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Single molecule spectroscopy of terrylene in n-alkane mixtures
13 February 1998
Chemical Physics Letters 283 Ž1998. 345–349
Single molecule spectroscopy of terrylene in n-alkane mixtures Nico Caspary, Viktor Palm 1, Karl K. Rebane 1, Vladimir E. Bondybey
2
Institut fur Lichtenbergstraße 4, D-85748 Garching, Germany ¨ Physikalische und Theoretische Chemie, Technische UniÕersitat ¨ Munchen, ¨ Received 14 October 1997; in final form 4 December 1997
Abstract To answer the question of whether special Shpol’skii guest–host combinations which exhibit well-defined trapping sites are essential for single molecule spectroscopy, we have reinvestigated at high-resolution the fluorescence of single molecules of terrylene. Our experiments have demonstrated that similar to the well-known terrylene–n-decane ‘‘Shpol’skii’’ system, spectra of comparable quality and with lifetime-limited linewidths of about 50 MHz can also be obtained in several mixtures of alkanes. In the light of these results we then discuss the prospects for single molecule Žor single ion. spectroscopy in rare gas matrices. q 1998 Elsevier Science B.V.
1. Introduction The field of single molecule spectroscopy ŽSMS. and the related area of photochemical hole-burning have received increasing attention in the last decade w1–3x. These SMS studies have been motivated, not only by the pure theoretical interest of being able to see single molecules, but also by potential applications for information storage. The hole-burning experiment allows, in principle, the use of wavelength multiplexing to enhance the storage capacity of the medium, with information being recorded and read independently at a number of different wavelengths from the inhomogeneously broadened spectral profile. In the limit, each molecule in the storage medium can provide one bit of information. Fluctuations on
1
Institute of Physics, Estonian Academy of Sciences, 142 Riia Str., EE2400 Tartu, Estonia. 2 Corresponding author.
the single molecule level, however, make the practical realization of such ‘‘single molecule devices’’ extremely difficult. Most of the few systems studied to date involved large aromatic polycyclic molecules like pentacene, perylene and terrylene as guests. Usually either single crystals of aromatic hydrocarbons Žp-terphenyl., polymers Žpolyethylene. or polycrystalline alkane matrices in so-called Shpol’skii systems are employed as the host matrix material. The latter systems involve special guest–host matrix combinations, which provide well-defined trapping sites for the guest molecule, and yield narrow inhomogeneous linewidths. A combination of the terrylene guest with the hexadecane host is such an extensively investigated system w4–6x. Well-defined sites and narrow inhomogeneous intensity distributions were important for studies by conventional absorption spectroscopy, where in non-Shpol’skii systems the spectra were broad and featureless due to spectral congestion and provided little spectral information.
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 3 9 9 - 7
346
N. Caspary et al.r Chemical Physics Letters 283 (1998) 345–349
Actually, a narrow inhomogeneous distribution is not really necessary for the purpose of single molecule spectroscopy and from the point of view of information storage it is an obstacle and disadvantage, rather than a desired feature. It leaves little room within the linewidth for wavelength multiplexing, and makes it more difficult to resolve and separate the spectral lines of individual molecules, which are all crowded into a narrow spectral interval. In our laboratory, we are in the process of designing and constructing an apparatus for studies of single molecules and molecular ions in low-temperature rare-gas matrices. In the initial experiments we have examined a previously known Shpol’skii system and decided to address the question as to what happens if one ‘‘spoils’’ the Shpol’skii matrix by using a mixture of solvents, thus making sure that the molecules are not in identical, homogeneous environments. We have therefore examined spectra not only of terrylene in n-decane, but also in 1:1 mixtures of n-decane with other alkanes. Perhaps somewhat surprisingly, in such mixed systems single molecules can be detected with equal ease. In the present Letter we describe and discuss our observations.
isolation table. The jitter was not a serious problem for the present experiments as the effective linewidth was still almost an order of magnitude narrower than the expected width of the investigated spectral lines. A dilute solution of terrylene in n-decane Žconcentration about 10y7 molrl. was prepared by Dr. I. Renge at ETH, Zurich, with chemical components ¨ kindly provided by Prof. U.P. Wild. The samples in mixed solvents were prepared by mixing this solution with another solvent in the desired volume ratio. A drop of the sample was then placed between two quartz plates to form a film with a thicknesses of the order of 1–5 mm. Prior to measuring the sample was rapidly quenched in liquid helium. The sample holder is similar to that described in Ref. w7x. Basically it consists of a 12.7 mm quartz lens to focus the laser beam onto the sample, a parabolic mirror to collect the fluorescence and a beamstop to block the laser light transmitted by the sample. The collected light was focused using two lenses onto a cooled photomultiplier ŽBurle C31034A-02. operating in the photon counting mode. A long-pass glass filter was used to prevent the scattered radiation at the laser frequency from reaching the detector. Electrical pulses generated by detected single photons were amplified, counted and registered in the usual manner as a function of the wavelength of the exciting laser light.
