Dissociation of small methanol clusters after excitation of the O–H stretch vibration at 2.7 μ

Publisher Website
Google Scholar
Abstract
Molecular beam depletion spectroscopy has been employed to study the dissociation of small methanol clusters upon excitation of the O–H stretch vibration at 2.7 μ. The tunable infrared radiation has been obtained from a Nd:YAG laser pumped optical parametric oscillator. Pure methanol dimer spectra, without contamination from larger clusters, have been measured using the scattering selection technique or working at reduced temperatures. The dimer spectrum features two absorption peaks at 3574.4 and 3684.1 cm−1. The lower frequency peak is redshifted by 106.6 cm−1 from the monomer value and is assigned to the excitation of the proton donor in the hydrogen bonded complex. The proton acceptor peak is blueshifted by only 3.1 cm−1. Methanol trimer dissociation is observed at 3462 cm−1, whereas larger clusters absorb at still lower frequencies. Absolute photodissociation cross sections were measured for both methanol dimer bands. The integrated dissociation cross sections were 5.59(25)×10−21 cm2 /molecule and 7.1(2.2)×10−22 cm2 /molecule for the proton donor and acceptor bands, respectively. The integrated dissociation cross sections are directly related to the O–H line strengths in the dimer. The increased line strength, for the proton donor band relative to the acceptor band by a factor of ∼8, is in accord with previous observations associated with hydrogen bond formation.