Photoionization of phosphorous trichloride clusters at gigawatt intense laser field

Abstract

Photoionization study of PCl₃ clusters has been carried out using time-of-flight mass spectrometry at selected wavelengths of UV (266, 355 nm) and Visible (532 nm) region employing nanosecond laser pulses of intensity ∼10⁹ W/cm². At these three laser wavelengths, dissociated fragments such as P⁺, Cl⁺, PCl⁺ and PCl₂⁺ are observed in the mass spectra. Formation of different fragments has been explained based on the multiphoton excitation at the respective wavelengths. Moreover, PCl₄⁺ ions are observed in 266 and 355 nm ionization of PCl₃ clusters. Generation of PCl₄⁺ is explained based on ion-molecule reaction within the ionized cluster. Using density functional theory (DFT), structure, ionization energy and absorption spectra of PCl₃ monomer and clusters have been calculated which helps to rationalize the experimental observation. Along with singly charged fragment ions, multiply charged atomic ions up to P⁵⁺ (I.E: 65.1 eV) and Cl⁴⁺ (I.E: 53.5 eV) are observed at 532 nm. Generation of multiply charged ions at such low laser intensity indicates efficient ionization of PCl₃ clusters. Formation of higher charge states can be explained based on “multiphoton ionization ignited-inverse bremsstrahlung heating–electron ionization”. Detailed study reveals that laser intensity, cluster size and ionization energy of the cluster constituents play an important role in governing the ionization dynamics under gigawatt laser intensity conditions. Thus, PCl₃ clusters exhibit diverse photochemical process as a function of laser wavelength at ∼10⁹ W/cm².