2. Experimental 3. Results The experiments were carried out in an optical liquid-helium cryostat, pumped to maintain the liquid at around 1.8 K, well below the 2 K superfluid transition temperature. The sample, immersed in the helium, was excited by a single-frequency Coherent CR-899 ring dye laser with Autoscan accessory, pumped by a Coherent Innova 200 argon ion laser. The output of the Rhodamine 6G dye laser was stabilized and controlled by an electrooptical modulator and the level of sample illumination was of the order of 1 Wrcm2 . The effective spectral linewidth of the dye laser, which nominally should have been about 1 MHz, was at least a factor of five larger due to frequency jitter. While we could not uniquely identify the source of the problem, we suspect that it is due to vibrations. The experiments were carried out on simple wooden laboratory benches, since we have been unable to obtain financing for a vibration
3.1. Terrylene in n-decane Broadband excitation spectra of terrylene in several alkanes including n-decane have been studied by Palewska et al. w8x and single molecule spectra of terrylene in n-decane by Ollikainen et al. w9x. Our measurements are in good agreement with their results. A total broadband fluorescence excitation spectrum of terrylene in n-decane is shown in the inset of Fig. 1. It exhibits two strong bands at 17612 and 17363 cmy1 , due to terrylene isolated in two different sites, denoted as ‘‘A’’ and ‘‘B’’ in Ref. w8x. Narrow band scans in the spectral regions of these bands reproducibly exhibit series of sharp lines, which could be identified as the fluorescence excitation spectra of individual terrylene molecules. We investigated, in some detail, the spectra taken
N. Caspary et al.r Chemical Physics Letters 283 (1998) 345–349
347
Fig. 1. Two consecutive high resolution excitation scans, exhibiting absorption lines of single terrylene molecules isolated in a mixed matrix of n-decanern-dodecane 1r1. For easier comparison the upper trace was offset by 500 ctsrs. The broadband excitation spectra in the mixed matrix Žsolid line. and in pure n-decane Ždashed line. are shown in the inset. The arrow marks the spectral position of the high resolution scans. The unit of the x-axes of these spectra is cmy1 Ž1 cmy1 f 30 GHz.. Three different sites of terrylene in n-decane are marked according to Ref. w8x with A, B, and C.
in the neighborhood of site ‘‘B’’ and then examined a number of the observed peaks at a higher resolution. We found that the lines of individual molecules can be reproducibly observed. In Refs. w9x and w6x a rather broad range of linewidths was reported for terrylene in Shpol’skii matrices. The linewidths of most of the molecules investigated in our experiments were around 50 MHz. The somewhat broader appearance of some of the lines is probably due to the overlap of two or more molecules emitting at almost the same excitation frequency or to high rate jumping of a single molecule line between different spectral positions with spectral separation less than the linewidth Žspectral diffusion.. This interpretation was also confirmed by the observation that while the narrower ‘‘single molecule’’ bands are symmetric and can be easily fitted by a Lorentzian lineshape, the broader lines are invariably asymmetric and exhibit more complex lineshapes. The somewhat larger average linewidths measured in our work compared to Ref. w9x are probably due to our problems with the laser linewidth. Variation of the excitation intensity over more than one order of magnitude had no observable effect on the linewidth and no saturation effects were detected.
