Photoionization And Photodissociation Of Pyrrolidine In Femtosecond Laser Field

The study of molecules in intense laser field with ultrashort pulse duration has been an area of considerable activity over the last several decades. Some phenomena, eg. above-threshold ionization ,and high harmonic generation, accelerate further experimental and theoretical works. There are many studies about the photoionization and photodissociation of polyatomic molecules in intense femtosecond laser field. We aim organic molecules in the study both because of their fundamental interest and important role in the generation of high harmonics.Control of molecular dissociation and alignment, and high harmonic generation of gas phase molecules, induced by short and intense laser pulse are some of the interesting phenomena that require a thorough understanding of the interaction mechanism between molecules and laser. Although the interaction of strong laser field with polyatomic molecules has a history of more than two decades, many fundamental questions remain unanswered. It seems that the ionization and dissociation processes are better understood in the limits of the intensity range higher than 10¹⁵ W/cm² and lower than 10¹² W/cm². In the intensity range lower than 10¹² W/cm², the ionization and dissociation can be considered by the multiphoton ionization mechanism(MPI), while for intensity above 10¹⁵ W/cm² molecular ionization is attributed to the so call field ionization. However, on the other hand, the ionization and dissociation mechanism involved in experiments with laser intensities in the intermediate range ( 10¹³ W/cm² - 10¹⁵ W/cm²) is less clear, although it has been the subjects of many recent publications. The motivation for the present work is to gain more insight about the interaction of molecules with laser field at this intermediate intensity range.In our research work, we study the photoionization and photodissociation of pyrrolidine molecule using femtosecond laser pulse with pulse duration of 90 fs and time-of-flight mass spectrometer. We use two different wave lengths of 400 and 800nm in our experiments and the intensity is in the range of 10¹³ - 10¹⁴ W/cm².In our experiment, we compare the degree of fragmentation with two different laser polarizations namely horizontal and circular polarization at 800 nm with the laser field intensity in the range of 1×10¹⁴ - 5×10¹⁴ W/cm². Based on the field dissociation introduced by Kong, in this intensity, the fragmentation will be caused by the field dissociation. In the model of field dissociation, only those molecules with the chemical bond that is parallel to the polarization direction of the laser field can dissociate along this direction. Only those chemical bonds that are parallel to the horizontal direction can dissociation in the interaction with horizontal polarized laser field and those aligned perpendicular can not. As for horizontal polarized laser field there is no perpendicular component, so the fragmentation of pyrrolidine parent ion is suppressed. At the same time, we compare the results with different polarization in 400 nm laser field. The result is similar with that obtained in 800 nm laser field. The fragmentation is suppressed in the horizontal polarized laser field.In the same field intensity and the same laser polarization, the resonance between parent ion energy level and laser wavelength is a key factor that effects the fragmentation. According to the absorption spectrum of the parent ion calculated, pyrrolidine molecule has no electronic transition in the cation at the excitation wave length 800 nm, results less fragment ions. In contrast, the pyrrolidine parent ion has absorption at 400 nm leading to stronger fragmentation.In our experiments, we also study the peak width of the parent ion. We found that the peak width of the parent ions is mainly caused by the size of the laser focus along the time-of-flight direction.