- Email: [email protected]
Single-molecules spectral lines ‘erasing’ under powerful infrared illumination
JOURNAL OF
LUMINESCENCE EISEYIER
Journal
of
Luminescence 72-74 (1997)22-24
Single-molecules spectral lines ‘erasing’ under powerful infrared illumination Taras Plakhotnik”, Ph.vsical Chemistry Laboratoqj,
Skss
Daniel Walser, Alois Renn, Urs P. Wild Federal Institute of’ Technology, ETH-Zentrum, CH-809.2 Zurich, Switzerland
Abstract
One- and two-photon excitation (OPE and TPE) spectra of single 1,8_diphenyloctatetraene molecules in n-tetradecane (transition at 444 nm) were recorded at cryogenic temperature. The high-power infrared light used for TPE changed the conditions in the sample so that the number of single molecule lines observed in OPE spectra was approximately twice the number in TPE spectra or OPE spectra with added infrared illumination. Single molecules detectable under OPE and TPE, only under OPE, and only under TPE were observed. Keywords:
Single-molecule spectroscopy; Two-photon
excitation; One-photon
In the electric dipole approximation, the onephoton transition between the SOand Sr electronic states in polyenes is forbidden by parity, since both states have A, symmetry, and only the two-photon transition between these states is allowed [l]. In some Shpol’skii matrices, both transitions become allowed due to distortion of the molecular symmetry, and the inhomogeneously broadened O-O zero-phonon line can be observed under onephoton excitation (OPE) and two-photon excitation (TPE). The similarity of the inhomogeneous line shape in the OPE and TPE spectra [2] may imply similarity in the properties of the single molecules (SMs) building up those lines. But what is true for the inhomogeneous band is not necessarily
*Corresponding address. [email protected].
Fax:
+ 41-l-632-1021;
e-mail:
0022-2313/97/$17.00 #(:I 1997 Elsevier Science B.V. All rights reserved PII SOO22-23 13(96)00358-4
excitation; Diphenyloctatetraene
true for a SM. Therefore, the observation of the same single molecule in the OPE and TPE spectra is of a fundamental interest. SM spectroscopy, recently developed first for OPE [3] and then for TPE [4], revealed significant differences between the two spectra. The experimental setup has been described in Refs. [2,4]. Light generated by a Ti: Sapphire single-mode laser and its second harmonic were focused by a microscope objective (MO) onto a spot of z 2 urn diameter. The sample, all-trans1,8_diphenyloctatetraene (DPOT) in n-tetradecane (TD) (So -+ S, transition at 444 nm) and the MO were immersed in a liquid He bath at 1.8 K. The luminescence, collected by the same MO, passed through a small aperture and was detected by a photomultiplier as a function of the laser frequency detuning. Due to the aperture, only luminescence from a volume of about 7 urn3
23
T. Plakhotnik et al. 1 Journal of Luminescence 72-74 (1997) 22-24
contributed to the signal, independent of the excitation wavelength. First, hole-burning techniques were applied to determine whether the same molecules could be observed in OPE and TPE spectra. Using OPE, spectral holes were burned in the center of the inhomogeneous band. These holes could then be observed in OPE spectra.The reduction of the hole depth during TPE readout, an effect needing further investigation, combined with low signal/noise ratio made the two-photon hole detection difficult, and only very deep holes (of about 50%) could be observed under both OPE and TPE (Fig. 1, inset). Except for a small shift to lower frequency of the hole position in TPE spectra [S], the holes are similar. This experiment shows that at least some of the molecules were sensitive to OPE as well as to TPE and confirms the supposition of similarity of OPE and TPE spectra. However, single-molecule spectroscopy reveals a surprising distinction between them (Figs. 1 and 2(c), (d)). The most remarkable difference is the number of observed SM lines, which is approximately double in OPE compared to TPE spectra. The same ratio was also estimated in the center of inhomogeneous band from the amplitudes of the statistical fine structure, which are proportional to the square root of the number of molecules per homogeneous line width. Further, the average SM line width is broader in TPE spectra. An even more striking result appears in a OPE spectrum under simultaneous illumination with the fundamental laser wavelength (Fig. 2(b)). It is important to emphasize that the contribution from TPE itself to the spectrum was only 25%, and Fig. 2(b) is not a superposition of Fig. 2(a) and 2(c). In fact, the infrared beam changed only the conditions for the observation of the OPE spectrum and this caused a cleaning effect. At present, we do not have a fully satisfying explanation for all these effects. We believe that they may be linked to the different sensitivity of SMs to local heating in the sample due to the high-power laser light necessary for efficient TPE. One may assume that a part of the molecules, corresponding to the ones detectable under TPE, has a relatively big thermal line shift and small line broadening [IS]. Other DPOT molecules, which
-5
E
O”.““““““““I”“““” 3000
2000
1000 2000 1000 0 2x Laser detuning [MHz]
3000
Fig. 1. A hole-burning experiment (inset) shows that some molecules can be detected in both OPE and TPE spectra, but two scans recorded with SM resolution in the red wing of the inhomogeneous band manifest a much more complicated picture. More molecules can be seen in the OPE spectrum and no obvious correlation between the spectral positions of SM lines in the two spectra is observed. The laser frequency was scanned by a triangular voltage ramp yielding a mirror symmetry in the recorded excitation spectra.
