The Interaction Mechanism Of Cyclohexanone And 2-methylcyclohexanone With Femtosecond Laser Field

The study of molecules in intense (I>1014 W/cm2) laser field with ultra-shortpulse duration (τ<10-13 s) has been an area of considerable activity over the last twodecades. The early observation of new phenomena, e.g. above-threshold ionizationand high order harmonic generation, stimulated further experimental and theoreticalworks. There are so many studies about photoionization and fragmentation processesinduced by ultra-intense lasers in organic molecules, both because of theirfundamental interest and because of they play an important role in the generation ofhigh harmonics of the fundamental radiation.Control of the dissociation process, molecular alignment, and high harmonicgeneration from molecular gases, induced by short and intense laser pulses, are someof the interesting phenomena that require a thorough understanding of the interactionmechanism between the molecular system and the laser beam. Although theinteraction of strong laser beams (1012 ~1017 W/cm2) with polyatomic molecules has ahistory of more than a decade, many fundamental questions remain unanswered. Itseems that the ionization/dissociation processes are better understood in the limits ofthe intensity range mentioned above. Thus, for the lower intensity limit (~1012 W/cm2)the ionization/dissociation processes can be understood by a multiphoton ionizationmechanism (MPI), while for intensities above 1015W/cm2 molecular ionization isattributed to so-called "field ionization" i.e., ionization induced by the electric field ofthe laser beam (tunneling, barrier suppression ionization, electron recollision, etc).However, on the other hand, the ionization/dissociation mechanism involved inexperiments with laser intensities in the intermediate range (1013~1015 W/cm2) is lessclear, although it has been the subject of many recent publications. The motivation forthe present work is to gain more insight about the interaction of molecules with lasersat high intensities.The interaction of cyclohexanone and 2-methylcyclohexanone with (2×1013W/cm2~2×1014 W/cm2) femtosecond laser fields is studied in the present thesis bymeans of a time-of-flight mass spectrometer. The spectra were obtained with 394 nmand 788 nm, 90 fs laser pulses respectively. These compounds are particularlyinteresting because they are often found as structural units in large biologicalsubstances such as steroids and polyether antibiotics. Furthermore, there are so manystudies about these two compounds.When the wavelength is 394 nm, the mass spectrum of each compound wasrecorded at typically five different laser intensities covering the range between 2×1013W/cm2 and 2×1014 W/cm2. From the mass spectra we found that there are somecommon characteristics among cyclohexanone and 2-methylcyclohexanone: theparent ion peak is far more less than the fragmental ion peaks. This implies that MPIis the main mechanism in ionization/dissociation processes, and the ionization-dissociation mechanism is dominated because of the pulse width (90 fs) is less thanthe time of parent dissociation for cyclic ketones (180 fs). However, when thewavelength is 788 nm, according to Keldysh theory, the non-adiabatic parameter isaround 1, and this value locates in that so-called borderline region between MPI andFI processes. During this region, the tunnel ionization mechanism cannot beneglected. In the mass spectra of both samples the parent ion is obvious, and thequantities of fragmental ions which focus on large ones are relative small. Thereby,we conclude that different laser wavelength contribute to distinct ionizationmechanism.Moreover, from the mass spectra of cyclohexanone at 394 nm, we observed thatthere exist two different dissociation channels and the one which produce m/z=58ions cannot be observed by Dewitt et al. However, when the wavelength is 788 nm,the dissociation channel which produce m/z=55 ions is extremely restrained andanother dissociation channel is appeared, namely the channel which produce m/z=84ions. Similarly, in the mass spectra of 2-methylcyclohexaone at 394 nm, we can seefour main dissociation channels, and one of these disappeared when wavelengthchanged to 788 nm while open other two dissociation channels. We thought thatdifferent laser wavelength lead to distinct ionization mechanism which affectsdissociation mechanism intensively.On the other hand, the degree of fragmentation is increased with laser intensitiesboth at 394 nm and 788 nm. The intensities of parent ion are also increased.However, when the laser intensity up to 1×1014W/cm2 (at 788 nm), the intensity ofparent ion decreased. This indicates that ionization is saturated. Furthermore, in thecase of cyclohexanone, we have seen that by increasing the laser intensity, the ratio ofC3H3O+/C3H6O+ is decreased. It should be mentioned that these two dissociationchannels are competitive.In addition, some of fragmental ions have the characteristic "double peak"profile that attributed to the function of laser field. This "double peak" changed to"one peak" profile when laser intensity increased and the "one peak" is wider than theother peaks. We thought this may due to polarization-induced dissociationmechanism.In order to describe the interaction mechanism of strong laser fields withpolyatomic molecules, some models have been proposed, we mention two of them,which have been published these years. Some people pointed out that the overalldissociation yield is depend on the degree of sigma-type bonding and the size ofmolecule, e.g. the extension of one carbon unit was sufficient to decrease theionization probability one order of magnitude. Besides, some researchers put forthanother explanation of the mechanism of the fragment/parent ion formation. Theythought the key factors in the fragmentation behavior of many molecules underintense pulsed laser irradiation are the wavelength of the laser excitation and thespectral characteristics of cations. Through compared the mass spectra ofcyclohexanone and 2-methylcyclohexanone with same conditions, we found the latterone can explain our results exactly, and the former one is not suitable.It should be mentioned that in some mass spectra of both two samples, weobserved a dominant H+ ion feature that is almost equal or excess in intensity to theparent ion peak. Such a dissociation mechanism is not normally seen in nanosecondphotodissociation/ionization experiments. We thought that the field-induceddissociation may produce such spectra either by H+ tunneling from the ends of themolecule or by a polarization-induced dissociation mechanism.To sum up, pulse, wavelength, and intensity of laser beam as well as thecharacteristics of the molecule will affect the ionization/dissociation processes deeply.