Characteristics Of High-order Harmonic Emission From CO₂Molecules

Molecular structure is the minimum unit of the matter, which keepsthe physicochemical characteristics, people can understand the materialcharacteristics and control the reaction process more better if it can beknown in term of dynamic and structure. With the development of thefemtosecond laser technology and the infrared optical parametricamplifier, we can generate high-order harmonics when they interact withatoms or molecules whose wavelengths locate in the XUV and X-rayregion and own the characteristics of ultrafast, low-dispersion, tunableand highly temporal and spatial coherence. By using high harmonicgeneration, people can study kinds of microscopic and ultrafast reactionprocess. Based on the different distance between the molecular nuclei, thehigh harmonic generation in the different aligned molecules, contains thericher physical information than that in atoms. With the development ofhigh harmonic from molecule, People find that the harmonic yield ofnon-alignment molecule is too low to continue study the feature ofharmonic from molecule, so the study of the HHG from aligned molecule becomes a hot area.This article investigates the angular distribution of high harmonicgeneration (HHG) from impulsively aligned CO2molecules driving bythe femtosecond laser, and observes the HHG spectra under the differentaligning angles and the different laser intensities. The angular distributionis found to be sensitive to both the harmonic order and the intensity of thedriving laser pulse. And then presents the molecular field-free alignmentand the polar plot of the weighted alignment distribution, gives theangular distribution of the molecular HHG theoretically andexperimentally, demonstrates the alignment dependence for the molecularHHG. At last, the article experimentally and theoretically investigates thealignment behaviors of polyatomic molecules under femtosecond laserpulses, observe the field-free alignment in C2H2and CO2molecules withHigh-Order Harmonic Generation (HHG) and discusses the evolution ofthe molecular rotational wave-packet with different laser parameters andionization potentials. The results show that the molecular alignmentdegree depends not only on the laser parameters, but also on themolecular ionization potential.By changing the laser pulse and ionization potential systematically andusing the high harmonic of molecule, this article studies the dependenceof the HHG from molecule to the alignment of molecule experimentallyand theoretically, and obtains the laser pulse intensity of the best alignment of molecules, it is useful to study the molecule dynamics inpractical application.