3. Reaction and Ionization of Polyatomic Molecules
1.4 Dissociative Ionization and Coulombic Explosion
1.4.1 Dissociative ionization of formic acid molecules 128
The hydrogen emissions and hydrogen migrations play important roles in the molecule decomposition and rearrangement dynamics because they move on a time-scale faster than most of other ion fragmentation processes.69The hydrogen migration channels of many organic molecules have been observed and the time scale of the hydrogen migrations have been discussed.70−72
As the simplest carboxylic acid, formic acid (HCOOH) attracted many researches. The ionic dissociation channels of formic acid have been studied by vacuum-ultraviolet (VUV) spectroscopy. The ground state 12A(IE= 11.33 eV) of singly charge ion is stable and its first excited ion state 12A (IE=12.37 eV) is associated with two dissociation channels, COOH++H and HCO++OH. For other lower excited state, 22A (IE = 14.81 eV), 22A(IE=15.75 eV) and 32A(IE=16.97 eV), the HCO++OH channel is also important. However, the HCO + OH+ channel is only open for excited states higher than the 32A state. In intense laser fields, the formic acid can decompose to several ion fragments [Wanget al., 2010]. Parent ions and fragments obtained by the loss of one H atom are dominant.
The main fragments are COOH+, COO+, HCOH+, HCO+, H2O+, OH+ and O+ from various dissociation channels. To identify these channels, information such as mass, kinetic energy released, angular distribution of these fragments produced, and even the correlation between the fragments are necessary. Therefore, a velocity map imaging apparatus with a 100fs pulsed 800 nm laser of∼1015W/cm2is employed for this study.
For singly charged parent ions, the experimental measurement suggests that the dissociation occurs with the channel of HCOOH→HCO++OH
Fig. 6. KERs of HCO+and OH+at 2.4×1014W/cm2(a) and 9×1013W/cm2(b) with a linear polarized laser at 800 nm.
+e−, from the excited states 12A, 22A, 22A and 32A of the parent ion HCOOH+, and the channel of HCOOH → HCO + OH+ + e−, mainly from the 32A state. Both channels lead to the dissociation of the same C–O single bond. The KERs of these fragment ions show clearly bimodal kinetic energy distributions (Fig. 1.6), suggesting that besides the dissociation channels described above the high-energy channels exist, i.e., from Coulomb explosion of double charged parent ions as discussed later in this section. Additional, the counts of low energy OH+ are much less than those of low energy HCO+, consistent with that OH+can be produced by the dissociation only from the higher excited state 32A while HCO+ may come from lower excited states.
Angular distributions of these fragments give the maxima along the laser polarization. These anisotropic angular distributions of fragmental ions commonly can be attributed to dynamic alignment, the molecular axis aligned along the laser polarization due to the interaction of laser
electric field and the induced molecular dipole moment, or geometric alignment with the angle dependent ionization rate. Normally, the dynamic alignment shows a dependence on the laser intensity.67 Because all the angular distributions obtained here do not depend on the laser intensity in the present intensity regime, the different anisotropic distributions HCO+ and OH+, dissociated from different excited state of the parent ion, may origin from the different angle-dependent ionization rate.
The double charged parent ions are produced with a significant yield in the 800 nm fs-laser intensity used. It is believed that sequential double ionization plays a dominate role in this case, as the data show that the circularly polarized laser pulse does not suppress the ion pair yield and the KER does not depend on the laser polarization and intensity, too. The coincidence maps of the TOF of fragment ions are shown in Fig. 7 at the laser intensities of 9×1013 and 2.4×1014W/cm2. These maps indicate that doubly charged formic acid ions are produced and decomposed from the COH+–OH+, COH+–O+, CO+–OH+, and CO+–O+ ion pairs. The H2O+–CO+ion pair is also observed as result of fast hydrogen migration with a low yield. These experimental observations of ion–ion coincidence momentum indicate that the two-body dissociative ionization process of doubly charged formic acid (HCOOH2+) is induced by intense fs-laser fields in the present intensity regime. Thus, the main channels of dissociative
Fig. 7. Ion–ion coincidence map at the intensities of 9×1013W/cm2(left) and 2.4× 1014W/cm2(right) with linearly polarized 800 nm laser.39
ionization identified are follows: (1) HCOOH → OH++ COH++ 2e−, (2) HCOOH→O++COH++H+2e−, (3) HCOOH→OH++CO++ H+2e−, and (4) HCOOH→O++CO++2H+2e−.
The Coulomb explosion channels with one neutral hydrogen emission have been identified using the ion–ion coincidence method. The different effective ionization potential of fragment ions and structure of precursor states have been discussed and are believed to arouse the different KER distributions. Another two weak two-body Coulomb explosion channels have been identified. One is from the hydrogen migration and the other is from the two-body Coulomb explosion of C=O double bond.