissociation energy, D o (CH 3 0-H), of 105::±: 1 kcal mol-I, in agreement with recent literature values. Translational energy release spectra from the three isotopomers exhibit progressions of 950::±:lOO cm-I , which are attributed to excitation in the v3 O-CH3 stretch of the methoxy product. The progressions peak at v = 1, with population out to at least v = 5. This differs from the results of a recent wave packet dynamics study on a calculated excited state potential energy surface [Marston et al., J. Chern. Phys. 98, 4718 (1993)], which predicted no O-CH3 stretch excitation in the methoxy fragment following photolysis of ground state methanol. The spatial anisotropy of the fragments ({3--0.7) implies a dissociation time ~ 1 ps. The impulsive model for rotational excitation is compared to the unresolved rotational contour of the vibrational peaks in the translational energy release spectra and is found to underestimate the extent of rotational excitation, though the model correctly predicts the increase in contour width observed for the O-deuterated species. The unresolved rotational contours are fit empirically. The inferred vibrational energy distributions are discussed in terms of a simple Franck-Condon model for the pseudotriatomic, Me-O-H. Implications of the vibrational and rotational photofragment distributions for the full 1 I A 1/ surface are discussed.Moreover, other spectroscopic studies have aSSigned this ab-J.

Infrared detection of gas phase formaldehyde towards the high mass protostar W33A

“…Ultraviolet photolysis of methanol, which requires photon energies ≥4.5 eV, yields the metoxy radical (CH 3 O) as the dominant channel (Wen et al 1994;Cheng et al 2002), whereas the threshold energy for ionization of methanol is 11 eV. Photodissociation of formaldehyde itself (at energies ≥ 3.8 eV) restricts the efficiency of its formation.…”

Exploring the Energy Disposal Immediately After Bond-Breaking in Solution: The Wavelength-Dependent Excited State Dissociation Pathways of para-Methylthiophenol

“…For example, Wittig and co-workers have determined that upon photoexcitation of methanol to its first nσ* state, O-H bond fission occurs in < 1 ps as inferred from recoil anisotropy, generating CH 3 O( X ) radicals with a modest amount of vibrational excitation and that most of the photon energy over and above that required to cleave the O-H bond is partitioned into H atom translation. 7 In the solution phase, there are many new dynamical complexities. For example, solvation can shift the potential energy curves (PECs) and introduce caging.…”

Product pair correlation in CH3OH photodissociation at 157 nm: the OH + CH3channel

“…14 In the photolysis of CD 3 OH at 193 nm, the ratio of H : D was determined to be 0.86 : 0.14; 82% of the total available energy was deposited into the translational degrees of freedom. Wen et al 15 detected H and D atoms from the photodissociation of three isotopic variants of methanol using the high-n Rydberg atom time-of-flight (HRTOF) technique. 16 Their results were in reasonable agreement with the previous result by Satyapal et al (ref.…”

“…In the C-O coordinate, there is a dissociation barrier of 0.58 eV, which prevents rupture of this bond on the S 1 PES at 193 nm. 15 Marston et al 25 performed a wave packet calculation to investigate the function of the initial vibrational excitation on S 1 . The relative branching ratio of CH 3 O + H versus CH 3 + OH depends on both the initial vibrational excitation of methanol and the photon energy.…”