Nonlinear intensity dependence in the infrared multiphoton excitation and dissociation of methanol pre-excited to different energies

We report quantitative dissociation yields for the reaction CH3OH (vOH) → nhn CH31OH induced by infrared multiphoton excitation of methanol pre-excited to various levels of the OH stretching vibration (vOH50, 1, 3, 5). The yields are measured by detecting OH using laser induced fluorescence. It is demonstrated that for low levels of pre-excitation (vOH50, 1, 3) there is a substantial nonlinear intensity dependence, as a higher yield is found for self mode-locked CO2 laser pulses ~with higher peak intensity! as compared to single mode pulses of the same laser fluence, but lower peak intensity. In contrast, at high levels of preexcitation (vOH55) this nonlinear intensity dependence is absent. Quantitative model calculations are carried out using a case B/case C master equation approach that takes nonlinear intensity dependence into account. The calculations are consistent with the experimental results and confirm the prediction that an important part of the selectivity of the CO2 laser excitation step in infrared laser assisted photofragment spectroscopy of CH3OH is due to this nonlinear intensity dependence. We discuss further consequences of these experimental observations and theoretical predictions, which are also extended to infrared multiphoton excitation of C2H5OH. Infrared ~C–O! chromophore band strengths are reported for CH3OH and C2H5OH. © 2002 American Institute of Physics. @DOI: 10.1063/1.1501280#