3.2. Terrylene in mixtures of n-decane with other n-alkanes To investigate the question of the necessity of a special choice of a guest–host pair for single molecule spectroscopic studies, we have attempted to ‘‘spoil’’ the Shpol’skii character of the n-decane solvent by using a mixture with n-dodecane. It was previously reported in Ref. w8x that the ‘‘B’’ site, which in n-decane occurs at 17363 cmy1 , is in all other n-alkanes Ž n s 8, 12, 14 and 16. shifted some 140 cmy1 to higher energies and occurs near 17500 cmy1 . The authors of Ref. w8x have also noted that the terrylene spectrum in n-decane exhibits narrower inhomogeneous linewidths than in all the other n-alkanes, and suggested that this may be due to the fact that terrylene and n-decane have an almost equal ‘‘length’’, see Shpol’skii w3x. Perhaps the same factor explains the unique spectral position of the 0–0 band in this solvent. The broadband excitation spectrum observed in the n-decanern-dodecane mixture is shown as an inset in Fig. 1. For comparison the trace of the spectrum of terrylene in pure n-decane is depicted as a dashed line. The absorption broadens considerably
348
N. Caspary et al.r Chemical Physics Letters 283 (1998) 345–349
and shifts to 17498 cmy1 , close to the position in pure n-dodecane. There is, however, no signal at the position expected for pure n-decane. This suggests that the n-alkane mixture forms upon rapid freezing an amorphous, glassy deposit and no phase segregation of the individual components occurs w10x. We made high resolution scans in several spectral regions searching for single molecule lines. Fig. 1 represents two such consecutive scans extending over about 5 GHz near the red edge of the above absorption band. The fluorescence of about a dozen of individual molecules can be seen reproducibly in the spectrum. All the lines exhibit similar lineshapes, with widths close to 50 MHz, the nearly lifetime limited value, observed also in n-decane. The variation in intensities of the individual molecule signals is partially due to the different excitation laser flux at different positions within the spot illuminated by the laser and partially an orientational effect reflecting varying angles of the molecule’s dipol transition moment with respect to the electric vector of the polarized exciting light. The observation of single molecule spectra in a glass-like matrix would be an interesting fact, because previous attempts to obtain such spectra for terrylene in the glassy phase of benzophenone were not successful w12x. We have also examined briefly another mixed matrix, containing approximately equal amounts of n-decane and n-nonane. This combination appeared to be interesting since terrylene was not previously studied in odd numbered n-alkanes and these are reported to crystallize in structures differing from the even numbered species w11x. However, even in this mixture single terrylene molecule signals could easily be detected. As in the other solvents, the individual linewidths were again near to 50 MHz. Since only a minute amount of terrylene solution in n-decane was available to us, we unfortunately could not examine its fluorescence in pure nonane.
neon – by a variety of methods. The overall neutrality of the matrix requires the presence of an equal number of counterions, but in general their identity and location in the matrix remain unknown. Our hope is to elucidate the ion–counterion interactions with the help of high-resolution single-ion fluorescence studies. It is therefore of interest to consider briefly the prospects for single-ion fluorescence studies in matrices in the light of our present results. More than twenty years ago we demonstrated that individual spectral ‘‘sites’’ in the matrix isolated ions can be selectively ‘‘burned out’’ of the inhomogeneously broadened line profile w13x. The present work suggests that a well-developed crystalline structure and a good ‘‘fit’’ of the guest molecule into the available host substitutional ‘‘sites’’ is not essential for single molecule studies. When considering the spectral properties of molecular ions such as fluorobenzene- w14x or polyacetylene- w15x radical cations in a solid neon matrix, one finds that in many respects they appear to be ideal systems for such work. They possess strong, fully allowed electronic transitions in the easily accessible visible region. They fluoresce in matrices with near unity quantum yields, with radiative lifetimes of the order of 50 ns. The transition is only weakly coupled to the lattice, and one observes an extremely well resolved vibrational structure characterized by sharp zero phonon lines, with little indication of phonon sidebands. A considerable advantage is also the absence of longlived lower lying states, with the quartet states being higher in energy than the fluorescing, lowest excited doublet state. The inhomogeneous linewidths of these species are typically relatively narrow Ž1–5 cmy1 ., so that even though the 0–0 transitions are rather strong, an extensive vibrational structure can usually be observed and resolved both in the absorption spectrum and in emission. This should easily permit to filter out the exciting laser radiation and detect the red-shifted fluorescence emission terminating on vibrationally excited ground-state levels.
