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On the role of spectral diffusion in single-molecule spectroscopy
Journal of Luminescence 86 (2000) 207}209
On the role of spectral di!usion in single-molecule spectroscopy V. Palm*, K. Rebane Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia
Abstract Described is a series of six single-molecule spectra of terrylene in Shpol'skii matrix n-decane recorded during 1.5 h at 1.7 K, after cooling the sample down from the room temperature. The spectra reveal single-molecule lines with di!erent rates of spectral jumps. One of these lines, remarkably unstable at the beginning of the measurements, became much more stable after relaxation of the sample at a constant temperature for about 1.5 h. Such a behaviour demonstrates that low-temperature relaxation of the nonequilibrium energy stored during the cooling of a sample can play an important role in processes of spectral di!usion. 2000 Elsevier Science B.V. All rights reserved. PACS: 42.62.Fi; 78.90.#t Keywords: Single molecule spectroscopy; Spectral di!usion; Shpol'skii matrix
1. Introduction The high sensitivity of the purely electronic zero-phonon lines (ZPL) in spectra of impurities in low-temperature solids makes it possible to observe the in#uence of temporal changes in the structure and state of the sample on the spectral position and shape of these lines } the processes of spectral di!usion (SD) [1]. Observation of these processes became quite common with single-molecule spectroscopy (SMS) [2}7]. SD is one of the reasons why SMS is not just an ordinary spectroscopy with a very high sensitivity, but it has some intrinsic di!erences. If the structural changes of a solid are not light induced, they can be caused not only by #uctuations, corresponding to the thermal equilibrium at * Corresponding author. Fax: #372-7-383033. E-mail address: viktor@".tartu.ee (V. Palm)
a given temperature, but also by the nonequilibrium energy stored during the cooling of a sample } in the course of a subsequent relaxation [8], whereas the characteristic times of di!erent relaxation processes can di!er by several orders of magnitude. This feature is illustrated by a series of SMS spectra, recorded for the same spectral interval for terrylene in Shpol'skii matrix n-decane (Tr-C ) during 1.5 h at 1.7 K.
2. Experimental An optical set-up with a sample located in super#uid liquid helium, at the joint focus of a parabolic mirror and a small focusing lens, was used in our SMS experiments. The experimental set-up is described in more detail in Refs. [9,10]. To prepare the sample, a small drop of Tr-C solution (concentration about 10\ mol l\) was
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 0 ) 0 0 1 6 5 - 4
208
V. Palm, K. Rebane / Journal of Luminescence 86 (2000) 207}209
placed between two 0.3 mm-thick fused silica plates to form a thin "lm due to the capillary forces. The plates with the Tr-C solution were mounted into the SMS optical cell [10] and inserted into the cryostat; the sample was cooled down to liquid helium temperature in about 3 min. The frozen solution formed an invisible 1}3 lm-thick transparent "lm. Actual SMS measurements started approximately an hour after the sample was cooled down. Six #uorescence excitation spectra (Sp. 1}Sp. 6) were recorded in the same spectral interval of 2 GHz in the region of the weak Shpol'skii site C (j+574 nm) [9,11] during &1.5 h (see Fig. 1). Only one laser frequency scan was used for recording every single #uorescence excitation spectrum to avoid a broadening of spectral features due to the limited accuracy in setting the absolute laser frequency by the laser spectrometer prior to every successive scan (error up to 60 MHz). To enable observation of small (10}60 MHz) spectral jumps of single-molecule (SM) ZPLs between two successive laser scans irrespective of the inaccuracy of the laser spectrometer, a spectral interval with two stable ZPLs was found and used for the measurements. If the spectral distance between the two SM ZPLs remains the same with an accuracy of some MHz in several successively recorded spectra, it makes it possible to bring the frequency scales of these spectra in conformity with each other with an accuracy of about 10 MHz (inaccuracy is due to the laser frequency drift). We assume here that simultaneous and nearly equal spectral distance jumps of two near-by SM ZPLs are quite improbable. The accuracy of the frequency scale during a single scan was provided by the laser CR-699-29 Autoscan design and was de"ned by the long-term drift of the laser frequency (up to 100 MHz h\). Signals of the two laser wavemeter etalons were simultaneously monitored to reveal possible laser mode hopes during the single scans.
3. Results and discussion Observation of relatively stable SM line near the maximum of the B site of the Shpol'skii system
Fig. 1. Fluorescence excitation spectra recorded in the same spectral region (zero laser detuning corresponds to 17422.13 cm\) for the same spot on a thin (1}3 lm) dilute (&10\ mol l\) sample of terrylene in a Shpol'skii matrix n-decane at 1.7 K, starting at di!erent time moments t (t"0 corresponds to the start of Sp. 1). Excitation intensity was &0.5 W cm\; laser scanning rate 10 MHz s\ with a spectral resolution of 2 MHz. (a) The initial sequence of three spectra (Sp. 1}Sp. 3) recorded during the "rst 18 min. of the measurements; (b) the second sequence of three spectra (Sp. 4}Sp. 6) recorded with about 1 h delay after the "rst one (start at t"82 min). Two relatively stable single-molecule lines (labelled a and b) can be observed in the spectra, as well as an initially unstable single-molecule line c.
