CO2 laser-induced multiphoton absorption of fluoroform-d: The effects of collisionsa)

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Abstract
Studies of the multiphoton absorption of the 10 μm R(26) CO2 laser line by fluoroform‐d have been extended to laser pulse widths (Δτ) of 60 ns over the range of pressures P=0.33 to 2.00 kPa. Comparison with earlier studies with Δτ=2 and 6 ns shows that for all fluences Φ the cross section σ=σ(Φ, PΔτ), which means that σ scales with the number of collisions during the pulse. Log–log plots of σ(Φ) vs Φ (P, Δτ fixed) show a bend at a fluence Φc. For Φ<Φc collisions appear mainly to induce rotational relaxation to fill the hole ‘‘burned’’ by the laser in the ground state rotational distribution. For Φ≳Φc the role of collisions is more complex and likely involves intramolecular relaxation processes.