4. Outlook for single ion spectroscopy in rare-gas matrices 5. Summary As noted in the introduction, the interest of our group is directed towards studies of single molecular ion fluorescence in rare gas matrices. Molecular ions can be generated in rare gas matrices – preferably
We have examined single molecule fluorescence of terrylene in pure n-decane, as well as in several n-paraffine mixtures. The single molecule signals in
N. Caspary et al.r Chemical Physics Letters 283 (1998) 345–349
both the pure solvent and in the mixtures are easily detected and exhibit similar, lifetime-limited linewidths of around 50 MHz. In the light of our results and experience obtained in the organic solvents, we discuss the prospects for studies of single molecule fluorescence spectroscopy of molecules and ions isolated in solid neon.
Acknowledgements We thank the Deutsche Forschungsgemeinschaft ŽSFB 377. and the Fonds der Chemischen Industrie for research funding. We also gratefully acknowledge support by NATO ŽCRG 960009 and 940681 HTECH LG.. Two of us ŽVP and KR. are grateful for research support from the Estonian Science Foundation Žgrant 2268., the Alexander von Humboldt Foundation and the ICSC-World Laboratory.
References w1x M. Orrit, J. Bernard, R.I. Personov, J. Phys. Chem. 97 Ž1993. 10256. w2x J.L. Skinner, W.E. Moerner, J. Phys. Chem. 100 Ž1996. 13251.
349
w3x E.V. Shpol’skii, Usp. Fiz. Nauk 80 Ž1963. 255; W.E. Moerner ŽEd.., Persistent Spectral Hole Burning: Science and Applications, Springer, Berlin, 1988; O. Sild, K. Haller ŽEds.., Zero-Phonon Lines and Spectral Hole Burning in Spectroscopy and Photochemistry, Springer, Berlin, 1988; K.K. Rebane, Chem. Phys. 189 Ž1994. 139; Th. Basche, ´ W.E. Moerner, M. Orrit, U.P. Wild ŽEds.., Single-Molecule Optical Detection, Imaging and Spectroscopy, VCH, Weinheim, 1996. w4x W.E. Moerner, T. Plakhotnik, Th. Irmgartinger, M. Croci, V. Palm, U.P. Wild, J. Phys. Chem. 98 Ž1994. 7382. w5x B. Kozankiewicz, J. Bernard, M. Orrit, J. Chem. Phys. 101 Ž1994. 9377. w6x M. Vacha, Yi Liu, H. Nagatsuka, T. Tani, J. Chem. Phys. 106 Ž1997. 8324. w7x W.P. Ambrose, Th. Basche, ´ W.E. Moerner, J. Chem. Phys. 95 Ž1991. 7150. w8x K. Palewska, J. Lipinski, J. Sworakowski, J. Sepiol, H. Gygax, E. Meister, U.P. Wild, J. Phys. Chem. 99 Ž1995. 16835. w9x O. Ollikainen, V. Palm, K. Rebane, Proc. of Estonian Acad. Sci., Phys. Math., to be published. w10x G.-I. Asbach, H.G. Kilian, Ber. Buns. Ges. 74 Ž1970. 814. w11x Yu.V. Mnyukh, J. Phys. Chem. Solids 24 Ž1962. 631. w12x M. Croci, V. Palm, U.P. Wild, Mol. Cryst. Liq. Cryst. 283 Ž1996. 137. w13x L.E. Brus, V.E. Bondybey, J. Chem. Phys. 63 Ž1975. 3123. w14x T.A. Miller, V.E. Bondybey ŽEds.., Molecular Ions: Spectroscopy, Structure and Chemistry, North-Holland, Amsterdam, 1983. w15x V.E. Bondybey, A.M. Smith, J. Agreiter, Chem. Rev. 96 Ž1996. 2113.
Chemical Physics Letters 283 Ž1998. 345–349
Single molecule spectroscopy of terrylene in n-alkane mixtures Nico Caspary, Viktor Palm 1, Karl K. Rebane 1, Vladimir E. Bondybey
2
Institut fur Lichtenbergstraße 4, D-85748 Garching, Germany ¨ Physikalische und Theoretische Chemie, Technische UniÕersitat ¨ Munchen, ¨ Received 14 October 1997; in final form 4 December 1997
Abstract To answer the question of whether special Shpol’skii guest–host combinations which exhibit well-defined trapping sites are essential for single molecule spectroscopy, we have reinvestigated at high-resolution the fluorescence of single molecules of terrylene. Our experiments have demonstrated that similar to the well-known terrylene–n-decane ‘‘Shpol’skii’’ system, spectra of comparable quality and with lifetime-limited linewidths of about 50 MHz can also be obtained in several mixtures of alkanes. In the light of these results we then discuss the prospects for single molecule Žor single ion. spectroscopy in rare gas matrices. q 1998 Elsevier Science B.V.