Tr-C was reported in Ref. [9]. In this work we report the spectra measured in the region of the weak C site [11] of this Shpol'skii system. Three spectral features (labelled a, b, and c) can be observed in all the recorded spectra (Sp. 1}Sp. 6,
V. Palm, K. Rebane / Journal of Luminescence 86 (2000) 207}209
see Fig. 1); we interpret them as the ZPLs of three di!erent single terrylene impurity molecules. Lines a and b are well described as Lorentzians with stable parameters all through the measurements: the line widths (FWHM) of the Lorentzian "ts are within &5% interval, 65.4 $ 2.6 MHz for a and 54.6 $ 2.8 MHz for b. The spectral distance between the maxima of a and b remains the same with a precision better than 10 MHz in Sp. 1}Sp. 3 (615 MHz) and in Sp. 4} Sp. 6 (695 MHz), but it increases by &80 MHz during a 67 min pause between the measurements of Sp. 3 and Sp. 4, probably due to a spectral jump of the line a. Assuming that the spectral position of the line b was stable during the measurements, the frequency scales of spectra Sp. 2}Sp. 6 were slightly shifted in Fig. 1 to partially compensate for the inaccuracy of the laser spectrometer. In Sp. 1}Sp. 3 the spectral feature c is of a radically di!erent shape in every measurement, but it contains peaks that could be described as abruptly emerged and/or cut-o! pieces of Lorentzian curves } it seems to be an unstable ZPL of a single terrylene molecule that makes small spectral jumps with a random rate of about 0.1 s\; at least some of these jumps do not exceed 100 MHz. In Sp. 4}Sp. 6 (measurements performed about an hour later), the jump rate for this molecule is already much lower, observable are only small jumps (less than 50 MHz), causing an asymmetrical broadening of the line; in the last measurement (Sp. 6) the spectral feature c has quite a symmetrical shape, well "tted by a Lorentzian curve with FWHM of &54 MHz. In the authors' opinion these spectra can be viewed as a manifestation of the role of the nonequilibrium energy, stored during the cooling of a sample and released in the course of lowtemperature relaxation, in processes of SD. The initiated SD should be time dependent and gradually slow down with various characteristic times (from fraction of a second up to centuries). Although many measurements of SM spectra were performed and several series of scans revealed
209
unstable SM lines along with stable ones, we did not "nd another example of unstable SM line with clearly observable change of SD rate in the time scale of our helium experiment. This fact is attributed to an individual nature of every impurity molecule as determined by their slightly di!erent environments. Hole burning technique seems to be more e!ective in investigation of averaged structural relaxation processes in ensemble after the cooling of a sample [8]. The series of SMS spectra described above demonstrate that these processes can also be observed on the SM scale; this interpretation does not con#ict with the results of other SMS measurements we performed for Tr-C . Acknowledgements This work was supported in part by the Estonian Science Foundation grant C4003 and in part by the International Center for Scienti"c CultureWorld Laboratory.
References [1] C.A. Walsh, M. Berg, L.R. Narasimhan, M.D. Fayer, J. Chem. Phys. 86 (1987) 77. [2] Th. BascheH , W.E. Moerner, M. Orrit, U.P. Wild (Eds.), Single-Molecule Optical Detection, Imaging and Spectroscopy, VCH, Weinheim, 1996. [3] W.E. Moerner, L. Kador, Phys. Rev. Lett. 62 (1989) 2535. [4] M. Orrit, J. Bernard, Phys. Rev. Lett. 65 (1990) 2716. [5] L. Kador, Phys. Stat. Sol. B 189 (1995) 11. [6] V. Palm, K.K. Rebane, A. Suisalu, J. Phys. Chem. 98 (1994) 2219. [7] W.E. Moerner, T. Plakhotnik, T. Irngartinger, M. Croci, V. Palm, U.P. Wild, J. Phys. Chem. 98 (1994) 7382. [8] K. Fritsch, J. Friedrich, B.M. Kharlamov, J. Chem. Phys. 105 (1996) 1798. [9] K.K. Rebane, O. Ollikainen, V.V. Palm, Opt. Spectrosc. 84 (1998) 374. [10] V. Palm, Rev. Sci. Instrum. 70 (1999) 2957. [11] K. Palewska, J. Lipinski, J. Sworakowski, J. Sepiol, H. Gygax, E. Meister, U.P. Wild, J. Phys. Chem. 99 (1995) 16 835.
