The first observation of the free induction signals of OH radicals in the terahertz region
Hydroxyl radical OH is the main oxidizing agent in combustion processes and in atmospheric chemistry. The rotational spectrum of OH is comparatively simple - there are less than 10 absorption lines in the therahertz region, corresponding to transitions between the lower rotational states of the radical. The FID signal was observed at the OH absorption line at 83.8 cm-1, that corresponds to the transition from the lowest rotational state of OH. Hydroxyl radicals were generated in the following sequence of chemical reactions
O3 + (hν = 266 nm) --> O(1D) + O2 (1)
O(1D) + H2O --> 2 OH (2)
OH + OH --> H2O + O (3)
The concentration of OH radicals was ~ 10+15 cm-3, lifetime ~ 300 …. 700 ms. The pulse of UV laser (266 nm) was synchronized with the FEL pulses. Shape of the FEL pulse after the optical cell was recorded by an ultrafast Schottky diode detector and 30GHz oscilloscope. Absorption line of the OH radical is a - doublet with a splitting of 4.3 GHz therefore the FID signal contains characteristic beats with a period of 0.23 ns. During the lifetime of the OH radical, many pulses of FEL pass through the cell, each of which initiates a FID signal. We could register a sequence of 10 pulses simultaneously, obtaining separate frames of the birth and death of the radical The influence of the magnetic field on the FID signal was studied by numerical simulation. A longitudinal magnetic field leads to a rotation of the polarization plane of the signal. The angle of rotation depends on the time after the FEL pulse. The effect of a nonmonotonic rotation of the polarization was predicted - after a certain time after the pulse, the direction of rotation changes.