1. Introduction The field of single molecule spectroscopy ŽSMS. and the related area of photochemical hole-burning have received increasing attention in the last decade w1–3x. These SMS studies have been motivated, not only by the pure theoretical interest of being able to see single molecules, but also by potential applications for information storage. The hole-burning experiment allows, in principle, the use of wavelength multiplexing to enhance the storage capacity of the medium, with information being recorded and read independently at a number of different wavelengths from the inhomogeneously broadened spectral profile. In the limit, each molecule in the storage medium can provide one bit of information. Fluctuations on
1
Institute of Physics, Estonian Academy of Sciences, 142 Riia Str., EE2400 Tartu, Estonia. 2 Corresponding author.
the single molecule level, however, make the practical realization of such ‘‘single molecule devices’’ extremely difficult. Most of the few systems studied to date involved large aromatic polycyclic molecules like pentacene, perylene and terrylene as guests. Usually either single crystals of aromatic hydrocarbons Žp-terphenyl., polymers Žpolyethylene. or polycrystalline alkane matrices in so-called Shpol’skii systems are employed as the host matrix material. The latter systems involve special guest–host matrix combinations, which provide well-defined trapping sites for the guest molecule, and yield narrow inhomogeneous linewidths. A combination of the terrylene guest with the hexadecane host is such an extensively investigated system w4–6x. Well-defined sites and narrow inhomogeneous intensity distributions were important for studies by conventional absorption spectroscopy, where in non-Shpol’skii systems the spectra were broad and featureless due to spectral congestion and provided little spectral information.
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 3 9 9 - 7
346
N. Caspary et al.r Chemical Physics Letters 283 (1998) 345–349
Actually, a narrow inhomogeneous distribution is not really necessary for the purpose of single molecule spectroscopy and from the point of view of information storage it is an obstacle and disadvantage, rather than a desired feature. It leaves little room within the linewidth for wavelength multiplexing, and makes it more difficult to resolve and separate the spectral lines of individual molecules, which are all crowded into a narrow spectral interval. In our laboratory, we are in the process of designing and constructing an apparatus for studies of single molecules and molecular ions in low-temperature rare-gas matrices. In the initial experiments we have examined a previously known Shpol’skii system and decided to address the question as to what happens if one ‘‘spoils’’ the Shpol’skii matrix by using a mixture of solvents, thus making sure that the molecules are not in identical, homogeneous environments. We have therefore examined spectra not only of terrylene in n-decane, but also in 1:1 mixtures of n-decane with other alkanes. Perhaps somewhat surprisingly, in such mixed systems single molecules can be detected with equal ease. In the present Letter we describe and discuss our observations.
isolation table. The jitter was not a serious problem for the present experiments as the effective linewidth was still almost an order of magnitude narrower than the expected width of the investigated spectral lines. A dilute solution of terrylene in n-decane Žconcentration about 10y7 molrl. was prepared by Dr. I. Renge at ETH, Zurich, with chemical components ¨ kindly provided by Prof. U.P. Wild. The samples in mixed solvents were prepared by mixing this solution with another solvent in the desired volume ratio. A drop of the sample was then placed between two quartz plates to form a film with a thicknesses of the order of 1–5 mm. Prior to measuring the sample was rapidly quenched in liquid helium. The sample holder is similar to that described in Ref. w7x. Basically it consists of a 12.7 mm quartz lens to focus the laser beam onto the sample, a parabolic mirror to collect the fluorescence and a beamstop to block the laser light transmitted by the sample. The collected light was focused using two lenses onto a cooled photomultiplier ŽBurle C31034A-02. operating in the photon counting mode. A long-pass glass filter was used to prevent the scattered radiation at the laser frequency from reaching the detector. Electrical pulses generated by detected single photons were amplified, counted and registered in the usual manner as a function of the wavelength of the exciting laser light.