On the role of spectral di!usion in single-molecule spectroscopy V. Palm*, K. Rebane Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia
Abstract Described is a series of six single-molecule spectra of terrylene in Shpol'skii matrix n-decane recorded during 1.5 h at 1.7 K, after cooling the sample down from the room temperature. The spectra reveal single-molecule lines with di!erent rates of spectral jumps. One of these lines, remarkably unstable at the beginning of the measurements, became much more stable after relaxation of the sample at a constant temperature for about 1.5 h. Such a behaviour demonstrates that low-temperature relaxation of the nonequilibrium energy stored during the cooling of a sample can play an important role in processes of spectral di!usion. 2000 Elsevier Science B.V. All rights reserved. PACS: 42.62.Fi; 78.90.#t Keywords: Single molecule spectroscopy; Spectral di!usion; Shpol'skii matrix
1. Introduction The high sensitivity of the purely electronic zero-phonon lines (ZPL) in spectra of impurities in low-temperature solids makes it possible to observe the in#uence of temporal changes in the structure and state of the sample on the spectral position and shape of these lines } the processes of spectral di!usion (SD) [1]. Observation of these processes became quite common with single-molecule spectroscopy (SMS) [2}7]. SD is one of the reasons why SMS is not just an ordinary spectroscopy with a very high sensitivity, but it has some intrinsic di!erences. If the structural changes of a solid are not light induced, they can be caused not only by #uctuations, corresponding to the thermal equilibrium at * Corresponding author. Fax: #372-7-383033. E-mail address: viktor@".tartu.ee (V. Palm)
a given temperature, but also by the nonequilibrium energy stored during the cooling of a sample } in the course of a subsequent relaxation [8], whereas the characteristic times of di!erent relaxation processes can di!er by several orders of magnitude. This feature is illustrated by a series of SMS spectra, recorded for the same spectral interval for terrylene in Shpol'skii matrix n-decane (Tr-C ) during 1.5 h at 1.7 K.
2. Experimental An optical set-up with a sample located in super#uid liquid helium, at the joint focus of a parabolic mirror and a small focusing lens, was used in our SMS experiments. The experimental set-up is described in more detail in Refs. [9,10]. To prepare the sample, a small drop of Tr-C solution (concentration about 10\ mol l\) was
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 0 ) 0 0 1 6 5 - 4
208
V. Palm, K. Rebane / Journal of Luminescence 86 (2000) 207}209
placed between two 0.3 mm-thick fused silica plates to form a thin "lm due to the capillary forces. The plates with the Tr-C solution were mounted into the SMS optical cell [10] and inserted into the cryostat; the sample was cooled down to liquid helium temperature in about 3 min. The frozen solution formed an invisible 1}3 lm-thick transparent "lm. Actual SMS measurements started approximately an hour after the sample was cooled down. Six #uorescence excitation spectra (Sp. 1}Sp. 6) were recorded in the same spectral interval of 2 GHz in the region of the weak Shpol'skii site C (j+574 nm) [9,11] during &1.5 h (see Fig. 1). Only one laser frequency scan was used for recording every single #uorescence excitation spectrum to avoid a broadening of spectral features due to the limited accuracy in setting the absolute laser frequency by the laser spectrometer prior to every successive scan (error up to 60 MHz). To enable observation of small (10}60 MHz) spectral jumps of single-molecule (SM) ZPLs between two successive laser scans irrespective of the inaccuracy of the laser spectrometer, a spectral interval with two stable ZPLs was found and used for the measurements. If the spectral distance between the two SM ZPLs remains the same with an accuracy of some MHz in several successively recorded spectra, it makes it possible to bring the frequency scales of these spectra in conformity with each other with an accuracy of about 10 MHz (inaccuracy is due to the laser frequency drift). We assume here that simultaneous and nearly equal spectral distance jumps of two near-by SM ZPLs are quite improbable. The accuracy of the frequency scale during a single scan was provided by the laser CR-699-29 Autoscan design and was de"ned by the long-term drift of the laser frequency (up to 100 MHz h\). Signals of the two laser wavemeter etalons were simultaneously monitored to reveal possible laser mode hopes during the single scans.
3. Results and discussion Observation of relatively stable SM line near the maximum of the B site of the Shpol'skii system
Fig. 1. Fluorescence excitation spectra recorded in the same spectral region (zero laser detuning corresponds to 17422.13 cm\) for the same spot on a thin (1}3 lm) dilute (&10\ mol l\) sample of terrylene in a Shpol'skii matrix n-decane at 1.7 K, starting at di!erent time moments t (t"0 corresponds to the start of Sp. 1). Excitation intensity was &0.5 W cm\; laser scanning rate 10 MHz s\ with a spectral resolution of 2 MHz. (a) The initial sequence of three spectra (Sp. 1}Sp. 3) recorded during the "rst 18 min. of the measurements; (b) the second sequence of three spectra (Sp. 4}Sp. 6) recorded with about 1 h delay after the "rst one (start at t"82 min). Two relatively stable single-molecule lines (labelled a and b) can be observed in the spectra, as well as an initially unstable single-molecule line c.
