Enhanced fluorescence emission of helium atoms by seeded optically driven impact excitation

We demonstrated enhancement of fluorescence emission from helium atoms in terms of ultraviolet and infrared filaments nonlinear interaction in a mixture of argon and helium atomic gases. The seed electrons produced by ultraviolet pulses were accelerated by infrared filaments to kick a significant amount of helium atoms into high-lying excited states. This provided a method to efficiently excite helium atoms through electron collisions for further investigation on electron-ion collisions and their effects on atomic coherence behavior.

Figure 1

(a) Schematic representation of the magnified area where the three-color plasma filaments interact, showing the interference pattern of the two TH laser fields and the propagation of synchronized spatially overlapped SH and FW pulses across the generated plasma grating. These multicolor laser pulses were focused into the mixture of helium and argon atoms. (b) Far-field image of the SH and TH beams when they were synchronized.

Figure 2

(a) Experimentally observed fluorescence spectra from two TH pulses produced plasma grating [olive curve (I)], from plasma grating with a Bragg angle incident SH beam [black curve (II)], from plasma grating with an incident FW laser beam [blue curve (III)], and from the plasma grating with these two synchronized SH and FW laser beams [red curve (IV)]. (b) Typical emission from the excited helium atoms at 2${}^{3}p$-3${}^{3}d$ (587.5 nm).

Figure 3

(a) Experimentally measured intensity of integrated continuum background (black squares) and helium fluorescence spectra at 587.5 nm (blue circles) emission from the mixed gases as a function of helium gas pressure, while argon atoms were fixed at 0.5 atm. (b) The dependence of continuum background intensity (black squares) and helium fluorescence (blue circles) on argon gas pressure.

Figure 4

Experimentally measured dependence of helium fluorescence intensity on the time delay between the FW and plasma-grating-forming TH laser pulses in the mixture of helium and argon atoms, which is well described by function ${[1/n_e(1.5,t)+\beta (t_d-1.5)]}^{-1}$ as the time delay ranging from 1.5 to 100 ps, as shown by the red curve.