Observation of ion acceleration in nanosecond laser generated plasma on a nickel thin film under rear ablation geometry

In this article we report acceleration observed for the ions produced in a 50-nm-thick nickel film coated on a quartz substrate, under nanosecond laser ablation, in the rear ablation geometry. A detailed study with varying background pressure and laser energy is done. Spectroscopic study including spectroscopic time of flight (STOF) measurements of ionic and other neutral transitions from the plasma has been undertaken. The STOF spectra recorded for ionic transition clearly show an enhancement in the velocity of the slow component as the background pressure increases. In addition, a large asymmetric spectral broadening in the 712.22-nm neutral line is observed, which increases with background pressure. While these observations have similarity to some of the reported studies on the acceleration of ionic species through double-layer formation, the electric fields calculated from the measured acceleration appear to be anomalously higher, and a double-layer concept seems to be inadequate. Moreover, the large asymmetry observed in the neutral line profile is indicative of microelectric fields present inside the laser produced plasma plume, which may play a role in the continuous acceleration of the ions. Interestingly, this asymmetry in spectral broadening exhibits temporal and spatial dependence, which indicates that significant electric field is present in the plasma plume even for longer duration and larger distance from the target. These spectroscopic observations of acceleration have also been complemented by triple Langmuir probe measurements. To the best of our information, such observations regarding large ion acceleration for the rather low laser intensities as used in this experiment have not been reported in literature so far.

Figure 1

Schematic diagram of experimental setup showing the arrangement of sample (thin film), triple Langmuir probe (TLP), and laser system aligned to the vacuum vacuum system (VS). TMP is the turbo molecular pump, L1 is the focusing lens, L2 and L3 are the lens system to image the plasma plume to fiber array. F is the band pass interference filter, Hr460 is the high-resolution spectrometer, PMT is the fast photomultiplier Tube, DSO is the fast digital storage oscilloscope, TCU is the trigger and control unit which synchronizes the instruments with laser pulses, and the data are acquired on a personal computer (PC)

Figure 2

Evolution of STOF spectrum of ionic line (362.68 nm) at 3 mm from the sample for different background pressures for 10 ns, 1064 nm laser pulse at laser energies of (a) 100 mJ and (b) 50 mJ.

Figure 3

Peaking time of fast and slow peaks of STOF spectra of ionic species and the delay between fast and slow peaks with varying pressures for 1064 nm laser wavelength with 100 mJ of energy at 3 mm from sample.

Figure 4

Evolution of STOF spectrum of neutral lines 3 mm away from the sample for different background pressures and laser energies 100 mJ (a)–(c) and 50 mJ (d)–(f) for10 ns, 1064 nm laser. The neutral lines are 361.94 nm (a), (d), 508.11nm (b), (e), and 712.22 nm (c), (f).

Figure 5

Comparison of peaking time of slow components of STOF spectra of ionic and neutral species with varying background pressure for 1064 nm laser wavelength with 100 mJ of energy at 3 mm from the sample.

Figure 6

Temporal evolutions of neutral (361.94 nm) and ionic (362.68 nm) lines at 3 mm (a)–(c) and 7 mm (d)–(f) for different background pressures for laser energy of 100 mJ. The background pressures used are 0.1 mbar (a), (d), 10 mbar (b), 9e), and 20 mbar (c), 9f).

Figure 7

Temporal evolution of STOF spectra of ionic and neutral lines (362.68 and 361.94 nm) for two background pressures for 1064-nm laser wavelength with 50 mJ of energy at 0.5 mm from the sample.

Figure 8

Temporal evolution of neutral lines 361.94 nm (a), 508.11 nm (b), and 712.22 nm (c) of nickel at very close (1 mm) to the sample for 1064-nm laser ablation using 50 mJ energy. F.P is for fast peak and S.P for slow peak.

Figure 9

Temporal evolution of neutral lines 361.94 nm (a), 508.11 nm (b), and 712.22 nm (c) of nickel at very close (1 mm) to the sample for 1064-nm laser ablation using 100 mJ energy. F.P is for fast peak and S.P for slow peak.

Figure 10

Temporal evolution of ion saturation current at 10 mm from the sample for different background pressures and laser energies of (a) 100 mJ and (b) 50 mJ.

Figure 11

Temporal evolution of floating potential at 10 mm from the sample for different background pressures and laser energies of (a) 100 mJ and (b) 50 mJ.

Figure 12

Zoomed section of the floating potential (a) and ion saturation current (b) recorded at 10 mm away from sample at early stages of plasma plume for 50 mJ energy, showing the traces of oscillations.

Figure 13

Asymmetric broadening of 712.22 nm neutral line of nickel at nearer points for delay times of (a) 200 ns, (b) 300 ns, and (c) 500 ns for different background pressures and laser energy of 100 mJ.

Figure 14

Asymmetry of 712.22 nm neutral line of nickel estimated as per Eq. (1) and the FWHM of the same line for 200 ns delay for different background pressures and distance up to 4.5 mm for laser energy of 100 mJ.