Role of electron heating in efficient interaction of a nanosecond laser with the cluster media: a case study on tetrahydrofuran cluster system

Interaction of tetrahydrofuran clusters with nanosecond laser pulses has been investigated at 532 and 1064 nm, using a time-of-flight mass spectrometer and home-built electron analyzer setup. An efficient laser–cluster interaction has been observed at both the laser wavelengths, based on detection of multiply charged atomic ions. At 532 nm, multiply charged atomic ions up to C4+ and O4+ have been observed, while at 1064 nm multiply charged atomic ions up to C5+ and O6+ have been detected. Such efficient laser cluster interaction is supposed to be facilitated by coupling of laser energy into the cluster by collisional heating of electrons confined within the cluster. Accordingly, kinetic energy distribution of electrons liberated upon interaction of tetrahydrofuran clusters with nanosecond laser pulses has been quantified using a home-built electron analyzer setup. A good correlation between the ionization energy of the highest multiply charged atomic ions observed at the two laser wavelengths and the measured kinetic energy of electrons (up to ∼80–100 eV at 532 nm and up to ∼300 eV at 1064 nm) has been obtained. Present studies suggest that upon initial ionization of the cluster by a multiphoton ionization mechanism, which is predominant under nanosecond laser conditions, further enhanced ionization of the cluster constituents during the time-span of the laser pulse is dominated by energetic electrons. These electrons are energized via inverse Bremsstrahlung absorption process, causing step-wise electron ionization of cluster constituents and augmentation of charge on the cluster, over a time scale where cluster expansion can be considered to be insignificant.