Experimental and theoretical study of free induction decay of water molecules induced by terahertz laser pulses

We experimentally investigate the free induction decay in the asymmetric top molecule H2O after interaction with short terahertz pulses. Two different experimental techniques are used to perform measurements in the gas phase under different experimental conditions: electro-optical detection and terahertz field-induced second-harmonic generation. Simulations of the free induction decay signals are performed with a theoretical approach in which the absorption and dispersion coefficients are evaluated from spectroscopic data pertaining to the water molecule. Experimental and theoretical signals are compared directly in the time domain in two limiting cases. For a short propagation length of similar to 40 cm, both signals show a slightly altered electric field consistent with the approximation of weak absorption and dispersion. For a longer propagation length of similar to 300 cm, the electric field is heavily distorted. A good agreement between experimental and calculated signals is obtained in both cases. Finally, we discuss the advantages and/or disadvantages of the two experimental techniques.