Persistent infrared spectral hole burning of NO−2 ions in potassium halide crystals. I. Principle and satellite hole generation

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Abstract
New features are resolved within the internal vibrational mode spectra of NO−2 defects in KCl, KBr, and KI crystals at low temperatures using high‐resolution Fourier transform spectroscopy and persistent infrared spectral hole (PIRSH) burning separately and together. With interferometry it has been discovered that the vibrational linewidths of the different modes range over a factor of 300—from 0.01 cm−1 to ∼3 cm−1 and, with PIRSH burning, it has been demonstrated that the narrowest lines are inhomogeneously broadened while the broadest ones are homogeneously broadened. PIRSH’s have been found in some internal modes and combination bands of the NO−2 molecule when pumped with low‐intensity single‐mode lead salt diode lasers; however, detectable persistent holes are not produced in all of the modes because of a competition between hole production and relaxation by tunneling at low temperatures. This competition results in a hole burning intensity, below which hole relaxation overwhelms hole production and only small holes may be produced. The most unusually shaped absorption features are the V‐shaped notches in the reorientational tunneling fine structure at the NO−2 bending mode frequency in KCl and KBr. Of all the internal modes that do show pronounced PIRSH burning, these V‐notched absorption bands exhibit the most striking behavior. Multiple satellite PIRSH’s are detected at frequencies away from the single‐mode laser burn frequency with a broadband probe beam produced by a high‐resolution Fourier transform interferometer. An explanation for these satellite holes is derived from temperature, plastic deformation, and uniaxial stress dependence measurements on the KCl: NO−2 absorption spectrum. We find that the inhomogeneous broadening of the KCl: NO−2 ν2 reorientational tunneling fine structure is dominated by degenerate rotor level splitting produced by random crystal strains. Degenerate perturbation theory of the rotor level splitting in the strain field is found to match very closely the V‐shaped inhomogeneous distribution of levels associated with the KCl: NO−2 reorientational tunneling fine structure. The general conclusion is that whenever strain splitting of a doubly degenerate level dominates the inhomogeneous broadening, then the absorption spectrum displays zero strength in the distribution at zero splitting and a linear increase in absorption coefficient away from this frequency generating the observed V‐shaped notch in the absorption profile.