Experimental And Theoretical Investigations Of Photoionization And Photodissociation Mechanisms Of Cycloketones And Pyrrolidine Molecules

Recently, the photoionization and photodissociation processes of atoms and molecules in intense femtosecond (fs) laser fields have attracted much attention of physicists and chemists. The mechanism of ionization/dissociation of the molecules interacted with fs laser pulses is not only meaningful for many dynamics, but also useful for understanding of the basic processes of chemical reactions. If the laser intensity is high enough, the strength of laser-atom/molecule interaction is comparable with the electron-nucleus binding energy, which attributes to the phenomena unobservable by using lower laser intensity. The works related to the interaction of atoms and intense laser pulses have been carried out for a long time. In molecules, however, more interesting phenomena can be measured because of the additional degrees of freedom the systems have, such as high harmonic generation, alignment and orientation, Coulomb explosion and so on. Accordingly, it is important to further investigate the mechanisms of ionization/dissociation of polyatomic molecules in intense fs laser field experimentally and theoretically.In this thesis, the ionization/dissociation mechanism of cyclopentanone has been experimentally measured in molecular beam by irradiating with intense 394 and 788 nm laser fields with pulse duration of 90 fs. For cyclohexanone we only use the 788 nm wavelength for investigation. Different polarizations (linear and circular) have been applied for pyrrolidine molecule with 800 nm wavelength only. The range of laser intensities varied from 3×10¹³ to 4×10¹⁴ W/cm² for all three polyatomic molecules. Actually, for all three molecules (cyclopentanone, cyclohexanone and pyrrolidine) we consider that the ionization is followed by dissociation process being the dominant reaction channel in our work because for neutral dissociation of a molecule to take place, the molecule has to absorb energy from the laser first and then either predissociate or decompose after internal conversion. The decomposition products would ionize afterward, and all these steps have to take place within the laser pulse duration of 100 fs, making this scenario rather unlikely.Firstly we focus on the photoionization process of cyclohexanone in 788 nm intense fs laser field. By comparing the experimental measurements and the ADK theory predictions we found that although ADK theory overestimates the rate constants in lower intensity region, it predicts the saturation intensity well with experimental observations. This method is recognized to be of useful for providing a reasonable standard by comparing the the experimental observations and the theoretical predictions.Subsequently the effect of different laser wavelengths on the ionization process of cyclopentanone in intense fs laser field. It should be noted that the fragment ions obtained by 394 nm are more abundant than that obtained by 788 nm at low laser intensities. Moreover, with increasing the laser intensity, smaller fragment ions appear and increase for both wavelengths. Interestingly, in all mass spectra the singly-charged parent ion is clearly seen while the doubly or multiply ones cannot be assigned definitely. By fitting the laser dependence of the ion yields of cyclopentanone parent ion and the main fragment ions below the saturation intensity, we measured the values of the slopes are coincidence with the minimum photon numbers necessary for ionizing the molecule, which is tenable for both wavelengths. In order to quantitatively understand the ionization process of cyclopentanone, we have calculated the ionization rate constants using ADK, Keldysh and KFR theory based on a hydrogen-like atom model. By comparing the results, we believe that the g-KFR theory, which is based on the original KFR theory combined with MO theory and Born-Oppenheimer approximation, is the best choice for calculating the ionization rate constants for real cyclopentanone. The results illustrate that in experimental laser intensity range, the doubly-charged parent ion may not be found due to much lower rate of the second ionization process compared with the first ionization. Furthermore, we compare the experimental ion yields with theoretical results find that in the lower intensity region, experimental observations for both wavelengths show a well agreement with theoretical ones. In the higher intensity range, however, an obvious departure is observed between the experimental and calculated results for 788 nm. The two main reasons as follow: first of all, it may due to the non-negligible contribution from the tunneling ionization. The internal energy deposited in the parent ion forms a distribution function. It should be noted that the detailed analysis of the KFR theory can provide the information of the most probable multiphoton ionization process involved in intense fs laser ionization of molecules. Secondly, in the case of intense laser ionization/dissociation, there might be some contribution from the laser field. Thus, a qualitative understanding of cyclopentanone ion has been achieved using the RRKM theory based on the ab initio surface.Except for the wavelength and intensity of laser pulses, the polarization is another important factor in the processes of laser induced polyatomic molecules ionization/dissociation. Thus, another part of our work is investigating the ionization and dissociation mechanism of pyrrolidine molecule in intense linearly and circularly polarized fs laser fields. Considering the ionization process is independent with the external electric field, we scaled the intensity of linear and circular polarization. Interestingly, the order of the laser dependence of cyclopentanone parent ion is smaller than both the minimum photon numbers ionized pyrrolidine need and that predicted by g-KFR theory. According to the TDDFT calculation of the absorption spectrum of neutral pyrrolidine molecules, in which the absorption band nearby 200 nm is found, we suggest that a resonant excitation of pyrrolidine in intense fs laser field followed by ionization process may act as a possible ionization– dissociation channel in our work. Furthermore, in order to further understanding the importance of the excited states, we also apply the Floquet method to investigate the collective excitation process of neutral pyrrolidine in intense LP fs laser field. Total 30 states are included in calculation of the excitation probabilities, and the results show that these probabilities are almost independent on the energy of the excited states; however, they are strongly dependent on the magnitude of the transition dipole moments between the ground and the excited states. It should be noticed that the slope can be roughly estimated as 4 5, showing a reasonable agreement with the experimental observations. Therefore, we propose that the excitation process provides another important ionization channel when pyrrolidine molecule interacts with intense fs laser pulses in the present condition.With regard to dissociation processes according to the characteristics of the calculated cation absorption spectrum provided by TDDFT, the weak fragmentation of the pyrrolidine can be interpreted by the absence of the single photon resonance at 800 nm. A qualitative understanding of the fs ionization/dissociation mechanisms of pyrrolidine has been achieved by using the RRKM theory based on the ab initio surfaces. Based on the calculations, the dissociation patterns from the ground state of the molecular cation cannot explain the experimental mass spectrum. However, the dissociation via the excited state of the pyrrolidine cation is recognized be significant to interpret the mass spectra. Thus, if the active energies of the molecular ions are different after absorbing additional photons, the dissociation channels and fragmentation patterns should be corresponding different. It is also considered that the difference of the available active energies between linearly and circularly polarized laser fields can explain the enhancement of the fragmentation observed in circularly polarized laser field.Furthermore, by comparing of the fs laser mass spectra with conventional electron impact ones we can see that the peak positions of the two kinds of mass spectra are identical but the relative peak intensities differ. This is due to the fact that the energy deposited in the parent ion depends on the experimental method. Intact molecular ion formation with only slight fragmentation by an intense fs pulse has been demonstrated in the present work, which is expected to be of great advantage for sensitive analytical purposes. To sum up, considering the different characteristics of the mass spectra provided by intense fs laser fields with various conditions (wavelength, polarization, pulse duration, intensity and so on), this kind of technique has the potential to develop a new type of mass spectrometry in the future.