Tr-C was reported in Ref. [9]. In this work we report the spectra measured in the region of the weak C site [11] of this Shpol'skii system. Three spectral features (labelled a, b, and c) can be observed in all the recorded spectra (Sp. 1}Sp. 6,
V. Palm, K. Rebane / Journal of Luminescence 86 (2000) 207}209
see Fig. 1); we interpret them as the ZPLs of three di!erent single terrylene impurity molecules. Lines a and b are well described as Lorentzians with stable parameters all through the measurements: the line widths (FWHM) of the Lorentzian "ts are within &5% interval, 65.4 $ 2.6 MHz for a and 54.6 $ 2.8 MHz for b. The spectral distance between the maxima of a and b remains the same with a precision better than 10 MHz in Sp. 1}Sp. 3 (615 MHz) and in Sp. 4} Sp. 6 (695 MHz), but it increases by &80 MHz during a 67 min pause between the measurements of Sp. 3 and Sp. 4, probably due to a spectral jump of the line a. Assuming that the spectral position of the line b was stable during the measurements, the frequency scales of spectra Sp. 2}Sp. 6 were slightly shifted in Fig. 1 to partially compensate for the inaccuracy of the laser spectrometer. In Sp. 1}Sp. 3 the spectral feature c is of a radically di!erent shape in every measurement, but it contains peaks that could be described as abruptly emerged and/or cut-o! pieces of Lorentzian curves } it seems to be an unstable ZPL of a single terrylene molecule that makes small spectral jumps with a random rate of about 0.1 s\; at least some of these jumps do not exceed 100 MHz. In Sp. 4}Sp. 6 (measurements performed about an hour later), the jump rate for this molecule is already much lower, observable are only small jumps (less than 50 MHz), causing an asymmetrical broadening of the line; in the last measurement (Sp. 6) the spectral feature c has quite a symmetrical shape, well "tted by a Lorentzian curve with FWHM of &54 MHz. In the authors' opinion these spectra can be viewed as a manifestation of the role of the nonequilibrium energy, stored during the cooling of a sample and released in the course of lowtemperature relaxation, in processes of SD. The initiated SD should be time dependent and gradually slow down with various characteristic times (from fraction of a second up to centuries). Although many measurements of SM spectra were performed and several series of scans revealed
209
unstable SM lines along with stable ones, we did not "nd another example of unstable SM line with clearly observable change of SD rate in the time scale of our helium experiment. This fact is attributed to an individual nature of every impurity molecule as determined by their slightly di!erent environments. Hole burning technique seems to be more e!ective in investigation of averaged structural relaxation processes in ensemble after the cooling of a sample [8]. The series of SMS spectra described above demonstrate that these processes can also be observed on the SM scale; this interpretation does not con#ict with the results of other SMS measurements we performed for Tr-C . Acknowledgements This work was supported in part by the Estonian Science Foundation grant C4003 and in part by the International Center for Scienti"c CultureWorld Laboratory.
References [1] C.A. Walsh, M. Berg, L.R. Narasimhan, M.D. Fayer, J. Chem. Phys. 86 (1987) 77. [2] Th. BascheH , W.E. Moerner, M. Orrit, U.P. Wild (Eds.), Single-Molecule Optical Detection, Imaging and Spectroscopy, VCH, Weinheim, 1996. [3] W.E. Moerner, L. Kador, Phys. Rev. Lett. 62 (1989) 2535. [4] M. Orrit, J. Bernard, Phys. Rev. Lett. 65 (1990) 2716. [5] L. Kador, Phys. Stat. Sol. B 189 (1995) 11. [6] V. Palm, K.K. Rebane, A. Suisalu, J. Phys. Chem. 98 (1994) 2219. [7] W.E. Moerner, T. Plakhotnik, T. Irngartinger, M. Croci, V. Palm, U.P. Wild, J. Phys. Chem. 98 (1994) 7382. [8] K. Fritsch, J. Friedrich, B.M. Kharlamov, J. Chem. Phys. 105 (1996) 1798. [9] K.K. Rebane, O. Ollikainen, V.V. Palm, Opt. Spectrosc. 84 (1998) 374. [10] V. Palm, Rev. Sci. Instrum. 70 (1999) 2957. [11] K. Palewska, J. Lipinski, J. Sworakowski, J. Sepiol, H. Gygax, E. Meister, U.P. Wild, J. Phys. Chem. 99 (1995) 16 835.