A. Laisaar (a), V. Hizhnyakov (a,b), J. Kikas (b), An. Kuznetsov (a) and A. Suisalu (a)
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(a) Institute of Physics, University of Tartu, Riia Str. 142, 51014 Tartu, Estonia;
(b) Department of Physics, University of Tartu, Tähe Str. 4, 51010 Tartu, Estonia
_____________________________________ One of the most powerful methods of high-resolution optical spectroscopy is persistent spectral hole burning (PSHB). This method provides accurate values of homogeneus width, gamma (homog), of zero-phonon electronic transitions in impurity molecules or atoms doped into a solid (crystalline or glassy) matrix, since in the hole-burning zero-dose limit spectral holes have a width gamma (hole) = 2 gamma (homog).
Using a Coherent single-frequency tunable ring dye laser of 2 MHz bandwidth and a UNIPRESS 15-kbar helium gas compressor, we studied the effect of pressure on spectral holes burned in the 0-0 absorption band of chlorin (7,8-dihydroporphin) molecules embedded in a glassy polystyrene. Holes were burned and thereafter measured at the same fixed P,T values (P = 0 - 8.6 kbar; T = 4.2 - 15 K) for solidified helium used as a pressure transmitter. It was found that under pressure the width of holes essentially decreased, the effect being more pronounced at higher temperatures. For example, at 4.2 K holes burned under pressure of 5.1 kbar were 1.8 times narrower than those burned at 1 atm (hole widths 2.4 and 4.3 GHz, respectively), whereas at 15 K a 3-fold reduction occurred (hole widths about 15 GHz at 5.3 kbar and about 45 GHz at 1 atm).
To explain this phenomenon, a theory based upon the soft anharmonic potential model was developed [1]. According to this theory, the effect of the hole narrowing is mainly caused by decrease in the number of soft localized vibrational modes (SLMs) with increasing external pressure. This happens because among SLMs with various positive and negative values of cubic anharmonicity, which are present in any glassy material, SLMs with a negative anharmonicity are more abundant, as was shown theoretically for the first time. For that reason an applied pressure transforms some part of SLMs into tunneling two-level systems (TLSs), while the simultaneous opposite process of transforming available TLSs into SLMs is less probable. Since at liquid helium temperatures SLMs make an essential contribution to the width of spectral holes in a glass, decrease in their number must lead to the narrowing of holes with pressure, the effect being stronger at higher temperatures.
Another recently observed effect - pressure-induced suppression of the low-energy part of a broad pseudophonon sideband appearing in the PSHB spectrum of chlorin-doped polystyrene glass on the red side of the purely electronic hole as a result of prolonged laser irradiation - is also in agreement with the above theory.
[1] V. Hizhnyakov, A. Laisaar, J. Kikas et al., Phys. Rev. B 62, 11296 (2000).
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