High resolution Coulomb explosion spectra and angular distributions of fragment ions of N ₂ in a femtosecond laser field

Abstract

Femtosecond laser field-induced ionization and Coulomb explosion are systematically investigated using high-resolution time-of-flight mass spectroscopy. Meanwhile a good alignment of the $\text{N}_{2}$ is achieved geometrically. Based on the energy and momentum conservation laws, the events from different Coulomb explosion channels are identified accurately and further used to obtain the Kinetic Energy Release (KER) by the created molecular ion pairs and the angular distributions of the fragment ions. The KERs measured at laser intensities varying from $\text{4 }\times {10}^{14}\,\text{W}/\mathrm {cm}^{2}$ to $\text{2 }\times {10}^{15}\,\text{W}/\mathrm {cm}^{2}$ are found to stay constant. The angular distributions are measured at laser intensity of $\text{9 }\times {10}^{14}\,\text{W}/\mathrm {cm}^{2}$. The atomic ions $\text{N}^{+}$, $\text{N}^{2+}$ and $\text{N}^{3+}$ exhibit highly anisotropic distributions and for higher charge state, the angular distributions become narrower. With good exclusion of channel N(1,0), the non-zeroes normal to the laser polarization vector in channel N(1,1) still exist, which indicates the presence of geometric alignments (GA). The elusive shrink structure at $\uptheta$=0$^\circ$ for channels N(1,1), N(1,2) and N(2,3) is observed, which implies that the non-sequential process exists, and the electron rescattering plays role in the ionization process.