2. Experimental 3. Results The experiments were carried out in an optical liquid-helium cryostat, pumped to maintain the liquid at around 1.8 K, well below the 2 K superfluid transition temperature. The sample, immersed in the helium, was excited by a single-frequency Coherent CR-899 ring dye laser with Autoscan accessory, pumped by a Coherent Innova 200 argon ion laser. The output of the Rhodamine 6G dye laser was stabilized and controlled by an electrooptical modulator and the level of sample illumination was of the order of 1 Wrcm2 . The effective spectral linewidth of the dye laser, which nominally should have been about 1 MHz, was at least a factor of five larger due to frequency jitter. While we could not uniquely identify the source of the problem, we suspect that it is due to vibrations. The experiments were carried out on simple wooden laboratory benches, since we have been unable to obtain financing for a vibration
3.1. Terrylene in n-decane Broadband excitation spectra of terrylene in several alkanes including n-decane have been studied by Palewska et al. w8x and single molecule spectra of terrylene in n-decane by Ollikainen et al. w9x. Our measurements are in good agreement with their results. A total broadband fluorescence excitation spectrum of terrylene in n-decane is shown in the inset of Fig. 1. It exhibits two strong bands at 17612 and 17363 cmy1 , due to terrylene isolated in two different sites, denoted as ‘‘A’’ and ‘‘B’’ in Ref. w8x. Narrow band scans in the spectral regions of these bands reproducibly exhibit series of sharp lines, which could be identified as the fluorescence excitation spectra of individual terrylene molecules. We investigated, in some detail, the spectra taken
N. Caspary et al.r Chemical Physics Letters 283 (1998) 345–349
347
Fig. 1. Two consecutive high resolution excitation scans, exhibiting absorption lines of single terrylene molecules isolated in a mixed matrix of n-decanern-dodecane 1r1. For easier comparison the upper trace was offset by 500 ctsrs. The broadband excitation spectra in the mixed matrix Žsolid line. and in pure n-decane Ždashed line. are shown in the inset. The arrow marks the spectral position of the high resolution scans. The unit of the x-axes of these spectra is cmy1 Ž1 cmy1 f 30 GHz.. Three different sites of terrylene in n-decane are marked according to Ref. w8x with A, B, and C.
in the neighborhood of site ‘‘B’’ and then examined a number of the observed peaks at a higher resolution. We found that the lines of individual molecules can be reproducibly observed. In Refs. w9x and w6x a rather broad range of linewidths was reported for terrylene in Shpol’skii matrices. The linewidths of most of the molecules investigated in our experiments were around 50 MHz. The somewhat broader appearance of some of the lines is probably due to the overlap of two or more molecules emitting at almost the same excitation frequency or to high rate jumping of a single molecule line between different spectral positions with spectral separation less than the linewidth Žspectral diffusion.. This interpretation was also confirmed by the observation that while the narrower ‘‘single molecule’’ bands are symmetric and can be easily fitted by a Lorentzian lineshape, the broader lines are invariably asymmetric and exhibit more complex lineshapes. The somewhat larger average linewidths measured in our work compared to Ref. w9x are probably due to our problems with the laser linewidth. Variation of the excitation intensity over more than one order of magnitude had no observable effect on the linewidth and no saturation effects were detected.
3.2. Terrylene in mixtures of n-decane with other n-alkanes To investigate the question of the necessity of a special choice of a guest–host pair for single molecule spectroscopic studies, we have attempted to ‘‘spoil’’ the Shpol’skii character of the n-decane solvent by using a mixture with n-dodecane. It was previously reported in Ref. w8x that the ‘‘B’’ site, which in n-decane occurs at 17363 cmy1 , is in all other n-alkanes Ž n s 8, 12, 14 and 16. shifted some 140 cmy1 to higher energies and occurs near 17500 cmy1 . The authors of Ref. w8x have also noted that the terrylene spectrum in n-decane exhibits narrower inhomogeneous linewidths than in all the other n-alkanes, and suggested that this may be due to the fact that terrylene and n-decane have an almost equal ‘‘length’’, see Shpol’skii w3x. Perhaps the same factor explains the unique spectral position of the 0–0 band in this solvent. The broadband excitation spectrum observed in the n-decanern-dodecane mixture is shown as an inset in Fig. 1. For comparison the trace of the spectrum of terrylene in pure n-decane is depicted as a dashed line. The absorption broadens considerably
348
N. Caspary et al.r Chemical Physics Letters 283 (1998) 345–349
and shifts to 17498 cmy1 , close to the position in pure n-dodecane. There is, however, no signal at the position expected for pure n-decane. This suggests that the n-alkane mixture forms upon rapid freezing an amorphous, glassy deposit and no phase segregation of the individual components occurs w10x. We made high resolution scans in several spectral regions searching for single molecule lines. Fig. 1 represents two such consecutive scans extending over about 5 GHz near the red edge of the above absorption band. The fluorescence of about a dozen of individual molecules can be seen reproducibly in the spectrum. All the lines exhibit similar lineshapes, with widths close to 50 MHz, the nearly lifetime limited value, observed also in n-decane. The variation in intensities of the individual molecule signals is partially due to the different excitation laser flux at different positions within the spot illuminated by the laser and partially an orientational effect reflecting varying angles of the molecule’s dipol transition moment with respect to the electric vector of the polarized exciting light. The observation of single molecule spectra in a glass-like matrix would be an interesting fact, because previous attempts to obtain such spectra for terrylene in the glassy phase of benzophenone were not successful w12x. We have also examined briefly another mixed matrix, containing approximately equal amounts of n-decane and n-nonane. This combination appeared to be interesting since terrylene was not previously studied in odd numbered n-alkanes and these are reported to crystallize in structures differing from the even numbered species w11x. However, even in this mixture single terrylene molecule signals could easily be detected. As in the other solvents, the individual linewidths were again near to 50 MHz. Since only a minute amount of terrylene solution in n-decane was available to us, we unfortunately could not examine its fluorescence in pure nonane.
