For the first time the effect of high pressure on the irreversible, residual broadening (RB) of spectral holes in temperature cycling (TC) experiments was studied. A setup consisting of a UNIPRESS 15-kbar helium gas compressor, a high-pressure optical cell and a temperature-controlled liquid-helium cryostat was used. Holes were burned and recorded in the impurity (0 - 0) absorption band of chlorin (dihydroporphin)-doped glassy polystyrene peaked at about 636 nm, employing a very high resolution Coherent single-frequency tunable ring dye laser of 2 MHz linewidth. The TC was performed at various fixed pressures of solidified helium up to 5 kbar, according to the scheme T_b -- T_c -- T_b, where T_b = 5 K was the hole burning temperature and T_c the cycling temperature varied between T_b and 18 K.

It was found that at any pressure a hole studied became broader and broader as T_c was raised step by step. This RB, i.e. a further broadening after each elevation of T_c , was rather sensitive to pressure, being smaller at higher pressures (e.g., at 4.9 kbar the observed RB made up less than 2/3 of its initial value at 1 atm).

The RB of spectral holes was interpreted as a result of irreversible thermally induced spectral diffusion caused by interaction of the electronic transition in an impurity molecule with two-level systems (TLSs) which make thermally activated overbarrier jumps between two possible states. The observed pressure effects were treated theoretically within the scope of the soft anharmonic potential model. It was shown that the majority of TLSs in a glass has a positive asymmetry, which means that more abundant are TLSs with such double-well potentials where the right minimum, corresponding to a larger local volume, is placed at a higher energy than the left one. In this case, applied pressure reduces the number of nearly symmetric TLSs (having almost equally deep minima), which make the main contribution to the RB of holes in TC experiments.

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