have significant thermal broadening, are observable only in OPE spectra. Adding of infrared illumination at 888 nm produces a cleaning effect in the OPE spectra (Fig. 2(b)), in agreement with this explanation. The SMs suitable for OPE and TPE should obey different selection rules and hence, the strongest molecules in the OPE spectra are not necessarily the strongest ones under TPE. This was actually observed (Fig. 2, molecule 1). The gradual transformation from the spectrum in Fig. 2(c) to the spectrum in Fig. 2(b) while steadily increasing the laser power could be observed only qualitatively, because the presence of spectral jumps in the Shpol’skii matrix [6] affected SM spectra in this time-consuming experiment. Further measurements
24
T. Plakhotnik
et al. / Journal
of Luminescence
72-74 (1997) 22-24
direct temperature dependence study of the OPE spectra could clarify the problem. This work was supported by the Swiss National Science Foundation and by ETH Ziirich.
References [II For a polyenes spectroscopy
200
0 1000
2000
1000 0 1000 2000 2 x Laser detuning [MHz]
3000
Fig. 2. Excitation spectra of single molecules measured under different conditions. (a) TPE with 350 mW power at the fundamental laser wavelength (z 888.4 nm); (b) OPE with 500 nW at second harmonic wave length (A z 444.2 nm) when the sample was simultaneously illuminated with 350 mW at the fundamental laser wavelength; (c) OPE (500 nW); (d) TPE (350 mW), a reproducibility check of scan (a). Spectra (a)-(c) are shifted up for clarity. The spectra were measured in the order (a)(d). Comparison of (a) and (d) shows that there was no laser frequency drift (see SM lines 3 and 4). The two-photon excitation spectra (a) and (d) have the vertical scale expanded by a factor 4. The contribution from TPE itself to the spectrum (b) is less than 25%. The number of SMs manifested in the OPE spectrum(c) is approximately twice the number in the spectra (a), (b), and (d). The 0 line of the molecules 3 and 4 can be observed under both OPE (b) and TPE (a), (d). Taking into account the lightinduced shift, molecule 3 most likely originates from one of the two molecules labeled by asterisks in pure the OPE spectrum(c). Molecule 1, the strongest peak under OPE (b), did not manifest itself in the pure TPE spectra. The shoulder marked by 2 and the molecule 5 in the spectra (a) and (d) were not observed in the spectrum (b). The vertical lines give a view guide for convenience. Places where corresponding lines are missing are labeled by arrows.
review see: B.E. Kohler, Electronic Properties of linear polyenes, in: Conjugated Polymers, eds. J.L. Bredas and R. Silbey (Kluwer Academic Publishers, Netherlands, 1991) pp. 4055434. VI T. Plakhotnik, D. Walser, A. Renn and U.P. Wild, Chem. Phys. Lett. 262 (1996) 379. [31 W.E. Moerner and L. Kador, Phys. Rev. Lett. 62 (1989) 2535; M. Orrit and J. Bernard, Phys. Rev. Lett. 65 (1990) 2716. D. Walser, M. Pirotta, A. Renn and U.P. M T. Plakhotnik, Wild, Science 271 (1996) 1703. frequency shift of the [51 The infrared light power-dependent O-O line was observed in TPE spectra of DPOT single molecules. The shift of about 600 MHz/W for the laser spot of about 2 urn diameter was caused most likely by local thermal heating and phonon excitation of a low-frequency vibration, though the optical quadratic Stark effect and the electrostriction effect could give a relatively small contribution (estimated of about 20%). It is remarkable that the corresponding line broadening was much smaller than the shift. T. Plakhotnik, D. Walser, A. Renn and U.P. Wild, Phys. Rev. Lett. 77 (1996) 5365. T. Irngartinger, M. Croci, [61 W.E. Moerner, T. Plakhotnik, V. Palm and U.P. Wild, J. Phys. Chem. 98 (1994) 7382.