neon – by a variety of methods. The overall neutrality of the matrix requires the presence of an equal number of counterions, but in general their identity and location in the matrix remain unknown. Our hope is to elucidate the ion–counterion interactions with the help of high-resolution single-ion fluorescence studies. It is therefore of interest to consider briefly the prospects for single-ion fluorescence studies in matrices in the light of our present results. More than twenty years ago we demonstrated that individual spectral ‘‘sites’’ in the matrix isolated ions can be selectively ‘‘burned out’’ of the inhomogeneously broadened line profile w13x. The present work suggests that a well-developed crystalline structure and a good ‘‘fit’’ of the guest molecule into the available host substitutional ‘‘sites’’ is not essential for single molecule studies. When considering the spectral properties of molecular ions such as fluorobenzene- w14x or polyacetylene- w15x radical cations in a solid neon matrix, one finds that in many respects they appear to be ideal systems for such work. They possess strong, fully allowed electronic transitions in the easily accessible visible region. They fluoresce in matrices with near unity quantum yields, with radiative lifetimes of the order of 50 ns. The transition is only weakly coupled to the lattice, and one observes an extremely well resolved vibrational structure characterized by sharp zero phonon lines, with little indication of phonon sidebands. A considerable advantage is also the absence of longlived lower lying states, with the quartet states being higher in energy than the fluorescing, lowest excited doublet state. The inhomogeneous linewidths of these species are typically relatively narrow Ž1–5 cmy1 ., so that even though the 0–0 transitions are rather strong, an extensive vibrational structure can usually be observed and resolved both in the absorption spectrum and in emission. This should easily permit to filter out the exciting laser radiation and detect the red-shifted fluorescence emission terminating on vibrationally excited ground-state levels.
4. Outlook for single ion spectroscopy in rare-gas matrices 5. Summary As noted in the introduction, the interest of our group is directed towards studies of single molecular ion fluorescence in rare gas matrices. Molecular ions can be generated in rare gas matrices – preferably
We have examined single molecule fluorescence of terrylene in pure n-decane, as well as in several n-paraffine mixtures. The single molecule signals in
N. Caspary et al.r Chemical Physics Letters 283 (1998) 345–349
both the pure solvent and in the mixtures are easily detected and exhibit similar, lifetime-limited linewidths of around 50 MHz. In the light of our results and experience obtained in the organic solvents, we discuss the prospects for studies of single molecule fluorescence spectroscopy of molecules and ions isolated in solid neon.
Acknowledgements We thank the Deutsche Forschungsgemeinschaft ŽSFB 377. and the Fonds der Chemischen Industrie for research funding. We also gratefully acknowledge support by NATO ŽCRG 960009 and 940681 HTECH LG.. Two of us ŽVP and KR. are grateful for research support from the Estonian Science Foundation Žgrant 2268., the Alexander von Humboldt Foundation and the ICSC-World Laboratory.
References w1x M. Orrit, J. Bernard, R.I. Personov, J. Phys. Chem. 97 Ž1993. 10256. w2x J.L. Skinner, W.E. Moerner, J. Phys. Chem. 100 Ž1996. 13251.
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