"Intense electromagnetic fields in the Terahertz (THz, 10^-12 Hz) frequency range have recently become tabletop available. In this proposal I offer to advance this unique range of frequencies into the world of gas phase coherent control and molecular dynamics. THz fields interact with molecules via their permanent electric dipoles, and induce molecular orientation, thus providing a new molecular handle (in addition to the commonly used visible/near-IR optical frequencies), for inducing and studying the rotational and molecular dynamics in the gas phase. I will utilize both THz and optical fields as two distinct molecular handles to induce unique molecular rotational responses that are inaccessible otherwise, and are invaluable for various spectroscopic techniques in the realm of molecular dynamics (such as high harmonic generation, molecular frame Photo-electron and Photo-ion angular distributions and ultrafast X-ray diffraction). THz induced inversion-asymmetry (orientation) enables nonlinear optical interrogation techniques that are forbidden in isotropic gas samples (such as second harmonic generation). Combining the polarizability-selective and dipole-selective interactions of optical and THz fields respectively, will enable far improved control of 3D molecular angular distributions – a long standing goal in chemistry and physics. The efficient THz-dipole interaction require field intensities that are orders of magnitude lower than those required by optical fields, and will be implemented for studying fragile samples such as bio-molecules. The dipole-selectivity of the THz field will be used for probing dynamic processes involving changes in the molecular dipole (such as trans<-->cis isomerization in azobenzene). The broadband resonant interaction of THz with the multilevel rotational system reveals collective light-matter phenomena that are fundamental to spectroscopy and will be further studied."
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