LUMINESCENCE EISEYIER
Journal
of
Luminescence 72-74 (1997)22-24
Single-molecules spectral lines ‘erasing’ under powerful infrared illumination Taras Plakhotnik”, Ph.vsical Chemistry Laboratoqj,
Skss
Daniel Walser, Alois Renn, Urs P. Wild Federal Institute of’ Technology, ETH-Zentrum, CH-809.2 Zurich, Switzerland
Abstract
One- and two-photon excitation (OPE and TPE) spectra of single 1,8_diphenyloctatetraene molecules in n-tetradecane (transition at 444 nm) were recorded at cryogenic temperature. The high-power infrared light used for TPE changed the conditions in the sample so that the number of single molecule lines observed in OPE spectra was approximately twice the number in TPE spectra or OPE spectra with added infrared illumination. Single molecules detectable under OPE and TPE, only under OPE, and only under TPE were observed. Keywords:
Single-molecule spectroscopy; Two-photon
excitation; One-photon
In the electric dipole approximation, the onephoton transition between the SOand Sr electronic states in polyenes is forbidden by parity, since both states have A, symmetry, and only the two-photon transition between these states is allowed [l]. In some Shpol’skii matrices, both transitions become allowed due to distortion of the molecular symmetry, and the inhomogeneously broadened O-O zero-phonon line can be observed under onephoton excitation (OPE) and two-photon excitation (TPE). The similarity of the inhomogeneous line shape in the OPE and TPE spectra [2] may imply similarity in the properties of the single molecules (SMs) building up those lines. But what is true for the inhomogeneous band is not necessarily
*Corresponding address. [email protected].
Fax:
+ 41-l-632-1021;
e-mail:
0022-2313/97/$17.00 #(:I 1997 Elsevier Science B.V. All rights reserved PII SOO22-23 13(96)00358-4
excitation; Diphenyloctatetraene
true for a SM. Therefore, the observation of the same single molecule in the OPE and TPE spectra is of a fundamental interest. SM spectroscopy, recently developed first for OPE [3] and then for TPE [4], revealed significant differences between the two spectra. The experimental setup has been described in Refs. [2,4]. Light generated by a Ti: Sapphire single-mode laser and its second harmonic were focused by a microscope objective (MO) onto a spot of z 2 urn diameter. The sample, all-trans1,8_diphenyloctatetraene (DPOT) in n-tetradecane (TD) (So -+ S, transition at 444 nm) and the MO were immersed in a liquid He bath at 1.8 K. The luminescence, collected by the same MO, passed through a small aperture and was detected by a photomultiplier as a function of the laser frequency detuning. Due to the aperture, only luminescence from a volume of about 7 urn3
23
T. Plakhotnik et al. 1 Journal of Luminescence 72-74 (1997) 22-24
contributed to the signal, independent of the excitation wavelength. First, hole-burning techniques were applied to determine whether the same molecules could be observed in OPE and TPE spectra. Using OPE, spectral holes were burned in the center of the inhomogeneous band. These holes could then be observed in OPE spectra.The reduction of the hole depth during TPE readout, an effect needing further investigation, combined with low signal/noise ratio made the two-photon hole detection difficult, and only very deep holes (of about 50%) could be observed under both OPE and TPE (Fig. 1, inset). Except for a small shift to lower frequency of the hole position in TPE spectra [S], the holes are similar. This experiment shows that at least some of the molecules were sensitive to OPE as well as to TPE and confirms the supposition of similarity of OPE and TPE spectra. However, single-molecule spectroscopy reveals a surprising distinction between them (Figs. 1 and 2(c), (d)). The most remarkable difference is the number of observed SM lines, which is approximately double in OPE compared to TPE spectra. The same ratio was also estimated in the center of inhomogeneous band from the amplitudes of the statistical fine structure, which are proportional to the square root of the number of molecules per homogeneous line width. Further, the average SM line width is broader in TPE spectra. An even more striking result appears in a OPE spectrum under simultaneous illumination with the fundamental laser wavelength (Fig. 2(b)). It is important to emphasize that the contribution from TPE itself to the spectrum was only 25%, and Fig. 2(b) is not a superposition of Fig. 2(a) and 2(c). In fact, the infrared beam changed only the conditions for the observation of the OPE spectrum and this caused a cleaning effect. At present, we do not have a fully satisfying explanation for all these effects. We believe that they may be linked to the different sensitivity of SMs to local heating in the sample due to the high-power laser light necessary for efficient TPE. One may assume that a part of the molecules, corresponding to the ones detectable under TPE, has a relatively big thermal line shift and small line broadening [IS]. Other DPOT molecules, which
-5
E
O”.““““““““I”“““” 3000
2000
1000 2000 1000 0 2x Laser detuning [MHz]
3000
Fig. 1. A hole-burning experiment (inset) shows that some molecules can be detected in both OPE and TPE spectra, but two scans recorded with SM resolution in the red wing of the inhomogeneous band manifest a much more complicated picture. More molecules can be seen in the OPE spectrum and no obvious correlation between the spectral positions of SM lines in the two spectra is observed. The laser frequency was scanned by a triangular voltage ramp yielding a mirror symmetry in the recorded excitation spectra.
