Multiphoton absorption of HF laser photons by molecules containing a hydroxyl group

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Abstract
Multiphoton absorption (MPA) of HF laser radiation has been studied, as a function of pressure (15 Pa to 1.3 kPa) and fluence (2 mJ/cm2 to 75 J/cm2) for the series: water, methanol, methan‐d3‐ol, ethanol, and 2,2,2‐trifluoroethanol. As the group attached to the –OH is made more complex, the quasicontinuum occurs after fewer excitation steps, and under ’’collisionless’’ conditions, the same degree of multiphoton excitation is found to require a lower fluence. For water, at pressures between 73 Pa and 1.3 kPa, the cross sections are considerably lower than those for the other molecules, and MPA requires fluences in excess of ∼75 J/cm2. The remaining molecules divide into two groups, the ’’small’’ molecules (CH3OH and CD3OH) and the ’’large’’ molecules (C2H5OH and CF3CH2OH). For the small molecules at low pressures, the cross sections decrease with increasing fluence, an effect which is thought to be due to anharmonic bottlenecking. As pressure increases, the fluence dependence of the cross sections disappear. For the large molecules, anharmonic bottlenecking appears to be reduced, due to the greater density of states, and cross sections increase with increasing fluence according to the empirical form: σ(E, P)=K′Eb′Pa (where P is pressure, E is fluence and b′, a, and K′ are constants). The facility of HF laser‐induced collisionless multiphoton dissociation of the –OH containing molecules is discussed in light of these results.