Electronic Energy Relaxation and Transition Frequency Jumps of Single Molecules at 30 mK

Abstract

Transition frequency jumps for single terrylene molecules in a polyethylene matrix caused by resonant laser irradiation are investigated at 30 mK. These jumps are not accompanied by substantial sample heating. A model for the effect is proposed, based on the interaction of tunneling two-level systems (TLSs) surrounding the single molecule with high-energy nonthermal phonons emitted by the molecule during electronic energy relaxation. The radius of the effective interaction volume is estimated to be r m ഠ 12.5 nm, and the interaction cross section for nonequilibrium phonon -TLS scattering is estimated as ϳ10 222 cm 2 . DOI: 10.1103/PhysRevLett.87.015504 PACS numbers: 61.43.Fs, 78.40. -q The low-temperature properties of glasses have strong anomalies as compared to crystals [1][2][3]. These anomalies are successfully described by the tunneling two-level system (TLS) model, which assumes the existence of a special kind of local low-energy excitations with a very broad distribution of energies and relaxation rates. Phonon assisted TLS dynamics is thought to be responsible for many anomalous properties of glasses at low temperatures and has been the subject of extensive investigation by various methods for almost 30 years. One tool used for the investigation of these dynamics is optical spectroscopy, in particular, photon echo, hole burning, and single-molecule spectroscopy (SMS) (see reviews [4 -13]). All of these methods use a chromophore impurity as a probe whose resonance frequency is subject to spectral diffusion (SD) because of interactions between the probe and TLSs. In principle, impurities can disturb the matrix structure, but there is no indication that impurities at low and moderate concentrations affect the TLS density. An increase of the rate of SD has been detected in samples only at chromophore concentrations above 10 22 mol͞l [14]. Another expected effect of impurity molecules on their local environments could arise from light absorbed by the chromophore. Through subsequent decay processes the absorbed energy is inevitably partially converted to high-energy phonons, which spread into the surrounding space, exciting TLSs and finally decaying into thermal equilibrium phonons. TLS excitation via high-energy phonons was suggested in [14] as a reason for a specific SD time dependence in highly concentrated samples, but no direct evidence for this effect was obtained. The advantage of SMS in measurements of such a kind is that one and the same molecule serves as a phonon generator and as a probe. Therefore, the measurements can be carried out at a low concentration of chromophores, which does not disturb the structure of the matrix. The present work is a first attempt at direct investigation of the influence of highenergy phonons emitted by impurity molecules on their local environments.Experiments were performed on the sample used in previous works [15,16]. The sample has a sandwich structure. A thin layer (about 1 mm thick) of polyethylene (PE) doped with terrylene at a concentration of ϳ10 26 M was p...

Charge hopping revealed by jitter correlations in the photoluminescence spectra of single CdSe nanocrystals

“…The individual probabilities, P n ͑͒ can be provided by a microscopic model for SD. 26 In Figs. 2͑a͒ and 2͑b͒ we plot the spectral jump histograms obtained from two single NCs with R ϳ 2.5 nm and R ϳ 4 nm cores, respectively.…”

Stretched Polyethylene Films Probed by Single Molecules

“…[26,27] Most studies have so far been concerned with probing local dynamics of the host at cryogenic temperatures. [28][29][30][31][32] In 2006, Butter et al reported on the orientation of single terrylene molecules in PE. [33] We have studied drawn PE films at the single-molecule level in order to determine the distribution of the orientations of individual, embedded chromophores and its dependence on stretching.…”