have significant thermal broadening, are observable only in OPE spectra. Adding of infrared illumination at 888 nm produces a cleaning effect in the OPE spectra (Fig. 2(b)), in agreement with this explanation. The SMs suitable for OPE and TPE should obey different selection rules and hence, the strongest molecules in the OPE spectra are not necessarily the strongest ones under TPE. This was actually observed (Fig. 2, molecule 1). The gradual transformation from the spectrum in Fig. 2(c) to the spectrum in Fig. 2(b) while steadily increasing the laser power could be observed only qualitatively, because the presence of spectral jumps in the Shpol’skii matrix [6] affected SM spectra in this time-consuming experiment. Further measurements
24
T. Plakhotnik
et al. / Journal
of Luminescence
72-74 (1997) 22-24
direct temperature dependence study of the OPE spectra could clarify the problem. This work was supported by the Swiss National Science Foundation and by ETH Ziirich.
References [II For a polyenes spectroscopy
200
0 1000
2000
1000 0 1000 2000 2 x Laser detuning [MHz]
3000
Fig. 2. Excitation spectra of single molecules measured under different conditions. (a) TPE with 350 mW power at the fundamental laser wavelength (z 888.4 nm); (b) OPE with 500 nW at second harmonic wave length (A z 444.2 nm) when the sample was simultaneously illuminated with 350 mW at the fundamental laser wavelength; (c) OPE (500 nW); (d) TPE (350 mW), a reproducibility check of scan (a). Spectra (a)-(c) are shifted up for clarity. The spectra were measured in the order (a)(d). Comparison of (a) and (d) shows that there was no laser frequency drift (see SM lines 3 and 4). The two-photon excitation spectra (a) and (d) have the vertical scale expanded by a factor 4. The contribution from TPE itself to the spectrum (b) is less than 25%. The number of SMs manifested in the OPE spectrum(c) is approximately twice the number in the spectra (a), (b), and (d). The 0 line of the molecules 3 and 4 can be observed under both OPE (b) and TPE (a), (d). Taking into account the lightinduced shift, molecule 3 most likely originates from one of the two molecules labeled by asterisks in pure the OPE spectrum(c). Molecule 1, the strongest peak under OPE (b), did not manifest itself in the pure TPE spectra. The shoulder marked by 2 and the molecule 5 in the spectra (a) and (d) were not observed in the spectrum (b). The vertical lines give a view guide for convenience. Places where corresponding lines are missing are labeled by arrows.
review see: B.E. Kohler, Electronic Properties of linear polyenes, in: Conjugated Polymers, eds. J.L. Bredas and R. Silbey (Kluwer Academic Publishers, Netherlands, 1991) pp. 4055434. VI T. Plakhotnik, D. Walser, A. Renn and U.P. Wild, Chem. Phys. Lett. 262 (1996) 379. [31 W.E. Moerner and L. Kador, Phys. Rev. Lett. 62 (1989) 2535; M. Orrit and J. Bernard, Phys. Rev. Lett. 65 (1990) 2716. D. Walser, M. Pirotta, A. Renn and U.P. M T. Plakhotnik, Wild, Science 271 (1996) 1703. frequency shift of the [51 The infrared light power-dependent O-O line was observed in TPE spectra of DPOT single molecules. The shift of about 600 MHz/W for the laser spot of about 2 urn diameter was caused most likely by local thermal heating and phonon excitation of a low-frequency vibration, though the optical quadratic Stark effect and the electrostriction effect could give a relatively small contribution (estimated of about 20%). It is remarkable that the corresponding line broadening was much smaller than the shift. T. Plakhotnik, D. Walser, A. Renn and U.P. Wild, Phys. Rev. Lett. 77 (1996) 5365. T. Irngartinger, M. Croci, [61 W.E. Moerner, T. Plakhotnik, V. Palm and U.P. Wild, J. Phys. Chem. 98 (1994) 7382.