3. Reaction and Ionization of Polyatomic Molecules
1.3 Fragmentation of Molecules in Intense Laser Fields
Fragmentation of molecules has shown a rich complexity in intense laser fields. In intense 10∼102fs-laser pulses, molecular decomposition occurs through various channels that can be from the neutral or charged species and within or after laser irradiation. Dissociation of a molecule can also happen at the nearly same time as its ionization irradiated by an intense laser pulse, i.e., dissociative ionization, a typical case of non Born–Oppenheimer approximation process in which the excitation energy is shared both in electronic and vibrational degrees of freedom.50 It is very valuable to understand these processes for such applications as controlling fragment ratio and “soft” ionization of large molecules.51
1.3.1. Ionization-dissociation of molecules in intense laser fields and statistical theoretical description
In many cases of polyatomic molecules, it is assumed that ionization followed by dissociation is the main reaction channel since the rate of the electron motion is faster than that for the nuclear motion. The neutral dissociation requires a relative long time to take place (predissociation or decomposition after internal conversion) after absorbing the energy from laser fields. This timescale is normally longer than 10 fs. Therefore, ionization is the primary process for molecules in intense laser fields.
After ionization, dissociation of molecular ions can be subdivided into two categories accordingly whether it occurs within or outside the laser pulse duration. Under intense laser fields, the potential surfaces of molecular ions can be modified by the laser electric field and further induce the ion dissociation during the laser duration. This type of ion dissociation
is usually referred to as field assisted dissociation (FAD).52 The second type of ion dissociation takes place outside the temporal influence of fs-laser pulse. This often happens in the case of polyatomic molecules in which the ions become hot and unstable, by accumulating certain energy from photoexcitation into the vibrational degree of freedom, and finally decomposite. Thus, in theoretical treatment for the molecular dissociation these two categories of dissociation differ on the account of whether such dissociation can or cannot be described in terms of statistical theory.53
1.3.2. Effects of cation absorption on molecular dissociation
In a “moderate” laser intensity range, molecular ionization can be caused through different mechanisms. Though multiphoton ionization cannot be excluded, field ionization of polyatomic molecules can play a role with an increasing rate constant in this relative “weak” field, for example, the field ionization is dominant, rather than the multiphoton process, for benzene in a laser field of 3.8×1013W/cm2.9
After this first ionization, the produced parent ions is able to gain more excitation energy by further photoabsorption in the same laser pulse for sequential steps, either decompositing or secondary ionizing.53,54Thus, it is expected that a role of cation photoabsorption may be significant in this sequential model of the molecular ionization/dissociation in intense laser fields with several 10 fs pulse duration.
The effect of molecular cation resonance was clearly observed in several polyatomic molecules such as aromatic, hydrocarbons, and cycloketones.37,55−58 Figure 5 gives the mass spectra from a series of cycloketone molecules produced in a 90 fs laser field with 6×1013W/cm2 (Ref. 37). Table 1 shows the data for the different photoabsorptivities of these molecular cations and the ratios of the parent ion yield to the total ion yield (P+/T+) at both the laser wavelengths used, correspondingly.
The quite different characters exhibited in the mass spectra for different cycloketones can be explained by the different photoabsorptivities of these molecular cations. It appears that there is a qualitative agreement between absorbency and degree of fragmentation as shown in the case of ketones.
It is obvious that the appearance of a parent ion peak is related to the
Fig. 5. Mass spectra of cycloketone molecules at 788 nm (left) or 394 nm (right) laser field with the intensity of 6×1013W/cm2. Form the top to bottom: cyclopentanone, cyclohexanone, cycloheptanone, and cyclooctanone. The peaks of parent ions are denoted by asterisks.37
Table 1. Photoabsorption of cyclokenones cations calculated and their corresponding ratios of parent ion yield to total ion yield measured at both 788 and 394 nm laser wavelengths (the calculation is done by using B3LYP method with 6-31++G(d,p)). From Ref. 37.
Molar absorptivity
λ(nm) (mol−1Lcm−1) P+/T+(%)
Cyclopentanone 394 520.17 5.33
788 <0.01 81.6
Cyclohexanone 394 570.56 1.62
788 6.74 52.6
Cycloheptanone 394 2846.74 2.5
788 880.10 5.5
Cyclooctanone 394 429.20 0.75
788 3139.13 0.1
absence of the resonant absorption of a single photon by the cations and the fragmentation of the molecular cations increases significantly in the case of resonance.
The resonance effects in these mass spectra were interpreted by the absorption spectra calculated from taking the optimized ionic molecular structures.37 The optimization is resulted from a nuclear rearrangement from the equilibrium structure, mainly contributed by theH atom move- ment. Yamanouchiet al.have demonstrated that this kind ofH migration in organic molecules is as fast as several tens of femtoseconds.59,60And therefore within a pulse duration of fs-laser the performation of this rearrangement can be achieved.