Summary of studies of emission spectra from XTL of NaCl. In all cases, XTL was obtained by precipitation of near-saturated aqueous solutions of NaCl by mixing with concentrated HCl. Studies include those of Weiser,6 Longchambon,16 Racz,17 Garten and Head,12 Walton and co-workers,18 and Barsanti and Maccarrone.1 In earlier studies by Bandrowski and by Farnau, the colour of the XTL was reported to be “blue–green” and “bluish–white,” respectively.5,19 The wavelength range of each study is shown by the box, the shaded region represents the emission range of XTL observed; wavelengths of nominal peaks are also noted. Two of the studies used intentional doping with metal cations, viz. Garten and Head (Pb2+) and Barsanti and Maccarrone (Cu+).
Schematic diagram illustrating the experimental apparatus. The sample was mixed in a tube held between a mirror and a lens. Light is collected onto one end of a fiber-optic bundle; the other end was connected to the input slit of the commercial spectrograph. The detector is an intensified charge-couple device (ICCD). The light-collection setup was covered in a dark enclosure (shown by the dashed line). The lens, fiber, and sample tube were made of UV-grade fused silica, and the mirror had a broad-band UV-visible enhanced-aluminium coating.
Image of XTL of NaCl captured using a compact digital camera (Canon PowerShot S90, ISO 3200, 0.5 s exposure, f/2 aperture). The metal dopant used was Cu2+. This picture has been enhanced to improve contrast. The outline of the round-bottom tube can be seen clearly, as can the meniscus of the solution at the top. The XTL appears as rapid, localized flashes.
XTL emission spectrum for NaCl doped with Dy3+. The experimental data (solid line) have been fitted with a set of Gaussian functions (dashed lines) giving 4 peaks at 253 ± 8, 267 ± 14, 409 ± 44, and 504 ± 12 nm.
XTL emission spectrum for NaCl doped with Cu2+. The experimental data (solid line) have been fitted with a set of Gaussian functions (dashed lines) giving 4 peaks at 252 ± 6, 259 ± 9, 359 ± 14, and 424 ± 40 nm.
Schematic diagram illustrating the shapes of the ground (1A1g) and low-lying excited triplet (3Eg) potential energy surfaces of NaCl:Ag+ as a function of the Ag–Cl bond length (dAg–Cl). This figure is based on electronic structure calculations (MSXα) of (AgCl6)5− by Pedrini et al. 31 The bond-length coordinate can be imagined to be representative of the reaction coordinate during collapse of a cluster during the XTL nucleation event. The similarities between the XTL and photoluminescence spectra suggest, however, that the timescale for XTL emission from the 3Eg states is much slower than the re-organization of the cluster; the emission then takes place near the equilibrated crystalline geometry, i.e., the minimum of the excited PES (point N). The long-wavelength tail in the emission (λtail) results from the displacement between the minima in the ground and excited states.
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